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Sandberg, Mats
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Hayati, A., Mattsson, M. & Sandberg, M. (2019). A wind tunnel study of wind-driven airing through open doors. The International Journal of Ventilation, 113-135
Open this publication in new window or tab >>A wind tunnel study of wind-driven airing through open doors
2019 (English)In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, p. 113-135Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Wind-driven flow, Single-sided ventilation, Cross flow, Tracer gas decay method, Wind turbulence, Churches
National Category
Building Technologies
Identifiers
urn:nbn:se:hig:diva-25112 (URN)10.1080/14733315.2018.1435027 (DOI)000469880200003 ()2-s2.0-85042108561 (Scopus ID)
Projects
Church project
Funder
Swedish Energy Agency, 2011-002440
Available from: 2017-08-28 Created: 2017-08-28 Last updated: 2019-08-22Bibliographically approved
Kabanshi, A. & Sandberg, M. (2019). Entrainment and its Implications on Microclimate Ventilation Systems: Scaling the Velocity and Temperature Field of a Round Free Jet. Indoor Air, 29(2), 331-346
Open this publication in new window or tab >>Entrainment and its Implications on Microclimate Ventilation Systems: Scaling the Velocity and Temperature Field of a Round Free Jet
2019 (English)In: Indoor Air, ISSN 0905-6947, E-ISSN 1600-0668, Vol. 29, no 2, p. 331-346Article in journal (Refereed) Published
Abstract [en]

Research on microclimate ventilation systems, which mostly involve free jets, point to delivery of better ventilation in breathing zones. While the literature is comprehensive, the influence of contaminant entrainment in jet flows and its implications on the delivery of supplied air is not fully addressed. This paper present and discuss entrainment characteristics of a jet issued from a round nozzle (0.05 m diameter), in relation to ventilation, by exploring the velocity and temperature fields of the jet flow. The results show a trend suggesting that increasing the Reynolds number (Re) reduces ambient entrainment. As shown herein, about 30% concentration of ambient air entrained into the bulk jet flow at Re 2541 while Re 9233 had about 13% and 19% for Re = 6537/12026 at downstream distance of 8 diameters (40 cm). The study discusses that “moderate to high” Re may be ideal to reduce contaminant entrainment, but this is limited by delivery distance and possibly the risk of occupant discomfort. Incorporating the entrainment mixing factor (the ratio of room contaminants entrained into a jet flow) in performance measurements is proposed and further studies are recommended to verify results herein and test whether this is general to other nozzle configurations.

Place, publisher, year, edition, pages
John Wiley & Sons, 2019
Keywords
Entrainment, Round free jet, Microclimate ventilation systems, Temperature field, Velocity field, Entrainment mixing factor
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:hig:diva-28736 (URN)10.1111/ina.12524 (DOI)000459637200016 ()30500986 (PubMedID)2-s2.0-85059514934 (Scopus ID)
Available from: 2018-12-02 Created: 2018-12-02 Last updated: 2019-08-12Bibliographically approved
Yin, S., Li, Y., Fan, Y. & Sandberg, M. (2019). Experimental investigation of near-field stream-wise flow development and spatial structure in triple buoyant plumes. Building and Environment, 149, 79-89
Open this publication in new window or tab >>Experimental investigation of near-field stream-wise flow development and spatial structure in triple buoyant plumes
2019 (English)In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 149, p. 79-89Article in journal (Refereed) Published
Abstract [en]

This paper presents a systematic experimental study on stream-wise flow development and spatial structure of triple buoyant plumes. Two-dimensional Particle Image Velocimetry (2-D PIV) is employed to resolve velocity fields. Stream-wise axial velocity profiles, flow structure, flow region parameters, and self-similarity properties are analyzed at different configurations that are characterized by spacing ratios S/W (source spacing S divided by source width W). From velocity fields and axial velocity profiles, a similar stream-wise developing trend is identified in different source configurations. When near-field plumes travel downstream, axial velocities increase rapidly, off-center velocity peaks get merged with the central peak, and the number of velocity peaks reduces with the downstream distance. A compact source layout, comparing with the wide one, could enhance the near-field plumes interaction and promote the plumes deflection significantly. Fundamentally, the stream-wise spatial structure of the triple plumes initially consists of a converging region, followed by a merging region, and finally a combined region. By examining the averaged velocity fields, flow recirculation with negative axial velocities is found to scarcely exist in the converging region. Merging level Zm and quasi-combined level Zqc are analyzed quantitatively and statistically. Within the studied S/W range, the normalized Zm shows a linear increase with S/W in the formula of Zm/H=2.007(S/W)+1.173 and the normalized Zqc gives a power law increase with S/W in the formula of Zqc/H=6.035(S/W)0.4959. In addition, triple plumes are found to establish self-similarity approximately at Z = 3H with S/W of 0.2 and at Z = 4.5H with S/W of 0.5.

