# Reply To: Gas-liquid turbulent flows with LBM

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Dear Marc,

Thank you very much for your reply. Mi simulation setup is the following:

– Geometry: straight, cylindrical pipe in 3D.

– Boundary conditions:

– Inlet: plug velocity profiles for liquid and gas, which enter separated (liquid in the lower-half of the inlet, and gas in the upper half). I use a ‘velocity-BB’ rule like the one implemented in OpenLB.

– Outlet: constant pressure, zero gradient for velocity, using extrapolation rules.

– Walls: no-slip condition via full-way bounce-back.

Resolution and time steps are estimated so that velocity at the inlet is lower than 0.01, and so that the smallest tau of the two phases is higher than a certain value, admittedly very close to 0.5. Increasing grid size is of course always convenient for stability, but I have not found any significant improvement upon using very high resolutions while respecting the stability restrictions.

Contrary to what I thought last week, turbulence does not seem to be the main problem since the model works well with high Reynolds numbers in single-phase flows, even with such small taus.

In fact, following your suggestions I have found that instabilities occur at the interface. Given that I use a free-surface VOF algorithm that is somewhat cumbersome, I will search for possible sources of instability there.

Again, thank you for your answer. Your idea of knowing where exactly instabilities occur has been very helpful.

Aurelio