A Field Study of the Mean Pressure About a Windbreak

Abstract

To provide additional field data for assessingwindbreak flow models, mean ground-level pressurehas been measured upstream and downstream from along porous fence (height H = 1.25 m, resistancecoefficient k _r = 2.4). Measurements were madeduring periods of near-neutral stability and near-normallyincident flow, with the fence standing on bare soil(roughness length, z ₀ ≈ 0.8 cm;H/z ₀ ≈ 160), or within a plant canopy. The mean pressure field,measured far from the ends of the fence, was foundto be quite insensitive to mean wind direction($\bar \alpha$, zero for perpendicular flow), for|$\bar \alpha$| less than about 25°.

In the absence of a canopy, during each measurementperiod the minimum pressure occurred at the closestsampling location to leeward of the windbreak, thepressure-gradient in most cases beingmaximally-adverse in the immediate lee, and decayingwith increasing downwind distance (x). On one day ofmeasurements, however, the pressure gradient over2 ≈ x/H ≈ 6 (H = windbreak height) resembled theleeward ‘plateau’ identified by Wang and Taklein their numerical studies. Perhaps this‘occasional’ feature was only due to instrumenterror. Nevertheless a ‘plateau’ of sorts wasindicated in similar measurements by Judd andPrendergast (with H = 1.92 m, z ₀ ≈ 1.2 cm;H/z ₀ ≈ 160, k _r ≈ 3). Therefore,existence of a leeward pressure plateau behind athin fence cannot be definitely ruled out.

When the windbreak was placed in a canopy, minimumsurface pressure was displaced downwind. Thisagrees with the wind-tunnel study of Judd, Raupach and Finnigan,and is consistent with a simple simulation reported here.