### 7.2.3. Gas-Solid and Liquid-Solid Flow

It is possible to model the motion of a large number of solid particles in a gas or a liquid as an `Eulerian-Eulerian` two phase flow. Examples occur in pneumatic conveying, sedimentation in rivers, and fluidized beds.

For example, the two phases may be `Water`, the continuous fluid (Continuous Fluid), and `Sand`, the dispersed fluid (Dispersed Fluid).

In such problems, you should assign the solid phase a small insignificant molecular viscosity. This is permissible, as the physics are dominated by interphase drag and turbulence effects. The solid phase should be assigned free slip boundary conditions at walls. If viscosity is not set, the solid material will not be available for selection in the Fluids and Particle Definitions... list on the Basic Settings tab of the Domains details view in CFX-Pre.

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