We investigate the effect of an ambient fluid on the dynamics of collapse and spread of a granular column simulated by means of the contact dynamics method interfaced with computational fluid dynamics. The runout distance is found to increase as a power law with the aspect ratio of the column, and, surprisingly, for a given aspect ratio and packing fraction, it may be similar in the grain-inertial and fluid-inertial regimes but with considerably longer duration in the latter case. We show that the effect of fluid in viscous and fluid-inertial regimes is to both reduce the kinetic energy during collapse and enhance the flow by lubrication during spread. Hence, the runout distance in a fluid may be below or equal to that in the absence of fluid due to compensation between those effects.

Interface models have been developed over the past 10 years for simulating various scales of composite debonding eects, such as decohesion between matrix and ®bers and delamination in laminates, formulated with and without rate dependencies. We give several examples of how sudden``solution jumps'' can occur in rate-independent models like the Tvergaard depending on the geometry, interface behavior, and ®nite element discretization. This kind of instability disappears for the often-used viscous models, but the result is a high rate dependency that deviate from experimental trends. A new kind of viscous regularization is proposed here, which applies to the Tvergaard debonding model or modi®ed ones, and has a limited rate dependency. A few simple examples are given, including the delamination of a double cantilever beam, to show the capabilities of the technique proposed.