Simulation of Effect of Wind Barriers on Airflow

Abstract

ABSTRACT LACK of a quantitative theory to explain airflow near wind barriers in the atmospheric boundary layer has hindered experimental programs in barrier research and made optimum barrier design for practical applications difficult. Our objectives were to develop a quantitative, theoretical simulation of airflow normal to narrow wind barriers of various porosities and, when possible, verify the results using experimental data. To simulate the airflow near wind barriers, we used five linked, partial differential equations. The differen-tial equations described the conservation of horizontal momentum, vertical momemtum, mass, turbulence energy, and dissipation rate of turbulence energy. Final-ly, we used an algebraic turbulence model to relate the turbulent viscosity to the turbulent energy and to the tur-bulent energy dissipation rate. We used finite difference methods having a combination of upwind and central difference schemes to solve the equations. As a barrier boundary condition, the porous wind barriers were treated as sources of horizontal velocity. The source strengths for 20-, 40-, and 60-percent-porous slat-fence barriers were determined by measuring the windspeed profiles at 0.5 to 1.0 barrier heights (H) leeward. For experimental verification of the simulation model, windspeed reduction was measured leeward of 20-, 40-, and 60-percent-porous barriers having a ratio of H to a roughness parameter (z0) of H/z0 = 75 and compared with the simulated results. Windspeed reduction data in the literature also were compared with simulated wind-speed reduction with H/z0 = 300. Finally, vertical pro-files of turbulence energy were measured near a 40-percent-porous wind barrier and compared with the simulated results. Treating porous barriers as a source of horizontal velocity appears to be a valid method to obtain useful simulation results because the leeward simulated and measured windspeed patterns generally agreed well. The windspeed profiles measured at 0.5 to 1.0 H leeward of porous barriers can be used as a measure of the source strength; however, it was necessary to average several profiles to obtain an adequate estimate of source strength.