Comparison Between Large-Eddy Simulation and Reynolds-Averaged Navier–Stokes Computations for the MUST Field Experiment. Part II: Effects of Incident Wind Angle Deviation on the Mean Flow and Plume Dispersion

Abstract

Large-eddy simulations (LES) and Reynolds-averaged Navier–Stokes (RANS)

computations of pollutant dispersion are reported for the Mock Urban Setting Test (MUST)

field experiment flow. In particular we address the effects of incident wind angle deviation on

the mean velocity and on the mean concentration fields. Both computational fluid dynamical

methods are assessed by comparing the simulation results with experimental field data. The

comparative analysis proposes to relate the plume deflection with the flow channelling effects.

The results show that the plume deflection angle varies with the altitude. As the ground is

approached the plume is shown to be almost aligned with the street canyon direction and

independent of the incident wind directions considered. At higher altitudes well above the

obstacles, the plume direction is aligned with the mean wind direction as in dispersion over

flat terrain. The near-ground plume deflection is the consequence of a strong channelling

effect in the region near the ground. The mean concentration profiles predicted by LES and

RANS are both in good qualitative agreement with experimental data but exhibit discrep-

ancies that can be partly explained by the influence of small incident wind angle deviation

effects. Compared to RANS, LES predicts a higher channelling and thus a higher deflection of

the plume. Results on the fluctuating intensity of the concentration obtained from LES show

a satisfactory agreement with experiments. This information is not available from RANS for

which only the mean concentration modelling is considered.