Realistic solar heating in urban areas: Air exchange and street-canyon ventilation

• The differential heating is parameterized with horizontal and vertical Ri numbers.
• Rih and Riv indicate atmospheric instability and solar tilt with respect to the wind.
• Flow is significantly affected by the superposition of wall, roof and ground heating.
• Convective heat transfer coefficient is the highest at the windward building wall.
• Air exchange rate is enhanced by the wall heating in the north-south canyon.

Numerical fluid flow and heat transfer simulations of a three-dimensional idealized urban environment are performed to investigate the effect of realistic non-uniform thermal forcing that is caused by solar insolation and inter-building shadowing. Simulations at different times of day are performed using Large Eddy Simulation (LES) and mean flow and turbulence statistics are investigated as determinants for urban canyon ventilation. The differential surface heating of the building canyon is parameterized using sets of horizontal and vertical Richardson numbers indicating atmospheric instability and solar tilt with respect to the wind direction, respectively. Roof heating, in combination with building walls and ground heating, is shown to be important in the strength and location of the canyon vortex. For example, in case of weak vertical stratification (Riv) and high horizontal temperature gradient opposing the wind direction above the canyon (Rih), the combination of roof heating and ground heating decreases the strength of the canyon vortex. The distribution of local convective heat transfer coefficients (CHTC) on building facets are analyzed. Throughout the day, the windward walls exhibit larger CHTCs and leeward heating enhances CHTC from the roof and windward walls. Additionally, the wall heating in the cross-stream canyon (north-south in our case) enhances the air exchange rate from the canyon, especially when the leeward wall is heated.