question about inlet & outlet boundaries?

hamidciv · September 11, 2015, 03:20

hi dear friends
What is the main reason for taking away input & output boundaries from upstream and downstream?
i know this work done in order to Prevention of warning under title: a wall has been placed........ or same reversed flow in boundary.
whether top warning appear only in case the boundary condition inplet or outlet of type static pressure or total pressure is defined Or it may occur as an example in the mass flow or specifiv velocity boundary conditions in inlet & outlet?
also I really do not know this action (my mean is distance within inlet & outlet boundaries and upstream,downstream) is a recommendation or force in ansys cfx and other software?

thanks a lot

ghorrocks · September 12, 2015, 06:21

I do not really understand your question. But I think you are asking why do the "a wall has been placed.." messages come up for inlets and outlets with back flow. It is because the numerics of that boundary cannot handle reversed flow, so the solve artificially stops these reverse flows and allows the rest of the boundary to proceed.

Antanas · September 12, 2015, 06:38

Quote:

Originally Posted by hamidciv

hi dear friends
What is the main reason for taking away input & output boundaries from upstream and downstream?
i know this work done in order to Prevention of warning under title: a wall has been placed........ or same reversed flow in boundary.
whether top warning appear only in case the boundary condition inplet or outlet of type static pressure or total pressure is defined Or it may occur as an example in the mass flow or specifiv velocity boundary conditions in inlet & outlet?
also I really do not know this action (my mean is distance within inlet & outlet boundaries and upstream,downstream) is a recommendation or force in ansys cfx and other software?

thanks a lot

The reason is that conditions on boundaries may affect results. This conditions in most cases are known approximately. That is why you should place boundaries far away from the region of interest. Rule for placing boundaries: place 'em at locations where conditions are known precisely. Otherwise move them away from region of interest. You can find info in CFX/Fluent training materials.

hamidciv · September 12, 2015, 07:22

dear glenn & antanas thanks for answer
whether this warning no have relation with type of boundaries (i.e specific velocity or mass flow rate,...)?
dear antanas , are you mean of conditions on boundaries that affect on results could be for example rotating flow?
Then I concluding that if boundary conditions on boundary (inlet or outlet) in my model be known, other we dont need we placed boundaries far away from the region of interest? if your possible please guide me where i can find More information in cfx? whether it exist in help cfx , if it exist please tell me in Which part?
whether this warning creates only in inlet or outlet boundaries that defined of pressure type? my mean is if I define inlet boundary of specific velocity and outlet boundary of total or static pressure , whether it is possible this warning appear for inlet or it can also create in inlet?
whether this problem is independent of the type of defined boundary (i.e pressure, specific velovity, mass flow,...)?

many thanks

Antanas · September 12, 2015, 10:58

Quote:

Originally Posted by hamidciv

dear glenn & antanas thanks for answer
whether this warning no have relation with type of boundaries (i.e specific velocity or mass flow rate,...)?
dear antanas , are you mean of conditions on boundaries that affect on results could be for example rotating flow?
Then I concluding that if boundary conditions on boundary (inlet or outlet) in my model be known, other we dont need we placed boundaries far away from the region of interest? if your possible please guide me where i can find More information in cfx? whether it exist in help cfx , if it exist please tell me in Which part?
whether this warning creates only in inlet or outlet boundaries that defined of pressure type? my mean is if I define inlet boundary of specific velocity and outlet boundary of total or static pressure , whether it is possible this warning appear for inlet or it can also create in inlet?
whether this problem is independent of the type of defined boundary (i.e pressure, specific velovity, mass flow,...)?

many thanks

Sorry I don't understand everything you wrote. Solver places artificial walls on pressure/massflow specified boundaries when there's backflow. When this happens corresponding message appear. If it stays to the end of calculation it often indicates pure placed boundary. Read CFX Modelling Guide item 2.3.3. Also here some slides from CFX training materials.

hamidciv · September 12, 2015, 11:21

dear antanas, that was exactly what i need.
sorry for bad speaking english , because it is not first my language.
my impression is : If either inlet and outlet boundary condition Of the type mass flow or pressure be defined, happens corresponding message appear, THIS IS CORRECT?
thanks in advance

ghorrocks · September 13, 2015, 06:19

No, the image you show means that if you have either a mass flow OR a pressure boundary if the flow reverses it places a wall. It does not mean it has a boundary where you specify mass flow and pressure.

hamidciv · September 13, 2015, 09:55

dear glenn
thanks in advance
best wishes

September 11, 2015, 03:20	question about inlet & outlet boundaries?	#1
hamidciv Senior Member Hydreaulic structures Join Date: Sep 2012 Posts: 283 Rep Power: 14	hi dear friends What is the main reason for taking away input & output boundaries from upstream and downstream? i know this work done in order to Prevention of warning under title: a wall has been placed........ or same reversed flow in boundary. whether top warning appear only in case the boundary condition inplet or outlet of type static pressure or total pressure is defined Or it may occur as an example in the mass flow or specifiv velocity boundary conditions in inlet & outlet? also I really do not know this action (my mean is distance within inlet & outlet boundaries and upstream,downstream) is a recommendation or force in ansys cfx and other software? thanks a lot Last edited by hamidciv; September 11, 2015 at 15:59.

September 12, 2015, 06:21		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,701 Rep Power: 143	I do not really understand your question. But I think you are asking why do the "a wall has been placed.." messages come up for inlets and outlets with back flow. It is because the numerics of that boundary cannot handle reversed flow, so the solve artificially stops these reverse flows and allows the rest of the boundary to proceed. hamidciv likes this.

September 13, 2015, 06:19		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,701 Rep Power: 143	No, the image you show means that if you have either a mass flow OR a pressure boundary if the flow reverses it places a wall. It does not mean it has a boundary where you specify mass flow and pressure. hamidciv likes this.

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September 12, 2015, 07:22		#4
hamidciv Senior Member Hydreaulic structures Join Date: Sep 2012 Posts: 283 Rep Power: 14	dear glenn & antanas thanks for answer whether this warning no have relation with type of boundaries (i.e specific velocity or mass flow rate,...)? dear antanas , are you mean of conditions on boundaries that affect on results could be for example rotating flow? Then I concluding that if boundary conditions on boundary (inlet or outlet) in my model be known, other we dont need we placed boundaries far away from the region of interest? if your possible please guide me where i can find More information in cfx? whether it exist in help cfx , if it exist please tell me in Which part? whether this warning creates only in inlet or outlet boundaries that defined of pressure type? my mean is if I define inlet boundary of specific velocity and outlet boundary of total or static pressure , whether it is possible this warning appear for inlet or it can also create in inlet? whether this problem is independent of the type of defined boundary (i.e pressure, specific velovity, mass flow,...)? many thanks

September 13, 2015, 09:55		#8
hamidciv Senior Member Hydreaulic structures Join Date: Sep 2012 Posts: 283 Rep Power: 14	dear glenn thanks in advance best wishes