Wavenumber in L.E.S.

derz · August 11, 2011, 20:39

Hi guys,

I am trying to analyse the SGS of my L.E.S.. I want to plot Energy versus wavenumber, to see at what eddy size the modelling kicks in.

I can't exactly figure out the best way to do this in CFX - unfortunately I haven't seen wavenumber ever mentioned in the L.E.S. section of the Ansys documentation.

Does it involve taking the FFT of velocity at a single point? And should I be plotting against turbulent kinetic energy?

Any help would be greatly appreciated.

Thanks

Judith · July 27, 2015, 03:57

Hi Derwin,

I just googled your post from 2011 because I'm facing the same problem right now. Did you find a solution or does somebody else can help?

Thank you

ghorrocks · July 27, 2015, 06:30

Yes, the normal way to do this is to use a monitor point to output a velocity at a point. The take that into a mathematical software like matlab, python or whatever and do an FFT on it. You will need to do additional processing to get wavenumber.

derz · July 27, 2015, 08:05

Hi Judith,

As Glen says, you need a velocity monitor and then to use MATLAB or some other processing tool. You can actually do it (somewhat less elegantly) in Excel if you are desperate (google FFT in excel) and have enough data points in time.

Make sure you insert your monitor point in the correct location you are interested - I used more than 10 monitor points throughout the domain to get an idea of the dynamics in different locations.

Also, for more info, check out Pope's "Turbulent Flows" book.

August 11, 2011, 20:39	Wavenumber in L.E.S.	#1
derz Member Derwin Parkin Join Date: Feb 2010 Posts: 35 Rep Power: 16	Hi guys, I am trying to analyse the SGS of my L.E.S.. I want to plot Energy versus wavenumber, to see at what eddy size the modelling kicks in. I can't exactly figure out the best way to do this in CFX - unfortunately I haven't seen wavenumber ever mentioned in the L.E.S. section of the Ansys documentation. Does it involve taking the FFT of velocity at a single point? And should I be plotting against turbulent kinetic energy? Any help would be greatly appreciated. Thanks

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July 27, 2015, 03:57		#2
Judith New Member Judith Richter Join Date: Jul 2014 Posts: 8 Rep Power: 11	Hi Derwin, I just googled your post from 2011 because I'm facing the same problem right now. Did you find a solution or does somebody else can help? Thank you

July 27, 2015, 06:30		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Yes, the normal way to do this is to use a monitor point to output a velocity at a point. The take that into a mathematical software like matlab, python or whatever and do an FFT on it. You will need to do additional processing to get wavenumber.

July 27, 2015, 08:05		#4
derz Member Derwin Parkin Join Date: Feb 2010 Posts: 35 Rep Power: 16	Hi Judith, As Glen says, you need a velocity monitor and then to use MATLAB or some other processing tool. You can actually do it (somewhat less elegantly) in Excel if you are desperate (google FFT in excel) and have enough data points in time. Make sure you insert your monitor point in the correct location you are interested - I used more than 10 monitor points throughout the domain to get an idea of the dynamics in different locations. Also, for more info, check out Pope's "Turbulent Flows" book.