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-   -   gravity driven rigid body motion in fluids (http://www.cfd-online.com/Forums/cfx/127833-gravity-driven-rigid-body-motion-fluids.html)

 tsh850227 December 22, 2013 21:55

gravity driven rigid body motion in fluids

Hi all,

is there anybody know whether a good cfd method could be used to model behivor of rigid body's motion in fluids driven by gravity? I want to study its terminal velocity under different condition.

 ghorrocks December 23, 2013 06:34

Rather than use a rigid body approach wouldn't it be better to do flow past a stationary object, and when the drag equals the mass you have found the terminal velocity? This will be much easier and faster than modelling it as a falling rigid body.

 tsh850227 December 23, 2013 06:53

Quote:
 Originally Posted by ghorrocks (Post 467402) Rather than use a rigid body approach wouldn't it be better to do flow past a stationary object, and when the drag equals the mass you have found the terminal velocity? This will be much easier and faster than modelling it as a falling rigid body.

Hi,

Thank you very much for your reply. You mean I need to do several simulations and get the terminal velocity until the force on the particle equals to the net force of gravity and buoyancy?

thank you again for your guidance~~

 ghorrocks December 23, 2013 21:15

Yes, that is correct.

 tsh850227 December 23, 2013 21:28

Quote:
 Originally Posted by ghorrocks (Post 467472) Yes, that is correct.
hi,

actually, i want to study the velocity of particle in a vessel full with various fluids, then impose mechanical vibration on the vessel and measure particlel's average terminal velocity. so I wonder is there any directly method can be used to medel this case

 ghorrocks December 23, 2013 21:54

Can't you vibrate the inlet/outlet and impose the various fluids with the flow over a stationary object model too?

It is MUCH easier to have the object fixed and the fluid flowing over it then to have the fluid fixed and the object falling in it.

 tsh850227 December 23, 2013 22:19

Quote:
 Originally Posted by ghorrocks (Post 467480) Can't you vibrate the inlet/outlet and impose the various fluids with the flow over a stationary object model too? It is MUCH easier to have the object fixed and the fluid flowing over it then to have the fluid fixed and the object falling in it.
hi,
thank you very much for your guidance,

when i imposed the vibration on inlet outlet and wall, the geometry will change because particle is still stationary. and the mesh always deform obviously. also, when vessel is being shaked, the drag force is changing, then how to measure it?

regards,

 ghorrocks December 24, 2013 03:50

You can measure the drag force with a monitor point. You can output it every time step and average to get the avergae drag.

You should do everything possible to run this with a stationary mesh. If you can replace your outer wall vibrating (which requires moving mesh) with a time dependant inlet and/or outlet then the simulation will be much easier. But if it cannot be done then you are forced to use moving mesh.

 tsh850227 January 6, 2014 05:20

Quote:
 Originally Posted by ghorrocks (Post 467499) You can measure the drag force with a monitor point. You can output it every time step and average to get the avergae drag. You should do everything possible to run this with a stationary mesh. If you can replace your outer wall vibrating (which requires moving mesh) with a time dependant inlet and/or outlet then the simulation will be much easier. But if it cannot be done then you are forced to use moving mesh.
Hi,thank you very much, I followed your suggestion, but some problem appears, the relative velocity between partcle and fluid are not a constant value, I also checked the drag and compared with result from moving mesh, the difference is a little large although the model of moving mesh is weak due to the deformation of mesh.

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