Velocity - Velocity BC in CFX

saisanthoshm88 · August 26, 2014, 01:56

Could some one please comment on the velocity at inlet and velocity at outlet boundary condition in CFX. Here are the recommended set of boundary conditions from the CFX manual and the velocity-velocity type is not mentioned there.

Most Robust: Velocity/mass flow at an inlet and static pressure at an outlet. The inlet total pressure is an implicit result of the prediction.
Robust: Total pressure at an inlet and velocity/mass flow at an outlet. The static pressure at the outlet and the velocity at the inlet are part of the solution.
Sensitive to Initial Guess: Total pressure at an inlet and static pressure at an outlet. The system mass flow is part of the solution.
Very Unreliable: Static pressure at an inlet and static pressure at an outlet. This combination is not recommended, as the inlet total pressure level and the mass flow are both an implicit result of the prediction (the boundary condition combination is a very weak constraint on the system).
Not Possible: Total pressure cannot be specified at an outlet. The total pressure boundary condition is unconditionally unstable when the fluid flows out of the domain where the total pressure is specified.

ghorrocks · August 26, 2014, 05:20

You cannot do velocity in and out, at least not in a steady state simulation. Then the pressure is not set and mass conservation cannot be achieved. This is known as a badly posed system.

But note you can do it in a transient simulation with a system which can accumulate or loose mass, such as compressible gas.

August 26, 2014, 01:56	Velocity - Velocity BC in CFX	#1
saisanthoshm88 Senior Member --------- Join Date: Oct 2010 Posts: 303 Rep Power: 17	Could some one please comment on the velocity at inlet and velocity at outlet boundary condition in CFX. Here are the recommended set of boundary conditions from the CFX manual and the velocity-velocity type is not mentioned there. Most Robust: Velocity/mass flow at an inlet and static pressure at an outlet. The inlet total pressure is an implicit result of the prediction. Robust: Total pressure at an inlet and velocity/mass flow at an outlet. The static pressure at the outlet and the velocity at the inlet are part of the solution. Sensitive to Initial Guess: Total pressure at an inlet and static pressure at an outlet. The system mass flow is part of the solution. Very Unreliable: Static pressure at an inlet and static pressure at an outlet. This combination is not recommended, as the inlet total pressure level and the mass flow are both an implicit result of the prediction (the boundary condition combination is a very weak constraint on the system). Not Possible: Total pressure cannot be specified at an outlet. The total pressure boundary condition is unconditionally unstable when the fluid flows out of the domain where the total pressure is specified. __________________ Best regards, Santhosh.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pros and Cons for CFX, CFdesign, COMSOL	Val	Main CFD Forum	3	June 10, 2011 02:20
PhD using CFX	Rui	CFX	9	May 28, 2007 05:59
maintaining a logarithmic velocity distribution	Morten Andersen	CFX	1	January 8, 2007 11:37
CFX Post - "Calculate Velocity Components"	Paul Lewis	CFX	0	July 26, 2005 07:48
WALL VELOCITY IN CFX	SS	CFX	1	February 3, 2004 01:05

August 26, 2014, 05:20		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,740 Rep Power: 143	You cannot do velocity in and out, at least not in a steady state simulation. Then the pressure is not set and mass conservation cannot be achieved. This is known as a badly posed system. But note you can do it in a transient simulation with a system which can accumulate or loose mass, such as compressible gas.