An analysis of drag force and moment for upright porous wind fences

Abstract

Upright porous wind fences have been studied extensively because of their wide use. This paper analyzes the drag force and moment of fences to provide corroborating evidence for design of the optimal porosity that has been suggested in previous studies. The analysis of drag force and moment was based on the velocity measurements from scaled simulation tests in a wind tunnel. Particle image velocimetry was employed to provide detailed measurements of the instantaneous velocity fields around fences with different levels of porosity at different free‐stream wind velocities. The data enabled us to calculate drag force and moment of fences using the integral form of momentum equation and the integral form of angular momentum equation. The drag force and moment were converted to dimensionless drag coefficient and moment coefficient. Drag force, moment, drag coefficient and moment coefficient revealed a critical fence porosity of around 0.3, which was consistent with the optimal porosity suggested in previous studies. At this critical porosity, the vertical drag force was zero, and the means of drag coefficients and moment coefficients for different wind velocities attained their maximum. The vertical drag force changed direction when the fence porosity became greater or less than the critical porosity. When the porosity was increased beyond the critical value the drag coefficient and moment coefficient decreased rapidly. Empirical equations were developed to relate drag force and moment to wind velocity, fence porosity and height.