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Axisymmetric force on a non-axisymmetric simulation

Posted Aug 11, 2011, 3:05 p.m. EDT Geometry, Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics 6 Replies

dnh37
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Hi,

I did the following:
New > 2D > Solid > Frequency Domain.
Draw a circle
Right click Solid Mechanics and add Boundary Load, select the perimeter of the circle

Now I have to define the force (load) F_load, which is always in the radial direction. Problem is, I don't want to define it in x and y, I want to define it in radial coordinates because it is in the radial direction -- e.g. as F_load = (Fmagnitude)*(r_hat).

I can convert that to cartesian coords and define it as F_load = (Fmagnitude)*(cos[pi*xposition/Radius])+(Fmagnitude)*(cos[pi*yposition/Radius]) but the problem is that I don't know how to enter xposition and yposition into that little box.

I can't use an axisymmetric simulation because other parts of my geometry are not axisymmetric, only the force is.

[Note: xposition and yposition here are the positions of each point on the geometry]

Thanks
6 Replies Last Post Aug 17, 2011, 4:36 a.m. EDT
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Aug 11, 2011, 5:33 p.m. EDT
If you want a radial force change the load type of the Boundary Load to Pressure.

Nagi Elabbasi
Veryst Engineering
If you want a radial force change the load type of the Boundary Load to Pressure. Nagi Elabbasi Veryst Engineering
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Aug 13, 2011, 1:33 p.m. EDT
Hi

You can probably also define, in your case, a cylindrical coordinates check the Definitions node

--
Good luck
Ivar
Hi You can probably also define, in your case, a cylindrical coordinates check the Definitions node -- Good luck Ivar
dnh37
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Posted: 1 decade ago Aug 16, 2011, 6:21 a.m. EDT
Thanks, this is exactly what I needed.

Now a different problem, but related: I have defined a cylindrical coordinate system and added a Body Load. I need the Body Load to have an r-component that depends on r.

By which I mean, Body Load has three components, r, phi, a. For the r component, I have some expression that consists of a bunch of constants multiplied by r. So the force per volume element is (constants)*r.

How do I put "r" into the Body Load force components box?

Thanks
Thanks, this is exactly what I needed. Now a different problem, but related: I have defined a cylindrical coordinate system and added a Body Load. I need the Body Load to have an r-component that depends on r. By which I mean, Body Load has three components, r, phi, a. For the r component, I have some expression that consists of a bunch of constants multiplied by r. So the force per volume element is (constants)*r. How do I put "r" into the Body Load force components box? Thanks
dnh37
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Posted: 1 decade ago Aug 16, 2011, 6:38 a.m. EDT
[continued]

I should tell you what I have tried.

In the Force-in-r-direction box, I tried typing r, and it doesn't recognize it. I then realized it needs to be in units of N/m^3 as it says to the right of that box, so I multiply r by 1*[N/m^4] in order to just get it in the correct units (so the final expression reads r*1[N/m^4], and it doesn't recognize it. Note that I am leaving out the constants for now, I'm just multiplying by 1 for the sake of getting it working.

Next I try u*1[N/m^4] just for the heck of it even though it's not what I want. It recognizes it! However I don't want u, I want r*1[N/m^4], or sqrt(u^2+v^2)*1[N/m^5] -- where I am again multiplying by 1 with some units to make the final expression be in N/m^3. However now it appears to not like the sqrt.

So in summary:

WORKS BUT INCORRECT:
u*1[N/m^4]
u^2*1[N/m^4]
(u^2+v^2)*1[N/m^4]

DOESN'T WORK BUT WANT:
r*1[N/m^4] --- doesn't work due to the "r"
or
sqrt(u^2+v^2)*1[N/m^4] --- doesn't work due to the "sqrt"
[continued] I should tell you what I have tried. In the Force-in-r-direction box, I tried typing r, and it doesn't recognize it. I then realized it needs to be in units of N/m^3 as it says to the right of that box, so I multiply r by 1*[N/m^4] in order to just get it in the correct units (so the final expression reads r*1[N/m^4], and it doesn't recognize it. Note that I am leaving out the constants for now, I'm just multiplying by 1 for the sake of getting it working. Next I try u*1[N/m^4] just for the heck of it even though it's not what I want. It recognizes it! However I don't want u, I want r*1[N/m^4], or sqrt(u^2+v^2)*1[N/m^5] -- where I am again multiplying by 1 with some units to make the final expression be in N/m^3. However now it appears to not like the sqrt. So in summary: WORKS BUT INCORRECT: u*1[N/m^4] u^2*1[N/m^4] (u^2+v^2)*1[N/m^4] DOESN'T WORK BUT WANT: r*1[N/m^4] --- doesn't work due to the "r" or sqrt(u^2+v^2)*1[N/m^4] --- doesn't work due to the "sqrt"
dnh37
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Posted: 1 decade ago Aug 16, 2011, 9:05 a.m. EDT
Never mind! Figured it out.

I had to use x and y rather than u and v, and then it works

Also, My "sqrt" was not incorrect, rather, I typed N/m^5 rather than N/m^4 into comsol.
Never mind! Figured it out. I had to use x and y rather than u and v, and then it works Also, My "sqrt" was not incorrect, rather, I typed N/m^5 rather than N/m^4 into comsol.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Aug 17, 2011, 4:36 a.m. EDT
Hi

careful with your variables names:

if YOU define a variable r = sqrt(x^2+y^2) you can refer to "r", but if you define a spherical coordinate (default settings) you must refer to "sys2.r" (if its the first coordinate defined else 2 becomes 3, 4 ...)

And do not mess up the "frames" sqrt(x^2+y^2) is different from sqrt(X^2+Y^2), check the doc and play with some simple examples

--
Good luck
Ivar
Hi careful with your variables names: if YOU define a variable r = sqrt(x^2+y^2) you can refer to "r", but if you define a spherical coordinate (default settings) you must refer to "sys2.r" (if its the first coordinate defined else 2 becomes 3, 4 ...) And do not mess up the "frames" sqrt(x^2+y^2) is different from sqrt(X^2+Y^2), check the doc and play with some simple examples -- Good luck Ivar

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