Volume Flow Rate and Area * Velocity values different

narendra.gadwal · July 9, 2013, 01:55

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

RodriguezFatz · July 10, 2013, 04:41

Is it multiphase?

narendra.gadwal · July 10, 2013, 05:41

No .... the fluid is air

RodriguezFatz · July 10, 2013, 05:46

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

RodriguezFatz · July 10, 2013, 05:49

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!

blackmask · July 10, 2013, 05:54

(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|.

narendra.gadwal · July 10, 2013, 05:54

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, 05:57

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"!

narendra.gadwal · July 10, 2013, 06:10

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

RodriguezFatz · July 10, 2013, 06:12

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

blackmask · July 10, 2013, 06:29

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.

narendra.gadwal · July 10, 2013, 12:27

Thanks !
Will do that.

mz_uon · January 23, 2022, 18:24

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

July 9, 2013, 01:55	*Volume Flow Rate and Area Velocity values different**	#1
narendra.gadwal Member Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 17	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, 04:41		#2
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 28	Is it multiphase? __________________ The skeleton ran out of shampoo in the shower.

July 10, 2013, 05:46		#4
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 28	That sounds strange... How much do they differ? __________________ The skeleton ran out of shampoo in the shower.

July 10, 2013, 05:49		#5
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 28	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, 05:57		#8
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 28	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, 05:41		#3
narendra.gadwal Member Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 17	No .... the fluid is air

July 10, 2013, 05:54		#6
blackmask Senior Member Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 23	(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\|.

July 10, 2013, 06:10		#9
narendra.gadwal Member Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 17	It is a simple rectangular surface. So don't you think the velocity magnitude is normal to that ?

July 10, 2013, 06:12		#10
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 28	Not necessarily... Why don't you post pictures? __________________ The skeleton ran out of shampoo in the shower.

July 10, 2013, 06:29		#11
blackmask Senior Member Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 23	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 (Unx + Vny + W*nz) should not be differ much from the volumetric flow rate divided by the area.

July 10, 2013, 12:27		#12
narendra.gadwal Member Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 17	Thanks ! Will do that.

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