# How to find out k and epsilon

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 December 31, 2013, 07:51 How to find out k and epsilon #1 Senior Member   Join Date: Sep 2013 Location: Bangalore India Posts: 134 Rep Power: 5 Sponsored Links 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 Senior Member   HECKMANN Frédéric Join Date: Jul 2010 Posts: 237 Rep Power: 10 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 ?). Antimony likes this.

 January 2, 2014, 05:45 #3 Senior Member   Join Date: Sep 2013 Location: Bangalore India Posts: 134 Rep Power: 5 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 Senior Member   HECKMANN Frédéric Join Date: Jul 2010 Posts: 237 Rep Power: 10 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 Senior Member   Join Date: Sep 2013 Location: Bangalore India Posts: 134 Rep Power: 5 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 ?

 January 4, 2014, 20:37 #6 Senior Member   Joachim Herb Join Date: Sep 2010 Posts: 422 Rep Power: 12 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

 January 6, 2014, 05:08 #7 Senior Member   Join Date: Sep 2013 Location: Bangalore India Posts: 134 Rep Power: 5 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 Senior Member   HECKMANN Frédéric Join Date: Jul 2010 Posts: 237 Rep Power: 10 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" ... sam.ho likes this.

 January 6, 2014, 06:29 #9 Senior Member   Join Date: Sep 2013 Location: Bangalore India Posts: 134 Rep Power: 5 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
Senior Member

HECKMANN Frédéric
Join Date: Jul 2010
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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 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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