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  • 1.
    Andersson, Daniel
    et al.
    Radarbolaget.
    Björsell, Niclas
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för elektronik, matematik och naturvetenskap, Elektronik.
    Ottoson, Patrik
    Radarbolaget.
    Rönnow, Daniel
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för elektronik, matematik och naturvetenskap, Elektronik.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Radar Images of Leaks in Building Elements2015Inngår i: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 78, s. 1726-1731Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Through leakage in the building envelope there is a penetration of air, water vapor and particles. The degree of leakage of air can be quantified by existing methods. However, the location of adventitious openings is often not known. In order to overcome the limitations in existing methods, a non-contact and non-destructive method based on ultra-wide bandwidth radar technology is suggested. A test-bed is designed that can measure with different polarization to be able to detect flaws in different directions. Initial measurements shows promising results for further development of the method of using radar images to find leaks in building elements.

  • 2.
    Antoniou, Nestoras
    et al.
    Department of Civil and Environmental Engineering, University of Cyprus, Nicosia, Cyprus; Department of the Built Environment, Eindhoven University of Technology, Eindhoven, The Netherlands.
    Montazeri, Hamid
    Department of the Built Environment, Eindhoven University of Technology, Eindhoven, The Netherlands; Department of Civil Engineering, KU Leuven, Leuven, Belgium.
    Wigö, Hans
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Neophytou, Marina
    Department of Civil and Environmental Engineering, University of Cyprus, Nicosia, Cyprus.
    Blocken, Bert
    Department of the Built Environment, Eindhoven University of Technology, Eindhoven, The Netherlands; Department of Civil Engineering, KU Leuven, Leuven, Belgium.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    CFD and wind-tunnel analysis of outdoor ventilation in a real compact heterogeneous urban area: evaluation using “air delay”2017Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 126, s. 355-372Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Outdoor urban ventilation in a real complex urban area is investigated by introducing a new ventilation indicator – the "air delay". Computational Fluid Dynamics (CFD) simulations are performed using the 3D steady Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. The up-to-date literature shows the lack of detailed evaluations of the two approaches for real compact urban areas. This study further presents a systematic evaluation of steady RANS and LES for the assessment of the ventilation conditions in a dense district in Nicosia, Cyprus. The ventilation conditions within the urban area are investigated by calculating the distribution of the age of air. To better assess the outdoor ventilation, a new indicator, the "air delay" is introduced as the difference between the local mean age of air at an urban area and that in an empty domain with the same computational settings, allowing the comparison of the results in different parts of the domain, without impact of the boundary conditions. CFD results are validated using wind-tunnel measurements of mean wind speed and turbulence intensity performed for the same urban area. The results show that LES can accurately predict the mean wind speed and turbulence intensity with the average deviations of about 6% and 14%, respectively, from the wind-tunnel measurements while for the steady RANS, these are 8% and 31%, respectively. The steady RANS simulations overestimate the local mean air delay. The deviation between the two approaches is 52% at pedestrian level (2 m).

  • 3.
    Blomqvist, Claes
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    A Note on Air Movements through Horizontal Openings in Buildings2002Inngår i: The 8th International Conference on Air Distribution in Rooms: Inividual Controlled Environment, 2002Konferansepaper (Fagfellevurdert)
  • 4.
    Blomqvist, Claes
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Air Movements through Horizontal Openings in Buildings: A Model Study2004Inngår i: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 3, nr 1, s. 1-10Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A building contains a number of large openings like doors and staircases. When the temperature of the spaces connected by these openings differs, the difference in density will cause air movements through them. Horizontal air movements through vertical openings in buildings like doors and windows are well investigated while studies of air movements through horizontal openings like stairwells are less frequent and therefore this work is focusing on this case.

    The paper reports on an experimental study of the possibility of using buoyancy forces to distribute air and heat through horizontal openings. The experiments have been carried out in a scale model with water as the operating fluid.

    The result of the study shows that the flow rate through a horizontal opening is roughly half of the flow rate through a vertical opening for the same conditions, probably caused by the more complex flow pattern in the horizontal opening. A staircase below the horizontal opening will guide the flow somewhat and will cause a small increase of the fluid exchange through the opening.

  • 5.
    Blomqvist, Claes
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Measurements and Control of Air Movements within a Building1997Inngår i: AIVC 18th Conference, proceedings of "Ventilation and Cooling", 1997, s. 427-436Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    There are a number of methods available concerning with distribution of air in buildings. Within control research, one can find new control algorithms which have not yet been used in practice. These new algorithms open the possibility of developing and implementing of new demand controlled ventilation systems.

    In a building the internal air motions are due both to differences in temperature and pressure differences caused by the ventilation system. Therefore, one fundamental question is to what extent it s possible to control the air motions within a building using fan powered ventilation in combination with temperature control.

    The aims of this paper is to report on measurements done to examine the influence of temperature differences between rooms on the air exchange through open doors in a building and to explore the use of modern control technique to minimise the temperature difference.

    The result of the measurements shows that even very small (0.1-0.2°C) temperature differences between rooms cause bi-directional air flows in the doorways of a magnitude that exceed the flow rates caused by the mechanical ventilation system. Therefore it is necessary to control the temperatures in the rooms to make it possible for the ventilation system to distribute the air to those parts of the building where it is needed.

  • 6.
    Blomqvist, Claes
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Spread of Gravity Currents in Multi Room Buildings2000Inngår i: The 7th International Conference on Air Distribution in Rooms: Ventilation for Health and Sustainable Environment, 2000Konferansepaper (Fagfellevurdert)
  • 7. Blomqvist, Claes
    et al.
    Sandberg, Mats
    KTH, Inst för byggd miljö.
    To what extent can one with mechanical ventilation control the air motions within a building1996Inngår i: ROOMVENT'96: 5th International Conference on Air Distribution in Rooms, 1996, s. vol 1 265-272Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    There are a number of methods available concerning with air distribution in buildings. Within control research, one can find new control algorithms which have not been used in practice yet. These new algorithms open the possibility of developing and implementing of new demand controlled ventilation systems.

    In a building the internal air motions are due both to differences in temperature and due to pressure differences induced by the ventilation system. Therefore, one fundamental question is to what extent one can with fan powered ventilation control the air motions within a building.

    The aims of this paper is to report on development of methods to study the air motions in a multi room residence apartment using various combinations of exhaust and supply air management. The experimental work includes measurements of air flow rates in door openings in both directions and use of various tracer gas methods to determine the supply air flow to each room, and identifying flow paths.

    In an accompanying paper (Björsell 1996) is reported on the results from a simulation of the performance of different control algorithms.

