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February 14, 2007, 15:26 
6dof solver in 2d

#1 
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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! 

February 15, 2007, 01:30 
Re: 6dof solver in 2d

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Even if your model is 2D, 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 

February 15, 2007, 04:17 
Re: 6dof solver in 2d

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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


February 15, 2007, 13:55 
Re: 6dof solver in 2d

#4 
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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! 

February 16, 2007, 03:09 
Re: 6dof solver in 2d

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February 16, 2007, 14:36 
Re: 6dof solver in 2d

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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! 

February 16, 2007, 20:55 
Re: 6dof solver in 2d

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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 2D motion or 3D motion. I don't know how Fluent is simulating the solid body motion. I have experince with CFDFASTRAN. I presume that both the approaches are same In this case, 1) There will be a moment of inertia tensor for each 3D body having 9 components of mass moment of inertia.. 2) We assign the motion to that 3D body only. Its a different issue whether the body is having 3 degrees of freedom (2D motion) or 6 degrees of freedom. However, if its a 2D MOTION, the solver will not consider the forces in zdirection and the acceleration in that dierction will be zero. "That doesn't mean that the 3D body is having only two moments of inertia!" Rajesh 

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