Friction and convection in a vertically vibrated granular system.

The effect of friction in the thermal convection instability of granular fluids is studied by means of molecular dynamics simulations. It is found that the transitions between different convective states (zero, one, and two rolls) are primarily governed by the average energy loss per collisions and not by the friction and restitution coefficients separately, and can be roughly described in terms of a single effective restitution coefficient. The average energy loss per collisions, for a fixed value of the restitution coefficient, shows a maximum for a friction coefficient kappa approximately 0.3. The presence of this maximum manifests itself as a reentrant behavior in the transition lines in parameter space when the value of the friction coefficient is increased beyond 0.3.