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 June 26, 2012, 01:54 Some questions about CFX #1 Member   Stefan Join Date: May 2012 Posts: 48 Rep Power: 6 As illustrated by the picture,does the arrows direction of "outlet" mean the fluid flow direction or pressure direction? (if I set the "boundary type" as 'OUTLET' on the left side and its "mass and momentum" as "total static pressure:20MPa") Last edited by shanxuewenjdx; June 26, 2012 at 21:19.

June 26, 2012, 01:56
#2
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Stefan
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And this the picture metioned above.
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 June 26, 2012, 09:40 #3 Senior Member   Join Date: Jul 2011 Location: Berlin, Germany Posts: 165 Rep Power: 7 I'm sorry if I didn't get the question right, but if I did...how do you want a Fluid Flow going in an another Direction than the pressure??? Meaning if the Pressure is oriented to a specific direction then the fluid flow will also be oriented that way....the fluid flowing from the high to the low pressure domain... Or am I just not getting the question or just being stupid and forgetting my physics?

June 26, 2012, 09:57
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Peter Galimutti
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Quote:
 Originally Posted by shanxuewenjdx As illustrated by the picture,does the direction of "outlet" mean the fluid flow direction or pressure direction? (if I set the "boundary type" as 'OUTLET' and the "mass and momentum" as "total static pressure:20MPa")
how do you know total pressure at outlet. total pressure is sum of static plus dynamic pressures, you get that info after flow field is solved not before. you can only get for inlet. it leads to instabilities, see cfx help on boundary conditions.

The pressure and velocity are coupled, unless you know the velocity or pressure at the outlet it's always set as static pressure and the velocity is implicitly calculated.

You may need to rephrase your question in a better way if this is not the answer you are looking for!

 June 26, 2012, 10:07 #5 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 12 I believe that in CFX-Pre, the direction shown is symbolic of the boundary condition, rather than what would actually happen for that situation. So, for "outlet" you see an arrow going out, for "opening" - arrows towards both sides (as back flow is allowed in "opening"), for wall - no arrows and so on. Essentially, in your case, even if you specify velocity at outlet as say (u,v,w) = (0,0,-2 m/s), you still will see the arrow going out.

June 26, 2012, 21:45
another question
#6
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Stefan
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Could these four fluid areas be set in one "Fluid Domain"?
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 June 27, 2012, 04:17 #7 Member   Peter Galimutti Join Date: May 2012 Posts: 37 Rep Power: 6 outlet velocity is calculated by the solver Last edited by p.galimutti; June 27, 2012 at 09:11.

June 27, 2012, 04:25
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Stefan
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Quote:
 Originally Posted by p.galimutti how do you know total pressure at outlet. total pressure is sum of static plus dynamic pressures, you get that info after flow field is solved not before. you can only get for inlet. it leads to instabilities, see cfx help on boundary conditions. The pressure and velocity are coupled, unless you know the velocity or pressure at the outlet it's always set as static pressure and the velocity is implicitly calculated. You may need to rephrase your question in a better way if this is not the answer you are looking for!
Thanks.
But if I only set parameters at inlet,and neglect the outlet.(And if I choose the outlet, I have to set paramerters at outlet) How would I know the situation at outlet?You know, it won't be nearly as same as the real situation.

June 27, 2012, 22:48
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Stefan
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Quote:
 Originally Posted by p.galimutti outlet velocity is calculated by the solver
You mean the outlet should not be set as anything? The cfx-help told me that it should be set as nothing only on the condition that it's supersonic,but in reality it is subsonic.As recommended in cfx-help, I set inlet:0mpa,outlet:2.667kg/s according to real situation.But it still can't be right.

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