Unexpected difference between velocity / mass-flow inlet

jpo · December 7, 2010, 23:47

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, 02:10

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, 07:17

-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, 08:44

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, 08:49

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, 10:37

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

jpo · December 9, 2010, 11:42

that is correct

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

-mAx- · December 10, 2010, 02:27

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

jpo · December 10, 2010, 08:20

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

December 7, 2010, 23:47	Unexpected difference between velocity / mass-flow inlet	#1
jpo Member Join Date: Apr 2009 Posts: 94 Rep Power: 17	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
-mAx- Super Moderator Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 41	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, 08:44		#4
-mAx- Super Moderator Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 41	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, 10:37		#6
-mAx- Super Moderator Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 41	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 10, 2010, 02:27		#8
-mAx- Super Moderator Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 41	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 9, 2010, 07:17		#3
jpo Member Join Date: Apr 2009 Posts: 94 Rep Power: 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...

December 9, 2010, 08:49		#5
jpo Member Join Date: Apr 2009 Posts: 94 Rep Power: 17	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, 11:42		#7
jpo Member Join Date: Apr 2009 Posts: 94 Rep Power: 17	that is correct I only know their approximate massflow percentages (e.g. parts of the mass entering the system)

December 10, 2010, 08:20		#9
jpo Member Join Date: Apr 2009 Posts: 94 Rep Power: 17	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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