  • 8.
    Blomqvist, Claes
    et al.
    KTH, Inst för byggd miljö.
    Sandberg, Mats
    KTH, Inst för byggd miljö.
    Transition from Bi-directional to Unidirectional Flow in a Doorway1998Inngår i: ROOMVENT '98 : proceedings: 6th International Conference on Air Distribution in Rooms / [ed] Elisabeth Mundt, Tor-Göran Malmström, Stockholm, 1998, s. vol 2, 539-546Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The air flow in a doorway is governed by density difference caused by temperature difference and pressure difference caused by mechanical ventilation. Tests have been carried out in a unique indoor test house where the room to room to temperature difference could be controlled very accurately with a new control system. In addition to these tests some tests were carried out in a scale model with water as the operating fluid. Two main criteria of unidirectional flow in a doorway have been explored:

     

    1a.     The recorded mean velocity is unidirectional

    1b.     The neutral height is equal to the height of the door

    2.       Unidirectional flow in the sense that

     

    there is no transfer of contaminant from one room to another. To explore condition one the velocity profile in the doorway have been recorded by transversing the door opening. Condition two has been explored by using tracer gas technique.

  • 9. Brinkworth, BJ
    et al.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    A validated procedure for determining the buoyancy-induced flow in ducts2005Inngår i: Building Services Engineering Research & Technology, ISSN 0143-6244, E-ISSN 1477-0849, Vol. 26, nr 1, s. 35-48Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A procedure is set out for calculating the buoyant flow induced in a duct with heat input at the wall, as in the cooling ducts used behind photovoltaic arrays. In these, buoyancy is opposed by various pressure losses, due to obstructions at the inlet and outlet, fluid friction at the walls and structural support members passing transversely through the duct. New methods are developed for calculating these losses, and each is validated separately by tests in a purpose-built isothermal rig. Measurements are also reported for some further losses, not yet amenable to calculation, due to nets and hoods at the duct ends, as might be used to exclude rain and wildlife. Finally, the whole procedure is validated by measurement of a duct with one heated wall.

    Practical application: Verified by measurements at every stage of its development, the method reported gives greater confidence in the routine calculation of the flow induced in ductwork where there is heat gain, as in systems for PV cooling and natural ventilation. The new treatments given for the hydrodynamic losses at basic components apply wherever these are used in the field of HEVAC.

  • 10. Brinkworth, BJ
    et al.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Design procedure for cooling ducts to minimise efficiency loss due to temperature rise in PV arrays2006Inngår i: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 80, nr 1, s. 89-103Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The principal variable to be fixed in the design of a PV cooling duct is its depth, and hence the hydraulic diameter of its cross-section D. Analysis of the flow and heat transfer in the duct under still-air (buoyant flow) conditions, when the temperature rise is greatest, is validated by measurements on a full-scale test rig. It is shown that there is an optimum value of this design variable, such that for an array of length L the minimum temperature occurs when the ratio L/D is about 20. The optimum value is not affected much by other quantities, including the slope of the array.

    In practical situations, the flow is obstructed by devices across the duct inlet and outlet to exclude insects, birds and rain, and by structural support members crossing the duct interior. It is shown that the latter are no cause for concern, since the effect of the reduction in the flow-rate due to their presence is more than offset by an increase in heat transfer through additional turbulent mixing.

    It is also shown that array temperatures are strongly reduced by wind effects, which increase both the heat lost from the front surface of the array and by enhancement of the flow in the duct. Though the trends are clear, limitations are encountered in the present state of knowledge in both areas. (c) 2005 Elsevier Ltd. All rights reserved.

  • 11.
    Broström, Tor
    et al.
    Gotland University, Department of Building Conservation.
    Linden, Elisabet
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Lindström, Svante
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Mattsson, Magnus
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Convective heating in a medieval church: Effects of air-to-air heat pumps on air movements, particle deposition and temperature distribution2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In Europe many historic buildings use direct electric heating. Air-to-air heat pumps are an interesting alternative, in particular for conservation heating.  However, the convective heating may accelerate soiling of walls and artefacts by increasing the velocity and turbulence.

     

    The objective of the present paper is to discuss the general problem, the methodology for studying air motions and temperature distribution, and to present the results from a case study where air-to-air heat pumps and bench heaters were used for heating in a medieval church. The temperatures, velocities and humidity in the church have been measured for four different heating modes.

     

    The present study does not indicate any major disadvantages of using heat pumps for background heating in stone churches of the studied kind. More detailed long term studies are needed to ascertain the effects over time.

  • 12.
    Buccoliere, Riccardo
    et al.
    Universita di Lecce.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Study of the effect of building density and overall shape of a city on pollutant dispersion by combination of wind tunnel experiments and CFD simulations2008Inngår i: Proc. 12th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, 2008Konferansepaper (Fagfellevurdert)
  • 13.
    Buccoliere, Riccardo
    et al.
    Universita di Lecce.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Di Sabatino, Silvana
    Universita di lecce.
    An application of ventilation efficiency concepts to the analysis of building density effects on urban flow and pollutant concentration2011Inngår i: International Journal of Environment and Pollution, ISSN 0957-4352, E-ISSN 1741-5101, Vol. 47, nr 1-4, s. 248-256Artikkel i tidsskrift (Fagfellevurdert)
  • 14.
    Buccoliere, Riccardo
    et al.
    Univ Salento, Dipartimento Sci Mat, Lecce, Italy, and Univ Ca Foscari Venezia, Dipartimento Informat, Venice, Italy.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Di Sabatino, Silvana
    Univ Salento, Dipartimento Sci Mat, Lecce, Italy .
    City breathability and its link to pollutant concentration distribution within urban-like geometries2010Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 44, nr 15, s. 1894-1903Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper is devoted to the study of pollutant concentration distribution within urban-like geometries. By applying efficiency concepts originally developed for indoor environments, the term ventilation is used as a measure of city “breathability”. It can be applied to analyse pollutant removal within a city in operational contexts. This implies the evaluation of the bulk flow balance over the city and of the mean age of air. The influence of building packing density on flow and pollutant removal is, therefore, evaluated using those quantities. Idealized cities of regular cubical buildings were created with packing density ranging from 6.25% to 69% to represent configurations from urban sprawl to compact cities. The relative simplicity of these arrangements allowed us to apply the Computational Fluid Dynamics (CFD) flow and dispersion simulations using the standard k turbulence model. Results show that city breathability within the urban canopy layer is strongly dependent from the building packing density. At the lower packing densities, the city responds to the wind as an agglomeration of obstacles, at larger densities (from about 44%) the city itself responds as a single obstacle. With the exception of the lowest packing density, airflow enters the array through lateral sides and leaves throughout the street top and flow out downstream. The air entering through lateral sides increases with increasing packing density.

    At the street top of the windward side of compact building configurations, a large upward flow is observed. This vertical transport reduces over short distance to turn into a downward flow further downstream of the building array. These findings suggest a practical way of identifying city breathability. Even though the application of these results to real scenarios require further analyses the paper illustrates a practical framework to be adopted in the assessment of the optimum neighbourhood building layout to minimize pollution levels.

