[SardarMech]

Hi all
I am modelling a mixing tank with a single fluid inside. The top of the tank is shown in real model to be at atmospheric pressure. The tank is 5.5 meters high. When filling the tank with 5.5 meters of water (i.e fully filled), I start getting "reverse flow in [such-and-such number of] cells" warning during iterations. I can read online that this can happen in case of a rotating flow at the boundary (top of the tank), which is my case. Also it is suggested that this warning can be remedied by extending the domain a bit further.

My questions are :

1. if I extend the tank up to, say, 6 meters, should I assign a second fluid like air to the extended domain, hence choose multiphase model, and yet be doing all this while I am not interested in the water-air interaction at all or can the domain remain empty, i.e no material assigned to it?

2. Do I need to worry about the warning at all?!

As probably apparent from my question, I am not fully mastering the software, so I would be glad if you would put your explanations in layman's terms please.
Thanks a lot.