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-   -   Volume Flow Rate and Area * Velocity values different (http://www.cfd-online.com/Forums/fluent/120522-volume-flow-rate-area-velocity-values-different.html)

Volume Flow Rate and Area * Velocity values different

Hello,

I have a fluent model, where the fluid is of constant density.
Structured mesh, well converged.

But the volume flow rate what fluent shows is different when I multiply the area and area weighted average of velocity magnitude.

Did not understand ? Why ?

(Volume Flow Rate) Fluent != (Area) fluent * (Area weighted average of velocity magnitude) fluent

 RodriguezFatz July 10, 2013 03:41

Is it multiphase?

No .... the fluid is air

 RodriguezFatz July 10, 2013 04:46

That sounds strange... How much do they differ?

 RodriguezFatz July 10, 2013 04:49

Ahh maybe this:
What do you integrate for the "Area weighted average of velocity magnitude"? You need to integrate only the perpendicular (to the surface) component of the velocity vector - not the length of the complete velocity vector!

(Volume Flow Rate) Fluent == (Area) fluent * (Area weighted average of normal component of velocity) fluent

Note the difference between <vector u> dot <vector A> and |magnitude u|*|magnitude A|.

I tried with Velocity magnitude, X, Y and Z velocities .... still they differ.
When I calculate velocity = (volume flow rate/area) fluent and
area weighted avg of (velocity magnitude/X-vel/Y-vel/Z-vel), both differ by almost 4-5m/s

 RodriguezFatz July 10, 2013 04:57

But this isn't what I wrote:
You need the perpendicular velocity component. If your surface is no "simple" cartesian surface, the cartesian (x,y,z) components won't help you.
What you need is "A dot v"!

It is a simple rectangular surface. So don't you think the velocity magnitude is normal to that ?

 RodriguezFatz July 10, 2013 05:12

Not necessarily... Why don't you post pictures?