# Discussion Forum

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

## Specifying a rotating boundary condition

I would like to simulate a rotating wall condition.

For simplicity, imagine a square box. On the top surface of that box there is a circle. Inside of the box is a viscous fluid. I would like to impose a rotational velocity on said circle and observed the effect on the fluid inside the box.

I am defining the velocity as follows:

Ux = -(sqrt(x^2+y^2))*omega*sin(omega*t)*rect1(t[1/s])
Uy = (sqrt(x^2+y^2))*omega*cos(omega*t)*rect1(t[1/s])
Uz = 0

where omega is angular velocity.

I then apply this velocity to the circular boundary on top of the box.

I am trying to use a moving wall in my simulation but this does not seem to work. When I check my results there is no motion anywhere in the fluid or along the boundary.

Any thoughts on what I might be doing wrong?

12 Replies Last Post 2017年6月7日 GMT-0400上午2:06
Posted: 5 years ago
Hi

there must be something with your formula, have you tried to plot the velocity vector your get ? That often helps to check you got the cylindrical projection correctly set-up

On the other hand, you can define a local cylindrical coordinate system, and refer to the radius as sys2.r etc (it's sys2. or 3. or ... depending on how many coordinate systems you define)

--
Good luck
Ivar

Posted: 5 years ago
I have two physics modules in my simulation; Heat Transfer (the circle imparts heat into the system) and Non-Isothermal flow (the fluid is non-Newtonian) and has a dynamic viscosity (I am using the carreau model). The only thing that I can see being updated from the nitf is kinematic viscosity. I am fairly certain that my velocity projection is accurate.

Does that shed any light on things?

Posted: 5 years ago
Hi

my only advice is : use the postprocessing to analyse your initial conditions (plot rotation vectors , different values and check how they look w.r.t. the expected results. Then do the same after solving (or after some solving) using the "plot while solving all solver steps" to check that it evolves in an understandable way

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Good luck
Ivar

Posted: 5 years ago
Hi,

Comsol will not find any motion unless you have inlet and outlet or you are using either FSI or rotating machinery interface or you are coupling equations of motion with boundary conditions via mathematics interface.

Cheers

Posted: 5 years ago
Hi

I would say, yes you can, with a sliding wall
my rectangle case is not converging to the default value, but the result is still OK, the concentric circle is OK

Files are 4.3a (if I manage to attach the files, ... now it works ;)

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Good luck
Ivar

Posted: 5 years ago
Ivar,

Thank you so much for your help with this. Unfortunately, I am not able to open the files you posted.

I define the rotating boundary as specified. Everywhere else I have set as no-slip.

Are there any other boundary conditions required for the NITF? I am still not observing any rotation.

Thanks again.

Posted: 5 years ago
Hi

here is a screen view, very simple set-up (oops have a Opera <-> Java update issue cannot upload from here anymore, checking with IE)

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Good luck
Ivar

Posted: 5 years ago
Ivar,

I don't think I explained my problem well enough. I have included a simplified version of the model if you'd like to take a closer look.

Thanks again.

edit:

I got it to work.

Posted: 5 years ago
Hi

just had some minutes to take a closer look, My main question: why do you add HT and NITF as the latter contains all HT anyhow, no reason for doubling up, and now you need to equate the two T that represents the same dependent variable ?

And for the cylindrical coordinates, add a Definition cylindrical coordinate system, then you can refer to the components sys2.r etc, instead of the full equations, unfortunately COMSOL has not foreseen that you add your own coordinates system to define the mowing wall, sad, it would be so much easier ;)

And I would avoid the variable name omega, as it's used by COMSOL for frequency domain and other solvers, use rather some "omega_" or "my_omega"

I cannot get any arrow plot to show up, anyhow you must ensure that t>0.2 to avoid the rect is sending all velocities to =0

--
Good luck
Ivar

Posted: 5 years ago
Thank you Ivar.

I have re-worked my model to use a 2-D axis symmetric geometry. It is working well now. Thanks for the help.

Posted: 3 years ago
Dear All,

I am just wondering would it be possible to rotate a sphere (3d) at a specified angular velocity about a pre determined axis by providing BC on the surface of the sphere as done with the discs in the problem being discussed above. I am solving for a sphere rotating in a fluid filled channel.

Any help is greatly appreciated.

Thanks,
Seemit

Posted: 4 months ago
In the original post:

===============
Ux = -(sqrt(x^2+y^2))*omega*sin(omega*t)*rect1(t[1/s])
Uy = (sqrt(x^2+y^2))*omega*cos(omega*t)*rect1(t[1/s])
===============

What is the "rect1(t[1/s])" term? The equations seem properly defined without that last term.