How is the free surface shape changing with time? How large is the tank relative to the impellor size: ie if you look at the flow speed is it reasonable to expect the free surface to react within 1.5s? How well resolved is the free surface?
The set up looks mostly OK. For high swirl you may need to consider the Reynolds Stress Model (read the theory to see why). Similarly, geo-recon may be overkill here, so review Compressive.
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