Learning Forum

[alrifai]

Hello,

My setup consists of a mass-flow inlet and a pressure outlet. I know both the inlet mass flow and inlet total pressure, while I have 0 information at the outlet. I start with an initial guess of what the outlet pressure may be and use a targeted mass-flow for my outlet. However, I never manage to achieve my desired inlet pressure. How would I go about updating my outlet pressure until I get my desired inlet pressure? Is it a UDF that I have to write? And if that's the case, I have no idea where to start from.

[Rob]

Is the fluid compressible? 

[alrifai]

It's incompressible

[Rob]

OK, so what do you gain from altering the outlet pressure to find the inlet pressure?  You know the inlet pressure relative to outlet for a given mass flow. As an aside, for incompressible flows use a velocity inlet, it's generally more stable and can be slightly quicker. 

[alrifai]

I want to alter the outlet pressure to obtain the total inlet pressure that is calculated

[Rob]

And you can't look at the inlet pressure against a known outlet pressure and use the domain dP? 

[alrifai]

We dont have a known outlet pressure, we start off with a guess and update that value until we're somewhat close to the value we want for inlet pressure, but I dont want to do that manually, I wanna be able to specify that in fluent. 

[alrifai]

I've written the following expression:
IF(inlet_pressure < Inlet_total_pressure, Outlet_gauge_pressure + abs(inlet_pressure-Inlet_total_pressure), Outlet_gauge_pressure + abs(inlet_pressure-Inlet_total_pressure)), but I'm unsure if this is getting updated iteration for iteration

[Rob]

OK, so we start with an outlet pressure of 0 Pa. You run the model and get an inlet pressure. If I increase the outlet pressure by (for example 100,000Pa) what might I expect the inlet pressure to be? If you're not sure feel free to run the model. 

[alrifai]

I ran it with the expression and it seems to work now, I had forgotten to turn off targeted-mf and it seems to be fine now. I get the mass flow I want at the outlet and the pressure I want at the inlet