Stadsventilation
The overall aim of the research project is to create a basis for a new method for planning of a healthy air quality in cities taking into consideration the release of contaminants within the street network of a city.
When assessing air quality, it is not enough to have a local perspective and study a single street. The air that flows into the city flows through the spaces of the city; streets, squares, intersections, etc. and dilutes pollutants and eventually flows out of the city, either via the street network or in an upward air current. Air currents in a vertical direction are positive and are affected by how you build.
The methodology of this project is based on seeing the entire city as a building consisting of rooms without a roof, which is ventilated by the wind that blows towards and through the city. Quantification of the ventilation and removal of pollutants takes place with the same methods and concepts that were developed for buildings.
The advantages are a) methods for quantifying complicated flow with recirculation that also occurs in a city can be used b) ventilation of the individual buildings and the entire city is a connected system dependent on each other and a coherent system view is therefore useful. New methods for benchmarking based on evenly distributed emission of pollutants will be developed. Special focus will be on studying the removal of pollutants, which occurs through the open roofs via velocity fluctuations.
Research Tools and Methods
The project has improved the research infrastructure significantly at University of Gävle in the field of city and urban ventilation research. The wind tunnel has been update with new equipment and new tracer gas techniques, methods and models.
Physical models have been developed that were crusial for the project and will be very useful for continued research in the field of city ventilation (final part of the PhD project, joint research with other research groups):
- Drag force model
- Box and cylinder model
- Ground level surface source with model clusters
Drag force model
Drag force model has been developed to study model clusters in different building area density and directions. Small cubes are mounted on a plate connected to a load cell that can measure the total drag force. With fixed lot area, different amount of the cubes set on the test plate provide different building area densities. The whole plate is rotatable to study various angles compared to the coming flow, which influenced the flow rates, catchment and mean age of air in the street networks within the blocks. This measurement shows how the resistance to the wind vary according to the building area density and the orientations of the street networks.
Box and cylinder model
To study the bi-directional dispersion process at the roof height in densely built cities, models with a single opening on the top were built for fundamental studies. One is a rectangular cuboid box, whose geometry is 12*41*30 cm (B*W*H). Another one is a cylinder, which is 30 cm tall, and the diameter for its top and bottom side is 20 cm. Both models have replaceable openings on their top. With different opening settings, the purging efficiency for the tracer gas inside is measured quantitatively, with the goals to investigate the shear ventilation and the eddy penetration at the roof level within the roof vortex above the building. To ensure the homogeneous distribution of the tracer gas (N2O) inside the model, a long-time constant releasing tracer gas method was developed. The tracer gas is released through a porous tube in the lower part of the model and sampled from the upper part of the model. The fine holes of the porous tube diminished the momentum from the slow releasing of N2O, making it work as a passive source. Additionally, to avoid mass loss of N2O due to the sampling, the sample gas is sent back to the model after it went through the analyzer.
We normalized the ventilation parameter, Purging Flow Rate, with the area of the opening and the upcoming flow velocity at the model height to derive the Normalized Purging Velocity, which indicates the equivalent flow velocity that contributes to the purging process (both by advection and turbulence) the opening.
Ground level surface source with model clusters
The regional dispersion conditions were investigated with the application of the mean age of air and time scalar for removal of contaminants. An idealized condition that the contaminants evenly originated everywhere within an urban region at the pedestrian level is modeled by a homogenous surface emission mounted at the ground level. The surface emission source consists of a porous cover and a cavity under it. The high-pressure tracer gas is injected into the cavity beneath the porous cover, and with the large pressure difference under and above the cover, the tracer gas penetrated through it and smoothly release with negligible initial velocity. The whole source is mounted under a turntable in the work section inside the wind tunnel.
Model clusters with different building area densities and frontal area indexes were built on the source surface, and their influences on the dispersion and the flow field were studied. The distribution of tracer gas is sampled at different heights and locations. The age of air and time scalar for removal at different horizontal planes can be derived based on the concentration measured, presented as contour plots at different levels. Also, the model is suitable for validation of numerical models.
Database/Collection of data
New dataset of wind tunnel measurements with physical model at University of Gävle. The dataset includes the interaction between urban geometries and wind on drag force and
urban air flow. The datasets and findings were presented at the RoomVent 2020 conference in February 2021, Turin, Italy.
