# Move rigid body in a fluid

Hi all,

I’m dealing with problem in which I want to get force on a rigid body (rigid sphere) in a fluid as a function of its position (in just one dimension). I formed the rigid body and made it be stationary (it can rotate though) by turning the force keyword “off” in all directions. I want to move this body to a new position so I’m using “fix move” with linear option and I set the required velocity for moving the body in the desired direction and I set NULL for two other directions. As I check the center of mass of the rigid body group (using “compute com”) it actually is moving to the specified position and that’s fine but the rigid body is deforming! it’s sort of an ellipsoidal shape at the end! So would you please give me a hint on where the problem could arise?

Thanks,
Kasra.

Fix move is moving particles, not adding a force. You cannot mix it with any other time integration fix. You need to use fix addforce. Too obvious??

Axel.

Axel,

I just want to move the rigid body to a new position and let it stay there to get the forces due to its surrounding. I don’t want to add a force on the particle.
Do you mean that I need to use addforce in order to move the particle? Cause in that case I don’t know how much force I should apply to move it to the desired position.

Best,
Kasra.

Axel,

I just want to move the rigid body to a new position and let it stay there
to get the forces due to its surrounding. I don't want to add a force on the
particle.
Do you mean that I need to use addforce in order to move the particle? Cause
in that case I don't know how much force I should apply to move it to the
desired position.

your question was a bit confusing.

nevertheless, you cannot mix two fixes that do time integration.
you can move your particle using fix smd. this will not move it
directly like fix move (which collides with fix rigid), but add a harmonic
constraint the reference point of which can be moved with a constant
velocity. the is actually the proper way to implement what you are
aiming to do anyway.

axel.