The artificial farfield boundary conditions for the 2-D airfoil

ljnpu · October 25, 2011, 01:37

I am a new learner in CFD field.
When I read the classical book named ''Computational fuild dynamics

rinciples and applications'' which is edited by J.Blazek, I have a problem about the farfield conditions.
My question is that how to calculate the Mach number on the boundary surface.
Because in the book,we can just obtain the flow parameters after we get the Mach number on the boundary surface.
After we know the Mach number,we can confirm that the boundary flow is supersonic or subsonic,inflow or outflow.
Without the Mach number on the boundary, I think i can get nothing.
My question is how to calculate the Mach number on the boundary surface!!
Many thanks

I think it is a hen and egg problem:
without the Mach number we can not calculate the boundary velocity,pressure,density and so on.
But without the parameter such as velocity,pressure,density on the boundary, how can we calculate the Mach number on the boundary surface?
which one is the first? hen or egg?

ljnpu · October 25, 2011, 02:24

waiting for your help!
many thanks

ljnpu · October 26, 2011, 08:42

Maybe i did not explain my question very well.
The farfield boundary conditions:
1.subsonic inflow
2.subsonic outflow
3.supersonic inflow
4.supersonic outflow
But before we can make sure which condition we should use,we have to calculate the local Mach number.
I don't understand how we can calculate the local Mach number on the boundary.
Because we DO NOT know the pressure,the velocity and the density on the farfeild boundary. and the parameters we need to calculate the Mach number can not be obtained before we can choose the boundary condition.
I am very CONFUSED.
Please help me!
Many THANKS!

October 25, 2011, 01:37	The artificial farfield boundary conditions for the 2-D airfoil	#1
ljnpu New Member liujie Join Date: Oct 2011 Posts: 8 Rep Power: 14	I am a new learner in CFD field. When I read the classical book named ''Computational fuild dynamicsrinciples and applications'' which is edited by J.Blazek, I have a problem about the farfield conditions. My question is that how to calculate the Mach number on the boundary surface. Because in the book,we can just obtain the flow parameters after we get the Mach number on the boundary surface. After we know the Mach number,we can confirm that the boundary flow is supersonic or subsonic,inflow or outflow. Without the Mach number on the boundary, I think i can get nothing. My question is how to calculate the Mach number on the boundary surface!! Many thanks I think it is a hen and egg problem: without the Mach number we can not calculate the boundary velocity,pressure,density and so on. But without the parameter such as velocity,pressure,density on the boundary, how can we calculate the Mach number on the boundary surface? which one is the first? hen or egg?

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October 25, 2011, 02:24		#2
ljnpu New Member liujie Join Date: Oct 2011 Posts: 8 Rep Power: 14	waiting for your help! many thanks

October 26, 2011, 08:42		#3
ljnpu New Member liujie Join Date: Oct 2011 Posts: 8 Rep Power: 14	Maybe i did not explain my question very well. The farfield boundary conditions: 1.subsonic inflow 2.subsonic outflow 3.supersonic inflow 4.supersonic outflow But before we can make sure which condition we should use,we have to calculate the local Mach number. I don't understand how we can calculate the local Mach number on the boundary. Because we DO NOT know the pressure,the velocity and the density on the farfeild boundary. and the parameters we need to calculate the Mach number can not be obtained before we can choose the boundary condition. I am very CONFUSED. Please help me! Many THANKS!