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December 31, 2013, 07:51 
How to find out k and epsilon

#1 
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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 

December 31, 2013, 10:39 

#2 
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HECKMANN Frédéric
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You have a tool on this website to help you: http://www.cfdonline.com/Tools/turbulence.php
After, it depends of your upstream geometry and properties (if you consider it fully turbulent ?). 

January 2, 2014, 05:45 

#3 
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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 

January 2, 2014, 11:06 

#4 
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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)... 

January 3, 2014, 02:21 

#5 
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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 komega 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 ? 

January 4, 2014, 20:37 

#6 
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Joachim Herb
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A few thoughts:
Have you tried to use the results of your simulation with the kepsilon turbulence model (especially the pressure and velocities) as initial conditions for your simulation with komega(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 

January 6, 2014, 05:08 

#7 
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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 . 

January 6, 2014, 06:09 

#8 
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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" ... 

January 6, 2014, 06:29 

#9 
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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 

January 6, 2014, 17:35 

#10  
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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:


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