### 7.9.1. Solid Pressure Force Model

The `Solid Pressure Force Model` is available for dispersed solid phases in a multiphase flow. The forces due to solid collisions are taken into account by introducing additional solids pressure and solids stress terms into the solid phase momentum equations based on either the Gidaspow model or by specifying the elasticity modulus directly. Additional theoretical information on these models is available in Solid Particle Collision Models in the CFX-Solver Theory Guide.

You can select the Gidaspow Model or specify the Elasticity Modulus for solids pressure on the Fluid Specific Models tab, for a particular dispersed solid, when creating a domain in CFX-Pre. For details, see Multiphase Options in the CFX-Pre User's Guide. The Gidaspow model requires the Reference Elasticity Modulus and the Compaction Modulus to be specified. These are used to calculate an Elasticity Modulus. For details, see Solid Particle Collision Models in the CFX-Solver Theory Guide. There are no universally accepted values for these. The values used by Bouillard et al. [17] are:

- Reference Elasticity Modulus = 1 Pa

- Compaction Modulus = 20 to 600

For information on setting the Maximum Packing parameter in CFX-Pre, see Multiphase Options in the CFX-Pre User's Guide.

Results tend to be insensitive to the details of the solids pressure model. The solids pressure gradient is only activated in regions close to the maximum packing, where its tendency is to prevent solid volume fractions from becoming too large.

Using a solid pressure force model in conjunction with the Gidaspow drag model, it is possible to model the large scale features of bubbling fluidized beds. For details, see Densely Distributed Solid Particles.

This `Solid Pressure Force Model` is very numerically stiff, prone to convergence problems and may cause divergence. It is needed in some two-phase flow situations, such as a fluidized bed simulation. It should be used with care as it has known issues with robustness. This behavior will be improved in future releases.

There is a more complex set of models that use the `Kinetic Theory of Granular Flow` to model solid pressures and stresses. For details, see Kinetic Theory Models.

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