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.

WEng estimates the shear exponent from calculations at just one height. The flow is modelled in Fourier space, so a convenient way to calculate fields of velocity gradients is to multiplying wave numbers with wave vectors and then do inverse FFT. You can get the full matrix of velocity gradients - from dU/dx to dW/dz - in this way. Having found the velocity and velocity gradient fields, WEng is able to extract velocities and vertical gradients at any site and estimate shear parameters by the formula alpha=dU/dz*z/U Here, the gradient and velocity corresponds to the selected height. This alpha exponent is a good fit near the selected height. However, if the real profile is not an exact power law you will get slightly different alpha values if you select other calculation heights.

WAsP 8.4 is OK, but I think your WAT scripts are out of date, please download the latest WAT 2.5 install package. Then check that you are able to run either WAT2Excel or WAT2Launcher. If not, then please report to wengsupport@risoe.dk. When the software is installed correctly you can find a little guidance in the start of the section called 'working with WAT' at http://www.wasp.dk/Products/wat/WAThelp/

As you probably suspect, this problem is related to program versions. However, the WAT2Excel script is not distributed with WEng, but with WAT. Please go to http://www.wasp.dk/Download/Software.html and download WAT version 2.5.0.125

There can be several reasons for a strange extreme-wind estimate. I would start by checking the input data in the input OEWC or REWC files. If you analysed your data by the WACA program they should include an uncertainty estimate, and a record of yearly extreme winds observations. Check that these are OK. In case of trouble you may have to check the data used to produce the OEWC file. Older files may not include this information, but you can assume that when an alpha parameter is very large >>1, the Gumbel fits will have high uncertainty.

The WEng extreme-wind estimates depend on terrain-induces speed-up factors, which are calculated by the flow model. Using a too low resolution will typically result in too little terrain speed-up factors. Setting up a terrain model in WEng is a compromise between grid resolution and domain size with restrictions imposed by the available computer memory. Read more about this at http://www.wasp.dk/Support/FAQ/WENGmaps.html

WEng is working with terrain data in grid format and therefore needs to convert the WAsP contour map to grid format before calculate anything. We have two alternative methods for this and they can both fail. This is the most common user problem with WEng. There can be several possible reasons: First, you have to use ASCII format. There could also be a small map inconsistency, which did not disturb your WAsP project, but makes one or both WEng map conversions routines crash. I must also admit that sometimes we see problems even with seemingly perfect maps. Best advice is first to check with the WAsP Map Editor, maybe try the alternative WEng Map conversion routine, and if you still have problems, then mail the map to wengsupport@risoe.dk plus a screen dump showing the grid coordinate and resolution settings just before the crash.

Hi Old Forum Archivist, The way WEng calculates TI is first to run a flow model, which predicts fields of wind speed and 3D velocity gradients. For specific sites WEng then predicts local turbulence spectra by the deformations by the mean flow and gradual adjustments to surface roughness changes. The variance of the local velocity perturbations and the turbulence intensity is then found by integration. The calculations are repeated for a number of wind directions. More details are found in: J Mann (2000) The spectral velocity tensor in moderately complex terrain, J. Wind Eng. Ind. Aerodyn. Vol. 94(2006) p. 581–602 There are certain limitations. WEng TI is for neutral stability only and it does not include turbulence produced in wakes of upstream turbines, obstacles, or behind steep terrain. Observed turbulence would include such effects and especially at low wind speeds you would see some scatter around the mean. Sorry about the late reply. I am relatively new to this forum. Aim to be more active in the future.

*.wtg files can have multiple performance tables, which you can add with the WASP turbine editor. If you want to model wake turbulence in WAT you may also need to modify the thrust-coefficient curves in the performance tables - although this effect could be small if you are only changing air density and the turbine operates with the same tip-speed ratio and pitch regulation (better consult a specialist on this). Certain WAT versions have a known problem reading *.wtg files (a work around methodis to input data in *.pow format). But once I get this fixed, you should again be able to right-click on the 'table' child object to the turbine object and set the appropriate performance table in the pop-up window.