On the unsteady flow.

mini

In simulating a kind of unsteady flow similar to the jet, I need to simulate the steady flow first, which can be used to initialize the unsteady flow. But it is very difficult to converge, the residuals for continuity equation, momentum equation and energy equation don¡¯t drop, but go up slowly.. Please give me some advices. Thanks. In addition, I don¡¯t know how to determine the time step. This is the first time that I use fluent to simulate the unsteady flow. Help me, please.