Speed Up Effects: Hill Effect
The wind in passing the summits of mountains becomes swift and dense and as it blows beyond the mountains it becomes thin and slow, like water that issues from a narrow channel into the wide sea. Notebooks of Leonardo da Vinci (1452-1519)
Hill Cartoon
A common way of siting wind turbines is to place them on hills or ridges overlooking the surrounding landscape. In particular, it is always an advantage to have as wide a view as possible in the prevailing wind direction in the area.
On hills, one may also experience that wind speeds are higher than in the surrounding area. Once again, this is due to the fact that the wind becomes compressed on the windy side of the hill, and once the air reaches the ridge it can expand again as its soars down into the low pressure area on the lee side of the hill.
Hill movie with wind arrows
You may notice that the wind in the picture starts bending some time before it reaches the hill, because the high pressure area actually extends quite some distance out in front of the hill.
Also, you may notice that the wind becomes very irregular, once it passes through the wind turbine rotor.
As before, if the hill is steep or has an uneven surface, one may get significant amounts of turbulence , which may negate the advantage of higher wind speeds.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 9 June 2003
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