# Volume Flow Rate and Area * Velocity values different

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 July 9, 2013, 00:55 Volume Flow Rate and Area * Velocity values different #1 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 15 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

 July 10, 2013, 03:41 #2 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26 Is it multiphase? __________________ The skeleton ran out of shampoo in the shower.

 July 10, 2013, 04:41 #3 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 15 No .... the fluid is air

 July 10, 2013, 04:46 #4 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26 That sounds strange... How much do they differ? __________________ The skeleton ran out of shampoo in the shower.

 July 10, 2013, 04:49 #5 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26 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! __________________ The skeleton ran out of shampoo in the shower.

 July 10, 2013, 04:54 #6 Senior Member   Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 21 (Volume Flow Rate) Fluent == (Area) fluent * (Area weighted average of normal component of velocity) fluent Note the difference between dot and |magnitude u|*|magnitude A|.

 July 10, 2013, 04:54 #7 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 15 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

 July 10, 2013, 04:57 #8 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26 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"! __________________ The skeleton ran out of shampoo in the shower.

 July 10, 2013, 05:10 #9 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 15 It is a simple rectangular surface. So don't you think the velocity magnitude is normal to that ?

 July 10, 2013, 05:12 #10 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26 Not necessarily... Why don't you post pictures? __________________ The skeleton ran out of shampoo in the shower.

 July 10, 2013, 05:29 #11 Senior Member   Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 21 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.

 July 10, 2013, 11:27 #12 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 15 Thanks ! Will do that.

January 23, 2022, 17:24
#13
Member

Join Date: Apr 2020
Posts: 76
Rep Power: 6
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
Attached Images
 funtion difference.jpg (88.9 KB, 17 views)