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Old   September 21, 2007, 20:29
Default Turbulent viscosity limited to viscosity...
  #1
Cyril
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Hi,

I'm studying 3D external aerodynamics (subsonic M0.8, SA viscosity, coupled implicit) and I have a message saying that "Turbulent viscosity limited to viscosity ratio of 1.00000e+05 in xxx cells (and xx keeps on increasing)

What does this message means and how to fiwx that prolem ? (I set up the Curent number to 15 (but i don't know what it really is)

Thanks!
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Old   September 23, 2007, 10:54
Default Re: Turbulent viscosity limited to viscosity...
  #2
Cyril
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I tried to adapt the mesh where the turbulent visosity ratio was high.. the number of cells started to decrease, but then started to increase again...

Do you think i'm using the right viscosity/solver scheme?

Thanks
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Old   September 24, 2007, 03:12
Default Re: Turbulent viscosity limited to viscosity...
  #3
mAx
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Hi Cyril, Did you well described the turbulence quantities in your BC?
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Old   September 24, 2007, 04:14
Default Re: Turbulent viscosity limited to viscosity...
  #4
Cyril
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I defined the BC as follow:

- fuselage: wall

- Symetry: symetry

- External surfaces (in the fron, behind, below..): pressure far field - gauge pressure 101325 - Mach 0.8 - Temperature 300.

- Operating conditions: 0

- Viscosity: Spalart allmaras

- Air: ideal-gas - sutherland three-coefficient-method 1.716e-05 - 273.11 - 110.56

I've never been working with viscosity before, but I based my BC and on the fluent tutorial (wing in M0.8).. Do you think they are good ?
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Old   September 24, 2007, 04:16
Default Re: Turbulent viscosity limited to viscosity...
  #5
Cyril
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I also ran a first set of iterations using the first order for both flow and viscosity, and then i tried to put the second order.. (but here again, i tried to find some explanations on the internet, but i still dont understand the difference between second order, power law, quick...)
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Old   September 24, 2007, 05:35
Default Re: Turbulent viscosity limited to viscosity...
  #6
mAx
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I'll check out tomorrow, I have to go. But for your BC, you should fill the turbulence quantities below Mch- Number etc...

For the first and second order, the difference is the truncated error of the discretization.

In french: c est les developpements limites (1er et 2e ordre)
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Old   September 24, 2007, 05:41
Default Re: Turbulent viscosity limited to viscosity...
  #7
Cyril
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I set up the Turbulent viscosity ratio to 10 as proposed in the tutorial.

But honestly... I dunno what that means (10.. why not less or more..)

Thanks for help
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Old   September 24, 2007, 07:50
Default Re: Turbulent viscosity limited to viscosity...
  #8
mAx
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switch on Turbulence Intensity and Hydraulic Diameter (Method). *Then you calculate your Hydraulic Diameter by Dh = 4.A/P , where A is the Aera of your Inlet and P the Perimeter *Turbulence Intensity is given by: I = 0.16*Re**(-1/8), with Re is your Reynold's Number

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Old   September 24, 2007, 08:15
Default Re: Turbulent viscosity limited to viscosity...
  #9
Cyril
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My case is external flows... I dont have a proper inlet.. should I use the the front face of my mesh as an inlet for the area/perimeter?
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Old   September 24, 2007, 10:53
Default turbulent BC.. TVR or Intensity/Hydraulic Diam?
  #10
Cyril
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Arf... i'm dumb or unlucky..

Tried with Dh=14m and Intensity=1.41% and the residuals diverge and computation stops after 42 iterations :'(
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Old   September 25, 2007, 01:30
Default Re: turbulent BC.. TVR or Intensity/Hydraulic Diam
  #11
mAx
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ok, but you don't have the warning regarding turbulent viscosity anymore, right? With M=0.8 you are in transsonic flowfield,eg: compressible Did you enable energy equation?
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Old   September 25, 2007, 01:58
Default Re: turbulent BC.. TVR or Intensity/Hydraulic Diam
  #12
Cyril
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Yeah, the energy equation is on. But as the residuals diverge, the calculation stops by itself..
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Old   September 25, 2007, 02:42
Default Re: turbulent BC.. TVR or Intensity/Hydraulic Diam
  #13
mAx
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ok, and which variable is diverging first? (do you receive an error message)? I am not familiar with pressure far field BC,so try to switch your front inlet with velocity Inlet. I will send you an document about CFD and extern-aero (best practices)

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Old   October 2, 2007, 10:08
Default Re: turbulent BC.. TVR or Intensity/Hydraulic Diam
  #14
PratiK Mehta
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i was goin throught the discussion and so i want to ask Cyrll r u still having trouble in solving or u have solved it ?
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Old   October 2, 2007, 13:28
Default Re: turbulent BC.. TVR or Intensity/Hydraulic Diam
  #15
Cyril
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Still have some problems... mainly Turbulent viscosity ratio limited to ... even if I adapt my mesh on the TVR gradient it doesn't really help.

I tried to run my simulation for Mach 0.2, (nearly no pb) then Mach 0.3 ("TVR limited..." appears as the residuals converge :s)

And when I try to adapt the y+ gradient, the residuals diverge (about all at the same time). (don't have enough ressources (RAM) to refine the mesh with gambit, that's why i do it with fluent.

But my report is to be finished by tomorow..... so there's not a lot i can do at this point!

And when I display the turbulent viscosity ratio, it seems good at the rear of the fuselage, but it's strange there are no turbulences behind the cockpit....
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Old   October 3, 2007, 07:44
Default Re: Turbulent viscosity limited to viscosity...
  #16
Oscar
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Hello! I have the same problem, but my case is about a car, at 200 km/h. I've configured all correctly, (i think) using the K-epsilon model for turbulence, without any further complication, and at the second iteration, Fluent say "Turbulent viscosity limited to viscosity ratio of 1e5 in xxx cells" i don't use the energy equations, and my fluid is non-compressible, you know, an easy car analysis. So, where is my problem? in the Mesh? in the configuraiton?

Thanks for your help!
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Old   October 4, 2007, 06:12
Default Re: Turbulent viscosity limited to viscosity...
  #17
PratiK Mehta
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hi Cyrll, Make sure you have set your turbulence intensity value to about 10e-3, which i suppose u already did ,if not then make it now. Try to switch to unsteady state with small timesteps likei generally use the thumb rule as timestepsize=(L/V), L= reference lenght V=free stream velocity. Value u get should be used as timestep, if u get divergence ,just try to reduce this value.

Keep the pressure overrelaxation to 0.8, just use 1st order scheme for all variable

Use PISO for pressure.velocity coupling

I hope this helps

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