# Initial Turbulence Conditions

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 April 2, 2013, 22:31 Initial Turbulence Conditions #1 Member     Anonymous Join Date: Dec 2011 Location: Everywhere Posts: 77 Rep Power: 6 I am doing an automotive analysis using the k-omega SST turbulence model and I have a question on initial conditions. The two values: turbulentKE turbulentOmega So to calculate these first I calculated Reynolds Number: Re = U * L / nu = 45 * 4.16 / 1.5e-5 = 12.5 E6 Then initial turbulence %: I = 0.16 Re ^-1/8 = 0.0208 Now to turbulentKE: k = 1.5 * (UI)^2 = 1.5 * (45 * 0.0208)^2 = 1.309 Now is the real question. I found 2 equations for epsilon with one being an approximation. After calculating both a get a big difference in numbers and wonder which one I should use to calculate omega. epsilon ~ (k^1.5) / L = (1.309^1.5) / 4.16 = 0.360 epsilon = (cmu^.75) * (k^1.5) * (l^-1) cmu = 0.09 (seems to be most commonly used) l = 0.07 * L = 0.07 * 4.16 = 0.2912 epsilon = (0.09^0.75) * (1.309^1.5) * (0.2912^-1) epsilon = 0.845 :::: :::: :::: epsilon = 0.360 epsilon = 0.845 So am I doing this right? If so, which epsilon value should I be using? :::: :::: :::: turbulentOmega = k / epsilon immortality and JR22 like this.

 April 3, 2013, 21:55 #2 Member     Anonymous Join Date: Dec 2011 Location: Everywhere Posts: 77 Rep Power: 6 Anybody know?

 April 4, 2013, 11:12 #3 Senior Member     Jose Rey Join Date: Oct 2012 Posts: 128 Rep Power: 10 I am also interested in some good input for this question. Although the openFoam forum has more traction, and this question is extremely relevant for openFoam, it might be appropriate for it to be in this forum: http://www.cfd-online.com/Forums/main/ If you do repost the question there, let me know as I am very interested.

 April 4, 2013, 12:17 #4 Member     Anonymous Join Date: Dec 2011 Location: Everywhere Posts: 77 Rep Power: 6 I thought about posting there but since this part of the forum (OpenFOAM) gets more foot traffic, I decided to post it here. I just made a new post however in the main. Initial Turbulence Conditions

 April 5, 2013, 17:45 #5 Senior Member   sivakumar selvaraju Join Date: Mar 2009 Location: Cape Town - South Africa Posts: 186 Rep Power: 9 Hi, I assume that your are doing external aerodynamics, in that case your problem is similar to motorBike case in OF. According to OF training manual, you need to use the value corresponds to the following formula epsilon = (Cmu^0.75 K^1.5)/ L Thanks, Sivakumar Last edited by sivakumar; April 8, 2013 at 04:55.

April 7, 2013, 08:39
#6
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Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 52
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Quote:
 Originally Posted by plucas epsilon ~ (k^1.5) / L = (1.309^1.5) / 4.16 = 0.360
Hi !

Could you tell us where you found this relation ? I've always seen the other one you talked about.

Quote:
 Originally Posted by plucas l = 0.07 * L = 0.07 * 4.16 = 0.2912
For the turbulent length scale value, you choosed the same value for L as the characteristic length used for the Reynolds. If you are doing external aerodynamics, those two lengths could be different.

The characteristic length used for turbulent length scale is a length which depends on your wind tunnel's dimensions.

Could you tell us the meaning of the length L you used for the reynolds and the turbulent length scale ?

I guess it was a dimension of your obstacle. If it is right, then your calculations may not be right : the epsilon values you are calculating are the values of the freestream, before the fluid meets the obstacle : this is a boundary condition. You should have chosen a domain large enough so that this freestream value is not influenced by your obstacle. Then the epsilon value for the free stream should not be influenced by a dimension of your obstacle (the 4.16 value you showed us)

Have a nice day !

April 27, 2013, 21:05
#7
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Tenglubao
Join Date: Feb 2013
Posts: 4
Rep Power: 5
Quote:
 Originally Posted by sivakumar Hi, I assume that your are doing external aerodynamics, in that case your problem is similar to motorBike case in OF. According to OF training manual, you need to use the value corresponds to the following formula epsilon = (Cmu^0.75 K^1.5)/ L Thanks, Sivakumar

In the user guide epsilon = (Cmu^0.75 K^1.5)/ l ，l is different from L

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