A WIND-TUNNEL STUDY OF TURBULENT-FLOW AROUND SINGLE AND MULTIPLE WINDBREAKS .1. VELOCITY-FIELDS

This paper describes wind-tunnel experiments on the flow around single and multiple porous windbreaks (height H), sheltering a model plant c anopy (height H/3). The mean wind is normal to the windbreaks, which s pan the width of the wind tunnel. The incident turbulent flow simulate s the adiabatic atmospheric surface layer. Five configurations are exa mined: single breaks of three solidities (low, medium, high; solidity = 1 - porosity), and medium-solidity multiple breaks of streamwise spa cing 12H and 6H. The experimental emphases are on the interactions of the windbreak flow with the underlying plant canopy; the effects of so lidity; the differences in shelter between single and multiple windbre aks; and the scaling properties of the flow. Principal results are: (1 ) the ''quiet zones'' behind each windbreak are smaller in multiple th an single arrays, because of the higher turbulence level in the very r ough-wall internal boundary layer which develops over the multiple arr ays. Nevertheless, the overall shelter effectiveness is higher for mul tiple arrays than single windbreaks because of the ''nonlocal shelter' ' induced by the array as a whole. (2) The flow approaching the windbr eak decelerates above the canopy but accelerates within the canopy, pa rticularly when the windbreak solidity is high. (3) A strong mixing la yer forms just downwind of the top of each windbreak, showing some of the turbulence and scaling properties of the classical mixing layer fo rmed between uniform, coflowing streams. (4) No dramatic increase in t urbulence levels in the canopy is evident at the point where the deepe ning mixing layer contacts the canopy (around x/H = 3) but the charact eristic inflection in the canopy wind profile is eliminated at this po int.