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Sandberg, Mats
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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)2-s2.0-85058185482 (Scopus ID)
Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-01-07Bibliographically 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)2-s2.0-85042402784 (Scopus ID)
Available from: 2017-03-15 Created: 2017-03-15 Last updated: 2019-01-08Bibliographically approved
Hayati, A., Mattsson, M. & Sandberg, M. (2018). A wind tunnel study of wind-driven airing through open doors. The International Journal of Ventilation
Open this publication in new window or tab >>A wind tunnel study of wind-driven airing through open doors
2018 (English)In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044Article in journal (Refereed) Epub ahead of print
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, 2018
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)2-s2.0-85042108561 (Scopus ID)
Projects
Church project
Available from: 2017-08-28 Created: 2017-08-28 Last updated: 2018-04-25Bibliographically approved
Kabanshi, A. & Sandberg, M. (2018). Entrainment and its Implications on Microclimate Ventilation Systems: Scaling the Velocity and Temperature Field of a Round Free Jet. Indoor Air
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
2018 (English)In: Indoor Air, ISSN 0905-6947, E-ISSN 1600-0668Article in journal (Refereed) Epub ahead of print
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, 2018
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)30500986 (PubMedID)
Available from: 2018-12-02 Created: 2018-12-02 Last updated: 2019-01-07Bibliographically 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
Ma, D., Omer, I., Osaragi, T., Sandberg, M. & Jiang, B. (2018). Why Topology Matters in Predicting Human Activities. Environment and Planning B: Urban Analytics and City Science
Open this publication in new window or tab >>Why Topology Matters in Predicting Human Activities
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2018 (English)In: Environment and Planning B: Urban Analytics and City Science, ISSN 2399-8083Article in journal (Refereed) Epub ahead of print
Abstract [en]

Geographic space is best understood through the topological relationship of the underlying streets (note: entire streets rather than street segments), which enabales us to see scaling or fractal or living structure of far more less-connected streets than well-connected ones. It is this underlying scaling structure that makes human activities or urban traffic predictable, albeit in the sense of collective rather than individual human moving behavior. This power of topological analysis has not yet received its deserved attention in the literature, as many researchers continue to rely on segment analysis for predicting urban traffic. The segment-analysis-based methods are essentially geometric, with a focus on geometric details such as locations, lengths, and directions, and are unable to reveal the scaling property, which means they cannot be used for human activities prediction. We conducted a series of case studies using London streets and tweet location data, based on related concepts such as natural streets, and natural street segments (or street segments for short), axial lines, and axial line segments (or line segments for short). We found that natural streets are the best representation in terms of traffic prediction, followed by axial lines, and that neither street segments nor line segments bear a good correlation between network parameters and tweet locations. These findings point to the fact that the reason why axial lines-based space syntax, or the kind of topological analysis in general, works has little to do with individual human travel behavior or ways that human conceptualize distances or spaces. Instead, it is the underlying scaling hierarchy of streets – numerous least-connected, a very few most-connected, and some in between the least- and most-connected – that makes human activities or urban traffic predictable.

Keywords
Topological analysis, space syntax, segment analysis, natural streets, scaling of geographic space
National Category
Other Engineering and Technologies Social and Economic Geography
Identifiers
urn:nbn:se:hig:diva-26166 (URN)10.1177/2399808318792268 (DOI)2-s2.0-85052568351 (Scopus ID)
Available from: 2018-02-18 Created: 2018-02-18 Last updated: 2018-09-24Bibliographically approved
Ma, D., Sandberg, M. & Jiang, B. (2017). A Socio-Geographic Perspective on Human Activities in Social Media. Geographical Analysis, 49(3), 328-342
Open this publication in new window or tab >>A Socio-Geographic Perspective on Human Activities in Social Media
2017 (English)In: Geographical Analysis, ISSN 0016-7363, E-ISSN 1538-4632, Vol. 49, no 3, p. 328-342Article in journal (Refereed) Published
Abstract [en]

Location-based social media make it possible to understand social and geographic aspects of human activities. However, previous studies have mostly examined these two aspects separately without looking at how they are linked. The study aims to connect two aspects by investigating whether there is any correlation between social connections and users' check-in locations from a socio-geographic perspective. We constructed three types of networks: a people–people network, a location–location network, and a city–city network from former location-based social media Brightkite and Gowalla in the U.S., based on users' check-in locations and their friendships. We adopted some complexity science methods such as power-law detection and head/tail breaks classification method for analysis and visualization. Head/tail breaks recursively partitions data into a few large things in the head and many small things in the tail. By analyzing check-in locations, we found that users' check-in patterns are heterogeneous at both the individual and collective levels. We also discovered that users' first or most frequent check-in locations can be the representatives of users' spatial information. The constructed networks based on these locations are very heterogeneous, as indicated by the high ht-index. Most importantly, the node degree of the networks correlates highly with the population at locations (mostly with R2 being 0.7) or cities (above 0.9). This correlation indicates that the geographic distributions of the social media users relate highly to their online social connections.

National Category
Other Social Sciences Other Civil Engineering
Identifiers
urn:nbn:se:hig:diva-24868 (URN)10.1111/gean.12122 (DOI)000405108800004 ()2-s2.0-85011710648 (Scopus ID)
Note

Funding agency:

Key Laboratory of Eco Planning & Green Building, Ministry of Education (Tsinghua University), China

Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2018-03-22Bibliographically approved
Antoniou, N., Montazeri, H., Wigö, H., Neophytou, M., Blocken, B. & Sandberg, M. (2017). CFD and wind-tunnel analysis of outdoor ventilation in a real compact heterogeneous urban area: evaluation using “air delay”. Building and Environment, 126, 355-372
Open this publication in new window or tab >>CFD and wind-tunnel analysis of outdoor ventilation in a real compact heterogeneous urban area: evaluation using “air delay”
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2017 (English)In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 126, p. 355-372Article in journal (Refereed) Published
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).

Keywords
Air delay, City breathability, Computational Fluid Dynamics (CFD), Large Eddy Simulations (LES), Steady RANS, Urban microclimate
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hig:diva-25585 (URN)10.1016/j.buildenv.2017.10.013 (DOI)000417010000030 ()2-s2.0-85042085549 (Scopus ID)
Available from: 2017-11-24 Created: 2017-11-24 Last updated: 2018-03-15Bibliographically approved
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