A wind fence slows the wind in one place by deflecting it to another. The best windbreaks produce a zone behind the windbreak which will have wind of about a quarter of the speed. For a 50ft (15.24m) high windbreak this zone will extend about 250ft (76m) downwind. From 250ft (76m) to 500ft (152m) the windspeed will increase up to about half that of the incoming wind. Beyond that windspeeds increase quickly.
Dr. Ion Parashivoiu, Department of Mechanical and Industrial Engineering, Concordia University
Although our CEC funded study evaluated two dimensional wakes produced directly behind the VAWT array, our other work on VAWTs indicates that a region of turbulence will develop just above an array of closely spaced turbines. Above that, the increase in wind flowing over the top of the VAWT array will produce a region of wind moving faster than the incoming wind speed at that height. In order to take advantage of the speed-up effect, the VAWT arrays would need to be placed so that the HAWT rotors operate in the higher-speed wind zone without being impacted by the turbulent region.