Place, publisher, year, edition, pages
Elsevier Ltd, 2019
Keywords
Merging level, Quasi-combined level, Self-similarity, Source spacing, Spatial structure, Stream-wise flow development, Merging, Velocity, Velocity measurement, Flow development, Self-similarities, Buoyancy
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:hig:diva-29063 (URN)10.1016/j.buildenv.2018.11.039 (DOI)000457118300008 ()2-s2.0-85058185482 (Scopus ID)
Note

Funding Agency:

RGC GRF project of the Hong Kong SAR Government  Grant no: 17201817

Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-03-12Bibliographically approved
Sandberg, M., Kabanshi, A. & Wigö, H. (2019). Is building ventilation a process of diluting contaminants or delivering clean air?. Indoor + Built Environment
Open this publication in new window or tab >>Is building ventilation a process of diluting contaminants or delivering clean air?
2019 (English)In: Indoor + Built Environment, ISSN 1420-326X, E-ISSN 1423-0070Article in journal (Refereed) Epub ahead of print
Abstract [en]

The purpose of this paper is to discuss the performance of air distribution systems intended for dilution of contaminants (e.g. mixing ventilation) and those intended for delivery of clean air to local regions within rooms (e.g. personalized ventilation). We first start by distinguishing the systems by their visiting frequency behaviour. Then, the performance of the systems with respect to their possibility to influence contaminant concentration in the room or regions within the room is dealt with. Dilution capacity concept for mixing systems is discussed, and delivery capacity concept for systems intended to deliver clean air locally is introduced. Various ways for supply of clean air to regions within the room are presented and their pros and cons are discussed. In delivery capacity systems, the most important single parameter is the entrainment of ambient air into the primary supply flow. Therefore, methods of determining entrainment in these systems need to be defined and the results should be included when describing the performance of the air terminal devices.

Place, publisher, year, edition, pages
Sage Publications, 2019
Keywords
Probability to return, Visitation frequency, Dilution capacity, Delivery capacity, Entrainment, Entrainment mixing factor
National Category
Applied Mechanics
Identifiers
urn:nbn:se:hig:diva-29401 (URN)10.1177/1420326X19837340 (DOI)
Available from: 2019-03-21 Created: 2019-03-21 Last updated: 2019-08-09Bibliographically approved
Kabanshi, A., Yang, B., Sörqvist, P. & Sandberg, M. (2019). Occupants’ perception of air movements and air quality in a simulated classroom with an intermittent air supply system. Indoor + Built Environment, 28(1), 63-76
Open this publication in new window or tab >>Occupants’ perception of air movements and air quality in a simulated classroom with an intermittent air supply system
2019 (English)In: Indoor + Built Environment, ISSN 1420-326X, E-ISSN 1423-0070, Vol. 28, no 1, p. 63-76Article in journal (Refereed) Published
Abstract [en]

The study reported herein builds on occupant response to an intermittent air jet strategy (IAJS), which creates periodic airflow and non-isothermal conditions in the occupied zone.  Previous research has highlighted the benefits of IAJS on thermal climate and supports energy saving potential in view of human thermal perception of the indoor environment. In this study, the goal was to explore occupant acceptability of air movements and perceived indoor air quality, and to determine a way of assessing acceptable air movement conditions under IAJS. Thirty-six participants were exposed to twelve conditions: three room air temperatures (nominal: 22.5, 25.5 and 28.5 oC), each with varied air speeds (nominal: <0.15 m/s under mixing ventilation (MV), and 0.4, 0.6 and 0.8 m/s under IAJS) measured at the breathing height (1.1 m). The results show that participants preferred low air movements at lower temperatures and high air movements at higher temperatures. A model to predict percentage satisfied with intermittent air movements was developed, and predicts that about 87% of the occupants within a thermal sensation range of slightly cool (-0.5) to slightly warm (+0.5), in compliance with ASHRAE standard 55, will find intermittent air movements acceptable between 23.7 oC and 29.1 oC within a velocity range of 0.4 – 0.8 m/s.  IAJS also improved participants’ perception of air quality in conditions deemed poor under MV. The findings support the potential of IAJS as a primary ventilation system in high occupant spaces such as classrooms. 

