Wind tunnel measurements of buoyant flows in street canyons

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 0.68 m/s and 2.32 m/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.