LES or DES in wind turbine

mohammad · April 19, 2011, 10:41

Hi everybody,
I'm simulating the NREL wind turbine.My model is based on SST.For wind speed 5.4 m/s and Physical time scales as follow I have the following results;

Time scale: 0.1----> exactly Not converged (No. of Iteration 400)
Time scale: 0.06----> very hard to say converged (No. of Iteration 1000)
Time scale: 0.01----> ???? but very hard to say it will converge( still running)

As I know, I can use LES or DES for the situations that RANS-based models are not good because of large area of separation.I searched in CFX to run my model with LES or DES but I didn't find it....

Now is there anybody to tell my how I can start with LES or DES?

Thanks in advance

ghorrocks · April 19, 2011, 22:23

First work though the suggestions here:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria

Before you do any LES/DES stuff make sure you do some thorough reading into the approaches. If you do not understand what it is doing you have no hope of getting an accurate answer.

mohammad · April 19, 2011, 23:16

Dear Glenn Horrocks,
First of all thanks for your reply.
I have read and used this FAQ before and it's interesting that I have already got acceptable results (based of the information of this FAQ) for my model with velocities between 6 to 13 m/s with appropriate accuracy.
(not for v=5.4m/s as yet )

What I found during those runs is in contradiction with hint No.3 of FAQ notes.I did ,somewhat, a timescale sensitivity test .I run my model with wind speed 10.3 m/s with different Physical timescales of {0.01,0.02,0.05,0.06,0.07,0.08,0.1, 0.15}.
What I found was that when the model had a poor( say no) convergence with a large Physical Time scale it would have better convergence with smaller ones.This is against what is shown in FAQ.???!!!

The second question here is the suggestion to increase physical time scale presented in FAQ is also in contradiction with what is presented under the title of "Steady State Time Scale Control" in CFX help mannual which recommend use of small physical time scale.

The Last thing that I think it's useful, is that my model will have large area of separation and I think naturally it contains natural instabilities like vortexes and...that's why I think I need to test it with LES/DES models...

I will be really thankful to know your opinion.

April 19, 2011, 10:41	LES or DES in wind turbine	#1
mohammad Senior Member mohammad Join Date: Dec 2010 Location: UK Posts: 245 Rep Power: 16	Hi everybody, I'm simulating the NREL wind turbine.My model is based on SST.For wind speed 5.4 m/s and Physical time scales as follow I have the following results; Time scale: 0.1----> exactly Not converged (No. of Iteration 400) Time scale: 0.06----> very hard to say converged (No. of Iteration 1000) Time scale: 0.01----> ???? but very hard to say it will converge( still running) As I know, I can use LES or DES for the situations that RANS-based models are not good because of large area of separation.I searched in CFX to run my model with LES or DES but I didn't find it.... Now is there anybody to tell my how I can start with LES or DES? Thanks in advance

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April 19, 2011, 22:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	First work though the suggestions here: http://www.cfd-online.com/Wiki/Ansys...gence_criteria Before you do any LES/DES stuff make sure you do some thorough reading into the approaches. If you do not understand what it is doing you have no hope of getting an accurate answer.

April 19, 2011, 23:16		#3
mohammad Senior Member mohammad Join Date: Dec 2010 Location: UK Posts: 245 Rep Power: 16	Dear Glenn Horrocks, First of all thanks for your reply. I have read and used this FAQ before and it's interesting that I have already got acceptable results (based of the information of this FAQ) for my model with velocities between 6 to 13 m/s with appropriate accuracy. (not for v=5.4m/s as yet ) What I found during those runs is in contradiction with hint No.3 of FAQ notes.I did ,somewhat, a timescale sensitivity test .I run my model with wind speed 10.3 m/s with different Physical timescales of {0.01,0.02,0.05,0.06,0.07,0.08,0.1, 0.15}. What I found was that when the model had a poor( say no) convergence with a large Physical Time scale it would have better convergence with smaller ones.This is against what is shown in FAQ.???!!! The second question here is the suggestion to increase physical time scale presented in FAQ is also in contradiction with what is presented under the title of "Steady State Time Scale Control" in CFX help mannual which recommend use of small physical time scale. The Last thing that I think it's useful, is that my model will have large area of separation and I think naturally it contains natural instabilities like vortexes and...that's why I think I need to test it with LES/DES models... I will be really thankful to know your opinion.