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Simple BC question
Posted May 3, 2010, 5:57 a.m. EDT 3 Replies
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Hello,
I want to simulate a laminar fluid flow through a straight pipe (vertical cylinder)
Instead of an inlet velocity (as a number) I want to set a laminar velocity profile as inlet BC.
I found a Comsol example in the model lib.
Comsol Multiphysics >> Fluid dynamics >> Micromixer...
The inlet BC (Navier Stokes) is:
U0 = (9/4)*U_mean*16*s1*(1-s1)*s2*(1-s2)
This results in a laminar velocity profile.
What should be my BC?
Copy and pasting the above results in a kind of sickle shaped velocity profile.
I hope someone can help me out.
Thanks,
Greetings.
I want to simulate a laminar fluid flow through a straight pipe (vertical cylinder)
Instead of an inlet velocity (as a number) I want to set a laminar velocity profile as inlet BC.
I found a Comsol example in the model lib.
Comsol Multiphysics >> Fluid dynamics >> Micromixer...
The inlet BC (Navier Stokes) is:
U0 = (9/4)*U_mean*16*s1*(1-s1)*s2*(1-s2)
This results in a laminar velocity profile.
What should be my BC?
Copy and pasting the above results in a kind of sickle shaped velocity profile.
I hope someone can help me out.
Thanks,
Greetings.
3 Replies Last Post May 3, 2010, 11:04 a.m. EDT
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Posted:
1 decade ago
Hi
Well it depends if you are in 2D or 3D. The equation you are referrring to is a 3D equation to give a parabolic profile with maximum at the middle based on the relative local variables s1 s2 (tangeant vectors going from 0 to 1 over a area. In 2D ist simply v0*s(1-s)*4 (or 6) depending if you want the average or max value. "s" is defined in COMSOL as going from 0 to 1 along any edge (direction as with the arrow)
make the integration to check your average/max values
Good luck
Ivar
Well it depends if you are in 2D or 3D. The equation you are referrring to is a 3D equation to give a parabolic profile with maximum at the middle based on the relative local variables s1 s2 (tangeant vectors going from 0 to 1 over a area. In 2D ist simply v0*s(1-s)*4 (or 6) depending if you want the average or max value. "s" is defined in COMSOL as going from 0 to 1 along any edge (direction as with the arrow)
make the integration to check your average/max values
Good luck
Ivar
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Posted:
1 decade ago
Hello Ivar,
Thanks for the fast reply!
Apologizes for my triple post, I got three times an error after clicking 'post'.
My simulation is in 3D.
A vertical cylinder, with the inlet on top and the oulet (P=0) at the bottom.
radius=0.61e-3 [m]
height=5.5e-3 [m]
I want to do the simulation for an average velocity in 3D. So exactly as in the micromixer example.
But I want the profile in the x-y plane in stead of the y-z plane and through a circular cross-section instead of a rectangular one.
I don't know how to change the equation to accomplish this.
Can you help me with that?
Thanks for the fast reply!
Apologizes for my triple post, I got three times an error after clicking 'post'.
My simulation is in 3D.
A vertical cylinder, with the inlet on top and the oulet (P=0) at the bottom.
radius=0.61e-3 [m]
height=5.5e-3 [m]
I want to do the simulation for an average velocity in 3D. So exactly as in the micromixer example.
But I want the profile in the x-y plane in stead of the y-z plane and through a circular cross-section instead of a rectangular one.
I don't know how to change the equation to accomplish this.
Can you help me with that?
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Posted:
1 decade ago
Hi
well if you have a circular entrance in 3D (v3.5a) you can express it in spherical coordinates, either by setting up a work-plane and a circular coordinate system and/or by writing out a scalar variable
r = sqrt((x-X0)^2+(y-Y0)^2)
with X0, Y0 constants defining the on axis of your cylinder, or via a coupling integration variable. then it is up to you to define the profile, probably a parabolic one of the type
v = V0*6*(1-r/R0)^2
with R0 = sqrt(Area/pi) where Area is the integration coupling variable of the inlet area A = int 1*dxdy
If you have a doubt about the average or max value, run a "Solve - Get initial value" to fill the matrices and then integrate your boundary velocity value, and divide by the area to see the average velocity.
You have an example below for a parabolic force, check the boundary integration coupling variables on the top surface, and calcuate the reaction force on the fixed surface, compare with the constant F0.
Have fun Comsoling
Ivar
well if you have a circular entrance in 3D (v3.5a) you can express it in spherical coordinates, either by setting up a work-plane and a circular coordinate system and/or by writing out a scalar variable
r = sqrt((x-X0)^2+(y-Y0)^2)
with X0, Y0 constants defining the on axis of your cylinder, or via a coupling integration variable. then it is up to you to define the profile, probably a parabolic one of the type
v = V0*6*(1-r/R0)^2
with R0 = sqrt(Area/pi) where Area is the integration coupling variable of the inlet area A = int 1*dxdy
If you have a doubt about the average or max value, run a "Solve - Get initial value" to fill the matrices and then integrate your boundary velocity value, and divide by the area to see the average velocity.
You have an example below for a parabolic force, check the boundary integration coupling variables on the top surface, and calcuate the reaction force on the fixed surface, compare with the constant F0.
Have fun Comsoling
Ivar
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