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Markus A. October 10, 2001 20:33

Best BC constellation
Hallo, I simulate the compressible gas mixing in a an apparatus like a small room. I have two inlets and one outlet.

The air enters the first inlet with defined velocity. Air will be sucked through the "Jet" and go out through the outlet.

The pressure at the 2nd Inlet is known. Outlet conditions are not known !!

What are the best BC on the 2nd inlet and outlet, so that I get the the suction Velocity at inlet and the Velocity and pressure at outlet?

Thanks in forward.


Chetan Kadakia October 11, 2001 13:04

Re: Best BC constellation
I think Fluent will work better if you define 2 inlet pressure conditions, and 1 outlet pressure conditions.

Although I am uncertain why, Fluent seems to converge faster and give more accurate results with pressure boundary conditions.

But for your case, you may just define an outlet condition (this will maintain conitinuity; it lets the mass coming in equal the mass coming out), and the inlet conditions can be velocity for the first one, and pressure for the second one. Or better yet, back out the pressure if you can assume a value for it at stagnation or static, and use two pressure inlets.

Where is your mixture exiting? Are you sure you cannot find the exiting pressure, or is the goal of your simulation to determine exiting conditions?

Markus A. October 11, 2001 15:48

Re: Best BC constellation
Yes, the simulation aim is to define the outlet condititions. I have tried to define one veloctiy inlet, pressure inlet and a pressure outlet with all possible initialization and solver settings without convergange. I will tre it with two pressure inlets and tell You the result.

Any how thank You for responding.


Wolfgang October 19, 2001 09:00

Re: Best BC constellation
Hi Markus, I have the same problems as you. I am calculating a high compressible flow in a jet pump. My calculation is converging but the prediction for the mass flow of the suction flow is not true (I recieved a measured value). I reached convergence using the coupled solver, first order scheme for flow and turbulent flow and starting with lower values for the pressure inlets. I increased it very slowly to keep convergence. It might be also a solution for you to start with a constant density and change it later to ideal gas (same for viscosity). After a few gradient and Y+ adaptions and first convergence I changed the discretization schemes to second order. By the way, I used Spallart-Allmaras for turbulence moddeling. Maybe I made something comletely wrong that I didn't reach the correct suction flow? Please tell me if you find a solution for your problem. Greetings, Wolfgang

Markus October 19, 2001 11:09

Re: Best BC constellation
Hallo Wolfgang,

I am trying the same way You said. I think without UDF one can not exactly solve the problem and get the pressure rise in the apparatus. I jave such problem to calculate the venilation of a tunnel. but it was not so complicated.

I will tell You after I reach the final solution.

A lot of thenks,


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