SE--Structures and Environment Wind Tunnel Study on Wind Speed Reduction through Successive Synthetic Windscreens

A wind tunnel study was carried out to evaluate the effect of successive windscreens perpendicular to the wind direction on wind speed reduction. Two types of windscreens, differing in essence only in open area, were used in the experiments. Their open areas were respectively 35·7 and 57·5%. The windscreens were modelled in a wind tunnel and had a height H of 10 cm. Experiments were carried out with two and three consecutive screens of the same nature. Both screen types were used in the experiments with two consecutive screens at a mutual spacing of 24 H while the less-open windscreen was also used in experiments with two and three consecutive screens at a spacing of 12 H. A free wind speed of 6·7 m s-1was applied during the tests. Wind speed was measured midway between the tunnel walls with vane probes at one location windward and various locations downwind of the first screen. For each location, wind speed was measured at four heights above the bottom surface. The wind tunnel study illustrates clearly that the extension of the lee zone, when two successive screens are used instead of one, depends on the recovery of the wind speed reduction at the second screen. It is about half the screen spacing when the second screen is placed where the wind speed recovery is 0% or where the wind speed is minimum. The extension increases to about one screen spacing when the screen is placed where the wind speed recovery is about 60%. The extension of the lee zone when three screens are installed, also depends on the locations of the second and third screens with respect to the recovery of the wind speed. Furthermore, it follows that the type of screen has little or no effect on the lee distance where the wind velocity has recovered between 60 and 70%. A second screen placed downwind of the first where the maximum wind speed reduction occurs, is less effective than when it is placed where the wind speed is already recovering. Consequently, successive screens should not be spaced too closely in order to be most effective.