If you take a walk between tall buildings, or in a narrow mountain pass, you will notice that the same effect is working:
The air becomes compressed on the windy side of the buildings or mountains, and its speed increases considerably between the obstacles to the wind. This is known as a “tunnel effect”.
So, even if the general wind speed in open terrain may be, say, 6 metres per second, it can easily reach 9 metres per second in a natural “tunnel”.
Placing a wind turbine in such a tunnel is one clever way of obtaining higher wind speeds than in the surrounding areas.
To obtain a good tunnel effect the tunnel should be “softly” embedded in the landscape. In case the hills are very rough and uneven, there may be lots of turbulence in the area, i.e. the wind will be whirling in a lot of different (and rapidly changing) directions.
If there is much turbulence it may negate the wind speed advantage completely, and the changing winds may inflict a lot of useless tear and wear on the wind turbine.