A calibrated and validated computational fluid dynamics (CFD) model has been developed to calculate and predict flows features and city ventilation parameters such as catchment, updraft and pressure footprint in street junctions. Wind tunnel measurements were used to validate the CFD model. The CFD predictions provide complete flow, mean age of air and pressure fields for different configurations and wind directions. Models and predictions have so far been published as an abstract paper at ICFEE2020 conference, Kyoto, Tokyo. http://www.icfee.org/2020.html
Also, validated numerical models has been developed to investigate and predict effective mean age of air (air quality). Different urban morphology parameters influence on the effective mean age of air are studied such as equivalent frontal area, building area density, height variability and width variability.
Education and Training
Educational material regarding theories, methods, ventilation indices, and critical urban morphology parameters, based on the project are now used as educational materials in the masterlevel course Sustainable Cities as well as in the post-graduate course Energy systems at University of Gävle. Several lectures are based on the project. Also, some laboratory works in the courses are based on models developed in the project.
https://www.researchgate.net/publication/373807126_City_Ventilation
Yearly demonstrations of the wind tunnel and the models used in the project have been held for high school students, e.g. Internationella Engelska skolan in Gävle.
Communication and activities
Presentation titled Urban Morphology and City Ventilation at the conference ICFEE2020 in Kyoto, Japan.
https://www.researchgate.net/publication/338555825_Urban_Morphology_and_City_Ventilation
Presentation of article in international conference COBEE2022, July 2022, Montreal, Canada. https://www.researchgate.net/publication/373821473_Evaluation_of_the_Equivalent_Purging_Flow_Rate_for_Single-side_Ventilated_Model_with_Tracer_Gas_Measurements
Presentation 8th of May 2019 at the conference Östra Sveriges Luftvårdsförbund – Luftvårdsdagen. https://www.researchgate.net/publication/354211188_Stadsventilation
Presentation 16th of March 2021 at the Formas seminar: Hälsosamma samhällen - luftkvalitet, folkhälsa och grönområden. https://www.researchgate.net/publication/354209757_Formas_seminarium
Presentation titled From Meteorology to City Ventilation given to the high school students at Vasaskolan, Feb 2021 Vasaskolan.pdf Pdf, 4 MB.
Presentation 8th of January 2019 for researcher and students at Department of Architectural Engineering, Division of Global Architecture, Osaka University, Osaka, Japan. Osaka.pdf Pdf, 7 MB.
Presentation 25th of April 2019 for researcher and Master students at Department of Civil and Architectural Engineering, Aarhus University, Aarhus, Denmark. Aarhus.pdf Pdf, 9 MB.
Online presentation 4th February 2022 for Miljöforum, Region Gävleborg, about city ventilation and the competence center Resilient related to the strategic research area Urban Sustainability at HiG. Miljo.pdf Pdf, 3 MB.
Presentation 24th of January 2020 in Uppsala at start-up meeting in the industrial PhD school Future-Proof Cities that is coordinated by University of Gävle. The school builds on a social dialogue between academic community, business sector and public sector.
Presentation 29th of May 2019 about City ventilation within the strategic research area Urban Sustainability (Mathias Cehlin co-director) at University of Gävle. Urban.pdf Pdf, 4 MB.
Presentation of article in international conference Roomvent 2020 in February 2021, Turin, Italy. http://hig.diva-portal.org/smash/record.jsf?pid=diva2:1573818
Article in Hållbart Byggande. https://hallbartbyggande.com/mer-kunskap-om-stadsventilation-ska-ge-battre-luftkvalitet/
Interview Boverket. https://www.boverket.se/sv/samhallsplanering/stadsutveckling/halsa-forst/stadsmiljoer/luftens-omsattning/
Pressrelease HiG. https://hig.se/Ext/Sv/Nyheter-och-press/Press/2018-10-11-Redan-nar-en-ny-stadsdel-planeras-avgors-dess-luftkvalitet.html
Publications
Cehlin, M., et al. (2020). Urban Morphology and City Ventilation. ICFEE2020. https://urn.kb.se/resolve?urn=urn:nbn:se:hig:diva-40631
Buccolieri, R., et al. (2021). Drag force rose representing the interaction between urban geometries and wind. RoomVent2020. https://urn.kb.se/resolve?urn=urn:nbn:se:hig:diva-36559
Lin, Y., et al. (2022). Evaluation of the Equivalent Purging Flow Rate for Single-side Ventilated Model with Tracer Gas Measurements. COBEE2022. https://urn.kb.se/resolve?urn=urn:nbn:se:hig:diva-39649
Projektets löptid
Extern finansiering av Formas under projektets löptid 1 november 2018 - 31 oktober 2022. Fortsatt forskning pågår inom stadsventilation, som bl.a. kommer leda till doktorsavhandling.
Kontakt
Projektledare Mathias Cehlin, docent i energisystem
E-post mathias.cehlin@hig.se Telefon: +46 72 299 55 52