  • 15.
    Buccolieri, R.
    et al.
    Dipartimento di Informatica, Università Ca' Foscari Venezia, Mestre-Venezia, Italy; Dipartimento di Scienza Dei Materiali, University of Salento, Lecce, Italy.
    Sartoretto, F.
    Dipartimento di Informatica, Università Ca' Foscari Venezia, Mestre-Venezia, Italy.
    Giacometti, A.
    Dipartimento di Scienza Dei Materiali, University of Salento, Lecce, Italy.
    Di Sabatino, S.
    Dipartimento di Scienza Dei Materiali, University of Salento, Lecce, Italy.
    Leo, L.
    Dipartimento di Scienza Dei Materiali, University of Salento, Lecce, Italy.
    Pulvirenti, B.
    Dipartimento di Ingegneria Energetica, Nucleare e Del Controllo Ambientale, University of Bologna, Bologna, Italy.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Wigö, Hans
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Flow and pollutant dispersion within the Canal Grande channel in Venice (Italy) via CFD techniques2010Inngår i: HARMO 2010 - Proceedings of the 13th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes / [ed] Albergel, A., 2010, s. 760-764Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The present paper is aimed at the analysis of flow and pollutant dispersion in a portion of the Canal Grande (Grand Canal) in Venice (Italy) by means of both Computational Fluid Dynamics (CFD) FLUENT simulations and wind tunnel experiments performed at the University of Gävle (Sweden). For this application, Canal Grande can be viewed as a sort of street canyon where the bottom surface is water and bus boat emissions are the major source of pollution. Numerical investigations were made to assess the effect of the water surface on air flow and pollutant concentrations in the atmosphere. One of the challenges has been to deal with the interface between two immiscible fluids which requires ad-hoc treatment of the wall in terms of the numerical scheme adopted and the grid definition which needs to be much finer than in typical numerical airflow simulations in urban street canyons. Preliminary results have shown that the presence of water at the bottom of the street canyon modifies airflow and turbulence structure with direct consequences on concentration distribution within the domain.

  • 16.
    Buccolieri, R.
    et al.
    Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, University of Salento, Lecce, Italy.
    Wigö, Hans
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sabatino, S. D.
    Department of Physics and Astronomy, University of Bologna, Italy.
    On the drag force distribution over arrays of cubical buildings: Wind tunnel experiments2017Inngår i: HARMO 2017 - 18th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Proceedings, Hungarian Meteorological Service , 2017, s. 384-388Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this paper we discuss the distribution of drag force along aligned arrays of cubes of different packing density. The distribution is evaluated via wind tunnel measurements performed on individual cubes located along the middle column of the array using a balance provided by a standard load cell. Results are compared with the drag force estimated by a pressure-derived method and clearly show a change of the distribution of the drag force. The force is uniform at low packing densities, while mostly acting on first rows of the arrays at large packing densities. This work leaves room for research tailored to a better parameterization of urban effects in dispersion models.

  • 17.
    Buccolieri, Riccardo
    et al.
    Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, University of Salento, Lecce, Italy.
    Wigö, Hans
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Di Sabatino, Silvana
    Department of Physics and Astronomy, University of Bologna, Bologna, Italy.
    Direct measurements of the drag force over aligned arrays of cubes exposed to boundary-layer flows2017Inngår i: Environmental Fluid Mechanics, ISSN 1567-7419, E-ISSN 1573-1510, Vol. 17, nr 2, s. 373-394Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Wind tunnel measurements of the total drag force for aligned arrays of cubes exposed to two different boundary-layer flows at three flow velocities are discussed. The drag force for eight different building packing densities λp (from 0.028 to 1) is measured with a standard load cell generating a novel dataset. Different λp are reproduced by increasing the number of buildings on the same lot area; this represents a real situation that an urban planner is faced with when a lot area of a given (fixed) size is allocated to the development of new built areas. It is assumed that the surrounding terrain is uniform and there is a transition from a given roughness (smooth) to a new roughness (rough). The approaching flow will adjust itself over the new surface within a distance that in general may be larger than the horizontal length covered by the array. We investigate the region where the flow adjustment occurs. The wide range of packing densities allowed us to analyse in detail the evolution of the drag force. The drag force increases with increasing packing densities until it reaches a maximum at an intermediate packing density (λp = 0.25 in our case) followed by a slight decrease at larger packing densities. The value of the drag force depends on the flow adjustment along the array which is evaluated by introducing the parameter “drag area” to retrieve information about the drag distribution at different λp. Results clearly suggest a change of the distribution of the drag force, which is found to be relatively uniform at low packing densities, while most of the force acts on first rows of the arrays at large packing densities. The drag area constitutes the basis for the formulation of a new adjustment length scale defined as the ratio between the volume of the air within the array and the drag area. The proposed adjustment length scale automatically takes into account the change in drag distribution along the array for a better parameterization of urban effects in dispersion models. 

  • 18.
    Cehlin, Mathias
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för energi- och maskinteknik.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Computed tomography for indoor application2006Inngår i: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, nr 4, s. 349-364Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper deals with tomographic techniques for two-dimensional spatially resolved concentration measurements indoors. This represents a significant advance over the traditional point measuring method for mapping tracer gas and pollutants. Methods for recording of data are stressed as well as different types of tomographic reconstruction algorithms such as the Smooth Basis Function Minimization (SBFM) and the modified Low Third Derivative (LTDm) methods. Among the reconstruction algorithms available today, SBFM and LTDm are among the most promising. These algorithms show potential for reconstruction of gas concentration in rooms, since they are regularized to converge towards smooth concentration distributions. Using the LTD method and ‘snapshot’ configuration enables the examination and real-time monitoring of transient flows.

  • 19.
    Cehlin, Mathias
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för energi- och maskinteknik.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Computed Tomography for Indoor Applications2006Inngår i: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, nr 4, s. 349-364Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    This paper deals with tomographic techniques for two-dimensional spatially resolved concentration measurements indoors. This represents a significant advance over the traditional point measuring method for mapping tracer gas and pollutants. Methods for recording of data are stressed as well as different types of tomographic reconstruction algorithms such as the Smooth Basis Function Minimization (SBFM) and the modified Low Third Derivative (LTDm) methods. Among the reconstruction algorithms available today, SBFM and LTDm are among the most promising. These algorithms show potential for reconstruction of gas concentration in rooms, since they are regularized to converge towards smooth concentration distributions. Using the LTD method and ‘snapshot’ configuration enables the examination and real-time monitoring of transient flows.

