Conditions for thermal circulation in urban street canyons

• We introduce a field experiment involving a mock urban street canyon.
• We study the effect of non-uniform heating on the canyon circulation.
• We introduce a new buoyancy parameter to delineate inertial and buoyancy dominated regimes.
• Three dynamical flow regimes are proposed with clear evidence of the first two.
• Laboratory data from the literature is also considered.

Under conditions of low background winds and high solar radiation, non-uniform heating of building walls and the ground in an urban street canyon may induce thermally driven circulation that competes with inertially driven circulation due to overlying winds. These two types of circulation were studied using a field experiment, wherein a mock building canyon constructed with two rows of north–south aligned shipping containers were subjected to natural differential wall heating and overlying winds of varying magnitude. The site was carefully instrumented, and the measurements and flow visualization were conducted over nine days with varying environmental conditions. A buoyancy parameter, B=(gαΔTH)/(u02[1+(H/L)2]), where gαΔT is the horizontal anomaly of buoyancy arising from differential heating of canyon walls, H the canyon height, L the canyon width and u0 the background velocity, was derived to demarcate thermal and inertial circulation regimes. When B < Bc, where Bc(≈0.05) is a critical value, the inertial circulation prevails, and the canyon velocities scaled by u0 are approximately constant. When B > Bc, the thermal circulation becomes important and at B ≫ Bc, the flow is expected to be independent of u0. An intermediate regime is found in the proximity of Bc, where the scaled velocity is dependent both on overlying flow and buoyancy effects.