[RKE]

I am using Star-CCM+ to study the performance of a stirred tank (cylindrical vessel with baffles and a centrally-mounted propeller). The rotational speed of the propeller will vary in the range of 300 to 900 RPM. The tank will only be partially full of liquid, so the surface will vary rapidly over time.

I am interested in modeling the variation in the velocity field at small time intervals over a "representative" time period.

I have performed transient runs using various time increments, and find that the residuals for continuity, x-, y-, and z-momentum only decrease to the range of 0.01 to 0.008 before leveling out. With the next time increment, the residuals increase to values in the range of 0.2 to 0.8 and start to decrease as in the previous time increment.

My concern are the "high" minimum residual values occurring in each time step. Is this normal, or should I use a different time step size?

Thanks for any comments or suggestions.