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Calculation of Volume Flow Rate/ Average Velocity

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Hello everyone!

I am a new Comsol User and I have a question concerning the general calculation of a volume flow rate.
In my example, I want to calculate the volume flow rate or average velocity in a round jet.

I have used two different ways:

1. I've created a cut plane through the jet and then used the Average command on the velocity magnitude of that plane.
2. Integral of the boundary and then divide by cross section.

Which way/ solution is more accurate?

Thank you very much for any hints.

Rennan

4 Replies Last Post 15 janv. 2016, 03:44 UTC−5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago 11 juin 2012, 15:23 UTC−4
Hi

for me the average operator is exactly that integration (over your boundary) of your operand divided by the integration over the same boundary of argument 1 => hence equal to the boundary area

for a velocity vector, you should perhaps consider the scalar product (u,v,w)*(nx,ny,nz) to get the surface nomal velocity average.

If you are in 2D axi do not forget that the integration over the radial section "*2*pi*r" the "loop length, there is tick to add for the integrations (not all be aware) to automatically perform surface integration from a edge (2D-axi boundary integration

--
Good luck
Ivar
Hi for me the average operator is exactly that integration (over your boundary) of your operand divided by the integration over the same boundary of argument 1 => hence equal to the boundary area for a velocity vector, you should perhaps consider the scalar product (u,v,w)*(nx,ny,nz) to get the surface nomal velocity average. If you are in 2D axi do not forget that the integration over the radial section "*2*pi*r" the "loop length, there is tick to add for the integrations (not all be aware) to automatically perform surface integration from a edge (2D-axi boundary integration -- Good luck Ivar

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Posted: 1 decade ago 13 juin 2012, 07:48 UTC−4
Hi Ivar,

thank you very much for the help.
I was just wondering, how is the Average Command implemented in Matlab?
Does it just sum up the velocity magnitude on each node and then divide by number of nodes?
For example if I use the same model once without and once with boundary layer meshes, the average velocity changes.

Yeah I've already seen the 2*Pi*r trick in one of the discussion threads here. I am
using a 3D model.


Hi Ivar, thank you very much for the help. I was just wondering, how is the Average Command implemented in Matlab? Does it just sum up the velocity magnitude on each node and then divide by number of nodes? For example if I use the same model once without and once with boundary layer meshes, the average velocity changes. Yeah I've already seen the 2*Pi*r trick in one of the discussion threads here. I am using a 3D model.

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago 13 juin 2012, 09:28 UTC−4
Hi

you would write the average as ( integration - over a boundary - of the argument - function of (x,y,z,t) - *dx*dy ) / ( integration - over the same boundary of 1 * dx*dy ) the dx*dy or whatever applies is implicit and newer written out

and COMSOL uses, by default, a 4rth order integration algorithm, better tan a "summation over the elements"

check the doc and the litterture, and do some simple tests on a cube an a sphere to get the right feeling of all this

--
Good luck
Ivar
Hi you would write the average as ( integration - over a boundary - of the argument - function of (x,y,z,t) - *dx*dy ) / ( integration - over the same boundary of 1 * dx*dy ) the dx*dy or whatever applies is implicit and newer written out and COMSOL uses, by default, a 4rth order integration algorithm, better tan a "summation over the elements" check the doc and the litterture, and do some simple tests on a cube an a sphere to get the right feeling of all this -- Good luck Ivar

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Posted: 9 years ago 15 janv. 2016, 03:44 UTC−5

Hi

for me the average operator is exactly that integration (over your boundary) of your operand divided by the integration over the same boundary of argument 1 => hence equal to the boundary area

for a velocity vector, you should perhaps consider the scalar product (u,v,w)*(nx,ny,nz) to get the surface nomal velocity average.

If you are in 2D axi do not forget that the integration over the radial section "*2*pi*r" the "loop length, there is tick to add for the integrations (not all be aware) to automatically perform surface integration from a edge (2D-axi boundary integration

--
Good luck
Ivar


Hi Ivar,

If I want to calculate the average velocity for one direction [let say in z-axis] in surface boundary area, then which one is correct to choose: "surface average" or surface integration"?

Thanks
[QUOTE] Hi for me the average operator is exactly that integration (over your boundary) of your operand divided by the integration over the same boundary of argument 1 => hence equal to the boundary area for a velocity vector, you should perhaps consider the scalar product (u,v,w)*(nx,ny,nz) to get the surface nomal velocity average. If you are in 2D axi do not forget that the integration over the radial section "*2*pi*r" the "loop length, there is tick to add for the integrations (not all be aware) to automatically perform surface integration from a edge (2D-axi boundary integration -- Good luck Ivar [/QUOTE] Hi Ivar, If I want to calculate the average velocity for one direction [let say in z-axis] in surface boundary area, then which one is correct to choose: "surface average" or surface integration"? Thanks

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