Setting Time-Step Size to Match Motions

When simulating motions, the time-step size should be matched to the mesh movement.

The fluid field around a moving object changes with the movement of the object. Therefore, the time-step size should be small enough to capture the changes induced by the motion.

For this tutorial, to resolve the fluid in the small gaps during rotation, the time step-size is chosen to match one degree of rotation. When complex physics is involved, for example multiphase, the time step should be reduced further.

In this tutorial, the configurations of the two rotors are listed as follows:

Rotor Number of Teeth Geometry Sector Rotation Rate Time Period Time Steps/ Geometry Sector Time Step Size
Outer Rotor 12 30° 1000 rpm 0.005s 30 (to match 1 degree of outer rotor's rotation) 1/6000 s

You use a fraction to improve the rotational precision.

Inner Rotor 11 32.73° 1090.90909 rpm 0.005s 30 1/6000 s

You resolve one cycle with 30 time steps, which corresponds to 1° rotation of the outer rotor.

Select the Solvers > Implicit Unsteady node and set Time-Step to 1/6000 s.