  • 20.
    Cehlin, Mathias
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för energi- och maskinteknik.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Time evolution of gravity currents discharged from low velocity diffusers2007Inngår i: Roomvent 2007, 2007, s. 61-70Konferansepaper (Fagfellevurdert)
  • 21.
    Cehlin, Mathias
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för energi- och maskinteknik.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Time evolution of gravity currents discharged from low velocity diffusers2007Inngår i: Roomvent 2007: 10th International Conference on Air Distribution in Rooms, 13-15 June 2007, Helsinki, Finland, 2007, s. Vol. 3 61-70Konferansepaper (Fagfellevurdert)
  • 22.
    Chen, Lan
    et al.
    School of Amopheric Sciences, Sun Yat-sen University, HaiZhu, Guangzhou, China.
    Hang, Jian
    School of Amopheric Sciences, Sun Yat-sen University, HaiZhu, Guangzhou, China.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Claesson, Leif
    Högskolan i Gävle, Akademin för teknik och miljö, BMG-labbet.
    Di Sabatino, Silvana
    Department of Physics and Astronomy - DIFA, ALMA MATER STUDIORUM - University of Bologna, Bologna, Italy.
    The Influence of Building Packing Densities on Flow Adjustment and City Breathability in Urban-like Geometries2017Inngår i: Procedia Engineering, ISSN 1877-7058, E-ISSN 1877-7058, Vol. 198, s. 758-769Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    City breathability refers to the air exchange process between the flows above and within urban canopy layers (UCL) and that of in-canopy flow, measuring the potential of wind to remove and dilute pollutants, heat and other scalars in a city. Bulk flow parameters such as in-canopy velocity (Uc) and exchange velocity (UE) have been applied to evaluate the city breathability. Both wind tunnel experiments and computational fluid dynamics (CFD) simulations were used to study the flow adjustment and the variation of city breathability through urban-like models with different building packing densities. We experimentally studied some 25-row and 15-column aligned cubic building arrays (the building width B=72 mm and building heights H=B) in a closed-circuit boundary layer wind tunnel. Effect of building packing densities (λp=λf=0.11, 0.25, 0.44) on flow adjustment and drag force of each buildings were measured. Wind tunnel data show that wind speed decreases quickly through building arrays due to strong building drag. The first upstream building induces the strongest flow resistance. The flow adjustment length varies slightly with building packing densities. Larger building packing density produces lower drag force by individual buildings and attains smaller velocity in urban canopy layers, which causes weaker city breathability capacity. In CFD simulations, we performed seven test cases with various building packing densities of λp=λf=0.0625, 0.11, 0.25, 0.36, 0.44 and 0.56. In the cases of λp=λf=0.11, 0.25, 0.44, the simulated profiles of velocity and drag force agree with experiment data well. We computed Uc and UE, which represent horizontal and vertical ventilation capacity respectively. The inlet velocity at 2.5 times building height in the upstream free flow is defined as the reference velocity Uref. Results show that UE/Uref changes slightly (1.1% to 0.7%) but Uc/Uref significantly decreases from 0.4 to 0.1 as building packing densities rise from 0.0625 to 0.56. Although UE is induced by both mean flows and turbulent momentum flux across the top surface of urban canopy, vertical turbulent diffusion is found to contribute mostly to UE.

  • 23.
    Chen, Lan
    et al.
    School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, PR China.
    Hang, Jian
    School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, PR China; Institute of Earth Climate and Environment System, Sun Yat-sen University, Guangzhou, PR China.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Claesson, Leif
    Högskolan i Gävle, Akademin för teknik och miljö, BMG-labbet.
    Di Sabatino, Silvana
    University of Bologna, Bologna, Italy.
    Wigö, Hans
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    The impacts of building height variations and building packing densities on flow adjustment and city breathability in idealized urban models2017Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 118, s. 344-361Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Improving city breathability has been confirmed as one feasible measure to improve pollutant dilution in the urban canopy layer (UCL). Building height variability enhances vertical mixing, but its impacts remain not completely explored. Therefore, both wind tunnel experiments and computational fluid dynamic (CFD) simulations are used to investigate the effect of building height variations (six height standard deviations σH = 0%–77.8%) associated to building packing densities namely λp/λf = 0.25/0.375 (medium-density) and 0.44/0.67 (compact) on city breathability. Two bulk variables (i.e. the in-canopy velocity (UC) and exchange velocity (UE)) are adopted to quantify the horizontal and vertical city breathability respectively, which are normalized by the reference velocity (Uref) in the free flow, typically set at z = 2.5H0 where H0 is the mean building height. Both flow quantities and city breathability experience a flow adjustment process, then reach a balance. The adjustment distance is at least three times longer than four rows documented in previous literature. The medium-density arrays experience much larger UC and UE than the compact ones. UE is found mainly induced by vertical turbulent fluxes, instead of vertical mean flows. In height-variation cases, taller buildings experience larger drag force and city breathability than lower buildings and those in uniform-height cases. For medium-density and compact models with uniform height, the balanced UC/Uref are 0.124 and 0.105 respectively, moreover the balanced UE/Uref are 0.0078 and 0.0065. In contrast, the average UC/Uref in height-variation cases are larger (115.3%–139.5% and 125.7%–141.9% of uniform-height cases) but UE/Uref are smaller (74.4%–79.5% and 61.5%–86.2% of uniform-height cases) for medium-density and compact models. 

  • 24.
    Chung, Juyeon
    et al.
    Faculty of Engineering Sciences, Kyushu University, Fukuoka, Japan.
    Lim, Eunsu
    Faculty of Science and Engineering, Toyo University, Tokyo, Japan.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Ito, Kazuhide
    Faculty of Engineering Sciences, Kyushu University, Fukuoka, Japan.
    Returning and net escape probabilities of contaminant at a local point in indoor environment2017Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 125, s. 67-76Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The quantified recirculation of a contaminant in a local domain is an essential property of the ventilation efficiency in a room. The returning probability of a contaminant (α) generated in a local domain and its net escape probability (NEP) are essential information for understanding the structure of the contaminant concentration distribution in a room and for controlling the indoor air quality. Here, we propose the fundamental definitions of α and NEP and discuss their potential relation with the net escape velocity (NEV) concept. NEP is defined at a local point and/or local domain as the probability that a contaminant is exhausted directly through an exhaust outlet and does not re-circulate to the target local point/domain again. In a computational fluid dynamics (CFD) simulation, the minimum local domain in a room corresponds to the control volume (C.V.) of discretization; hence, NEP in a C.V. is assumed as the probability in a point without volume. In this study, the calculation results of α, NEP, and NEV distributions in a simple two-dimensional model room and a three-dimensional room with push-pull type ventilation system are demonstrated and discussed.

  • 25. Elvsén, Per-Åke
    et al.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Buoyant jet in ventilated rooms: velocity field, temperature field and airflow pattern analysed with three different whole field methods2009Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, nr 1, s. 137-145Artikkel i tidsskrift (Fagfellevurdert)
  • 26.
    Elvsén, Per-Åke
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Particle Streak Velocimetry for Room Air Flows-Some Improvements2004Inngår i: Roomvent, 2004Konferansepaper (Fagfellevurdert)
  • 27. Erell, E
    et al.
    Etizon, Y
    Carlstrom, N
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Molina, J
    Maestre, I
    Maldonado, E
    Leal, V
    Gutschker, O
    "SOLVENT": development of a reversible solar-screen glazing system2004Inngår i: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 36, nr 5, s. 467-480Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Preliminary experiments with a novel glazing system developed at the Desert Architecture and Urban Planning Unit of Ben-Gurion University of the Negev in Israel indicated that it may provide improved visual and thermal performance in buildings with large glazed areas located in sunny regions, regardless of orientation. In winter, it reduces glare, local over-heating and damage to furnishings caused by exposure to direct solar radiation, with only a small reduction in solar space heating. In summer, it reduces the penetration of unwanted radiation without obstructing the view through the window, to an extent that may render external shading devices unnecessary. The SOLVENT project was contracted to complete the development of the glazing system, which is based on the concept of converting short-wave solar radiation to convective heat and long wave radiation. The glazing system was modeled and evaluated experimentally; a suitable frame was developed for it; and a design tool required for its application was developed. The current paper reports on physical modeling and experimental evaluation of the glazing system.

