Vorticity cancellation

When there are marked variations in the height of adjacent obstacles, the wake vorticity shed from upwind buildings can produce sharp down-flows and increased trailing vorticity in the flow direction, (see Lawson, 1980 [360]). These effects contribute to mixing between the canopy and external flow, Figure 2.6. When tall cuboid obstacles are closely packed (i.e. H/w > 1, b/d > 1/2), as the air flow passes around them the wakes tend to disappear (because of cancellation of vorticity) and the streamlines are determined simply by the displacement or blocking effects of the buildings (Davidson et al., 1995 [143] Moulinec et al., 2003 [436]). 

In summary, the omega equation shows that ris-ing/sinking motion is proportional to the rate of increase with height of positive/negative vorticity advection plus warm/cold advection. A disadvantage of the omega equation expressed in the form of Eq. (112) is that the vorticity advection and the thermal advection are not independent and tend to cancel one another. 

---