Outlet Boundary Condition

ashtonJ · April 24, 2011, 19:30

Dear All

I want to simulate blood flow inside a curved artery with constant diameter along the artery (artery doesn’t have any constriction). I know the pressure value at inlet of artery but I do not know the mass flow and pressure at outlet. I want to find the pressure value at outlet. Does anybody know what type of boundary condition should be used at outlet in order to get the outlet pressure value by considering that mass flow rate is unknown.

Kind regards,
AshtonJ

juliom · April 25, 2011, 08:48

dear friend there you have a difficult problem, first of all it is a non newtonian flow, so you have to introduce the dependence between the sheear strees and the deformation of the fluid, and furthemore you will need at least the velocity at the outlet.
If i were you i would use, flow at the inlet and an outflow at the outlet

ashtonJ · April 25, 2011, 08:53

Dear juliom,

Thank you very much for your kind reply. The problem is that I do not know the flow rate. I have got the physiological pressure value at inlet of a healthy artery and I am trying to find how much pressure would be droped along the artery and that is why I need to get the pressure value at outlet.

Best wishes,
AshtonJ

juliom · April 25, 2011, 09:05

Ok, look my dear friend. If you use just pressure as a boundary condition you will have a system which is non robust. Remember that the equation system requires to be closed by the boundaries conditions and the only way to couple them is through the conservation law and the mometum law. I sugest you to read more about the SIMPLE algorithm. Then you will see why you should use the other type of boundaries conditions.!!! I would like to help you but is not easy in that way

ashtonJ · April 25, 2011, 19:36

Dear juliom,

Again thanks. Do you know how I can impose "outflow" boundary condition in CFX-Pre. I checked all available boundary conditions in CFX-Pre but I could not find "outflow".

Best wishes,
AshtonJ

Lance · April 26, 2011, 02:22

Quote:

Originally Posted by ashtonJ

Dear juliom,

Again thanks. Do you know how I can impose "outflow" boundary condition in CFX-Pre. I checked all available boundary conditions in CFX-Pre but I could not find "outflow".

Best wishes,
AshtonJ

If you checked all the boundary conditions in CFX you would have found "outlet" (and "opening"). Have a read in the documentation about them.

juliom · April 26, 2011, 08:09

Hello my friend, when you use a condition of "outflow", you are forcing the the system to take out all the fluid from the domain. However the "outlets" are one kind of outflow, since you have to set up different properties.
Indeed you can use pressure at the inlet and the outlet but I am quite sure that you will have problem with the solver, perhaps an overflow problem or it will not solve the problem and you will have convergence problem.
You should use at least one condition with velocity or flow.
Remember that Patankar defined, in his book, "outflow" as a condition where the gradient at the outlet is zero for all the propierties minus the V (vertical velocity).
I hope this will help you

April 24, 2011, 19:30	Outlet Boundary Condition	#1
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 16	Dear All I want to simulate blood flow inside a curved artery with constant diameter along the artery (artery doesn’t have any constriction). I know the pressure value at inlet of artery but I do not know the mass flow and pressure at outlet. I want to find the pressure value at outlet. Does anybody know what type of boundary condition should be used at outlet in order to get the outlet pressure value by considering that mass flow rate is unknown. Kind regards, AshtonJ

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April 25, 2011, 08:48		#2
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	dear friend there you have a difficult problem, first of all it is a non newtonian flow, so you have to introduce the dependence between the sheear strees and the deformation of the fluid, and furthemore you will need at least the velocity at the outlet. If i were you i would use, flow at the inlet and an outflow at the outlet

April 25, 2011, 08:53		#3
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 16	Dear juliom, Thank you very much for your kind reply. The problem is that I do not know the flow rate. I have got the physiological pressure value at inlet of a healthy artery and I am trying to find how much pressure would be droped along the artery and that is why I need to get the pressure value at outlet. Best wishes, AshtonJ

April 25, 2011, 09:05		#4
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Ok, look my dear friend. If you use just pressure as a boundary condition you will have a system which is non robust. Remember that the equation system requires to be closed by the boundaries conditions and the only way to couple them is through the conservation law and the mometum law. I sugest you to read more about the SIMPLE algorithm. Then you will see why you should use the other type of boundaries conditions.!!! I would like to help you but is not easy in that way

April 25, 2011, 19:36		#5
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 16	Dear juliom, Again thanks. Do you know how I can impose "outflow" boundary condition in CFX-Pre. I checked all available boundary conditions in CFX-Pre but I could not find "outflow". Best wishes, AshtonJ

April 26, 2011, 08:09		#7
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Hello my friend, when you use a condition of "outflow", you are forcing the the system to take out all the fluid from the domain. However the "outlets" are one kind of outflow, since you have to set up different properties. Indeed you can use pressure at the inlet and the outlet but I am quite sure that you will have problem with the solver, perhaps an overflow problem or it will not solve the problem and you will have convergence problem. You should use at least one condition with velocity or flow. Remember that Patankar defined, in his book, "outflow" as a condition where the gradient at the outlet is zero for all the propierties minus the V (vertical velocity). I hope this will help you