The SST k-u model uses an automated wall treatment [46] for the
ground and building surfaces, which automatically switches between
(1) low-Reynolds number modeling; and (2) standard wall
functions by Launder and Spalding [55] in conjunction with the
sand-grain based roughness (kS) modification defined by Cebeci
and Bradshaw [56]; depending on the mesh resolution near the
wall. For the ground surfaces, the values of the roughness parameters,
i.e. the sand-grain roughness height (ks ¼ 0.14 m) and the
roughness constant (Cs ¼ 7), are determined based on the relationship
with the aerodynamic roughness length z0 derived by
Blocken et al. [44]:
kS ¼ 9:793z0
CS
(5)
The sand-grain roughness height is set to zero (kS ¼ 0 m) for the
building surfaces (smooth walls). At the outlet plane, zero static
gauge pressure is applied and at the top and lateral sides of the
domain zero normal velocities and zero normal gradients of all
variables are imposed.