J. F. Boudet

The granular jump

When a fluid jet hits a solid surface, a hydraulic jumps occurs. This jump sharply delimits a thin film of liquid from a thicker film. We show here that a granular jet impinging on a solid surface also gives rise to several features reminiscent of the hydraulic jump and we refer to this situation as the granular jump. We describe, in detail, this phenomenon and show that if many of its features can be understood in analogy with the hydraulic jump, others are directly related to the granular nature of the medium and, in particular, the small-scale dynamics of the jump.

Intermittency of the velocity fluctuations in a granular surface flow

Velocity fluctuations in a granular surface flow are studied. These fluctuations are self-similar at small scales with an energy density spectrum showing a −5∕3 power law. The probability density functions of velocity increments are strongly non-Gaussian. The moments of velocity differences between two points separated by a distance r vary as a power law with the scale r; the exponents saturate at a value of 2∕3 showing very strong intermittency of the velocity fluctuations.

Structure of velocity distributions in shock waves in granular gases with extension to molecular gases.

Velocity distributions in normal shock waves obtained in dilute granular flows are studied. These distributions cannot be described by a simple functional shape and are believed to be bimodal. Our results show that these distributions are not strictly bimodal but a trimodal distribution is shown to be sufficient. The usual Mott-Smith bimodal description of these distributions, developed for molecular gases, and based on the coexistence of two subpopulations (a supersonic and a subsonic population) in the shock front, can be modified by adding a third subpopulation. Our experiments show that this additional population results from collisions between the supersonic and subsonic subpopulations. We propose a simple approach incorporating the role of this third intermediate population to model the measured probability distributions and apply it to granular shocks as well as shocks in molecular gases.