Hey, I've been trying top model granular phase flow and I cannot get the equations to converge. I keep getting unstable residuals that oscillate. I've tried calculating intial solutions by turning off the volume fraction calculator. Initial conditions are calculated well, but once I turn on the volume fraction calcualtor I get oscillations and instability again.

PLEASE HELP

What kind of system do you want to simulate? What equations do not converge?

As general suggestions:

1) Use an hexaedral mesh, if possible.

2) Use the second order upwind or the QUICK discretization scheme for the convective term of transport equations.

3) Set the under-relaxation factors as follows:


Pressure 0.6
Momentum 0.4
Turbulence (both k and eps) 0.8

Leave the other values unchanged.

4) If you do an unsteady calculation, try to reduce the size of the time step.

Hi

ap

Thank you, I will try those methods. I'm having especial difficulty in converging the contiuum and air velocity equations. I also have a lot of turbulent viscosities limited. I've heard this can be caused by an unstable problem.

Turbulent viscosity limitation usually is due to a too coarse grid or to bad boundary conditions for turbulence variables.

Hi

ap

The basic shape Im working with is a recatngular box with air blowing in one end and out the other. Along hte bottom of the box is another box where there is a bed of 'granular phase'.

It reflects two 'zones' representing the air and the granular phase(the granular phase on the bottom). I have a velocity inlet on one end and a pressure outlet on the other and walls every where else. I've tried many different mesh sizes with the same result: very unstable solutions with oscillation, especially in velocities and continuity equation. Thanks for your help.

With the granular model of FLUENT I always have the turbulent viscosity limitation.

For convergence, what kind of drag factor are you using? I obtained good results using Syamlal and O'Brien formulation both for the drag factor and the viscosity of the granular phase.

Hi

ap