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-   -   force on gmo with prescribed motion (http://www.cfd-online.com/Forums/flow-3d/87236-force-gmo-prescribed-motion.html)

jiko April 14, 2011 11:28

force on gmo with prescribed motion
 
I am analysing vertical force of water on a vertically (z, axis of gravity) oscillating GMO. the amplitude and frequency of the gmo is prescribed.
now since it is oscillating in still water I thought the initial force would be 0 at 0 time. but it is not. could someone please explain why the vertical force is not zero at zero time. or is something wrong in my simulation setup ?

thanks in advance.

jiko April 26, 2011 10:00

Quote:

Originally Posted by jiko (Post 303655)
I am analysing vertical force of water on a vertically (z, axis of gravity) oscillating GMO. the amplitude and frequency of the gmo is prescribed.
now since it is oscillating in still water I thought the initial force would be 0 at 0 time. but it is not. could someone please explain why the vertical force is not zero at zero time. or is something wrong in my simulation setup ?

thanks in advance.


it's quite surprising that no one seems to have any idea of buoyancy

mohammadhosainshfie July 25, 2011 13:45

Quote:

Originally Posted by jiko (Post 305124)
it's quite surprising that no one seems to have any idea of buoyancy

i think u mast do 2 work:
1. simulate your model and calculate time for stability
2. add this time to general time and told to software start your own problem after calculate time for stability.

JBurnham July 25, 2011 17:02

Force is calculated from acceleration (of the water on the object). If there is gravity, then the force is non-zero. If there is motion, then the force is non-zero. At t=0, when using the GMO harmonic oscillation model, acceleration is non-zero. Therefore, the initial state of the fluid is non-zero: although it is not 'in motion' (no advection is occurring), it is accelerating, correct? Therefore, the force on the GMO is also non-zero. Note that velocity works the same way, since it is also following a harmonic oscillation: GMO velocity at t=0 is not zero.

MuxaB July 29, 2011 00:21

Quote:

Originally Posted by jiko (Post 303655)
I am analysing vertical force of water on a vertically (z, axis of gravity) oscillating GMO. the amplitude and frequency of the gmo is prescribed.
now since it is oscillating in still water I thought the initial force would be 0 at 0 time. but it is not. could someone please explain why the vertical force is not zero at zero time. or is something wrong in my simulation setup ?

thanks in advance.

It is the buoyancy force, like you said, that is computed at t=0. It should be close to the weight of the fluid displaced by the object


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