  • 28.
    Fallenius, Bengt E.G.
    et al.
    KTH.
    Sattari, Amir
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Fransson, Jens
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Experimental study on the effect of pulsating inflow to an enclosure for improved mixing2013Inngår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, s. 108-119Artikkel i tidsskrift (Fagfellevurdert)
  • 29.
    Forsberg, Ann-Kristin
    et al.
    Högskolan i Gävle, Institutionen för matematik, natur- och datavetenskap, Ämnesavdelningen för datavetenskap.
    Winkler Pettersson, Lars
    Högskolan i Gävle, Institutionen för matematik, natur- och datavetenskap, Ämnesavdelningen för datavetenskap.
    Linden, Elisabet
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Seipel, Stefan
    Högskolan i Gävle, Institutionen för matematik, natur- och datavetenskap, Ämnesavdelningen för datavetenskap.
    An augmented-reality approach to co-located visual exploration of indoor climate data in real rooms2005Inngår i: Indoor Air 2005: Proceedings of the 10th International Conference on Indoor Air Quality and Climate, 2005, s. 2860-2860Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    We live in two spaces, the visible space and the non-visible but otherwise sensed space. Both spaces must satisfy our needs and there is a relation between them. If parts of the room are too cold this will lead to a restriction of the use of the room. We cannot endure draft for any longer time. Draft caused by a ventilation supply frequently leads to blockage of the supply device, which in turn gives rise to a reduction of the ventilation rate. The final result may be a deterioration of the air quality. Therefore, to be able to guarantee the air quality it is necessary to make the invisible thermal climate visible. In this paper a novel method based on Augmented Reality for presenting die thermal climate is presented and discussed. The data, e.g. temperatures and velocities, are shown on a lightweight display. Several people can walk around in a real room and see on a screen where the hot and/or cold spots may appear. Different ventilation solutions could in that way be compared in a dialogue between different actors in the building process.

  • 30.
    Fredriksson, J
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    The effect of false ceilings on the performance of passive chilled beams2007Inngår i: The 6th International Conference on Indoor Air Quality, Ventilation & Energy Conservation in Buildings, 2007, s. 13-Konferansepaper (Fagfellevurdert)
  • 31.
    Fredriksson, Jan
    et al.
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    The effect of false ceiling on the cooling capacity of passive chilled beams2009Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, nr 7, s. 1426-1430Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Passive chilled beams are often used to provide cooling or additional cooling when the ventilation system cannot cope with the whole cooling load. The advantage of passive cooling is that it is a silent cooling. Often the chilled beams are installed above a false ceiling and thereby the room is subdivided into two compartments. From the chilled beam a plume is generated. Make-up air (return air) needs to flow into the upper compartment to substitute the airflow generated by the chilled beam. Therefore openings for this purpose are installed in the false ceiling. Small openings constitute a resistance to the flow and the locations of the openings affect the flow pattern. The overall performance was studied in a mock-up of a real office by changing both the size and position of the openings for the make-up air. A uniform heating source was arranged by covering the floor with a heating foil. The best location and size of the openings were explored by both recording the heat absorbed by the beam and the temperature in the room. Minimum temperature attained in the room is the signature of the most efficient cooling. To achieve efficient cooling with a uniform floor-based heating source, two conditions must be fulfilled: a) the return opening area must be at least equal to the horizontal area of the chilled beam; b) the return air openings must be located at the perimeter of the room. In general we can expect conditions a) and b) to be applicable irrespective of type of heat, but for point sources we could achieve the best cooling by placing the return air opening above the heat source.

  • 32.
    Hang, J.
    et al.
    University of Hong Kong.
    Li, Y. G.
    University of Hong Kong.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Buccoliere, Riccardo
    Universita di Lecce.
    Di Sibatino, Silvana
    University of Salento.
    The influence of building height variability on pollutant dispersion and pedestrian ventilation in idealized high-rise urban areas2012Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 56, s. 346-360Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Studies are still required to understand how rural/marine wind remove ground-level pollutants released uniformly in street networks of high-rise urban areas. The link between building height variability and pollutant removal process still remains unclear. Several idealized urban-like neighbourhoods made of 9-row and 18-row small-scale high-rise square arrays (building width B = street width W, building packing density λp = 0.25) were first numerically studied with a parallel approaching wind and neglecting thermal effects. Normalized pollutant transport rates and pedestrian purging flow rate were applied to quantify the contribution of pollutant removal by mean flow and turbulent diffusion and their net purging capacity.

    Results show that the prediction of isothermal turbulent flows agreed generally well with wind tunnel data. For 9-row arrays with building height variations (standard deviation of 0–57.1%) and the same average canopy height (H0 = 2.33W), pollutant removal mainly depends on mean flows. Larger standard deviations tend to induce better pedestrian ventilation. In comparison to small and large standard deviations, medium values of 14.3–42.9% may experience smaller purging capacity by horizontal mean flows but significantly enhance that by vertical mean flows. For arrays with uniform heights, lowering aspect ratios (H/W = 2.33 and 2.67–1.5) or increasing street lengths (9-row to 18-row) may enhance the contribution of removing pollutants by turbulent diffusions across canopy roofs which may be similarly important as that by mean flows. Although further investigations are still required, this paper clarifies the relationship between building layouts, height variability and removal potential of ground-level pollutants in high-rise urban-like geometries.

  • 33.
    Hang, Jian
    et al.
    University of Hong Kong and Guangzhou University.
    Li, Yuguo
    University of Hong Kong.
    Buccolieri, Riccardo
    University of Salento, Italy.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Di Sabatino, Silvana
    University of Salento, Italy.
    On the contribution of mean flow and turbulence to city breathability: the case of long streets with tall buildings2012Inngår i: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 416, s. 362-373Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper analyses the contribution of mean flow and turbulence to city breathability within urban canopy layers under the hypothesis that winds from rural/marine areas are sources of clean air (inhale effect) and main contributors to local-scale pollutant dilution (exhale effect). Using Computational Fluid Dynamics (CFD) simulations, several idealized long streets flanked by tall buildings are investigated for wind flow parallel to the street axis. Aspect ratios (building height/street width) ranging from 2 to 4 and street lengths ranging from neighborhood scales (~. 1. km in full scale) to city scales (~. 10. km in full scale) are analyzed. To assess the inhale effect, the age of air concept is applied to quantify the time taken by a parcel of rural/marine air to reach a reference location within the urban canopy layer. To simulate the exhale effect, removal of pollutants released from a ground level source is considered. Numerical results agree with wind tunnel observations showing that a bulk portion of rural/marine air enters the streets through windward entries, a smaller part of it leaves through street roofs and the remaining fraction blows through the street aiding pollutant dilution. Substantial differences between neighborhood-scale and city-scale configurations are found. For neighborhood-scale models, pollutant removal by rural/marine air is mainly associated to mean flow along the streets. Breathability improves in streets flanked by taller buildings since in this case more rural/marine air is captured inside canyons leading to stronger wind along the street. For city-scale models, pollutant removal due to turbulent fluctuations across street roofs competes with that due to mean flows along the street. Breathability improves in streets flanked by lower buildings in which less rural/marine air is driven out and pollutant removal by turbulent fluctuations is more effective. Based on these findings, suggestions for ventilation strategies for urban areas with tall buildings are provided.

