Multiphase modeling
 

Hello.
I want to analyze sand water mixture flow from a pipe using star ccm+. The objective of project is to find the optimum velocity of mixture so that sand will not settle in pipe. Can anyone suggest me which multiphase model is suitable?

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i would use the lagrangian model but it does have restrictions on the percentage by volume of particles in each cell of around 40% so do this check first on your incoming volume fraction and this also means that accumulation of particles in dead zones can't occur but with your aim this wont happen since you will increase your velocity
the general purpose multiphase segregated model can also be used of course and has the advantage that 100% of each phase can occur but post processing information is not as useful as lagrangian
and dispersed multiphase could be good as well and is it will probably solve faster

I would suggest Eulerian Segregated Multiphase - with something like Solid Pressure turned on and one of components set to Particles.
At least - it allows to maintain maximum volume fraction of dispersed phase thus (comparing to Lagrangian approach) sand won't be lying in a thin layer but will occupy the real part of volume.