Oblique, Stratified Winds about a Shelter Fence. Part II: Comparison of Measurements with Numerical Models

Abstract

To evaluate Reynolds-averaged Navier–Stokes (RANS) models of disturbed micrometeorological winds, steady-state computations using a second-order closure are compared with observations (see Part I) in which the surface layer wind was disturbed by a long, thin porous fence (height h = 1.25 m; thickness dx ≈ 1 mm). Starting with the case of neutral stratification and normal incidence, it is shown that low-resolution RANS simulations (streamwise grid interval Δx/h = 1) produce reasonably good transects of mean wind speed, though with an ambiguity (or nonuniqueness) of at least 10%–15% of the amplitude of the relative wind curve, mainly arising from sensitivity to the choice of the solution mesh. For nearly perpendicular flows, the measured influence of stratification (stable or unstable) is to diminish the amplitude of the relative wind curve (i.e., windbreak is less effective), an effect that is replicated very well by the simulations. Obliquity of the incident wind, like stratification, also correlates with poorer shelter, but the computed response of the relative wind curve to obliquity is excessive. As for higher-order wind statistics, computed transects of velocity standard deviations compare poorly to those observed. Therefore, if this disturbed flow may be taken as representative, then caution must be recommended should it be thought that RANS-type models might be suitable (i.e., accurate, as well as convenient) for computing the disturbed wind statistics (typically mean velocity, shear stress tensor, and turbulent kinetic energy dissipation rate) that are needed to “drive” modern dispersion models in the complex wind regimes that must be confronted in such contexts as urban dispersion, or the wind migration of pollen.