Publikationen / Ergebnisse
(Englisch)
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The project deals with the modelling of urban microclimate in street canyons and urban neighbourhoods taking into account in particular combined effects of wind and solar radiation. Modelling and simulation are performed using both building energy simulation (BES) and computational fluid dynamics (CFD) techniques. Validation experiments in selected isothermal and non-isothermal cases are undertaken (air flow patterns, temperature distributions) in the Empa/ETH atmospheric boundary layer wind tunnel. With the enhanced understanding of the flow and heat transfer phenomena and the respective models developed, the impact of the urban climate on daytime and night-time (passive cooling) ventilation potentials as well as on heating and cooling demand of buildings are investigated.
In the present phase of the project, BES and CFD simulations were performed for typical street canyon situations. Methods to reduce the computational demand for the CFD simulations are crucial for coupled simulations. Therefore customized wall function modelling was developed and applied, thus better allowing for coupled BESCFD simulations. A wind tunnel street canyon model with heated walls was constructed and is presently used for time-resolved flow experiments in the wind tunnel using the particle image velocimetry system, for both forced and mixed convection (considering buoyancy along the heated building façades). Measured results were compared with results from CFD simulations. Furthermore the analysis of urban heat island (UHI) effect on heating and cooling degree days was completed. Major drivers for UHI effects for seasonal and diurnal patterns, and possible measure to mitigate such effects were identified.
In the year 2012, the wind tunnel tests will be continued and BES-CFD coupled simulations will be performed for typical street canyon situations. The results will then be used to quantify the energy impact of urban morphology and UHI effects for a number of configurations. The results of the project will be published in a PhD thesis, at conferences, and in scientific journal articles.
Auftragnehmer/Contractant/Contraente/Contractor:
EMPA
Basler & Hofmann AG
Autorschaft/Auteurs/Autori/Authors:
Dorer,Viktor
Allegrini,J.
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Schlussbericht
(Englisch)
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In the past decades the portion of the population living in urban areas has continuously increased. Due to the high building density, the microclimate in urban areas changed signi?cantly compared to rural areas. The temperatures measured in urban areas are, due to the urban heat island (UHI) e?ect, higher compared to the rural temperatures. The UHI intensities are increasing with higher building densities and growing cities. Space cooling and heating demands of buildings are strongly a?ected by the local microclimate at the building sites. Due to the climate change and the limited energy resources, energy saving and sustainability are nowadays important issues. A signi?cant part of the global energy consumption is used for space cooling and heating of buildings. Thus its minimization for buildings in urban areas has great energy saving potential. The aim of this project is to investigate the impact of the urban microclimate on the energy demand of buildings in an urban context by conducting detail building energy simulations (BESs). Most existing BES models were developed for stand-alone buildings and therefore do not consider e?ects of the urban microclimate. For this project a BES model is adjusted in such a way that it can account for the urban microclimate. The three main aspects of the urban microclimate are (in order of importance): (i) the radiation exchange between neighbouring buildings, (ii) the UHI e?ect and (iii) the reduced convective heat transfer due to wind sheltering. To consider the urban microclimate aspect of solar and longwave radiation exchange between neighbouring buildings in BES, the radiation model implemented in the BES model is used. This model was originally developed only for interior spaces. To be able to use this radiation model for outdoor spaces, the street canyons are modelled as outside ”atria” with open ceiling, imposing speci?c boundary conditions. The radiation model accounts for multiple re?ections of solar and longwave radiation between building envelope elements. To consider urban heat island e?ects, diurnal UHI intensity schedules are developed based on data measured in the city Basel (Switzerland) in the frame of the BUBBLE campaign. With these data, local urban heat intensities were quanti?ed and correlated with urban morphology types, typical primarily for the city of Basel but basically for many Swiss cities. These urban heat island e?ects, the relation to the urban morphology and the impacts on heating and cooling degree days were studied in the frame of a subproject and are documented in a separate report. To consider the third urban climate aspect, the reduced convective heat transfer at the building fa¸cades, computational ?uid dynamic (CFD) simulations are conducted. Correlations for convective heat transfer coe?cients (CHTCs) are derived as a function of the reference wind speed for di?erent stand-alone and urban geometric building con?gurations. Then BESs are conducted for the climate of Basel for di?erent stand-alone and street
canyon con?gurations and for di?erent o?ce and also residential building types. A strong in?uence of the urban microclimate on the space heating and especially on the space cooling demands of buildings can be observed. The changes of building energy demands for di?erent local microclimates may be in the same order of magnitude as the demands for the stand-alone building themselves. This shows the importance of accounting for the local microclimate, when predicting with BES the energy demands for buildings in urban areas. Finally BESs are conducted for di?erent climates. The general trends, of how the local microclimate in?uences the space cooling and space heating demands, are similar for di?erent climates. Following these BES related issues, the report gives a detailed coverage of the work done in relation to CFD modelling of convective heat transfer at building surfaces. This heat transfer is strongly dependent on the ?ow in the boundary layer of the surface. In the street canyons chosen as a generic urban con?guration, building fac¸ades are heated by solar radiation, thus inducing buoyancy, which has to be considered in the CFD simulations. When using Reynolds-averaged Navier-Stokes (RANS) CFD simulations, there exist mainly two approaches to model the boundary layers at walls. The ?rst approach is to resolve the boundary layers with low-Reynolds number modelling (LRNM) using a ?ne mesh in the near-wall region. Because LRNM is computationally expensive, for a wide range of applications wall functions (WFs) are preferred, which model part of the boundary layer with empirical equations. With WFs computational power can be saved using coarser meshes. However, standard wall functions may show important errors predicting the wall heat ?uxes. Therefore, in the frame of this project, a new temperature wall function is developed, improving substantially the heat transfer predictions of RANS CFD simulations. Then, as outlined above, for the BES, correlations for convective heat transfer coe?cients (CHTCs) are derived as a function of the wind speed for di?erent stand-alone and urban geometric building con?gurations. In addition to simulations using the CHTC correlations derived from the case speci?c CFD simulations, also coupled BES-CFD simulations are conducted, where at each BES time step a CFD simulation is conducted and the results are transferred to the BES, to determine the convective heat ?ux at the building fa¸cades. With coupled BES-CFD simulations the BES results can be improved, especially for cases where no case speci?c CHTC correlations are available. To validate the CFD simulations of buoyant ?ows in street canyons, measurements on a street canyon model with heated surfaces are conducted in the Empa/ETH atmospheric boundary layer wind tunnel. For the validation the velocities and turbulent kinetic energies of the CFD simulations are compared with the values of the ?ow ?elds measured by particle image velocimetry (PIV). With CFD the general ?ow structure can be captured. Di?erences in the detailed ?ow ?elds between the CFD and the measurements may originate from errors in the predictions of the wind velocities at the top plane of the street canyon. In conclusion it was demonstrated that the urban microclimate has to be considered in su?cient detail to accurately predict building energy demands for space cooling and heating of buildings in urban areas. With the approach developed in this project and documented in this report, most of the thermal e?ects of the urban microclimate can be captured and quanti?ed on street canyon scale. Finally, in the outlook, the extension of this approach to larger urban scales is discussed.
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