Momentum equation used by mixture model

George

Hello. I would like to know how FLUENT's momentum equation (eq.24.3-4) for the mixture model was derived. It seems that the 2/3mu term (which is found in the single phase Navier-Stokes equation eq. 9.2-4) has been eliminated in some way, when all the single-phase momentum equations were summed to derive the mixture momentum equation.

I know that the 2/3mu term dissapears when a single-phase flow is incompressible (del dot v =0). However, FLUENT states that one can have multiple compressible liquids in the model. Why then is the 2/3mu term missing from eq. 24.3-4?

(I am modelling density currents in reservoirs, driven by suspended sediment. The volumetric concentrations can vary from 1 to say 15%. The mixture model and Discrete phase model appear to be reasonable candidates.)

Regards George