Laboratory measurements of velocity and turbulence field behind porous fences

Flow characteristics of turbulent wake behind porous fences have been investigated experimentally. The velocity fields were measured using the two-frame PTV method in a circulating water channel. The fence models used in this study have geometric porosity (epsilon) of 0%, 20%, 40% and 65%, respectively. Each fence model was located in uniform flow whose boundary layer thickness (delta) at the fence location was about 0.1 of the fence height (H). Among the porous fences used in this study, the porous fence with porosity epsilon = 20% shows the maximum reduction of mean streamwise velocity, but it has the highest vertical mean velocity at about x/H = 1 location and large turbulence intensity in the near wake region. However, the porous fence with epsilon = 40% has good flow characteristics for abating wind erosion with small turbulent fluctuations and a relatively large reduction in mean velocity. Except for the solid fence (epsilon = 0%), two shear layers develop from the porous fences. As the fence porosity (epsilon) increases, the height of the shear layer and the streamline curvature decrease. When the porosity (epsilon) is greater than 40%, there is no re-circulation flow behind the fence due to the strong bleed flow, the Reynolds shear stress is nearly negligible in the entire near-wake region and relatively small turbulent kinetic energies are concentrated in the region just behind the fence (x/H < 0.5). When the fence porosity is less than 20%, the Reynolds shear stress and turbulent kinetic energy are strong over the fence and in the shear layer near the reattachment region. (C) 1999 Elsevier Science Ltd. All rights reserved.