Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.
Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.
Calculation of Volume Flow Rate/ Average Velocity
Posted 11 juin 2012, 08:18 UTC−4 Version 4.2a 4 Replies
Please login with a confirmed email address before reporting spam
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
Please login with a confirmed email address before reporting spam
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
Please login with a confirmed email address before reporting spam
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.
Please login with a confirmed email address before reporting spam
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
Please login with a confirmed email address before reporting spam
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