Keywords
Intermittent air jets, Air movement acceptability, Perceived air quality, High occupant density
National Category
Energy Engineering
Identifiers
urn:nbn:se:hig:diva-23753 (URN)10.1177/1420326X17732613 (DOI)000454140000006 ()2-s2.0-85042402784 (Scopus ID)
Available from: 2017-03-15 Created: 2017-03-15 Last updated: 2019-01-28Bibliographically approved
Yin, S., Fan, Y., Sandberg, M. & Li, Y. (2019). PIV based POD analysis of coherent structures in flow patterns generated by triple interacting buoyant plumes. Building and Environment, 158, 165-181
Open this publication in new window or tab >>PIV based POD analysis of coherent structures in flow patterns generated by triple interacting buoyant plumes
2019 (English)In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 158, p. 165-181Article in journal (Refereed) Published
Abstract [en]

The paper performs an investigation on multi-scale coherent structures that exist in different flow patterns of triple interacting plumes. Time-averaged and turbulent flow characteristics are first obtained by the two-dimensional (2-D) particle image velocimetry (PIV) technique. Proper orthogonal decomposition (POD) method is then applied to extract the dominant coherent structures. Energy contributions, POD modes, and POD coefficients are systematically explored to understand the spatial-temporal characteristics of the coherent structures. The first three POD modes are most important in the turbulent kinetic energy (TKE). Mode 1, as the most dominant mode, can contain about 10% TKE in an average sense. In addition, the first 12–14 POD modes can totally contribute 50% TKE. Hierarchical four-scale flow structures, independent of the flow patterns, are clearly identified when mode number increases, which are successively characterized by main flows (the 1st scale), larger-scale outer-side vortex rings (the 2nd scale), shear-layer vortex shedding (the 3rd scale), and smaller-scale homogeneous turbulent vortices (the 4th scale). Frequency spectra analysis confirms the co-existence of lower-frequency and higher-frequency periodic cycles in one certain POD mode. Dominant frequencies of the first three POD coefficient series (a1–a3) fall into the range of 0.025–0.187 Hz. Probability density analysis has identified negative/positive peak probability densities of the series a1. Cumulative probabilities of negative and positive coefficients are approximately equivalent during the sampling period. 

Keywords
Coherent structure, Energy contribution, Flow pattern, POD coefficient, POD mode, Triple interacting plumes, Flow fields, Kinetic energy, Kinetics, Principal component analysis, Probability, Shear flow, Turbulent flow, Velocity measurement, Vortex flow, POD modes, Flow patterns, decomposition analysis, particle image velocimetry, plume, probability density function, sampling, spatiotemporal analysis, spectral analysis, vorticity
National Category
Civil Engineering
Identifiers
urn:nbn:se:hig:diva-30534 (URN)10.1016/j.buildenv.2019.04.055 (DOI)000468894500015 ()2-s2.0-85065614068 (Scopus ID)
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-22Bibliographically approved
Buccolieri, R., Sandberg, M., Wigö, H. & Di Sabatino, S. (2019). The drag force distribution within regular arrays of cubes and its relation to cross ventilation – Theoretical and experimental analyses. Journal of Wind Engineering and Industrial Aerodynamics, 189, 91-103
Open this publication in new window or tab >>The drag force distribution within regular arrays of cubes and its relation to cross ventilation – Theoretical and experimental analyses
2019 (English)In: Journal of Wind Engineering and Industrial Aerodynamics, ISSN 0167-6105, E-ISSN 1872-8197, Vol. 189, p. 91-103Article in journal (Refereed) Published
Abstract [en]

