Oblique, Stratified Winds about a Shelter Fence. Part I: Measurements

Wind statistics were measured using cup and sonic anemometers, placed upwind and downwind from a porous plastic windbreak fence (height h 5 1.25 m, length Y 5 114 m, resistance coefficient kr0 5 2.4, and porosity p 5 0.45) standing on otherwise uniform land (short grass with roughness length z 0 ; 1.9 cm). Intercomparison with collocated two-dimensional sonic anemometers suggested that, except in strongly stratified winds, cup anemometers (distance constant 1.5 m), subjected to a uniform overspeeding correction (here ;10%), provide a reasonably accurate transect of the mean wind across the disturbed flow region. The measurements, binned with respect to mean wind direction and stratification, establish that the resistance coefficient of a windbreak of this type implies the maximum (or ‘‘potential’’) mean wind reduction, a potential that is realized in neutral, perpendicular flow and for which a semiempirical formula is derived. Obliquity of the approaching wind reduces actual shelter effectiveness below the potential value, as was already known. However, a systematic influence of stratification could only be discriminated in winds that were not too far (say, within about 6308) from perpendicular, under which conditions both stable and unstable stratification reduced shelter effectiveness. The ‘‘quiet zone,’’ in which velocity standard deviations ( su, sy) are reduced relative to the approach flow, was found to extend farther downwind for the normal velocity component (u) than for the parallel component (y).