[syavash]

Hi Foamers,

I think it is a quite important subject. I have tried to simulate channel395 through both dynamic Smagorinsky (local) and an explicit SGS stress tensor adding to momentum equations.

The first approach is a well-known eddy-viscosity method which calculates nuSgs and adds it to nu to constitute nuEff and finally calculates divDevReff from something like:

fvm::laplacian(nuEff, U)

The second approach does not calculate nuSgs directly, but instead calculates SGS stress tensor B from the following relation:

B = -2 * nu_t * S_ij

where nu_t is equal to nuSgs and S_ij is the resolved strain rate tensor. The only difference from the first approach is that divDevReff is now an explicit source term which is added to momentum equation as the following term:

fvc:: div(B) - fvm::laplacian(nu, U)

Well, I would expect to get the same results in term of viscous stress (u_tau) in the case of channel395 with default grid spacing, but it is not as thought. The first approach (currently used in OF) yields a reasonable Re_tau=370 but the second approach strongly over-predicts viscous stress, yielding a Re_tau=500!!

Does anyone have some sort of experience with this type of SGS stress modeling?!

I would highly appreciate if someone could give me insight. I wonder if this big difference should be expected when modelling SGS stress explicitly ??!

Syavash

[syavash]

Well, the issue was resolved. The problem was in using yPlusLES in calculating wall friction velocity.

For whoever that might be interested, to calculate friction velocity and y+ one must use wallShearStressLES instead of yPlusLES. The latter only accounts for eddy-viscosity models while the former suits both approaches.

Syavash