23.14.4 Eulerian Model

Calculating an Initial Solution

To improve convergence behavior, you may want to compute an initial solution before solving the complete Eulerian multiphase model. There are three methods you can use to obtain an initial solution for an Eulerian multiphase calculation:

• Set up and solve the problem using the mixture model (with slip velocities) instead of the Eulerian model. You can then enable the Eulerian model, complete the setup, and continue the calculation using the mixture-model solution as a starting point.

• Set up the Eulerian multiphase calculation as usual, but compute the flow for only the primary phase. To do this, deselect Volume Fraction in the Equations list in the Solution Controls panel. Once you have obtained an initial solution for the primary phase, turn the volume fraction equations back on and continue the calculation for all phases.

• Use the mass flow inlet boundary condition to initialize the flow conditions. It is recommended that you set the value of the volume fraction close to the value of the volume fraction at the inlet.

 You should not try to use a single-phase solution obtained without the mixture or Eulerian model as a starting point for an Eulerian multiphase calculation. Doing so will not improve convergence, and may make it even more difficult for the flow to converge.

Temporarily Ignoring Lift and Virtual Mass Forces

If you are planning to include the effects of lift and/or virtual mass forces in a steady-state Eulerian multiphase simulation, you can often reduce stability problems that sometimes occur in the early stages of the calculation by temporarily ignoring the action of the lift and the virtual mass forces. Once the solution without these forces starts to converge, you can interrupt the calculation, define these forces appropriately, and continue the calculation.

Using W-Cycle Multigrid

For problems involving a packed-bed granular phase with very small particle sizes (on the order of 10 m), convergence can be obtained by using the W-cycle multigrid for the pressure. Under Fixed Cycle Parameters in the Multigrid Controls panel, you may need to use higher values for Pre-Sweeps, Post-Sweeps, and Max Cycles. When you are choosing the values for these parameters, you should also increase the Verbosity to 1 in order to monitor the AMG performance; i.e., to make sure that the pressure equation is solved to a desired level of convergence within the AMG solver during each global iteration. See Section  23.12.2 for more information about granular phases, and Sections  25.6.2 and 25.22.3 for details about multigrid cycles.

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