A novel set of wind tunnel measurements of the drag force and its spatial distribution along aligned arrays of cubes of height H and planar area index λ p (air gap between cubes) equal to 0.028 (5H) to 0.69 (0.2H) is presented and analysed. Two different types of measurements are compared: one type where the drag force is obtained using the standard load cell method, another type where the drag force is estimated by measuring the pressure difference between windward and the leeward façades. Results show that the drag force is nearly uniformly distributed for lower λ p (0.028 and 0.0625), it decreases up to 50% at the second row for λ p = 0.11, and it sharply decreases for larger λ p (from 0.25 to 0.69) where the force mostly acts on the first row. It follows that for the lowest λ p the drag force typically formulated as a drag area corresponds to the total frontal area of the array, whereas for large λ p the drag area corresponds to the area of the first row. By assessing the driving pressure for ventilation from the drag force, the analysis is extended to estimate the cross ventilation as an example of application of this type of measurements. 

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Cross ventilation, Cubic building arrays, Drag area, Drag distribution, Interference factor, Standard load cell, Geometry, Ventilation, Wind tunnels, Aligned arrays, Cubic building, Experimental analysis, Pressure differences, Standard loads, Wind tunnel measurements, Drag
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:hig:diva-30516 (URN)10.1016/j.jweia.2019.03.022 (DOI)000467392500008 ()2-s2.0-85064004026 (Scopus ID)
Funder
EU, Horizon 2020, 689954
Available from: 2019-08-19 Created: 2019-08-19 Last updated: 2019-08-20Bibliographically approved
Sandberg, M., Wigö, H. & Kabanshi, A. (2018). Is Building Ventilation a Process of Diluting Contaminants or Delivering Clean Air?. In: Risto Kosonen, Mervi Ahola and Jarkko Narvanne (Ed.), Excellent Indoor Climate and High Performing Ventilation: . Paper presented at Roomvent and Ventilation 2018, 2-5 June 2018, Aalto University, Espoo, Finland (pp. 253-258).
Open this publication in new window or tab >>Is Building Ventilation a Process of Diluting Contaminants or Delivering Clean Air?
2018 (English)In: Excellent Indoor Climate and High Performing Ventilation / [ed] Risto Kosonen, Mervi Ahola and Jarkko Narvanne, 2018, p. 253-258Conference paper, Published paper (Refereed)
Abstract [en]

The purpose of the paper is to discuss the performance of air distribution systems intended for dilution of contaminants and those intended for delivery of clean air to local regions within rooms. At first the systems are distinguished by their visiting frequency behaviour. The performance of the systems with respect to their possibility to influence the concentration due to contaminants is dealt with by the concept dilution capacity for mixing systems and by introduction of the concept delivery capacity for systems intended for delivery of clean air locally. Various ways of realizing systems for supply of clean air to regions within a room are presented and their pros and cons are discussed.  The most important single parameter is the entrainment of ambient air into the primary flow that drives the airflow in the room.   

Keywords
probability to return, visitation frequency, dilution capacity, delivery capacity, entrainment, mixing factor due to entrainment
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hig:diva-26850 (URN)978-952-5236-48-4 (ISBN)
Conference
Roomvent and Ventilation 2018, 2-5 June 2018, Aalto University, Espoo, Finland
Note

Digital, password-protected proceedings

Available from: 2018-06-07 Created: 2018-06-07 Last updated: 2018-06-11Bibliographically approved
Kabanshi, A., Sandberg, M. & Wigö, H. (2018). Measurement of Entrainment into an Axisymmetric Jet using Temperature as a Tracer: A Pilot Study. In: Risto Kosonen, Mervi Ahola and Jarkko Narvanne (Ed.), Excellent Indoor Climate and High Performing Ventilation: . Paper presented at Roomvent and Ventilation 2018, 2-5 June 2018, Aalto University, Espoo,Finland (pp. 397-402).
Open this publication in new window or tab >>Measurement of Entrainment into an Axisymmetric Jet using Temperature as a Tracer: A Pilot Study
2018 (English)In: Excellent Indoor Climate and High Performing Ventilation / [ed] Risto Kosonen, Mervi Ahola and Jarkko Narvanne, 2018, p. 397-402Conference paper, Published paper (Refereed)
Abstract [en]

