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-   -   TurbulentInlet BC (https://www.cfd-online.com/Forums/openfoam-solving/60005-turbulentinlet-bc.html)

chris1980 September 5, 2006 05:09

I want to use the 'turbulentIn
 
I want to use the 'turbulentInlet' boundary condition. As far as I can see the use only inputs a fluctuation scale but what about the integral length scale or other appropiate scales because the flucation scale alone do not define a turublent inlet.

Ok I know it is anyway no real turbulent inlet because there is no turbulence strcuture etc.

Additionally, I was wondering what I have to set for k and epsilon at this turbulent boundary (using high Re k-epsilon turb model)?

chris1980 September 5, 2006 11:15

Is there nobody who is able or
 
Is there nobody who is able or want to answer my question?

pierre September 5, 2006 12:21

Hi, I think that that turbu
 
Hi,

I think that that turbulentInlet is to be used for LES/DNS, not RANS. It just produces a random noise at the inlet of given magnitude, which anyway is most lileky to be dissipated quickly.

For RANS, if you use K-Eps, just use fixed value boundary conditions and set k and Eps to what you like.
You can use something like
K_inlet = 3/2 (Uref*T)^2 and Eps = Cmu*k^(3/2)/L
where T is tubulence intensity (10% for example) and L is a turbulent lengthscale.

Hope this helps

daniel_mills September 20, 2006 13:50

Excuse me for the stupid quest
 
Excuse me for the stupid question. I did not know you could actually set BC's like this. What would an example of the actual inlet BC be for k and epsilon?

sihaqqi September 9, 2013 00:32

chris

if you have found answer to your query, can you please let me know. I want to use turbulentInlet BC.

regards

vut March 27, 2014 09:35

Hi,

I am searching what does "fluctuationScale" in turbulentInlet BC mean?

If I set it 0.01, for example? What is this value signify?

Thanks in advance,

vut

Garusen October 21, 2015 04:32

To get information, OpenFoam source code is the best place.

If you read comment in the .H file, you have in comments:
fluctuationScale
RMS fluctuation, provided as the fraction of the mean field

So 0.001 means the fluctuation will be 0.001 time the mean flow.

To be used in RANS, you need to create fields for k and epsilon as explained above, for example, see implementation of AtmBoundaryLayer.

beatlejuice July 4, 2017 10:02

Hello everybody,

I also want to use the turbulentInlet BC for an LES simulation. The case of interest is a flow over a cylinder - longitudinal orientated to the flow.

I'm not quite sure how to set the fluctuationSclae value. My idea was the following:

Since i know the turbulent intensity of the flow field (freestream), I want to use this to set the fluctuationScale.

Def. of turb. Intensity: I = \frac{u'}{\overline{U}}

with u' = \sqrt{\frac{1}{3}((u'_x)^2+(u'_y)^2+(u'_z)^2)}

where \overline{U} is the magnitude of the mean velocity @ inlet and u' is the root-mean-square of the turbulent velocity fluctuations.

By assuming that u'_x = u'_y = u'_z and \overline{U} = 1 i can rewrite the first equ. to I = u'_x = u'_y = u'_z.

Therefore, I want to set the fluctuationScale components equal to turbulence intensity.

Is this a way to set the fluctuationScale value or do i make a fundamental mistake by following this idea? How can I estimate the fluctuationScale values for a flow case like mine?

Any help or ideas are greatly appreciated =) thanks!


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