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Displacement from distance- coupled ODE

Posted 4 févr. 2012, 03:30 UTC−5 Computational Fluid Dynamics (CFD) Version 4.2a 2 Replies

Siva Kumar Manickam
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Hi,

I have used global equations in 'sand falling' tutorial:

X =Xt-Xdot
Xdot =Xdott-(F_z+F_g)/m_grain

These equations upon solving gives the distance (scalar).

How would I calculate displacement (vector)?

Please some one help me.
2 Replies Last Post 5 févr. 2012, 20:16 UTC−5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 4 févr. 2012, 04:29 UTC−5
Hi

I ave some prolems with your equation and the notation (variable naming)

If I read your right you have gravity along Z (probably in 3D) then you write an equation in X, but be aware x,y,z,r,t as X,Y,Z,R and many more are already used by COMSOL. So if you add something name it such to distinguis it from the COMSOL naming.

Then if you want a vector solution you need 3 global equations one for each direction

--
Good luck
Ivar
Hi I ave some prolems with your equation and the notation (variable naming) If I read your right you have gravity along Z (probably in 3D) then you write an equation in X, but be aware x,y,z,r,t as X,Y,Z,R and many more are already used by COMSOL. So if you add something name it such to distinguis it from the COMSOL naming. Then if you want a vector solution you need 3 global equations one for each direction -- Good luck Ivar
Siva Kumar Manickam
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Posted: 1 decade ago 5 févr. 2012, 20:16 UTC−5
Thank you very much Dr. Ivar.
Actually the problem was in axisymmetric. So using X did not affect R and Z ( I suppose).
In my 2D simulation I replaced equations with
Qt-Qdot
Qdott-(f_x)/m.

So Q did not affect any existing notations. My problem is as you know flapping like a pendulum. After simulation the graph Q versus time shows distance versus time.
My oscillation was one dimensional but involving to and fro motion.
Please see the attached picture. Actually when the body returns, displacement should be subtracted from distance. I expected a sine wave horizontal to time axis. But I get a sine wave along the diagonal. I suppose that this is distance versus time graph and not displacement versus time graph, which shows accumulated displacement which keeps increasing with respect to time. Am I right?

With lots of regards
Siva
Thank you very much Dr. Ivar. Actually the problem was in axisymmetric. So using X did not affect R and Z ( I suppose). In my 2D simulation I replaced equations with Qt-Qdot Qdott-(f_x)/m. So Q did not affect any existing notations. My problem is as you know flapping like a pendulum. After simulation the graph Q versus time shows distance versus time. My oscillation was one dimensional but involving to and fro motion. Please see the attached picture. Actually when the body returns, displacement should be subtracted from distance. I expected a sine wave horizontal to time axis. But I get a sine wave along the diagonal. I suppose that this is distance versus time graph and not displacement versus time graph, which shows accumulated displacement which keeps increasing with respect to time. Am I right? With lots of regards Siva

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