LIDAR correction

[windmueller]

Hi,

I would like to to correct RSD data due to the inhomogenity within the measuring area and I found the LIDAR error script.

Some questions:

-is the script the right one (there is also the script "wind speedup and deflection" and I also do not have any mast data)?

-I want to have correction factors for each 10° wind direction bin and I have time series LIDAR data. My assumption is that I have to input the bin-wise wind speeds for each of the 10° direction bin as "observed wind" at the site of the LIDAR, correct?

-when doing the above the results are given in 12° wind direction bins. How can I adjust that?

-there are also the results for "windcube" and "ZephIR". Is that due to the different cone angles?

-are the "Ratio Windcube" and "Ratio ZephIR" the correction factors?

Quite a lot questions.... thanks in advance for answering!

 

[Morten]

The "wind speed and deflection script" lists speedup factors and deflection from a reference mast to a turbine site, with possible height difference, for a number of wind directions. This is probably not what you need.

The idea of the 'Lidar Error' script is to simulate flow components along a number of lidar beams by flow solutions of WEng, combine these components as the lidar would do and, finally, compare the combined lidar response with the horizontal wind at the Lidar position. We used to distribute this script with the attached PDF file, and the basic research is described in an article available at Modeling conically scanning lidar error in complex terrain with WAsP Engineering — Welcome to DTU Research Database    

The lidar error will mostly depend on the flow-line inclination at the measurement points, the number of lidar beams, and their angle from zenith. The method works reasonably well with moderate terrain complexity but less so in really complex terrain, since the linear flow model of WEng cannot predict flow separation. The script calculates for two classic lidars, but it might be possible to modify it script for another lidar types, if you know the cone angle and number of beams. 

I think some lidars offers a similar correction internally in the instrument, so please make sure only to apply the method on data which are not already corrected.

LiDAR_Error_Manual.pdf

[windmueller]

Thanks Morten for the quick response!

There is still one question left: how can I get 10° wind direction bins instead of 12°?

And do you advice to use the "observed wind" as described above or shall I use "geostrophic wind" or doesn't it matter at all for this kind of analysis?

[Morten]

Creating new scripts for WEng can be complicated, but modifications of an existing script is feasible. Video demonstration no. 8 at https://panopto.dtu.dk/Panopto/Pages/Viewer.aspx?pid=686617ee-3a81-4381-9082-af9400ec5924 explains where these scripts are stored and how to make simple modifications. 

I never tried modifying the Lidar error script, but it seems like lines 63-70 defines the parameters for the calculations. Here, it seems like WDstep=12 determines the directional increment of the displayed table, so try to reduce this to 10. 

You should also be aware that the ConAngle and Step parameters represents the cone angle and horizontal angle between measurement points in the Lidar scanning pattern. Please check whether these parameters are correct for the Lidar you are using. Also note that the length of the parameter arrays near line 261-264 of the script, e.g. Dim nx( 60), must equal to the number of measurement points = 360/step. Thus, if you define Step=5 instead of 6, you should modify array dimensions, e.g. Dim nx( 72). 

If a script fails, it will not tell you what the problem was, so it is often better to test after small incremental change, rather than changing everything before the first test run. 
 

[windmueller]

Thanks again for this helpful input! I will try to modify the script and will report if it works.

But there is still the question about the wind(s) to be entered.... Up to now I entered for each 10°bin the measured wind speed ("observed wind") but I am asking myself if the script does need that. Or is independent from the wind?

[windmueller]

The change to 10° was successful. It works.

But I remarked that with "newer" versions of Excel (I tested Excel 2013) you need to modify the script in line 124 from "GetObject" to "CreateObject"

 

Now only the "wind topic" is pending...

[Morten]

Thank you for posting this. I forgot that I once made my own update of this script (tested by Excel 2016), where the difference between "GetObject" and "CreateObject" was not an issue. 

However, I had to delete two line which tries to remove extra worksheets. The reason is that Excel 2016 by default opens with only one blank worksheet, while earlier version used to open with three blank worksheets. 

Edited May 8, 2023 by Morten

[windmueller]

Regarding the wind issue I had a test where I calculated with Wind and one without any wind information. The differences are negligible (below 0,2% in the ratio). So, it seems that this questions is answered. Hope you can confirm that...