  • 34.
    Hang, Jian
    et al.
    University of Hong Kong.
    Li, Yuguo
    University of Hong Kong.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Experimental and numerical studies of flows through and within high-rise building arrays and their link to ventilation strategy2011Inngår i: Journal of Wind Engineering and Industrial Aerodynamics, ISSN 0167-6105, E-ISSN 1872-8197, Vol. 99, nr 10, s. 1036-1055Artikkel i tidsskrift (Fagfellevurdert)
  • 35. Hang, Jian
    et al.
    Li, Yuguo
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Understanding Air Exchange and Pollutant Dispersion in a Very Long Street with a RBG k-e Model2006Inngår i: International Conference Megacities 2006, conference proceedings, 2006, s. 442-451Konferansepaper (Annet vitenskapelig)
  • 36.
    Hang, Jian
    et al.
    University of Hong Kong.
    Li, Yuguo
    University of Hong Kong.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Claesson, Leif
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, BMG Labbet.
    Wind Conditions and Ventilation in high rise long Street Models2010Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 45, nr 6, s. 1353-1365Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We regarded high-rise cities as obstacles and channels to wind. We first studied wind conditions and ventilations in idealized high-rise long street models experimentally and numerically with a constant street width (W = 30 mm), variable street heights (H = 2 W, 2.5W, 3W, 4W), variable street lengths (L = 47.4W, 79W. 333W, 667W) and a parallel approaching wind. The flow rates penetrating into windward entries are a little larger than the reference flow rate in the far upstream free flow through the same area with windward entries in all models. The stream-wise velocity decreases along the street as some air leaves upwardly across street roofs. Near the leeward entry, there is a downward flow which brings some air into the street and results in an accelerating process. In the neighborhood scale long streets (L = 47.4W and 79W), wind in taller streets is stronger and the ventilation is better than a lower one. For the city scale long streets (L = 333W and 667W), a constant flow region exists where the vertical velocity is zero and the stream-wise velocity remains constant. In such regions, turbulent fluctuations across the street roof are more important to air exchange than vertical mean flows. In a taller street, the process to establish the constant flow conditions is longer and the normalized balanced horizontal flow rate is smaller than those in a lower street. In the city scale long streets, the turbulence exchange rate can be 5-10 times greater than the mean flow rate. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

  • 37.
    Hang, Jian
    et al.
    Department of Mechanical Engineering, The University of Hong Kong, Hong Kong.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Li, Yugo
    Department of Mechanical Engineering, The University of Hong Kong, Hong Kong.
    Claesson, Leif
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, BMG Labbet.
    Flow mechanisms and flow capacity in idealized long-street models2010Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 45, nr 4, s. 1042-1053Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    It is an open question whether a street network of a city has a certain flow capacity characterizing the flow that can pass through the street network. It s our hypothesis that at least the simple street network has a certain flow capacity. With the purpose of exploring this we studied numerically and experimentally the flow capacity in some idealized long-street models continuously lined with buildings and exposed to a parallel approaching wind. The height of all the models is the same (H = 69 mm). Three groups of models were studied: models with the same uniform street width (W = H) but different lengths (L = 21.7H, 43.5H, 72.5H); models with the same length (L = 43.5H) but different uniform width (W = H, 2H, 4H); and models with a change of width at half distance, L/2. In the last of the three cases, the width of the upstream half was always the same (W1 = H), but there was a wider (W2 = 1.25H, 1.5H, 2H) or narrower (W2 = 0.75H, 0.5H) downstream half. We normalized flow rates by a reference flow rate equal to the flow rate through an opening far upstream with the same area as the windward entry. The normalized flow rate through the windward entry was about 1.0 in all cases. For a sufficiently long-street models, a flow balance is established, creating a fully developed region with a constant horizontal flow (flow capacity) and zero vertical mean velocity. The street length does not affect the flow capacity but as expected the width of the street affects the flow capacity.

  • 38.
    Hang, Jian
    et al.
    epartment of Mechanical Engineering, The University of Hong Kong, Hong Kong.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    LI, Yuguo
    epartment of Mechanical Engineering, The University of Hong Kong, Hong Kong.
    Age of air and air exchange efficiency in idealized city models2009Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, nr 8, s. 1714-1723Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Wind can provide relevantly clean external (rural) air into urban street network, i.e. city ventilation. The local mean age of air denotes the time it takes for the external air to reach a location after entering the urban canopy layer. The air exchange efficiency denotes the efficiency of flushing the street network with external air. However, difficulties exist in calculating the local mean age of air in a city due to open boundaries. The traditional experimental homogeneous emission method is adapted here in a CFD method to predict the urban local age of air and analyze the air exchange efficiency for city ventilation. Three simple city models are considered, including a round city model, a square city model and a long rectangular city with one main street parallel to the approaching wind or with two crossing streets. The difference in the city shape results in significant difference in the local mean age of air. In the round city of one narrow street, two inflows through street openings converge close to the city centre and exits through the street roof, so the air close to the city centre is relatively old and the air exchange efficiency is low (30%). For a round city with two crossing streets, a slightly non-parallel wind to the main street generates younger air and the higher air exchange efficiency in the city.

  • 39.
    Hang, Jian
    et al.
    Department of Mechanical Engineering, TheUniversity of Hong Kong, Hong Kong SAR, Hong Kong.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Li, Yuguo
    University of Hong Kong, Hong Kong SAR, Hong Kong.
    Effect of urban morphology on wind condition in idealized city models2009Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, nr 4, s. 869-878Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Wind conditions in urban environments are important for a number of reasons. They can serve to transport air pollutants out of the urban environment and to moderate urban microclimatic conditions if satisfactory, yet can compromise pedestrian comfort and safety if not. We aim to study experimentally and numerically the effects of urban morphology (e.g., overall city form (skyline), street orientation, and street configuration) on wind conditions in cities. This report considers our initial investigations of two idealized city forms that are coincidentally similar to ancient Roman cities that were organized on one or two primary streets - a main north-south street, the cardus maximus, and a secondary east-west street, the decumanus maximus - and contained within a well-defined perimeter. We first consider round and square city models with one main street set parallel to the approaching wind and a secondary street producing an intersection at city centre. Not surprisingly, wind conditions in the two city models are dissimilar due to their shape differences. We then consider a long rectangular city model with a fully developed steady flow region along the main street. If the main street of the round city model is narrow, the parallel approaching wind cannot blow through the entire street and a penetrating inflow exists at the leeward opening. For the round city model with two crossing streets, a slightly non-parallel wind to the main street generates a stronger wind level in the entire street volume.

