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-   -   Unexpected difference between velocity / mass-flow inlet (http://www.cfd-online.com/Forums/fluent/82861-unexpected-difference-between-velocity-mass-flow-inlet.html)

jpo December 7, 2010 22:47

Unexpected difference between velocity / mass-flow inlet
 
Hello,

I am modeling a 3D incompressible inviscid airflow in a system. It has 1 velocity inlet and a number of outflow outlets. It converges fine.

Then I check the mass flux via inlet: let's say it is M_dot
and
switch velocity inlet to mass-flow inlet, request a flux equal to M_dot.

Case loses convergence right away and can't converge properly anymore. I don't understand why. Nothing really has changed, except the inlet type.


Where am I going wrong?
Thank you for your thoughts.

-mAx- December 9, 2010 01:10

try to switch from outflow to pressure outlet.
I always heard that combination velocity-inlet / pressure-outlet was more stable than mass-flow-inlet / pressure-outlet.
Don't ask me why...

jpo December 9, 2010 06:17

-mAx-, thank you for your reply and for the time you dedicate in helping others in this forum

I tried this, but there is a problem. How do I set the pressures?


My goal is to switch the whole thing to compressible flow. But velocity inlet/outflows do not work with pressure. I cannot make the transition...

-mAx- December 9, 2010 07:44

I don't understand what you mean with transition.
Set your outlet with Pressure-Outlet Boundary Condition.
Set your pressure with 0 Pa (outlet free)

jpo December 9, 2010 07:49

My system has a number of outlets. Compressible simulation is hard to converge. This is why I do incompressible first and try to find out the pressures on all terminals.

Once this is done, I can retry running compressible calculation with the new pressures.

-mAx- December 9, 2010 09:37

So you have number of outlets, but you don't know neither their pressure, nor their massfow?

jpo December 9, 2010 10:42

that is correct :)

I only know their approximate massflow percentages (e.g. parts of the mass entering the system)

-mAx- December 10, 2010 01:27

then try to work with pressure-outlet and enable target massflow option, with giving approximate massflow

jpo December 10, 2010 07:20

to be honest, whenever I have tried enabling target mass flow on pressure outlets, it hardly ever works :(
It didn't work in this case either


But it works with velocity inlet/outflow exits. I am trying to figure out how to use this successful calculation and set up the exits as pressure outlets


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