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-   -   Strange epsilon (http://www.cfd-online.com/Forums/openfoam-solving/59958-strange-epsilon.html)

chris1980 September 13, 2006 03:23

I am running a simulation with
 
I am running a simulation with the standart k-epsilon model. I have specified an initial condition for k=9.5 and epsilon=1750. However the volume averaged epsilon after the first time step is <epsilon> = 3700!

Can anybody shed some light into it why the volume averaged epsilon is some much different form the initial condition?

hjasak September 13, 2006 05:53

That's because your turbelence
 
That's because your turbelence is huge! Your velocity FLUCTUATION is 3.5 m/s - this is the speed I have trouble to walk on. Assuming turbulence intensity of 1%, your mean velocity would be the speed of sound (with a lot of turbulence).

I would revisit my initial condition, it does not look right.

Hrv

chris1980 September 13, 2006 06:12

Hrv, I think a velocity flu
 
Hrv,

I think a velocity fluctuation of 3.5 m/s corresponds not to k=9.4 m^2/s^2, instead a fluctuation of 2.52 m/s is right.

Do you agree ?

The turbulence data (velocity fluctuation, integral lengthscale) comes from a experimental turbulence generator (multihole plate) with an integral lenghtscale of 3mm.

In the experiment I want to simulate there is almost no mean flow but there is a sort of background turbulence created by the turbulence generator.

So do you still think there is a incorrect initial condition?

hjasak September 13, 2006 06:17

Ah, OK: if you have a turbulen
 
Ah, OK: if you have a turbulence generator and experiments, this may well be right. In that case, you should just let the flow develop and the model should do its job. As for k, I probably made a mistake:

k = 3/2 uPrime^2

I should have said 2.5 m/s, i.e walking speed :-)

Hrv

olwi September 13, 2006 07:58

Hi, Epsilon usually scales
 
Hi,

Epsilon usually scales something like K^1.5/(0.3*L). In your case this is 3e4. In that light the value after one time step is not very high, rather your initial value is very small.

/Ola

chris1980 September 13, 2006 09:03

ok but the same case running w
 
ok but the same case running with star give me averaged epsilons from 2000..6000! So I don't know where the big difference gomes into the game http://www.cfd-online.com/OpenFOAM_D...lipart/sad.gif

chris1980 October 24, 2006 09:05

What is resonable choice for e
 
What is resonable choice for epsilon (I use the compressible RNG-k-epilon model) in the case of no initial turbulence (i.e. k=1e-8)?


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