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Old   February 9, 2012, 16:46
Default Applying Body Forces in CFX
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Hi there, could anyone help with applying body forces to an object in CFX?

I have a coil shaped object, and I need to apply gravitational acceleration along the whole outer wall of the coil, acting inwards. This is essentially simulating angular velocity.

Any help would be greatly appreciated
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Old   February 9, 2012, 20:04
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Hi MJ55,

Can you further elaborate on:
"gravitational acceleration along the whole outer wall of the coil, acting inwards. This is essentially simulating angular velocity."

hard to advice without having a picture in mind of what you're trying to model
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Old   February 9, 2012, 20:45
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Hi



There's the picture. The circle represents my coil, and the arrows pointing inwards represent the forces.

The forces need to be acceleration of around 100g.

I'm not sure if its possible to even model this, if not could you advise on the best alternative?

Many thanks
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Old   February 9, 2012, 20:55
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Do you know the force or the acceleration? If you know the force then you need a FSI simulation to work out the motion. If you know the acceleration then you can do this as just a moving mesh simulation.

But either way this simulation is probably do-able.
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Old   February 9, 2012, 21:03
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I know the acceleration.

Forgive my ignorance but how would I go about doing a moving mesh simulation?

And also I'd want to have the geometry stationary during the simulation, so only the fluid inside is affected by the acceleration.
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Old   February 9, 2012, 22:10
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For moving mesh consult the documentation and tutorials.

But if you want to move the fluid but not the geometry then you do not want moving mesh but you want a source term in the momentum equation. Maybe you should explain what you are doing so we can understand what you are trying to do.
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Old   February 10, 2012, 05:33
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Basically I'm trying to simulate the effects of high g-forces on the fluid. I have been provided with the geometry of a coil, and told that I need to apply g-forces all over the coil and see what affect it has on the fluid.
I hope that makes more sense now.

How can I find out more on manipulating the momentum equation?
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Old   February 10, 2012, 06:28
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It makes no sense yet. What is applying the acceleration to the fluid?
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Old   February 10, 2012, 07:03
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In the real life part of this experiment, the coil as a whole is being rotated around a seperate axis, creating a planetary motion.

My theory is that by applying g-forces around the outside of the coil this will simulate planetary motion without the need to model the secondary motion.

The acceleration is the g-force induced through the planetary motion of the coil.




The picture shows the planetary motion, with the coil rotating abuot its own axis, whislt also rotating around the centre point in the picture.

The next picture describes how fluctuating acceleration field will develop around the coil as a result of this motion




It is this fluctuating acceleration field which I am trying to add to my coil in CFX.

I hope this makes more sense now
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Old   February 10, 2012, 08:11
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I would model in CFX the same way you carry your experiment. I'm assuming the coil is a rigid solid (no deformation and no water in it).

Suggestion:

A. Create a cylindrical fluid sub-domain containing the coil at centre. Locate the sub-domain within a larger fluid domain and use GGI to connect the two domains. Rotate the sub-domain around the coil axis.

B1. Rotate the whole domain above around a specified global axis.

B2. Specify an angular flow at relevant boundaries of the whole domain by using 'cyl. velocity components'

With A+B1, rotating one domain is easy. Tricky part is that you have to find if it's possible, and how, to rotate two domains in a simulation. (ideal)

With A+B2, the problem is that it's a local analysis. You won't be able to model the 'swirling or propeller' effect of the whole problem. The inflow will be a continuous clean angular flow rather than the 'memory' of the wake generated from the previous cycle
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Old   February 10, 2012, 08:49
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The coil is a rigid solid, but it has water flowing through it. That is the fluid which I am studying.

Unfortunately I have been told to simulate with body forces, as opposed to having two rotations.

What I need to know is it possible to apply acceleration forces similar to those shown in the second image?
I am guessing this would be achieved through the use of CEL, which I am trying to use at the moment.

Would anyone be willing to give me a head start on the functions to be used?
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Old   February 10, 2012, 09:44
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See if the following tutorial in CFX can be of help:
Chapter 32: Modeling a Buoy using the CFX Rigid Body Solver

You can apply multiple 'acceleration' forces or moments on the coil (shell body)

If the above doesn't allow the level of control you need for the forces, the next level would be FSI as suggested by Glenn.

Btw, i'm trying figure out the physics behind applying the forces as shown in the second image.

1. The second image indicates that the acceleration field is fluctuating. If so, how would you determine the appropriate force or acceleration to apply at a particular point and time on the coil to cause the desired motion?

2. If the forces are same in all direction as shown in your first pic post, wouldn't the potential be zero?

I'm guessing the system's a toroidal coil centrifuge

Last edited by Darren Leong; February 11, 2012 at 03:21.
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Old   February 11, 2012, 06:31
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Darren's comments are spot on. Forget about the body force approach.

You could model it as a rigid body with rotating frames of reference as Darren suggests. I do not think you can put a rotating frame of reference in another rotating frame. You will have to do this with moving mesh. This will be very slow.

I would recommend you do it as an immersed solid. Then you can specify any motion you like, and make the under lying mesh a high quality 1 aspect ratio hex mesh. And you can keep your supervisor happy by saying you are using a body force approach - jus tthe body forces are being applied to model the motion of the solid, not some weird guess as to what the coils effect on the fluid would be.
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Old   February 11, 2012, 07:34
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Thank you all for your replies. It is very much appreciated!
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Old   May 25, 2012, 06:15
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Hi Friends
I am also studying immersed boundary method and and try to simulate a block falling in the water. I am using "immersed solid" then rigid body 6DOF and I let it fall freely but it can't stoped in the bottom where the velocity must be zero. I give a density to the block and i let it fall freely under gravity. Noted that the rigid body is defined as an immersed solid. i have specified a stationary coordinate frame that has its origin at the center of mass of the physical rigid body. Another fixed coordinate frame was specified related to the water at rest.
What can I do to stopped the rigid body in the bottom where the potentiel energy must be zero?

thank you!
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