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How to find out k and epsilon

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Old   December 31, 2013, 07:51
Default How to find out k and epsilon
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Hi Foamers,

I am simulating flow through valve in which i know only pressure at inlet and outlet of a valve. In this scenario how to find out k and epsilon ?
Or how to give boundary condition to inlet , outlet and walls ?
Inlet pressure = 5 bar
outlet pressure = 1 bar ( atmosphere )
Density = 800 kg/m3
dynamic viscosity = 6.68 X 10^-3 Pa s
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Old   December 31, 2013, 10:39
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HECKMANN Frédéric
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You have a tool on this website to help you: http://www.cfd-online.com/Tools/turbulence.php

After, it depends of your upstream geometry and properties (if you consider it fully turbulent ?).
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Old   January 2, 2014, 05:45
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Hi Fredo,

I have cross checks all the parameters and all are correct. As explained below

Boundary Conditions :
Inlet pressure = 5 Bar
Outlet Pressure = 1 Bar ( atmospheric pressure)
Density of fluid = 800 kg/m3
Dynamic Viscosity = 6.68 X 10^-3 Pa s

As i did not knew how to calculate k and epsilon values from pressure so i got nominal flow rate from ANSYS 14.5 results and calculated k and epsilon as follows
k = 0.00036 m2/s2
epsilon = 0.00014 m2/s3
and nuT = 1.04 X 10^-7 m2/s Calculated as muT= C_mu X k^2/epsilon
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Old   January 2, 2014, 11:06
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HECKMANN Frédéric
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There is no direct relation between the pressure and k/epsilon. You should seriously learn more about the turbulence theory ! There is no defaut value that always works... Even Fluent asks for some parameters to estimate k and epsilon (at the bottom of the inlet condition panel).

Usually people use the turbulent intensity to estimate k and epsilon. The turbulent intensity at the inlet highly depends of your case/geometry before your domain. If your inlet is a long pipe, it can be considered as fully turbulent and you can use the hydrolic diameter; if the inlet is "laminar" you need to estimate the level of turbulence intensity (it is never 0 in real cases)...
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Old   January 3, 2014, 02:21
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Hi Fredo,

I calculated turbulent intensity as 3% and then proceeded to calculate k, epsilon and nuT.
And I got the values as mentioned above.
Yesterday I changed the fvSolutions and fvSchemes by taking reference of motorBike tutorial and ran this case with k-omega SST model. It ran well but the velocity and pressure and tremendously high.
As I gave inlet pressure as 625 m2/s2 and outlet pressure as 125 m2/s2 which i obtained by dividing pressures with density.

May any body know why this behaviour of the simulation ?
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Old   January 4, 2014, 20:37
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A few thoughts:

Have you tried to use the results of your simulation with the k-epsilon turbulence model (especially the pressure and velocities) as initial conditions for your simulation with k-omega(SST)?

There are special boundary conditions for the turbulence related fields to use turbulence intensity and turbulent mixing length:
https://github.com/OpenFOAM/OpenFOAM...hScalarField.H
https://github.com/OpenFOAM/OpenFOAM...hScalarField.H
https://github.com/OpenFOAM/OpenFOAM...ld.H?source=cc
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Old   January 6, 2014, 05:08
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Hi jehrb,

Could you please explain how to calculate Turbulent Mixing length. ?
Because mine is a Ball Valve simulation .

I have used turbulent viscosity ratio for the calculation of k, epsilon and omega .
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Old   January 6, 2014, 06:09
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HECKMANN Frédéric
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Please read about the turbulence theory... You missunderstand many things.

In your case, your flow comes from a "long" pipe and it is often a good assumption to say that the flow is fully turbulent.

Therefor you can use the turbulence length scale and the turbulence intensity to get all your variables (k, epsilon, omega).
- There is an empirical relation between the turbulence length scale and the pipe diameter.
-There is an empirical relation between the turbulence intensity and the hydraulic diameter

At least read some documents like this:
http://jullio.pe.kr/fluent6.1/help/html/ug/node178.htm

And look in google: "turbulence in tube" ...
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Old   January 6, 2014, 06:29
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Hi ,

Thank you for the explanation.
But my case is not like that.. I know only pressures at inlet and outlet as boundary condition so that leading me to a lot of confusion.
Time being i took the flow rate from ANSYS results and calculated k, epsilon and omega.
And now i will calculate the values according to your suggestion and test my case.
Thank a ton
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Old   January 6, 2014, 17:35
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HECKMANN Frédéric
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jherb gave you the suitable boundary conditions. Indeed you don't have the velocity at your inlet but you can run a first simulation with I (turbulence intensity) at 5% and correct the value once you get a first convergence.

k is function of the Reynolds (and then the velocity), but epsilon/omega are only function of the diameter of your pipe (turbulent length scale).

Just use a first rough estimation of k and correct it in a second simulation that will be more accurate.

Quote:
Originally Posted by jherb View Post
There are special boundary conditions for the turbulence related fields to use turbulence intensity and turbulent mixing length:
https://github.com/OpenFOAM/OpenFOAM...hScalarField.H
https://github.com/OpenFOAM/OpenFOAM...hScalarField.H
https://github.com/OpenFOAM/OpenFOAM...ld.H?source=cc
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