  • 40. Hang, Jian
    et al.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Li, Yuguo Li
    Claesson, Leif
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Pollutant dispersion in idealized city models with different urban morphologies2009Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, nr 38, s. 6011-6025Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The mechanism of pollutant dispersion in idealized city models is investigated numerically by the introduction of a uniformly distributed pollutant source at street pedestrian level. We first study three short city forms with a single main street or two crossing streets, characterized by street length/street height ratios of L/H = 6 or 7 and a street height/street width ratio of H/W = 1, including a sharp-edged round city model, a smooth-edged round city model, and a sharp-edged square city model. For short city models with a single street and a parallel approaching wind, pollutant dilution mainly depends on the horizontal flow rate which decreases along the street. This decreasing rate is smallest for the smooth-edged round city model, which results in the lowest street concentrations. For city models with two crossing streets and the approaching wind parallel to the main street, the differences in overall city form result in different dispersion processes. For a sharp-edged round city model with two crossing streets, an approaching wind slightly non-parallel to the main street generates a lower pollutant concentration in the entire street volume. We also studied a sharp-edged round city model with one narrow street (L/H = 6; H/W = 6.7), finding that the uniformly distributed pollutants are transported from two street entries to the city centre, and are then removed out across the street roof. In contrast to the short city models we studied a single-street sharp-edged long rectangular city model (L/H = 21.7; H/W = 1) in which the horizontal flow rate remained nearly constant in a region far from the two entries. Within this region the turbulence across the street roof contributed more to the pollutant removal than vertical mean flows.

  • 41.
    Hang, Jian
    et al.
    Department of Atmospheric Sciences, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, China.
    Wang, Qun
    Department of Atmospheric Sciences, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, China .
    Chen, Xieyuan
    Department of Atmospheric Sciences, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, China .
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Zhu, Wei
    Department of Atmospheric Sciences, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, China .
    Buccolieri, Riccardo
    Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, University of Salento, Lecce, Italy.
    Di Sabatino, Silvana
    Department of Physics and Astronomy, University of Bologna, Bologna, Italy .
    City breathability in medium density urban-like geometries evaluated through the pollutant transport rate and the net escape velocity2015Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 94, nr P1, s. 166-182, artikkel-id 4213Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper investigates pollutant removal at pedestrian level in urban canopy layer (UCL) models of medium packing density (λ<inf>p</inf> = λ<inf>f</inf> = 0.25) using computational fluid dynamics (CFD) simulations. Urban size, building height variations, wind direction and uniform wall heating are investigated. The standard and RNG k-ε turbulence models, validated against wind tunnel data, are used. The contribution of mean flows and turbulent diffusion in removing pollutants at pedestrian level is quantified by three indicators: the net escape velocity (NEV), the pollutant transport rate (PTR) across UCL boundaries and their contribution ratios (CR).Results show that under parallel approaching wind, after a wind-adjustment region, a fully-developed region develops. Longer urban models attain smaller NEV due to pollutant accumulation. Specifically, for street-scale models (~100 m), most pollutants are removed out across leeward street openings and the dilution by horizontal mean flows contributes mostly to NEV. For neighbourhood-scale models (~1 km), both horizontal mean flows and turbulent diffusion contribute more to NEV than vertical mean flows which instead produce significant pollutant re-entry across street roofs. In contrast to uniform height, building height variations increase the contribution of vertical mean flows, but only slightly influence NEV. Finally, flow conditions with parallel wind and uniform wall heating attain larger NEV than oblique wind and isothermal condition.The paper proves that by analysing the values of the three indicators it is possible to form maps of urban breathability according to prevailing wind conditions and known urban morphology that can be of easy use for planning purposes. 

  • 42.
    Hayati, Abolfazl
    et al.
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Mattsson, Magnus
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    A Study on Airing Through the Porches of a Historical Church – Measurements and IDA-ICE Modelling2016Inngår i: ASHRAE and AIVC IAQ 2016 - Defining Indoor Air Quality: Policy, Standards and Best Practices, 2016, ASHRAE, 2016, s. 216-223, artikkel-id C029Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In churches, intentional airing may be a measure to evacuate temporarily high levels of contaminants that are emitted during services and other occasions. Crucial contaminants include moisture and other emissions that may deteriorate and/or soil painted surfaces and other precious artefacts. Most old churches do not have any mechanical ventilation system or any purpose provided openings for natural ventilation, but the ventilation is governed by air infiltration. Enhanced airing may be achieved by opening external windows or doors. Thus, models provided in energy simulation programs should predict this kind of air flows correctly, also in order to get a proper estimation of the total energy use. IDA-ICE is examined here and the model for air flow through a large vertical opening used in the program is investigated. In the present study, field measurements were performed for airing rate in a historical church. In comparison with measured air flow rates, the simulated results were of the same magnitude, but the effect of wind direction was less considered by the simulation program.

  • 43.
    Hayati, Abolfazl
    et al.
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Mattsson, Magnus
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    A wind tunnel study of wind-driven airing through open doors2018Inngår i: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Temporarily enhanced natural ventilation of indoor environments can be achieved by opening windows and/or doors, i.e. airing. In this study, wind driven airing rate through doors was measured by tracer gas at a building model in a wind tunnel. Both single opening and cross flow airing was investigated, with doors placed in centrally on the long side of an elongated building model. It was found that cross flow airing yielded 4–20 times higher airing rate than single opening airing; lowest value occurring with opening surfaces perpendicular to wind direction. At single opening airing, windward positioned door yielded about 53% higher airing rate than leeward positioned. Inclusion of a draught lobby (extended entrance space) lowered airing rate by 27%, while higher wind turbulence increased it by 38%. Advection through turbulence appeared a more important airing mechanism than pumping. At cross flow, however, turbulence and draught lobby had practically no effect.

  • 44.
    Hayati, Abolfazl
    et al.
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Mattsson, Magnus
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Evaluation of the LBL and AIM-2 air infiltration models on large single zones: three historical churches2014Inngår i: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 81, s. 365-379Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Air infiltration in ancient churches and other historical and monumental buildings is of great importance considering moisture transfer, energy consumption, thermal comfort and air pollutants that induce surface soiling. Two of the most established models for predicting air infiltration rate in buildings are the Lawrence Berkeley Laboratory (LBL) model and the Alberta air Infiltration Model (AIM-2). Being originally developed mainly for dwellings, their applicability to large single zone buildings is evaluated in this study by comparing model predictions with field measurements in three historical stone churches that are naturally ventilated only through infiltration. The somewhat more developed AIM-2 model yielded slightly better predictions than the LBL model. However, an LBL version that allows inclusion of the Neutral Pressure Level (NPL) of the building envelope produced even better predictions and also proved less sensitive to assumptions on air leakage distribution at the building envelopes. All models yielded however significant overpredictions of the air infiltration rate. Since NPL may be difficult to attain in practice, the AIM-2 model was chosen for model modification to improve predictions. Tuning of this model by varying its original coefficients yielded however unrealistic model behaviors and the eventually suggested modification implied introducing a correction factor of 0.8. This reduced the median absolute prediction error from 25% to 11%. Thus, especially when the NPL is not at hand, this modification of the AIM-2 model may suit better for air infiltration assessment of churches and other buildings similar to the tested kind.

