# Unexpected difference between velocity / mass-flow inlet

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 December 7, 2010, 23:47 Unexpected difference between velocity / mass-flow inlet #1 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 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.

 December 9, 2010, 02:10 #2 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,149 Rep Power: 32 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... __________________ In memory of my friend Hervé: CFD engineer & freerider

 December 9, 2010, 07:17 #3 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 -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...

 December 9, 2010, 08:44 #4 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,149 Rep Power: 32 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) __________________ In memory of my friend Hervé: CFD engineer & freerider

 December 9, 2010, 08:49 #5 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 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.

 December 9, 2010, 10:37 #6 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,149 Rep Power: 32 So you have number of outlets, but you don't know neither their pressure, nor their massfow? __________________ In memory of my friend Hervé: CFD engineer & freerider

 December 9, 2010, 11:42 #7 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 that is correct I only know their approximate massflow percentages (e.g. parts of the mass entering the system)

 December 10, 2010, 02:27 #8 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,149 Rep Power: 32 then try to work with pressure-outlet and enable target massflow option, with giving approximate massflow __________________ In memory of my friend Hervé: CFD engineer & freerider

 December 10, 2010, 08:20 #9 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 9 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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