The Boundary Condition of a suction problem

gaozhengyu · September 27, 2012, 04:06

The model is a box and there is a inlet on both sides of the box and two outlets on the front wall of the box. The air get out through the outlet by suction (just like using a fan) and the air get in through the inlets from outside naturally.

I want to know how to set the BC of the inlet and outlet . !

I have tried that set the inlet as pressure-inlet (gauge total pressure=0Pa), set the outlet as pressure-outlet(gauge total pressure=1000Pa) but I didn't get the result I want.And can I set the outlet as inlet-velocity or mass-flow-inside and use the negative number?

I don't know ANSYS a lot. Someone please help me.thx

Maged_CFD · October 1, 2012, 11:29

According to CFX best practice guidelines, the following are the recommended combinations of boundary conditions, listed from the most robust option to the least robust:

• Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet. The Inlet total pressure is an implicit result of the prediction.
• Robust: Total Pressure at an Inlet; 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; Static Pressure at an Outlet. The system mass flow is part of the solution.
• Very Unreliable: Static Pressure at an Inlet; 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.
With more than two inflows or outflows, the other “openings” should be of the boundary condition type Opening. This is because the flow at these other boundaries could in general be in or out, and the direction will be part of the solution.
Hope this helps!

gaozhengyu · October 1, 2012, 12:55

I will try it. Thank you very much！

September 27, 2012, 04:06	The Boundary Condition of a suction problem	#1
gaozhengyu New Member Gao Join Date: Sep 2012 Posts: 13 Rep Power: 14	The model is a box and there is a inlet on both sides of the box and two outlets on the front wall of the box. The air get out through the outlet by suction (just like using a fan) and the air get in through the inlets from outside naturally. I want to know how to set the BC of the inlet and outlet . ! I have tried that set the inlet as pressure-inlet (gauge total pressure=0Pa), set the outlet as pressure-outlet(gauge total pressure=1000Pa) but I didn't get the result I want.And can I set the outlet as inlet-velocity or mass-flow-inside and use the negative number? I don't know ANSYS a lot. Someone please help me.thx Last edited by gaozhengyu; September 27, 2012 at 04:09. Reason: change the item to"?"

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October 1, 2012, 11:29		#2
Maged_CFD New Member Maged Join Date: Sep 2012 Posts: 7 Rep Power: 14	According to CFX best practice guidelines, the following are the recommended combinations of boundary conditions, listed from the most robust option to the least robust: • Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet. The Inlet total pressure is an implicit result of the prediction. • Robust: Total Pressure at an Inlet; 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; Static Pressure at an Outlet. The system mass flow is part of the solution. • Very Unreliable: Static Pressure at an Inlet; 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. With more than two inflows or outflows, the other “openings” should be of the boundary condition type Opening. This is because the flow at these other boundaries could in general be in or out, and the direction will be part of the solution. Hope this helps!

October 1, 2012, 12:55		#3
gaozhengyu New Member Gao Join Date: Sep 2012 Posts: 13 Rep Power: 14	I will try it. Thank you very much！