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# Why do Multiphase Simulations Take so Long?

Multiphase simulations have transient time-dependency.

The number of iterations in transient simulations depend on total time and timestep.

If you use default settings, simulation timestep will be dependent on Courant number. The following equation shows the relationship between the courant number, timestep, cell size, and velocity magnitude:

$Co&space;=&space;\frac{t&space;\cdot&space;u}{\Delta&space;x}$

Therefore, the smaller the cell size is, the smaller the timestep will be. This will increase the number of iterations, leading to the simulation time and computational expense being significantly more. To learn more about meshing a Multiphase Analysis , Visit our Knowledge Base Article here.

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