A wind tunnel simulation of the mean velocity fields behind upright porous fences

Porosity is the most important parameter that determines the efficiency of wind fences. The present study provided a deeper understanding of mean flow regime behind fences with different porosities at different wind velocities by means of a scaled wind tunnel simulation. Velocities were measured using particle image velocimetry and the mean velocity field was obtained and discussed. The mean velocity fields obtained at different wind velocities were similar. Analyzing the streamline patterns revealed an inherent link between fence porosity and mean airflow characteristics behind the fence. The optimal fence porosity is considered to be the critical porosity above or below which airflow characteristics differ strongly. According to the present study, the optimal porosity is found to be around 0.2 or 0.3, which corresponds to a critical porosity above which bleed flow dominates and below which reversed flow becomes significant. The parameters characterizing the reverse cell behind fences were well correlated with porosity. The velocity profiles revealed seven typical flow regions behind fences, characterized by different velocity gradients. The airflow becomes less complicated and the number of flow regions decreases as fence porosity increases. Some regions, especially the reverse cell and small vortex, disappear when the porosity exceeds a certain value. The flow regions gradually merge as the distance downwind increases, and eventually recover a single velocity profile due to downward transfer of momentum from overlying layers. The recovery distance decreases with increasing fence porosity.