WAsP-CFD speedups

[fabienfarella]

Hi WAsP Team!
I would like to compare the results of WAsP-CFD with another CFD software. For this, I intend to use the speedups as found in the files Angle_*_a_*m.grd. I could already compare the Inclination and DirectionDeflection grids, but face some problems comparing TurbulenceIntensity and SpeedUp.
The procedure would be to transform those speedups to actual wind speeds at certain heights (as I can obtain with my other CFD model).
As I understand, the speedups are defined relative to a logarithmic profile with 10m/s at height=10m. Given the far-field MesoscaleRoughnessLength found in TerrainAnalysisResultsManifest.xml, one should be able to calculate the friction velocity u* = (10 m/s) * 0.4/ ln (10m/MesoscaleRoughnessLength).
And then, convert each file Angle_*_a_*m.grd by taking for each direction the relevant MesoscaleRoughnessLength and foar each height the relevant scaling parameter (derived from u*)
U(z) = (1/0.4). u*(direction, MesoscaleRoughnessLength) * ln(z/ MesoscaleRoughnessLength) with z varying for each grd file. However, doing so, I find quiet unexpected results. So my question is?
- What is the reference speed used to convert actual windspeeds to speed-up factors for a given tuple (direction, height)? Is the logarithmic profile procedure described above correct, or am I miising something?

Regards Fabien ;)

[Duncan]

Hi Fabien,

Thanks for your post.

Lots of the team are on holday just now, but your question has been noted and I hope we can give you the information you need before too long.

Best wishes, Duncan.

[Mark Kelly]

Hello Fabien,
The WAsP-CFD results (in the implementations we've released up to 2015) are Reynolds-number independent; the speedup should not depend on the wind speed.
Indeed files such as

Angle_270_a_0100m.grd

give a grid of speedup (in this case for winds from the west, at z=100m).
To get the mean wind speed in a given direction you take the generalized mean wind speed in that direction (from the GWC), at the appropriate height over the appropriate roughness (e.g. 100m over z0=3cm mesoscale-roughness), and multiply it by the speedup for that {direction,height}. For non-standard meso-roughnesses, it gets a bit more complicated...

Why not just export the resource grid of mean wind speeds, from WAsP?