No abstract

At day-times building façades and ground surfaces are heated by solar radiation. Due to the increased surface temperatures, buoyancy is induced which changes the flow field around buildings significantly. Wind tunnel measurements were conducted to study the influence of buoyancy on the flow in a scaled urban street canyon with heated surfaces.Particle image velocimetry was used to measure the flow field in a section of the street canyon. The two wall and the bottom surfaces of the street canyon were heated either individually or all together. A wide range of Froude numbers between 0.65 and 17.3 was covered with surfaces temperatures raised up to 70°C -130°C and freestream velocities between 0.68m/s and 2.32m/s. The velocity and turbulent kinetic energy (TKE) fields were analysed, and for some cases also the air temperatures inside the street canyon were measured. For most cases one main vortex is formed in the centre of the street canyon. This main vortex is strengthened, and the TKE inside the street canyon increased by, heating of (in order of importance) the ground, the leeward wall, and all three surfaces for low freestream velocities. For windward wall heating a second counter-rotating vortex is formed due to buoyancy and the flow direction close to the windward wall changes from a downward to an upward motion. The centres of the main and secondary vortex change their position for different windward wall temperatures with increasing freestream velocities. For low Froude numbers the air exchange rate is increased due to buoyancy.

An important part of the world's energy is used for space cooling and heating of buildings. Its minimization has great energy saving potential. An important part of the heat exchange between buildings and the ambient surrounding is due to convective and radiative heat flows. In this study detailed building energy simulation (BES) is used to analyse the effect of neighbouring buildings on these heat flows and their influence on the space cooling and heating demand of buildings. BESs were conducted for stand-alone buildings and buildings in street canyon. This study demonstrates the importance of accounting for the urban microclimate for the prediction of the energy demand of buildings. With the proposed model most of the thermal effects of the urban microclimate can be captured and quantified on street canyon scale. Due to multiple reflections more solar and thermal radiation is absorbed at the façades of buildings in street canyons than at façades of stand-alone buildings. These effects cause higher surface temperatures in street canyons leading to higher space cooling and lower space heating demands. Other reasons are the lower convective heat transfer coefficients in street canyons, the reduced removal of heat from street canyons and the urban heat island effect.

Computational fluid dynamics (CFD) is often used to predict flow structures in urban areas for the determination of pollutant dispersion, human comfort or heat fluxes. During daytime building façades and ground surfaces are heated by solar radiation and thereby induce buoyancy, which changes the flow field around buildings significantly. The CFD models used to simulate buoyant flow fields in urban areas are not sufficiently validated. This study aims to validate CFD simulations for buoyant flows in urban street canyons by comparison with wind tunnel measurements. 2D steady RANS (Reynolds-Averaged Navier-Stokes) CFD simulations were conducted with different near-wall treatments. Velocity, turbulent kinetic energy and temperature profiles from CFD were compared with the measured flow fields (measurement technique: particle image velocimetry). Isothermal cases as well as cases with leeward wall or windward wall heating or all surfaces heated were considered. The results show that CFD can predict the general flow structures and the influence of buoyancy. The detailed flow field inside the street canyon is strongly dependent on the flow structure within the shear layer at the top of the street canyon. Therefore to get accurate results for the flow profiles inside the street canyon, the flow within the shear layer has to be predicted correctly.