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Chander June 11, 2012 09:53

Sign of massflow across a plane

I have a small doubt regarding CFX-Post. If I create a plane across my domain, say Plane 1, and ask CFX post to give me massFlow@(Plane 1), then how is the sign of the resulting quantity decided?
The sign of fluxes for boundaries is clear i.e. mass flux at inlet boundary is positive and negative at outlet boundaries. This is decided by the normal of the boundary ( that points outwards og domain ) and the direction of the velocity ( outwards at outlet and inwards at inlet).
So I think the sign of a flux across any plane is also decided similarly. i.e. based upon the normal of the plane and the direction of the velocity through the plane. But how does CFX-Post decide the direction of the normal to this newly created plane?

Thanks for your inputs!

rikio June 12, 2012 09:31

This has been explained in the help, Reference Guide > Functions in ANSYS CFX > Quantitative Function List > massFlow. "For planes and surfaces that cut through a domain, the normal of the plane or surface is determined by from the right-hand rule and the manner in which the plane or surface is constructed".
Not clear about how to use the right-hand rule. But could you tell me the meaning to know the sign of massflow?

Chander June 12, 2012 11:26

Hello Rikio,

Thanks for your response.
Actually, I was trying to find massflow through a part of my inlet boundary. While CFX post reports a positive massflow through the entire inlet boundary (as it should), it reports a negative massflow through planes that I create in CFX-Post at the location of the inlet boundary. The magnitude of massflow is ok. It is the sign that confused me. I thought that CFX would give the same sign of massflow on these planes as that on the inlet boundary.
Thinking on the same lines, how would CFX-Post decide in which direction the normal to a plane points if the plane is constructed somewhere inside the domain. As you already mentioned , the documentation seems a bit vague on this point.

rikio June 12, 2012 21:10

If you want to know the sign in the interior, generate a vector on the created location, and then get the sign of the plane by combining the direction of vectors. But all of these would be known after the calculation. Anyway, you can check with technical support to make it clear. :-)

Far June 13, 2012 04:21

shouldn't it follow the direction of flow? obviously at each location it checks the inlet and then finds the direction of flow through that plane.

Chander June 13, 2012 19:01

Nope. It does not seem to do that. I will check with Ansys support.
Thanks rikio and Far for your replies.

Far June 14, 2012 03:36

You can try the mixing elbow problem to clarify the things.

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