reversed flow

sivam · January 8, 2010, 11:16

Hi friends,
I am simulating the flow with certain complex geometries, in which while iterating in fluent, i encounter reversed flow in certain number of faces in the pressure outlet region. I use vel. inlet and pre. outlet condition. i want to know really what reversed flow indicates . Is my simulation work is valid for its results or is it a kind of error?? Thanks in advance.

meshkati · January 9, 2010, 08:30

Hey,
What is your test case?
Can you replace the pressure outlet with outflow boundary condition?
If you can, this will be useful test because in outflow reversed flow doesn't appear (and if appear it can be a kind of error).
Anyway changing of discretization scheme may control this problem and after some iteration the reversed flow will terminated (if it was an error).
Regards,
Hadi

sivam · January 9, 2010, 11:27

Thanks to your reply.
Can this condition(outflow) change my results any way?. I am letting my outlet of the flow to atmospheric condition in my experiments. What scheme should i use for heat transfer and fluid flow problems ? mention some.Thanks

AMusteykis · January 11, 2010, 04:31

Hello, sivam!
I also faced the same problem with reversed flow using pressure outlow in my cases. I guess it can happen phisically because if you you use velocity inlet bound. cond. static pressure adjusts to accommodate prescribed velocity distribution (it was written in fluent tutorial) and in some areas you can obtain pressure less than what you prescribed using pressure outflow.
In case of outflow bound. data at exit plane is extrapolated from interior so you can't have opportunity for reversed flow. And I don't think that it will affect on your solution.
Also outflow bond.:
Appropriate where exit flow is close to fully developed condition.
Intended for incompressible flows.
Cannot be used with a Pressure Inlet; must use velocity inlet.
Cannot be used for unsteady flows with variable density
I also have a question:
If I try to use outflow bound. with compressible flow the warning appeared. But if I use pressure outflow as recommended then reversed flow appeared and sometimes solution failed. I use as inlet bound. mainly mass flow inlet because I deal with it in real experiments. What to do if I don't know exactly exit pressure but want to simulate compressible flow with mass flow inlet?
M.b. some suggestions or expertises?
Thanks in advance!

January 8, 2010, 11:16	reversed flow	#1
sivam Member SIVALINGAM Join Date: May 2009 Location: Kodaikanal,Madurai,India Posts: 31 Rep Power: 16	Hi friends, I am simulating the flow with certain complex geometries, in which while iterating in fluent, i encounter reversed flow in certain number of faces in the pressure outlet region. I use vel. inlet and pre. outlet condition. i want to know really what reversed flow indicates . Is my simulation work is valid for its results or is it a kind of error?? Thanks in advance.

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January 9, 2010, 08:30		#2
meshkati New Member S.M. Hadi Meshkati Join Date: Nov 2009 Posts: 7 Rep Power: 16	Hey, What is your test case? Can you replace the pressure outlet with outflow boundary condition? If you can, this will be useful test because in outflow reversed flow doesn't appear (and if appear it can be a kind of error). Anyway changing of discretization scheme may control this problem and after some iteration the reversed flow will terminated (if it was an error). Regards, Hadi

January 9, 2010, 11:27		#3
sivam Member SIVALINGAM Join Date: May 2009 Location: Kodaikanal,Madurai,India Posts: 31 Rep Power: 16	Thanks to your reply. Can this condition(outflow) change my results any way?. I am letting my outlet of the flow to atmospheric condition in my experiments. What scheme should i use for heat transfer and fluid flow problems ? mention some.Thanks

January 11, 2010, 04:31		#4
AMusteykis New Member Anton Musteykis Join Date: Sep 2009 Location: Saint-Petersburg, Russia Posts: 12 Rep Power: 16	Hello, sivam! I also faced the same problem with reversed flow using pressure outlow in my cases. I guess it can happen phisically because if you you use velocity inlet bound. cond. static pressure adjusts to accommodate prescribed velocity distribution (it was written in fluent tutorial) and in some areas you can obtain pressure less than what you prescribed using pressure outflow. In case of outflow bound. data at exit plane is extrapolated from interior so you can't have opportunity for reversed flow. And I don't think that it will affect on your solution. Also outflow bond.: Appropriate where exit flow is close to fully developed condition. Intended for incompressible flows. Cannot be used with a Pressure Inlet; must use velocity inlet. Cannot be used for unsteady flows with variable density I also have a question: If I try to use outflow bound. with compressible flow the warning appeared. But if I use pressure outflow as recommended then reversed flow appeared and sometimes solution failed. I use as inlet bound. mainly mass flow inlet because I deal with it in real experiments. What to do if I don't know exactly exit pressure but want to simulate compressible flow with mass flow inlet? M.b. some suggestions or expertises? Thanks in advance!