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-   -   BC -> k e simpleFoam (https://www.cfd-online.com/Forums/openfoam-solving/98996-bc-k-e-simplefoam.html)

 klio March 23, 2012 07:47

BC -> k e simpleFoam

Hello!

I tried to simulate a simple airflow with the kEpsilon model, solver simpleFoam. a totalPressureInlet with 0.44 pa is given. Velocity has to be calculated. The residuals and the continuity looks good but the velocity much to small about 1e-06. the Solution should be about 0.35 m/s.

BC:

U:
internalField uniform (0 0 0);

inlet: type pressureInletVelocity;
value uniform (0 0 0);

wall: type fixedValue;
value uniform (0 0 0);
------------------------------------------------------------
p:
internalField uniform (0 0 0);

inlet: type totalPressure;
p0 uniform 0.44;
value uniform 0.44;
gamma 1;

------------------------------------------------------------
epsilon:

internalField 0.02247;

inlet: type turbulentIntensityKineticEnergyInlet;
intensity 0.001;
value uniform 0.02247;

wall: type epsilonWallFunction;
value uniform 0.02247;
---------------------------------------------------------------------------------------
k
internalField uniform (0 0 0);

inlet: type turbulentMixingLengthDissipationRateInlet;
mixingLength 0.01;
value uniform 1.356e-03;

wall: type kqRWallFunction;
value uniform 1.356e-03;

Has anyone a hint for me ?

 kmooney March 23, 2012 09:32

I'm guessing it is an issue with your p and U boundary conditions. Neither of them actually drive a flow. You'd need something like a fixed value U or a fixed pressure gradient to get some momentum into the domain.

 klio March 27, 2012 03:53

Hi!
With fixedGradent instead of fixedValue in my pressure inlet BC i got a flow in my flow simulation :rolleyes:.

 klio March 28, 2012 02:37

Ok but now i got the problem that the pressure decreases at the inlet. i thought fixed Gradient is NOT variable.
Do you have an idea what goes wrong there?

 soonic March 28, 2012 12:50

Perhaps nothing is wrong. You fixed the gradient, not the value.

 klio March 29, 2012 06:56

hmmm... Thanks!

But I have the Problem that I just know the fix pressure value at the inlet and if I only fix the Value there, there is nothing which drives my Flow.
so should I fix the value and the gradient at the inlet and how does it works ?

 soonic March 29, 2012 08:47

what is the dimension of the p variable you have prediscribed ?
check if it is not p/rho. And you have maybe correct solution for the wrong task.

 klio March 29, 2012 08:57

yes it is p/rho!
My pressure is about 0.5 pa, rho is 1.225 so my PressureValue is about 0.44
an the solution is about 2000 times smaller!
I also tried to set initial conditions with potentialFoam but either my velocities are much to small or much to high! :o

 klio March 29, 2012 09:16

I think I can't really imagine how the BC at inlet and outlet of U and P should be set correctly.
When I take a look at the examples I can only find BC where the velocity is known. Not so in my case!

 soonic March 29, 2012 09:43

1 Attachment(s)
try attached case with simpleFoam. It seems fine with your values on D=1cm pipe.

(it is based on case_pipe_viscous_heating for mySimpleFoam solver found on that forum)

edit: it is only laminar case.

 klio March 29, 2012 09:49

Ok I'll try it thank you very much. Hope that even works in k e model

 soonic March 29, 2012 11:33

yes, it do. Tried with 10Pa on inlet to have Re in turbulent zone.

 klio March 30, 2012 02:24

Thank you very much for this nice example. I took same BC for k-e-Model and it didn't worked!

 klio March 30, 2012 07:41

Hi at all!

now the Laminar model works!

But if I take the same BC as in the laminar model for the kE-Modell it doesn't work!

My BC are:

k & epsilon

interiorField: 1
pressure_inlet: 1
pressure_outlet: 1
wall: WllFunktion 1

p

interiorField: 0
pressure_inlet: 0.44
pressure_outlet: 0

U

interiorField: (0 0 0)
wall: (0 0 0)

Can anyone help me?

 soonic April 2, 2012 07:24

what is not working? descrbie more.
Is your Re in turbulent zone ? Try initialize the case with higher eps values and add approximate/correct values for k and eps at your BCs.

the test case with values of one "does not work" too. Correct values will make it.

 klio April 3, 2012 01:52

In my case i have to compare fluent with openFoam! So i got a simulation from my college with given BC. And to compare it correctly I have do set the same initial condition as he did. Which is k & epsilon is 1 the pressure is 0.5 (in my case 0.44 because of dimensions) and velocity is not known. so its not possible to change the values.

If I set the BC like shown before the simulation crashes with errors! I think because of the bounding variables (up to inf) -> velocities higher than 1e+20. I also tried to set BC of k eps at the walls to zeroGradient it also didn't work. Its also diverging

 soonic April 3, 2012 03:22

1 Attachment(s)
I tryed to set correct values for pressure to have turbulent flow and set calculated values for k and eps and there is the case again.
I still do not know, how big is your pipe. I just bet, that correct values for k and epsilon are thousand times smaller than one.

 lobstar April 3, 2012 04:47

Hi kilo, I'm having the exact same problem as you. I'm also trying to do a comparison between fluent and openfoam using a turbulent flow. I've also tried using simplefoam and the k-e model. I haven't been able to get the simulation to work using a set pressure drop and having velocity calculated, but used my results from the fluent to set the boundary conditions and I'm also getting velocities of 1e20. I haven't been able to get soonic's suggestions to work for me. Have you had any luck?

 klio April 3, 2012 09:56

hi!

@soonic:
hmm ... my Model is a cube with a 0.006 m thick pipe (0.06 m long ) at the inlet and same at the outlet.
I also have calculated the k and epsilon for the model but it must work somehow with the same input parameter a fluent gets. ive tried now :

k & epsilon

interiorField: 0
pressure_inlet: 1
pressure_outlet: 1

p

interiorField: 0
pressure_inlet: 0.44
pressure_outlet: 0

U

interiorField: (0 0 0)
wall: (0 0 0)

-----> IT WAS MUCH BETTER! but now the velocity is to small!! 0.002 instead of 0.35 :rolleyes:

@lobstar: