## Fluids

#### Help with free fall study of multiple 2D cylinders that fall collectively

• vishwatejakasoju
Subscriber

Hi,

I am trying to simulate free fall study of multiple cylinders that move collectively i.e., force one cylinder affect the motions of the other cylinders. Is this type of study possible in fluent? I am attaching the picture of my model below. The smallest cylinders are 1mm in diameter and largest cylinder is 18 mm.

I have been struggling on this type of study for months now. Any help is really appreciated

• Rob
Ansys Employee
Yes, using moving mesh, but you'll need to be very careful how you model this as it'll be a huge model. What do you actually want to know?n
• vishwatejakasoju
Subscriber
Can we do it using overset meshing? I tried using overset meshing by created a small fluid domain around each of the cylinders and created a 6dof object for each cylinder providing mass and moment of inertia with respect to center of big cylinder. Initially, I tried with two small cylinders each on left and right side of big cylinder at the center. Whenever I press calculate, fluent closes abruptly throwing me "MPI Application rank 0 exited before MPI_Finalize() with status -1073741571.n
• Rob
Ansys Employee
You can, but watch for orphan cells. Also if the two overset zones come together it may get more interesting. nThere should be something written above the mpi error, that's just a parallel core failing because something happened. n
• vishwatejakasoju
Subscriber
Yeah sure, i will lookout for orphan cells. What method do you suggest? Dynamic mesh with remeshing or overset mesh? nAlso, I wanted to know if we can have those cylinders move collectively i.e., have the small cylinders move relative to each other i.e., considering all cylinders as one part and rotating/translating together? n
• Rob
Ansys Employee
You can use either method. Overset is probably easiest to set up, but may have issues when running. MDM tends to be more difficult to set up but you may find it's slightly faster and you can use collision UDFs for when things get too close to each other. nThe cylinders can move as a single block or singly, it's for you do define during model set-up. n
• mese
Subscriber
According to your picture, the cylinders never touch each other, is that right? nAs Rob said, both overset mesh and dynamic mesh with remeshing are possible. For your case, I believe that overset mesh is good choice and I expect it to be computationally cheaper. With overset, your meshing will be simpler and nicer because each cylinder has its own mesh sorted around the circle. Also, you can easily introduce both rotational and translational motions. nJust make sure that around solid objects meshes are fine enough so that there are ideally at least 4 elements between the walls at each moment. That avoids formation of orphan cells.