Offshore Low Level Jet modelling

Wind & Water Inflow Turbulence
1

Hello,

I am currently looking into modelling an offshore Low Level Jet whose characteristics appear to be different from what TurbSim generates. However, similarities between the two exist. As such, there will be elements of the TurbSim model that I will be looking to replicate such as coherent turbulence (I’ve read some of Neil Kelley’s papers on their impact on turbines in the Great Plains and I have good reason to assume coherent turbulent structures are being produced offshore). That is for a bit later on however.

I looked at someone else’s script but decided to create the model from scratch. Currently, what I have is the original Sandia method; that is, I am only modelling the ‘u’ component. I think what I have done so far is correct: I have tested the spectra of the time series of wind speeds at various points on the grid and it looks sensible; I’ve also looked at cross-correlation and that looks sensible too. So I am now looking at generating 3-component wind; that is, I want to introduce v and w. Whilst I could very easily do a ‘rinse and repeat’ process, I am interested in making sure I have some degree of correlation between u, v and w (Neil Kelley calls this cross-axis correlation). I’ve read another paper by Neil Kelley on cross-axis correlation (nrel.gov/docs/legosti/old/5225.pdf). Does this mean that I generate the time series for three components and then modify the u-component using equation 11 at the bottom of page 7?

Next, is it possible to have a look at the code TurbSim uses for generating coherent turbulent structures? From another of Neil’s reports (interim report), I understand that the coherent turbulent structures are superimposed on a more random background turbulence. So I was looking to have something as an add on I suppose.

I am very grateful to those who have taken the time to read this message.

Many thanks,

Alex

2

Hi, Alex.

Sorry about the delay in answering this post. I’ve been swamped with other projects lately.

Neil’s SNLWIND-3D code originally used this method, based on his data. However, it didn’t consistantly produce the correlations he was looking for when implemented with random phases in the code. So, in TurbSim, we specify the correlation coefficients and calculate the scaling parameters required to get the desired correlations. You can read about how that is done in the discussion about PC_UW in the TurbSim User’s Guide (page 19). You can also look at the TurbSim.f90 code starting around line 1390.

All of the TurbSim source code is freely available. You can download it in the TurbSim archive, found here: http://wind.nrel.gov/designcodes/preprocessors/turbsim/. The code for generating coherent structures starts around line 2034 in TurbSim.f90. You can read a little bit about what the code is trying to do in the “Coherent Strutures” section of the TurbSim User’s Guide.

3

Hello Professor,

Many thanks for the reply. I will have a look at the suggested links and hopefully that will clear things up for me.

Yours,

Alex