Wind Obstacles

This movie was shot at a coastal wind site with the wind coming from the right side of the picture. It shows an interesting phenomenon:
We would really expect the wind turbine to the right (which is facing the wind directly) to be the one to start first when the wind starts blowing. But you can see, that the wind turbine to the right will not start at the low wind speeds which are sufficient to drive the other two wind turbines. The reason is the small wood in front of the wind turbines which shelters the rightmost turbine in particular. In this case, the annual production of these wind turbines is probably reduced by some 15 per cent on average, and even more in case of the rightmost turbine.
(The turbines are located some five rotor diameters apart, and the wood is located at a similar distance from the first wind turbine. The reason why the turbines look like they are standing very close together, is that the movie was shot from about a mile away with the equivalent of a 1200 mm lens for a 35 mm camera).

Side view of wind flow around an obstacle.
Note the pronounced turbulent airflow downstream

Obstacles to the wind such as buildings, trees, rock formations etc. can decrease wind speeds significantly, and they often create turbulence in their neighbourhood.
As you can see 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.

Top view of wind flow around an obstacle.

Shelter Behind Obstacles
Obstacles will decrease the wind speed downstream from the obstacle. The decrease in wind speed depends on the porosity of the obstacle, i.e. how "open" the obstacle is. (Porosity is defined as the open area divided by the total area of the object facing the wind).
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 kilometre away in one of the more important wind directions.

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