[atul1018]

Dear Community

I looked in the forum to see if there is a way to generate artificial turbulence in openfoam for RANS simulations but could not find a concrete answer.

Actually, I am simulation particle-laden flow with the RANS-DEM method. The fluid phase is solved using the RANS approach, and particles are tracked inlagrangian frame of reference. I know RANS gives only mean flow statistics, and turbulent fluctuations are lost due to RANS averaging. To recover the lost turbulent fluctuations and include their effects on the particle's trajectory, I am using simple dispersion models. Till now, I am not generating any additional turbulence, and my inlet turbulence parameters are based on turbulent intensity and length scale assuming flow is fully developed turbulent flow.

I want to increase the turbulence in the system artificially and see what happens to particle motion when there is more turbulence in the system. Unfortunately, I can't find a tutorial or source that can guide me in achieving my goals. Please guide me on whether I can do this in RANS simulations.

I saw some papers, where they have used synthetic turbulence but in my understanding, these bc (for synthetic turbulence) are made for LES, DNS, or hybrid RANS-LES. Can I use any kind of synthetic turbulence generator for RANS simulations to have some more randomness than simply a fully developed turbulent flow?

Best Regards
Atul Jaiswal

[Tobermory]

I saw some papers, where they have used synthetic turbulence but in my understanding, these bc (for synthetic turbulence) are made for LES, DNS, or hybrid RANS-LES. Can I use any kind of synthetic turbulence generator for RANS simulations to have some more randomness than simply a fully developed turbulent flow?

Yes and no. You can increase the turbulence levels, but not in the way that you are thinking. Forget about synthetic turbulence - as you noted earlier, it makes no sense in a RANS simulation. To boost the turbulence levels above the equilibrium levels, simply add a source term for k across the whole field, or a portion of the field - as desired.

[atul1018]

Originally Posted by Tobermory

Yes and no. You can increase the turbulence levels, but not in the way that you are thinking. Forget about synthetic turbulence - as you noted earlier, it makes no sense in a RANS simulation. To boost the turbulence levels above the equilibrium levels, simply add a source term for k across the whole field, or a portion of the field - as desired.

Thanks Tobermory

What do I need to do to have turbulence above equilibrium level? I mean, what should be the easiest and quickest way to do it? I would avoid modifying source codes of turbulence models to add the additional source term for turbulence generation. can it be done by fvOptions/fvModels?

Any reference (papers, cases, tutorials) would be very great to start.

Best Regards
Atul

[Tobermory]

can it be done by fvOptions/fvModels?

Exactly! You got it.

[atul1018]

Originally Posted by Tobermory

Exactly! You got it.

Thank you.

I am not sure how I can do that in fvOptions. I tried to look online, and the simplest way I could find (from:Add source term to KEpsilon model in given location) is in the form of below-mentioned code:

scalarSource
{
    type            scalarSemiImplicitSource;
    selectionMode   cellSet;
    cellSet              sourceDisk1;

    volumeMode      absolute;

    injectionRateSuSp
    {
        k           (30.7 0);
        epsilon     (1.5  0);
    }
}

This seems straightforward, but I am not sure how can I estimate the k and Epsilon. Also, do I have to add source terms in both k and epsilon/omega?

Best Regards
Atul Jaiswal

[Tobermory]

Yep - that's the right source to use. Just be careful that you understand the impact of the "volumeMode" flag. As for the value for epsilon - look at your existing field to see what the equilibrium value is, then try add say half of that in the source and see what impact it has - you may need to try a few times to get the effect that you want. No need to add an omega (dissipation) source, unless you want to adjust the turbulence lengthscale.

So long and short of it - you are on the right track. Now try running and it tuning the source value.