  • 45.
    Hayati, Abolfazl
    et al.
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Mattsson, Magnus
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Single-sided ventilation through external doors: measurements and model evaluation in five historical churches2017Inngår i: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 141, s. 114-124Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ventilation through open doors is a simple way to temporarily enhance ventilation of indoor spaces, with the purpose to evacuate indoor air pollutants or to adjust the indoor temperature. In churches and other historical buildings, which otherwise are ventilated only through air infiltration, temporarily enhanced ventilation through open doors or windows may be a prudent deed after e.g. services involving large congregations and burning of candles or incense. In the present study, the air exchange occurring at single-sided ventilation through the external doors of five historical churches is measured by tracer gas decay method. Further, air velocity measurements and smoke visualization in a doorway are performed. Measurement results are compared with predictions attained from four previously developed models for single‐sided ventilation. Models that include terms for wind turbulence yielded somewhat better predictions. According to the performed measurements, the magnitude of one hour single-sided open-door airing in a church is typically around 50% air exchange, indicating that this is a workable ventilation method, also for such large building volumes. A practical diagram to facilitate estimation of a suitable airing period is also presented. The study adds particularly knowledge to the issue of airing through doors, in large single zones.

  • 46.
    Hayati, Abolfazl
    et al.
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Mattsson, Magnus
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Byggvetenskap - installationsteknik.
    Linden, Elisabet
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, BMG Labbet.
    Evaluation of two air infiltration models on a church2013Inngår i: Conference proceedings: Cultural heritage preservation – 3rd European Workshop on Cultural Heritage Preservation, 2013, s. 47-53Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Air infiltration in ancient churches and other historical and monumental buildings is of great importance considering moisture transfer, energy consumption, thermal comfort and indoor surface soiling. Two of the most established models for simulatingand predicting air infiltration in buildings are the Lawrence BerkeleyLaboratory (LBL) model and the Alberta air Infiltration Model (AIM-2). The applicability of these models in superimposing wind and buoyancy driven infiltration in large single zone buildings such as churches are evaluated in this study by comparing model predictions with field measurements in a 19thcentury stone church. Both tested air infiltration models yielded significant positive correlations between measured and predicted data, and it is concludedthat the AIM-2 model works better than the LBL model for the studied church. Both models tend however to over-predict the air infiltration rate significantly. The over‑predictions were larger in cases with high wind speed and it seems that the models are more fragile in wind dominating conditions. Inclusion of crawl space coefficients in the AIM-2 model improved however the predictions, especially at high wind speeds. It seems that models of the tested kind can be useful in predicting air infiltration in churches and similar buildings, but that some empirically attained model coefficients might need some adjustment to suit this kind of buildings better.

  • 47. Heiselberg, P
    et al.
    Jensen, JJ
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Nielsen, PV
    Experimental and Numerical Analysis of Wind Driven Natural Ventilation in a Building Scale Model2004Inngår i: RoomVent 2004: 9th international conference in University of Coimbra : Portugal, 5-8th september 2004, 2004Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Airflow through openings in a cross ventilated building scale model was investigated in a wind tunnel and by numerical predictions. Predictions for a wind direction perpendicular to the building showed an airflow pattern consisting of streamlines entering the room, that originated from approximately the same upstream area in the undisturbed boundary layer and a direction of the flow into the room dependent on opening location with velocity vectors pointing away from the stagnation point. The measured internal pressure for various opening sizes was not the average pressure on the windward and leeward sides and the airflow rate was found to be a linear function of the local pressure difference across the building.

  • 48.
    Heiselberg, P.
    et al.
    Hybrid Ventilation Centre, Department of Civil Engineering, Aalborg University, Aalborg, Denmark.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Evaluation of Discharge Coefficients for Window Openings in Wind Driven Natural Ventilation2006Inngår i: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 5, nr 1, s. 43-52Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper describes the classical approach for calculation of wind driven airflow through large openings in buildings and discusses the fulfilment of the limiting assumptions. It is demonstrated that the limiting assumptions are not fulfilled for large openings in buildings for cross ventilation, and therefore, the classical approach is not appropriate for prediction of airflow through large openings in buildings in the cross ventilation case. Using the approach for real openings and estimating the discharge coefficient for window openings has also not been very successful. The discharge coefficient cannot be regarded as a constant and it is very difficult to estimate correct values resulting in less accuracy of prediction of natural ventilation.

  • 49. Holmberg, Sture
    et al.
    Sandberg, Mats
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Mattsson, Magnus
    Högskolan i Gävle, Institutionen för teknik och byggd miljö, Ämnesavdelningen för inomhusmiljö.
    Nilsson, Håkan
    Holmér, Ingvar
    Indoor Air Quality and climate control parameters in office environment – CFD calculaions and measurements2000Inngår i: Roomvent 2000 Conference, 2000Konferansepaper (Fagfellevurdert)
  • 50.
    Jiang, Bin
    et al.
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för Industriell utveckling, IT och Samhällsbyggnad, Samhällsbyggnad, GIS.
    Ma, Ding
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för Industriell utveckling, IT och Samhällsbyggnad, Samhällsbyggnad, GIS.
    Yin, Junjun
    Department of Geography and Geographic Information Science, University of Illinois at Urbana and Champaign, Illinois, USA.
    Sandberg, Mats
    Högskolan i Gävle, Akademin för teknik och miljö, Avdelningen för bygg- energi- och miljöteknik, Energisystem.
    Spatial Distribution of City Tweets and Their Densities2016Inngår i: Geographical Analysis, ISSN 0016-7363, E-ISSN 1538-4632, Vol. 48, nr 3, s. 337-351Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Social media outlets such as Twitter constitute valuable data sources for understanding human activities in the virtual world from a geographic perspective. This article examines spatial distribution of tweets and densities within cities. The cities refer to natural cities that are automatically aggregated from a country’s small street blocks, so called city blocks. We adopted street blocks (rather than census tracts) as the basic geographic units and topological center (rather than geometric center) to assess how tweets and densities vary from the center to the peripheral border. We found that, within a city from the center to the periphery, the tweets first increase and then decrease, while the densities decrease in general. These increases and decreases fluctuate dramatically, and differ significantly from those if census tracts are used as the basic geographic units. We also found that the decrease of densities from the center to the periphery is less significant, and even disappears, if an arbitrarily defined city border is adopted. These findings prove that natural cities and their topological centers are better than their counterparts (conventionally defined cities and city centers) for geographic research. Based on this study, we believe that tweet densities can be a good surrogate of population densities. If this belief is proved to be true, social media data could help solve the dispute surrounding exponential or power function of urban population density.

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