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-   -   6dof solver in 2d (http://www.cfd-online.com/Forums/fluent/43756-6dof-solver-2d.html)

Mekala Krishnan February 14, 2007 15:26

6dof solver in 2d
 
Hi

I am simulating a micron scale tile moving in water as a 2d simulation in FLUENT, for which I am using FLUENT's 6dof solver.

I was wondering how I should calculate the mass and moments of inertia (for the 6dof udf) for my tile since it is a 2d simulation. What value should I use for the depth (z direction)? Should I assume that the depth is 1m? (This means my tile has dimensions 500 micron X 500 micron x 1m)

Thanks!

Rajesh February 15, 2007 01:30

Re: 6dof solver in 2d
 
Even if your model is 2-D, it will have 3 moments of inertia if it is symmetrical (square, rectangular, circular, etc) and 6 moments of inertia if it is not symmetrical. Here are the formulae for the 3 moments of inertia about axii passing through the CG of the body.

Ix=0.5*mass*y^2

Iy=0.5*mass*x^2

Iz=(mass/12)*(x^2+y^2), where x and y are the respective x and y dimensions of the body...

check this link also

http://www.efunda.com/math/math_home/math.cfm

hope this helps..

Rajesh


Luca February 15, 2007 04:17

Re: 6dof solver in 2d
 
No! if you're running a 2d simulation you only have three degrees of freedom: translation along x and y and rotation around z axis. So consider to have unity depth. Luca

Mekala Krishnan February 15, 2007 13:55

Re: 6dof solver in 2d
 
Thanks for the responses! I am still confused though.

If I calculate moment of inertia according to the formula

Ix=0.5*mass*y^2

Iy=0.5*mass*x^2

Iz=(mass/12)*(x^2+y^2)

then I am assuming that the depth is 0. How do I calculate mass in that case? Since mass = density * x* y * depth, what should I take the depth as?

On the other hand, if I calculate the moment of inertia by considering unit depth, do I only need to specify mass and Izz (since it is a 2d problem)?

Thanks!

Rajesh February 16, 2007 03:09

Re: 6dof solver in 2d
 
http://ocw.mit.edu/NR/rdonlyres/Aero...E031/0/d18.pdf....


Mekala Krishnan February 16, 2007 14:36

Re: 6dof solver in 2d
 
Hi

Thanks - the link was very helpful!

I still have some doubts though:

1) Why does FLUENT need values of Ixx, Iyy if it is a 2d simulation? Shouldn't Izz be enough?

2) What value of mass should I input? Should I input m = density*x*y*1? (my dimensions are actually 500 micron X 500 micron X 30 micron)

Thanks again!

Rajesh February 16, 2007 20:55

Re: 6dof solver in 2d
 
You can read the page more carefully (especially the example section). Or you can try reading any classical mechanics text like the one by Etkins..How Euler's equations of motion are formulated..

if your dimensions are 500 x 500 x 30 ..then mass would be density x 500 x 500 x 30..whether or not the body is having 2-D motion or 3-D motion.

I don't know how Fluent is simulating the solid body motion. I have experince with CFD-FASTRAN. I presume that both the approaches are same

In this case,

1) There will be a moment of inertia tensor for each 3-D body having 9 components of mass moment of inertia.. 2) We assign the motion to that 3-D body only. Its a different issue whether the body is having 3 degrees of freedom (2-D motion) or 6 degrees of freedom. However, if its a 2-D MOTION, the solver will not consider the forces in z-direction and the acceleration in that dierction will be zero. "That doesn't mean that the 3-D body is having only two moments of inertia!"

Rajesh



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