[xyq916]

In CFX-post, how to obtain the mass flow rate of a cross section?

[ghorrocks]

Go to function calculator and choose the massflow function.

[mz_uon]

Originally Posted by ghorrocks

Go to function calculator and choose the massflow function.

Hi Glenn

I want to calculate the volume flow rate through a zone. I am unable to find a function to do that. Any suggestions?

Thanks

[ghorrocks]

I presume by "zone" you mean a boundary face.

For simple flat faces volume flow rate is areaInt(Normal Velocity)@Zone Face, where you replace Normal Velocity with the velocity direction and Zone Face with the appropriate name. You can do curved faces as well, but they will require a bit more thought.

[mz_uon]

Originally Posted by ghorrocks

I presume by "zone" you mean a boundary face.

For simple flat faces volume flow rate is areaInt(Normal Velocity)@Zone Face, where you replace Normal Velocity with the velocity direction and Zone Face with the appropriate name. You can do curved faces as well, but they will require a bit more thought.

Thanks for your reply. Correct, I have found this across several documents.
However, I am confused when it comes to actual selection of the surface and/or velocity. See the attached snapshot. The 2 methods give different answers, whereas I imagined it should give the same volume flow. What is the difference between them?

Regards

[Opaque]

In the first one, you are asking for

Area integral along Z direction of the velocity magnitude

on the second one, you are asking for the

Area integral of Velocity along the Z direction

[mz_uon]

Originally Posted by Opaque

In the first one, you are asking for

Area integral along Z direction of the velocity magnitude

on the second one, you are asking for the

Area integral of Velocity along the Z direction

So which one is the correct value for volume flow through the window? I observed two formulae in literature (see image), does any of my earlier expression calculate this?

[Opaque]

Neither of those is correct (generically). The formalism is

massFlow()@Boundary / Density

or you could also create an expression

VdotN = Velocity u * Normal X + Velocity v * Normal Y + Velocity w * Normal Z

then

areaInt (VdotN)@Boundary

I assume you are working with an incompressible fluid (constant density) since volume flow rate does not mean much (to me) for compressible fluids.

[pavlenty]

To calculate massFlow()@Boundary, CFX determine velocity, normal to boundary?

[pavlenty]

Originally Posted by Opaque

Neither of those is correct (generically). The formalism is

massFlow()@Boundary / Density

or you could also create an expression

VdotN = Velocity u * Normal X + Velocity v * Normal Y + Velocity w * Normal Z

then

areaInt (VdotN)@Boundary

I assume you are working with an incompressible fluid (constant density) since volume flow rate does not mean much (to me) for compressible fluids.

What is the point of VdotN ? This is the velocity. But you should give the velocity, normal to selected boundary due to determine mass flow rate.

[Opaque]

Originally Posted by pavlenty

What is the point of VdotN ? This is the velocity. But you should give the velocity, normal to selected boundary due to determine mass flow rate.

Perhaps you should read the documentation. The suggested expression VdotN would be evaluated at the boundary of interest for the function. For example,

areaInt(VdotN)@Inlet will use velocity and Normal at the boundary named Inlet

areaInt(density*VdotN)@Inlet will use density, velocity and normal vector at the boundary named Inlet.