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January 17, 2005, 13:38 
Converge problem of 3D free jet flow

#1 
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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~


January 17, 2005, 13:45 
Re: Converge problem of 3D free jet flow

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


January 18, 2005, 05:52 
Re: Converge problem of 3D free jet flow

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


January 19, 2005, 13:00 
Re: Converge problem of 3D free jet flow

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If its a freejet, try using Pressure farfield B.C. on your outer walls. It is a nonreflecting boundary condition,


January 20, 2005, 00:19 
Re: Converge problem of 3D free jet flow

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


January 20, 2005, 00:21 
Re: Converge problem of 3D free jet flow

#6 
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The outlet is not so far from the nozzle, then, can I specify it as pressure far field outlet BC?


January 20, 2005, 01:45 
Re: Converge problem of 3D free jet flow

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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.


January 20, 2005, 01:48 
Re: Converge problem of 3D free jet flow

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Oh~Good suggestion! I will try it.
Thanks very much~ 

January 20, 2005, 16:10 
Re: Converge problem of 3D free jet flow

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


January 20, 2005, 16:14 
Re: Converge problem of 3D free jet flow

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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?


January 20, 2005, 20:25 
Re: Converge problem of 3D free jet flow

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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!!!


January 24, 2005, 01:21 
Re: Converge problem of 3D free jet flow

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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 Pfar field, but the best way to use a pressure farfield 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 Pfar field all around, even at the outlet. Just make sure that the Pfar 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 

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