In the next two figures the side and top view of the wind flow around an obstacle is shown. As it can be seen from this drawing of typical wind flows around an obstacle, the turbulent zone may extend to some three time the height of the obstacle. The turbulence is more pronounced behind the obstacle than in front of it.
Therefore, it is best to avoid major obstacles close to wind turbines, particularly if they are upwind in the prevailing wind direction, i.e. in front of the turbine.
Shelter behind obstacles
Obstacles will decrease the wind speed downstream from the obstacle. The decrease in wind speed depends on the porosity, defined as the open area divided by the total area of the object facing the wind of the obstacle, i.e. how "open" the obstacle is. A building is obviously solid and has no porosity, whereas a fairly open tree in winter with no leaves may let more than half of the wind through. In summer, however, the foliage may be very dense, so as to make the porosity less than, say one third.
The slowdown effect on the wind from an obstacle increases with the height and length of the obstacle. The effect is obviously more pronounced close to the obstacle and close to the ground. When manufacturers or developers calculate the energy production for wind turbines, they always take obstacles into account if they are close to the turbine, say less than 1 km away in one of the more important wind directions.
This graph gives an estimate of how wind speeds decrease behind a blunt obstacle, i.e. an obstacle which is not nicely streamlined. In this case the obstacle is a seven story office building, 20 m tall and 60 m wide placed at a distance of 300 m from a wind turbine with a 50 m hub height.
The wind shade can be shown as different shades of grey. The blue numbers indicate the wind speed in per cent of the wind speed without the obstacle. At the top of the yellow wind turbine tower the wind speed has decreased by some 3%, thus to 97% of the speed without the obstacle. Note that this means a loss of wind energy of some 10%.