how to get the turbulence intensities, length scales, draw power spectra?

Stan_Mech · March 8, 2012, 23:13

Hello everyone,

how can I plot the turbulence intensity along a line, length scales and time scales and power spectra at certain location?

also anybody knows about turbulent production, how can i plot these too along a line?

Thanks,

ghorrocks · March 9, 2012, 04:12

Turbulence intensity - easy, just plot k along a line feature.
Length/time scales - you will have to define a variable for your length/time scale, but that is easy to plot along a line feature.
Turb production - again define a variable which describes this and plot it.

The above ones you can do in a 2 eqn turbulence model This next one requires an LES style model:

Power spectra - To do this you need to use transient statistics, but to do a proper spectra you will need to use a monitor point and export the data and do post-processing elsewhere.

Stan_Mech · March 9, 2012, 10:33

Quote:

Originally Posted by ghorrocks

Turbulence intensity - easy, just plot k along a line feature.
Length/time scales - you will have to define a variable for your length/time scale, but that is easy to plot along a line feature.
Turb production - again define a variable which describes this and plot it.

The above ones you can do in a 2 eqn turbulence model This next one requires an LES style model:

Power spectra - To do this you need to use transient statistics, but to do a proper spectra you will need to use a monitor point and export the data and do post-processing elsewhere.

thanks,

Thanks,

would K be representative of turbulence intensity? or should i process it because k=0.5(3*u^2) (assuming its isentropic) but the intensity is u/U_mean

for defining a variable for the length scales you mean from statistics equation, what would be the variable, where can i place it?

for the power spectra, why specially LES? ,can I use other models like SST or k epsilon

ghorrocks · March 10, 2012, 04:44

Sorry, I misunderstood your initial question. People often refer to k as turbulence intensity. But if you are looking for % turbulence intensity then define a variable and plot that. Still very easy.

k is directly linked to turbulence intensity.

In a 2 eqn turbulence model if you have two turbulence parameters (say k and epsilon) then you can derive all other turbulence parameters such as length scales from that. You just need to know how they are related. "Turbulence Modelling for CFD" by Wilcox is a good one and has all the relationships.

2 eqn turbulence models assume a single length and time scale represents the turbulence at that location. You cannot get a spectrum from that.

March 8, 2012, 23:13	how to get the turbulence intensities, length scales, draw power spectra?	#1
Stan_Mech New Member Join Date: Mar 2012 Posts: 4 Rep Power: 14	Hello everyone, how can I plot the turbulence intensity along a line, length scales and time scales and power spectra at certain location? also anybody knows about turbulent production, how can i plot these too along a line? Thanks,

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March 9, 2012, 04:12		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Turbulence intensity - easy, just plot k along a line feature. Length/time scales - you will have to define a variable for your length/time scale, but that is easy to plot along a line feature. Turb production - again define a variable which describes this and plot it. The above ones you can do in a 2 eqn turbulence model This next one requires an LES style model: Power spectra - To do this you need to use transient statistics, but to do a proper spectra you will need to use a monitor point and export the data and do post-processing elsewhere.

March 10, 2012, 04:44		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Sorry, I misunderstood your initial question. People often refer to k as turbulence intensity. But if you are looking for % turbulence intensity then define a variable and plot that. Still very easy. k is directly linked to turbulence intensity. In a 2 eqn turbulence model if you have two turbulence parameters (say k and epsilon) then you can derive all other turbulence parameters such as length scales from that. You just need to know how they are related. "Turbulence Modelling for CFD" by Wilcox is a good one and has all the relationships. 2 eqn turbulence models assume a single length and time scale represents the turbulence at that location. You cannot get a spectrum from that.