CFD Multiphase models - simulating mixing problems

Multiphase models describe the physical behaviour of multiple fluids. The liquid or gaseous fluids can be modelled mixable or unmixable. VOF-(Volume of Fluid)-models enable simulation of two or more unmixable fluids (e.g. air/water, oil/water). The interfaces between the fluids due to surface tension are sharply resolved and droplet formation and break-up are simulated unsteadily. Mixable fluids are modelled with similar approaches. Mixing and separation can be simulated.

The following video shows an unsteady multiphase simulation of two imiscible fluids in a fermenter. From left to right the visualisations are: phase boundaries coloured by absolute velocity, absolute velocity, velocity vectors. The fermenter diameter is 1m, the agitator rotates at 50 RPM.

The next video shows an unsteady simulation of a Rayleigh-Taylor instability. The red coloured liquid has the same viscosity but a density that is 0.5% higher the the density of the blue fluid. Due to gravitation the two liquids mix in a chaotic way driven by density gradients. The video plays 3-times faster than real time.