Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Displacement from distance- coupled ODE

Please login with a confirmed email address before reporting spam

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 Feb 5, 2012, 8:16 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Feb 4, 2012, 4:29 a.m. EST
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

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Feb 5, 2012, 8:16 p.m. EST
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

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.