On shelter efficiency of shelterbelts in oblique wind

The changes of shelter effects in oblique flows are studied by numerical simulations. The simulated results show that horizontal profiles of wind speed and the location of the minimum wind speed (i,e., maximum wind reduction) move toward the shelterbelt when approach flows depart from the normal, and wind speed may exceed the undisturbed wind speed in the middle lee because of the channeling effect of shelterbelts. With increasing wind incidence angle (IA), the minimum wind speed may decrease or increase, and the rate of decrease in the shelter distance may be faster or slower than cos(IA), both depending on the height of observation and the density and width of shelterbelts, The mean wind speed reduction over 30H (H is shelterbelt height) leeward shows similar characteristics to the shelter distance. The change of shelter effects in oblique flows may result from (i) change of effective shelterbelt density, (ii) different efficiencies in reducing wind speed in directions perpendicular and parallel to the belt, and (iii) change of horizontal wind direction as the flow recovers to the undisturbed direction. The relative importance of each factor, which depends on the height of observation and the density and width of shelterbelts, determines the variation of shelter effects. The simulations produced all the observed qualitative characteristics of shelter effects and for those previous reports giving measured values, the model produced results that are in good agreement.