Discussion Forum

All,

An interesting and fun problem was posed today that seemed suited to a COMSOL model. The basic problem is launching a ping pong ball (or any suitable projectile) down a plastic tube propelled by a bolus of compressed air.

The embodiment would consist of a tube of some length. A thin viscoelastic deformable ring disposed circumferentially just slightly smaller that the ball diameter, would prevent the ball from rolling out.

A sealed, sliding window would allow placement of the ball into a chamber. The proximal end of the tube would be sealed with a fitting to couple air into the chamber.

When a pressure pulse is applied to the chamber via the fitting, the ping pong ball would overcome the soft seal (viscoelastic ring) at a certain pressure progressing down the tube exiting at some velocity.

For modeling this seemingly simple scenario, several questions come to mind:

1) How many physics are involved?
2) Is CFD needed for modeling air resistance (compressible flow) against the ball as it travels along the tube length?
3) How to model the plastic deformation of the viscoelastic seal as the ball passes through?
4) Are equations of motion required similar to what is needed if a trace particle is released in a CFD flow model?
5) What if the gas is not air? Does that matter?
6) Is a moving mesh needed?
7) How to account for friction as the ball moves along the tube?

What the model could ultimately provide answers to would be:
- Exit velocity of the ball
- Exit energy of the ball
- How does the shape if not a ball effect the velocity and energy characteristics? Pointed projectile?

This probably sounds like a crazy model, however, any help in any way would be greatly appreciated. Certainly if anyone has some ideas and would like to start on a model, that would be really great.

Thanks,

Art