The current extended abstract is a pilot study of an ongoing experimental and theoretical investigation of ambient entrainment of room air into an axisymmetric free jet using temperature as a tracer. The project aims to investigate, by revisiting the concepts and fundamentals of axisymmetric free Jets and entrainment in ventilation applications, particularly focusing on how to optimize performance of low mixing air distribution systems and to test methods of measuring entrainment in such systems. The study aims to explore a scalar field method using temperature as a tracer to estimate entrainment in axisymmetric free Jets. The results obtained show jet characteristics that slightly differ from what is reported in velocity field measurements and other scalar field studies. Thus, a call is made herein for further investigations to understand entrainment and appropriate methods to determine jet characteristics and its mixing effect. Additionally, more studies are needed to verify whether earlier results are representative of entrainment conditions for low mixing ventilation systems whose operation mode depend on near-filed characteristics of jets.

Keywords
Entrainment, jets, near-field, passive tracer, temperature, delivery capacity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hig:diva-26849 (URN)978-952-5236-48-4 (ISBN)
Conference
Roomvent and Ventilation 2018, 2-5 June 2018, Aalto University, Espoo,Finland
Note

Digital, password-protected proceedings

Available from: 2018-06-07 Created: 2018-06-07 Last updated: 2018-06-11Bibliographically approved
Yin, S., Li, Y., Fan, Y. & Sandberg, M. (2018). Unsteady large-scale flow patterns and dynamic vortex movement in near-field triple buoyant plumes. Building and Environment, 142, 288-300
Open this publication in new window or tab >>Unsteady large-scale flow patterns and dynamic vortex movement in near-field triple buoyant plumes
2018 (English)In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 142, p. 288-300Article in journal (Refereed) Published
Abstract [en]

Unsteady flow patterns of interacting buoyant plumes are important for buoyant ventilation and particularly influence pollutant and heat transports in indoor and outdoor environments. This study reveals fundamental large-scale flow patterns in triple building plumes, investigates vortex moving trends during the pattern transition processes, and explores possible mechanisms of pattern diversity by two-dimensional (2-D) Particle Image Velocimetry (PIV). Total five tests are studied, including three different heat strengths Q (180, 90, and 30 W) and three source layouts characterized by the ratios of source spacing S to source width W (0.2, 0.5, and 1.0). Streamline distributions and axial velocity profiles clearly reveal three fundamental global flow patterns: a right-slanting asymmetrical flow pattern, a left-slanting asymmetrical flow pattern, and an axisymmetric flow pattern. Correspondingly, it indicates four basic transition processes, i.e., right-to-center, left-to-center, center-to-right, and center-to-left transitions (“center” represents the axisymmetric pattern). A novel vortex tracking method, based on lambda-2 (λ2) criterion and principles of the PIV technique, is developed and successfully applied to qualitatively track the vortex moving trends during the transition processes. The regular vortex moving trends are found to be reasonably consistent with the global pattern transition trends. The flow pattern diversity is speculated to be mainly driven by unstable heat source wall flows and downstream swaying motions in this study. These critical unstable motions are considered to probably relate to unstable lateral entrainment and vortex interaction, particularly beside the central plume. Consistently, the regular vortex moving trends are usually observed in and around the central plume.

Place, publisher, year, edition, pages
Elsevier Ltd, 2018
Keywords
Dynamic vortex movement, PIV measurement, Transition mechanism, Triple buoyant plumes, Unsteady flow patterns, Vortex tracking, Buoyancy, Unsteady flow, Velocity measurement, Vortex flow, Buoyant plume, Dynamic vortices, PIV measurements, Flow patterns
National Category
Energy Systems
Identifiers
urn:nbn:se:hig:diva-27515 (URN)10.1016/j.buildenv.2018.06.027 (DOI)000445440600026 ()2-s2.0-85048721494 (Scopus ID)
Note

This work is supported financially by a RGC CRF project (grant no: HKU9/CRF/12G ) of the Hong Kong SAR government.

Available from: 2018-07-04 Created: 2018-07-04 Last updated: 2018-11-23Bibliographically approved
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