[tobit]

Now I have the combination of Ladd’s method as an inlet and Guo’s non-equilibrium extrapolation method as an outlet and it works like a charm. Finally I can push my model to unrealistically high Reynolds numbers without a loss of stability.
Thank you very much and have a nice week!
tobit

[tobit]

Hey Marc!
Sorry for the late reply and thanks for your advice!
So far I have used the simple Zou/He boundaries with my D2Q9-lattice and Hecht/Harting with the D3Q19-lattice for pressure and velocity boundaries as well as anti-bounce-back for pressure and Ladd’s boundary condition for velocity inlets. When coupling them with turbulence models all of them but Ladd’s boundary condition yield nodes of unphysically high velocity near the boundary and become instable when under-resolved which makes them very little attractive. Thus so far I have a stable velocity inlet but no stable pressure outlet.
Bouzidi for the case of q=1/2 seems just to be a special case of Ladd’s method where the density of the model is assumed 1, isn’t it? Haserta et al. (ICCS 2011) mention that they use a Bouzidi pressure outlet as well but all I could find is a velocity inlet.
And by interpolated boundaries you intend extrapolated boundaries like Guo’s non-equilibrium extrapolation method?
Thanks a lot
tobit

[tobit]

Hey Marc,
another question. As it seems you have more experience with LES simulation with LBM: Which in- and outlet boundary conditions would you recommend for turbulent flow (in particular on a D3Q19 lattice with BGK and MRT Smagorinsky collision operators)?

• This reply was modified 5 years ago by tobit.

[tobit]

Thanks Marc for your detailed reply!
I am familiar with the TRT papers and the derivation of the optimal parameters but I did not consider this. Your argument makes perfectly sense: Setting the anti-symmetric relaxation parameter using the magic parameter lambda (with 1/4 as I did) and a very high Reynolds number and thus a very low positive relaxation time (tau+ very close to 0.5) will result in a very high value for tau- (a very small omega-) and therefore the influence of the anti-symmetric part will be (almost) neglectable. Fixing tau+ and tau- to similar values makes TRT though lose its advantages over BGK as you have pointed out.
I think turbulence modelling in LBM in general is quite appealing indeed due to its simplicity and computational efficiency. Some time ago i benchmarked it and I think my performance dropped by around 1/3 (compared to the simple BGK operator) to 75 Mlups with a D3Q19 lattice on an octa-core processor.
I will let you know if I stumble across something. In the meanwhile thanks again for your kind reply!
Tobit

[tobit]

Thanks a lot, that solved my problem!
Funny enough I had already tried something similar: I simply swapped omegap and omegam but doing so I also changed the rest node relaxation rate and therefore I still had a similar problem.
So for the advection-diffusion equation the rest node is still relaxed with omegap but in this case omegap is calculated with the magic parameter whereas omegam is calculated with the LBM diffusion coefficient.
Thanks again!

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