IEC Turbulence Simulator

[emilhedevang]

Good evening,

I have just downloaded an tried the IEC Turbulence Simulator from "http://www.wasp.dk/Products/weng/IECturbulenceSimulator.htm". The simulated turbulence does not seem depend on the Alpha parameter, indeed with the setting shown below I get identical results, byte for byte. In the first case the Alpha parameter is 0.01 while in the second it is 0.10.

I would expect the Alpha parameter to influence the spectrum and thus the simulated turbulence. Am I mistaken?

Best regards,
Emil Hedevang


********************************************************************************
IECTurbulenceSimulator : 07-09-2010 18:14:54

Type of simulation Basic
Mean horizontal wind speed 8m/s
AlphaEpsilon 0.01
l 280m
Gamma 0

Simulated turbulence field:
Velocity components (u,v,w) (1,0,0)
Number of points in each direction 16384, 8, 8
Fieldsize in meters 5200, 110, 110
dL1, dL2, dL3 0.3173828125m, 13.75m, 13.75m
Spectrum type Kaimal
Random seed 6
Output type HAWC
High frequency compensation applied
********************************************************************************
IECTurbulenceSimulator : 07-09-2010 18:53:02

Type of simulation Basic
Mean horizontal wind speed 8m/s
AlphaEpsilon 0.1
l 280m
Gamma 0

Simulated turbulence field:
Velocity components (u,v,w) (1,0,0)
Number of points in each direction 16384, 8, 8
Fieldsize in meters 5200, 110, 110
dL1, dL2, dL3 0.3173828125m, 13.75m, 13.75m
Spectrum type Kaimal
Random seed 6
Output type HAWC
High frequency compensation applied
********************************************************************************

[Morten]

Hi Emil,

The Alpha parameter, which refer to the group Alpha*Epsilon^(2/3) in the articles of Jakob Mann, is a spectral energy scale. So increasing this parameter with a factor of 10 should increase the velocity perturbations in your time series by a factor of sqrt(10). The turbulence simulator is a windows wrapper around Jakob’s software and I think the mistake is in this software layer. The convention of the FLEX format is to normalise data so here you should actually not see a difference – although I find it strange that the turbulence simulator can produce results when I set Alpha=-1. Something funny is going on here.

I am not a HAWC user but I have the HAWC2 manual. By default HAWC2 calls a DLL of Jakob Mann to produce turbulence files and conveniently take care of the file bookkeeping. However you can also use Mann files created by the Turbulence Simulator or from inside WEng, where you can include terrain and roughness-change effects on the turbulence. In either case the HAWC2 user also specifies a turbulence intensity and I am 99% sure that this implies a rescaling of the turbulence simulations. So you would see no effect of changing the alpha parameter in the ‘Turbulence simulator’.

The reason why load simulation programs like FLEX and HAWC2 wants to scale the time series is that the variance of simulated time series is random and will vary among realisations. With rescaling you avoid that kind of variability.

[emilhedevang]

Hi Morten,

Thanks you for the quick response.

My though was indeed to just scale the velocity fluctuations to achieve the target variance as Alpha just scaled the spectrum and hence the variance. I'm guessing the program just calls the DLL with some fixed value of the Alpha parameter and hence ignores the user input.

Best regards,
Emil

[emilhedevang]

Hi again,

I have now had another look at the turbulence simulated from the "IEC Turbulence Simulator". It appears that the lengths scale L is ignored as well. Shown below is the content of the txt-files generates as a part of the turbulence simulation. The only thing I have changed is the value of the turbulence length scale, in the first case it is 50m and in the second it is 100m. Still, the outputs form the turbulence simulator are identical, byte for byte.

Best regards,
Emil

********************************************************************************
IECTurbulenceSimulator : 08-09-2010 12:11:12

Type of simulation Basic
Mean horizontal wind speed 8m/s
AlphaEpsilon 0.01
l 50m
Gamma 1

Simulated turbulence field:
Velocity components (u,v,w) (1,0,0)
Number of points in each direction 16384, 8, 8
Fieldsize in meters 5200, 110, 110
dL1, dL2, dL3 0.3173828125m, 13.75m, 13.75m
Spectrum type Kaimal
Random seed 6
Output type HAWC
High frequency compensation applied
********************************************************************************
IECTurbulenceSimulator : 08-09-2010 12:07:38

Type of simulation Basic
Mean horizontal wind speed 8m/s
AlphaEpsilon 0.01
l 100m
Gamma 1

Simulated turbulence field:
Velocity components (u,v,w) (1,0,0)
Number of points in each direction 16384, 8, 8
Fieldsize in meters 5200, 110, 110
dL1, dL2, dL3 0.3173828125m, 13.75m, 13.75m
Spectrum type Kaimal
Random seed 6
Output type HAWC
High frequency compensation applied
********************************************************************************

[Morten]

These errors are probably related. Good you also found the error related to the length scale. This will make it easier for me to motivate the programmers.