I'm M.Anil, doing M.Tech Chemical, IIT, Roorkee, India.I'm new is to field of CFD, I'm doing my thesis on CFD of three phase bubble column using CFX-5.6, I'm getting satisfactory results for velocity profile and volume fraction but pressure profile is coming wrong ie; pressure at inlet is lower than pressure at outlet. my system is air ,water and glass beads. At inlet boundary ie sparger I'm specifying velocities and at outlet boundary I'm specifying as degassing condition. I'm also considering buoyancy in vertical direction as –g (-9.81 m/s2). So can anyone suggest my what is the problem in this ? My email id is mik_anil@rediffmail.com

M.Anil

Dear Anil,

the solution in CFX-5 (and most other CFD codes) uses a reduced pressure ie P* = P_thermodynamic - P_hydrostatic

The P_hydrostatic is the hydrostatic pressure that would occur if the fluid were all at density rho_ref.

This is useful numerically since the pressure and hydrostatic terms could nearly cancel themselves and it is only the net effect that acts on the fluid. In finite precision of a computer this can lead to round-off problems. This is also useful in specifying pressure boundary conditions because the hydrostatic contribution does not need to be added and constant P* at an outlet is a good approximation.

So what you need to do is either specify rho_ref = 0.0 or take the resulting pressure field (P*) and add the hydrostatic contribution back on in post-processing.

If you are a member of the cfx-community there is a useful tech tip on this entitiled "Setting boundary conditions at openings in buoyant flows with stagnant atmospheres"

Regards,

Bak_Flow