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Jonas Larsson April 29, 1999 02:58

UDF in Fluent to Match Mass Flow at Pressure Outlet
I'm running a case in Fluent where I need to match the outlet massflow to a certain given value. The outlet is a simple pressure outlet. Until now I have been doing this manually by iteratively adjusting the pressure until the massflow becomes correct. There must be a better way to do this automatically with a UDF (user defined function) in Fluent.

The UDF should compute the average outlet massflow and then adjust the outlet pressure based on Bernoulli to eventually obtain the right mass-flow. This could be done say every 100th iteration or so - not to often, that will be unstable.

Has anyone done this? I don't want to re-invent the wheel.

Thanks in advance!

The case btw is a fighter aircraft. I'm simulating the whole external flow including the air-intakes and the duct down to the engine inlet. The mass-flow I want to match is the engine inlet mass-flow.

John C. Chien April 29, 1999 10:44

Re: UDF in Fluent to Match Mass Flow at Pressure Outlet
(1). The only concern is the possibility of exit plane flow separation in the inlet diffuser. It can happen at the steady-state solution or during iteration. (2). In the steady-state case, the flow separation depends on the operating condition of the aircraft, that is the flow approaching the inlet locally. For the flow separation during the iteration, it depends on the initial flow field guess and solver. (3). Since the specification of the pressure is at the down stream station, you have to wait long enough for the inlet condition to change so that the whole solution is converged. This is especially important for the transient compressible flow solver. (4). You can try to make it automatic if there is no exit plane flow separation. ( I havn't done it)

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