I have successfully get the converged result of 3D jet flow, but I found that the chamber is not wide enough to get a completely FREE jet. So I increase the width and decrease the length, but the result is diverge. I have tried many size of the chamber, all of them are diverge. I don't know why. Would some one tell me the problem? Thanks so much~

Almost all the residuals are highly fluctuant, especially x&y velocity, k, and epsilon...

I suppose you made a steady calculation. The fluctuation of the residuals seems to indicate unsteady behavior, so you should try an unsteady calculation.

If its a free-jet, try using Pressure far-field B.C. on your outer walls. It is a non-reflecting boundary condition,

Yeah, I made it a steady problem. Do you think free jet is an unsteady problem?

The outlet is not so far from the nozzle, then, can I specify it as pressure far field outlet BC?

I would specify the domain walls parallel to your jet as Pressure Far Field, and the one perpendicular to it (ie.outlet) as a pressure outlet.

Oh~Good suggestion! I will try it.

Thanks very much~

I think so, otherwise your calculation should have converged. Or you did something wrong with your boundary conditions.

But when I set the air velocity as 20m/s, it converge. When I set it as 200m/s, it converge too. But when I set it as 130m/s, it doesn't converge. What's wrong?

I have set them as pressure far field BC's, but when I initialize, an error message appear"bc type not handled Update_Fluid_Density". What does this error mean? Thanks a lot!!!

Some more info from Jason on when and how to use Pressure Far Field:

The only B.C.s that should be touching a pressure far field condition is a symmetry or periodic condition. When a pressure far field contacts any other type of boundary condition (especially wall, but including pressure inlet/outlet, velocity inlet, mass flow inlet, etc...) there tends to be a discontinuity at the intersection. You can use other B.C.s that touch the P-far field, but the best way to use a pressure far-field condition is to use it for the entire "ambient" condition. This minimizes error, and improves convergence of solution. If you look at the Fluent tutorial for a 2D airfoil, they use P-far field all around, even at the outlet. Just make sure that the P-far field is far enough from the body impacting the flowfield that the constant mach and pressure assumptions are valid.

Hope this helps, and goodluck Jason