PERIODIC WAVES IN SHALLOW GRANULAR FLOWS

Authors: PAL, DIPANKAR - UNIV. OF MISSISSIPPI; PRASAD, S. - UNIV. OF MISSISSIPPI; Romkens, Mathias

Submitted to: American Society of Mechanical Engineers
Publication Type: Proceedings
Publication Acceptance Date: 3/1/2000
Publication Date: 5/1/2000
Citation: N/A

Interpretive Summary: Sediment movement in shallow overland flow has characteristics which are similar to those observed in dry granular flow. These characteristics consist of sediment movement in periodic waves. By studying the gravitational flows of dry materials, the complicating role of the liquid phase is eliminated and thus much can be learned for sediment movement in shallow overland flow. In granular gravitational flow, the granular material becomes self organized and exhibits wave-like motion. The reason for the organization is explained in terms of interparticle effects which affect the energy state of the flow. At low sediment concentrations, interparticle collisions are infrequent. At higher concentrations, the frequency of these collisions increase, and the resulting energy dissipation leads to a "pile-up" or wave front. This process repeats itself downslope leading to the occurrence of periodic waves. These waves were experimentally observed, and similar observations were made in shallow overland flow. This article discusses these findings as well as the impact of this energy cyclic process on the longitudinal variations in the concentration of the depth-average solid fraction.

Technical Abstract: Shallow gravitational flows of dry granular materials often exhibit the passage of periodic waves in the solid-gas mixture. The interparticle distances in most part of such dispersed flows are large compared to the mean diameter of the solid particles. The particles move large distances before they collide with other particles. As the particles roll on the inclined plane in an unsteady manner, they undergo cyclic changes in their velocity pattern as the progressive waves move through the two-phase medium. As a result of this energy cyclic process, the depth-averaged solid fraction of the mixture varies in the longitudinal direction. Though the gas-phase does not possess a mean transport velocity, it dissipates a part of the mean kinetic energy by exerting aerodynamic drag on the moving particles. The excess energy of the accelerating particles is dissipated through random inter-particle collisions at the discontinuities of the periodic structures.