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

Thanks in advance

Is it multiphase?

No .... the fluid is air

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

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

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 ?

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

Let the normal of the rectangle be [nx, ny, nz], and the area-weighted average of x-,y-,z- velocity be [U, V, W], then (U*nx + V*ny + W*nz) should not be differ much from the volumetric flow rate divided by the area.

Thanks !
Will do that.

Hi

I am stuck with the same problem now.
See attached the image of my model. I am trying to calculate the volume flow through this room. But the 2 methods are giving different answers. I am not sure if I understand it correctly.

Any suggestion/help?

Thanks