Displacement height vs. Obstacle vs. roughness

[Anil]

Hi there,

I'm using WindPRO 2.9 and WAsP 10.2

My WTG has 60 m hub height height. In 50 m distance in main wind direction is a very dense tree line with 15-20 m height.

I was now wondering, how to handle the tree line, as obstacle or as roughness. I'd consider it as obstacle. Should I further consider a displacement height for the WTG?

Regards,
Anil

[Niels Gylling Mortensen]

Hi Anil,
So, your WTG hub center is 2-3H away from the obstacle and at a height of 3-4H; where H is the height of the obstacle. The WAsP obstacle model was not designed for this set-up, where the distance is less than about 5H, see the WAsP help file. Therefore, the model may/will not give realistic results (I doubt it will give any shelter effect at all because of the height of the obstacle relative to the hub height).


Alternatively, you can specify the tree line as adding to the roughness of the terrain. However, because of the short distance between turbine site and roughness change line, the effect will likely be negligible.


So, in summary, WAsP was not really designed for an obstacle being so close to the turbine. In the real world there may be an effect of the tree line, but WAsP is not likely to pick that up. I think you should take a look in the literature and see if you can find a similar situation, or try to model the tree line with another model.


Best regards,
Niels
WAsP support