Static pressure boundary condition at outlet

jennz · March 28, 2011, 17:53

I have a simple flow problem with constant flow velocity at the inlet and static pressure (I use 0) at the outlet. However, when I monitor results in CFX-post, the pressure at the outlet is far from 0. It ranges from -30Pa to +1MPa. The average or area-average pressure is not 0 either. Does anyone knows why? One thing I notice is CFX imposed a wall boundary condition on 60% of the outlet to prevent inwards flow. Does this cause the incorrect pressure condition?
Appreciate any answer I may get!

taw · March 28, 2011, 21:18

Can you clarify more. I assume you are doing a laminar flow through a parallel plate (2Dduct flow). If so I may put my 5cent or I simply suggest play with the bc, since I was facing a similar problem when I do a plate flow problem. My main problem was that I tried to use the bc terminologies I normally use from fluent to cfx. Need to understand how cfx interprets bc.

jennz · March 29, 2011, 01:40

Thanks taw. When I specified a pressure BC at the inlet instead of a flow rate, the outlet pressure behaved much better, i.e., the result is closer to the pressure value I specified at the outlet. Don't understand why outlet pressure had such a huge variation when I used inlet flow rate. I don't think CFX document gives a clear explanation on "static pressure BC" either. By the way I have a wider inlet, narrower flow channel, and an outlet the same size as the inlet.

taw · March 29, 2011, 23:02

Hi,

I think you need to use opening boundary condition at the trailing edge and can assign a velocity like 0.001 at inlet to ensure the flow remains laminar, otherwise to whatever you wanted. That is why I say you may be using outlet bc as commonly used in Fluent. The reson using opening bc is that you may not apriori know whether there is a return flow at the trailing edge or not. So the solver will have the freedom to assign whatever is the case with the opening bc. But make sure you use the static pressure condition.

Hope things are fixed by now or let me know.

faizan · February 11, 2014, 04:29

What is the meaning of Relative Pressure when you choose Average Static Pressure Option in Outlet Boundary Details under Mass Momentum Section in CFX Pre? Please reply to my query I am a beginner in using ANSYS CFX.

March 28, 2011, 17:53	Static pressure boundary condition at outlet	#1
jennz New Member jennz Join Date: Mar 2011 Posts: 2 Rep Power: 0	I have a simple flow problem with constant flow velocity at the inlet and static pressure (I use 0) at the outlet. However, when I monitor results in CFX-post, the pressure at the outlet is far from 0. It ranges from -30Pa to +1MPa. The average or area-average pressure is not 0 either. Does anyone knows why? One thing I notice is CFX imposed a wall boundary condition on 60% of the outlet to prevent inwards flow. Does this cause the incorrect pressure condition? Appreciate any answer I may get!

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March 28, 2011, 21:18		#2
taw New Member Join Date: Oct 2009 Posts: 29 Rep Power: 17	Can you clarify more. I assume you are doing a laminar flow through a parallel plate (2Dduct flow). If so I may put my 5cent or I simply suggest play with the bc, since I was facing a similar problem when I do a plate flow problem. My main problem was that I tried to use the bc terminologies I normally use from fluent to cfx. Need to understand how cfx interprets bc.

March 29, 2011, 01:40		#3
jennz New Member jennz Join Date: Mar 2011 Posts: 2 Rep Power: 0	Thanks taw. When I specified a pressure BC at the inlet instead of a flow rate, the outlet pressure behaved much better, i.e., the result is closer to the pressure value I specified at the outlet. Don't understand why outlet pressure had such a huge variation when I used inlet flow rate. I don't think CFX document gives a clear explanation on "static pressure BC" either. By the way I have a wider inlet, narrower flow channel, and an outlet the same size as the inlet.

March 29, 2011, 23:02		#4
taw New Member Join Date: Oct 2009 Posts: 29 Rep Power: 17	Hi, I think you need to use opening boundary condition at the trailing edge and can assign a velocity like 0.001 at inlet to ensure the flow remains laminar, otherwise to whatever you wanted. That is why I say you may be using outlet bc as commonly used in Fluent. The reson using opening bc is that you may not apriori know whether there is a return flow at the trailing edge or not. So the solver will have the freedom to assign whatever is the case with the opening bc. But make sure you use the static pressure condition. Hope things are fixed by now or let me know.

February 11, 2014, 04:29		#5
faizan New Member Join Date: Feb 2014 Posts: 21 Rep Power: 12	What is the meaning of Relative Pressure when you choose Average Static Pressure Option in Outlet Boundary Details under Mass Momentum Section in CFX Pre? Please reply to my query I am a beginner in using ANSYS CFX.