Effects of Trees on Wind Flow Variability and Turbulence

This paper describes the results of a field experiment at the Goodnoe Hills, Wash. site to examine the effects of trees on wind flow variability and turbulence. Although vegetation at the site consisted primarily of grass, scattered areas of trees that penetrated the site provided an excellent opportunity to evaluate the effects of surface roughness changes on the wind flow characteristics. Wind data collected at nine towers across the site revealed that surface roughness changes in the upwind fetch caused pronounced variations in the wind flow over site. At two towers that were frequently 200 m to 300 m downwind of a grove of 10-m to 18-m trees, 20–30 percent reductions in wind speed and a factor of two to three increase in turbulence were measured at a height of 32 m. A substantial increase in the magnitude of the wind gusts, as well as a considerable decrease in the mean wind speed, was observed when a tower was downwind of the trees. Implications for a wind turbine located downwind of the trees, with a hub height near 30 m, would be reduced power output, more variable power output, more start/stop cycles, and increased stress caused by the tree-induced turbulence. The effects of the trees on the wind flow characteristics were considerably reduced at heights of 60 m and at distances greater than 500 m.