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-   -   Add Random value to velocity (http://www.cfd-online.com/Forums/openfoam/101417-add-random-value-velocity.html)

zxj160 May 3, 2012 09:41

Add Random value to velocity
 
Dear Foamers,

How to add random value to velocity in the solver using LES? I just want to see larger eddies in a shorter time? It may relate to Random.H function. But how to link it to velocity? I have no experience of using Random.H.

gschaider May 3, 2012 13:18

Quote:

Originally Posted by zxj160 (Post 359097)
Dear Foamers,

How to add random value to velocity in the solver using LES? I just want to see larger eddies in a shorter time? It may relate to Random.H function. But how to link it to velocity? I have no experience of using Random.H.

Have you had a look at the turbulentInlet-BC? That might just do what you need (quoted from the source code):
Code:

Description
    Generate a fluctuating inlet condition by adding a random component
    to a reference (mean) field.
    Input:
        referenceField
            Mean field.
        fluctuationScale
            RMS fluctuation, provided as the fraction of the mean field.
        alpha
            temporal correlation factor;
            the fraction of the new random component added to the previous
            time-step (defaults to 0.1).


zxj160 May 3, 2012 15:50

Thanks. But my BC is cyclic. I want to add a perturbation component to velocity vector. I noticed that perturbU utility can be useful. But I do not know how to do that.

nenazarian July 10, 2013 16:21

Hi Jian,

I was wondering if you were able to introduce the perturbation and how?.

I also have a LES simulation with periodic BC and I want to add random u,v,w fluctuations (having rms values equal to friction velocity) to trigger turbulence and I dont know how to do it in Fluent.

I would appreciate your help.
Cheers,
Negin

zxj160 July 11, 2013 11:29

Dear Negin,

I am sorry that I have not make it. I just use pressure gradient to drive my flow (just as the ChannelFoam tutorial) and select LES model in the turbulence directory. Also, I am not familiar with Fluent.

But, you may try......
U = U+Random()*FACTOR

My best regards,
Jian


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