eTheses Repository - University of Birmingham

ort
where U is the wind component orthogonal to the street direction at level IU and
IU
the constant drag coefficient Cd
is set to 0.4, as explained in Martilli et al. (2002).
• The impact of the surface is taken into account in the shear and buoyant production
terms in the TKE equation. The extra source term for TKE is due to the fact that the
presence of buildings increases the conversion of mean kinetic energy into TKE:
Fe
r
3
V
ort V
IU = Cd
U IU S IU
(Equation 3.11)
• Modification of the turbulent length scales used to calculate the dissipation term in
the TKE equation. This modification is needed because the presence of the
buildings generated vortices which have the same scale as the buildings. The
modification is equivalent to adding a second dissipation terms linked to the scale
of the buildings, which has the net effect of increasing the dissipation rate and the
cascade of mean kinetic energy to TKE.
3.2.2 Calculation of thermodynamic effects
The following thermodynamic effects are calculated:
• The turbulent fluxes of sensible heat from horizontal surfaces (roofs and canyon
floors) are calculated, as done for the momentum fluxes, using MOST, where fh is
from Louis (1979):
70