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Problem: 3D Laminar Flow Stationary Solver - Lift and Drag coefficient

Posted Sep 27, 2012, 5:19 a.m. EDT Computational Fluid Dynamics (CFD), Geometry, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2a 5 Replies

Max Mustermann
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Hi,
I habe a problem regarding the calculation of the lift and drag coefficient in a stationary solver.
I tried to work with the "Drag on a Cylinder" model:
The "Drag on a Cylinder" model gives the drag coefficient as -reacf(u)*2/(spf.rho*U_mean^2*0.2)
- I never defined "u", what's that?
- For me one problem of this formula is the "0,2", which should be the plane area. Would be great if Comsol could calulate this area on his own, is that possible
- I never defined any mass of inner part of the model. How can Comsol calculate Lift and Drag forces without knowing any material or mass.

Maybe you can give me some help with my problem.
Best regards,
Felix

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5 Replies Last Post Sep 27, 2012, 12:10 p.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Sep 27, 2012, 5:53 a.m. EDT
Hi

I believe you should read the doc more carefully, and check the dependent variables you are using, not to say all the other COMSOL variable names that are already defined (see options preferences Equation View and the Equation nodes)

normally a mas is obtained by COMSOL via the material density and the domain (volume) concerned, anyhow COMSOL and FEM is working on rho*dx*dy*dz = dm and not on full M being the integration over the domain of the densities

(u,v,w) in CFD are the three components of the vector field fluid velocity units [m/s]

--
Good luck
Ivar
Hi I believe you should read the doc more carefully, and check the dependent variables you are using, not to say all the other COMSOL variable names that are already defined (see options preferences Equation View and the Equation nodes) normally a mas is obtained by COMSOL via the material density and the domain (volume) concerned, anyhow COMSOL and FEM is working on rho*dx*dy*dz = dm and not on full M being the integration over the domain of the densities (u,v,w) in CFD are the three components of the vector field fluid velocity units [m/s] -- Good luck Ivar
Max Mustermann
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Posted: 1 decade ago Sep 27, 2012, 6:55 a.m. EDT
Thanks Ivar for your quick reply.
The thing is, that I created the geometry of the model with a difference between two blocks. After that I only have the opportunity to give the outer part a material but not to the inner part of the difference (which bascially is the thing which lift and drag I want to know). So comsol can't calculate the mass via rho*dx*dy*dz = dm because I can't specify a density of the inner part.

The other thing is, that I don't know why I get a lift force (although it's negative) but my drag force is always zero.
Any ideas?

Sincerly,
Felix
Thanks Ivar for your quick reply. The thing is, that I created the geometry of the model with a difference between two blocks. After that I only have the opportunity to give the outer part a material but not to the inner part of the difference (which bascially is the thing which lift and drag I want to know). So comsol can't calculate the mass via rho*dx*dy*dz = dm because I can't specify a density of the inner part. The other thing is, that I don't know why I get a lift force (although it's negative) but my drag force is always zero. Any ideas? Sincerly, Felix
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Sep 27, 2012, 9:38 a.m. EDT
Hi

for me it looks like you have an issue with your geometry, you must first get all domains and boundaries well defined, get the material correct, then the physics, then only the results will be coherent

--
Good luck
Ivar
Hi for me it looks like you have an issue with your geometry, you must first get all domains and boundaries well defined, get the material correct, then the physics, then only the results will be coherent -- Good luck Ivar
Max Mustermann
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Posted: 1 decade ago Sep 27, 2012, 11:50 a.m. EDT
I tried the same study 2Dimensional like it's in the tutorial "Flow after a cylinder" and it works perfectly for me.
So what's wrong with my geometry, what do you exactly mean with "get all domains and boundaries well definded"
Have I to add boundaries around my difference, or what else?
Thanks for your help already,
Sincerly,
Felix
I tried the same study 2Dimensional like it's in the tutorial "Flow after a cylinder" and it works perfectly for me. So what's wrong with my geometry, what do you exactly mean with "get all domains and boundaries well definded" Have I to add boundaries around my difference, or what else? Thanks for your help already, Sincerly, Felix
Max Mustermann
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Posted: 1 decade ago Sep 27, 2012, 12:10 p.m. EDT
Now I just managed it to give the boundaries of my inner part a material.
Additionally I gave the boundaries of my inner part the "slip" condition, while the walls have the "noslip" condition.
What else can be wrong.
My study gets solved, but I just don't get comsol to show me my drag and lift coefficients.
best regards,
felix
Now I just managed it to give the boundaries of my inner part a material. Additionally I gave the boundaries of my inner part the "slip" condition, while the walls have the "noslip" condition. What else can be wrong. My study gets solved, but I just don't get comsol to show me my drag and lift coefficients. best regards, felix

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