Dual role of friction in granular flows: attenuation versus enhancement of instabilities

Abstract

Flow instabilities driven by the dissipative nature of particle–particle interactions have been well documented in granular flows. The bulk of previous studies on such instabilities have considered the impact of inelastic dissipation only and shown that instabilities are enhanced with increased dissipation. The impact of frictional dissipation on the stability of grains in a homogeneous cooling system is studied in this work using molecular dynamics (MD) simulations and kinetic-theory-based predictions. Surprisingly, both MD simulations and theory indicate that high levels of friction actually attenuate instabilities relative to the frictionless case, whereas moderate levels enhance instabilities compared to frictionless systems, as expected. The mechanism responsible for this behaviour is identified as the coupling between rotational and translational motion. These results have implications not only for granular materials, but also more generally to flows with dissipative interactions between constituent particles – cohesive systems with agglomeration, multiphase flows with viscous dissipation, etc.

Kinetic-theory predictions of clustering instabilities in granular flows: beyond the small-Knudsen-number regime

“…One such term, viscous heating (P : ∇u), has been shown to be an important contributor to the clustering instability via continuum theory (Brey, Ruiz-Montero & Cubero 1999;Soto, Mareschal & Mansour 2000) and discrete-particle simulations (Soto et al 2000;Mitrano et al 2013). The neglect of nonlinear contributions to clustering, namely viscous heating, causes the over-prediction of L cluster (i.e.…”

Transport coefficients of a granular gas of inelastic rough hard spheres

“…5, 7, and 8, on the other hand, shows that the general dependencies of the transport coefficients η * , λ * , and µ * on both α and β (in particular, the maxima around β ∼ 0) are highly correlated to that of the cooling rate ζ * . This explains that fair qualitative estimates can be obtained by using the expressions for smooth spheres [11,12] with the cooling rate replaced by the one for rough spheres [46].…”

Numerical study of collisional particle dynamics in cluster-induced turbulence

“…In purely granular systems, clustering is enhanced via inelastic dissipation (Hopkins & Louge 1991;Goldhirsch & Zanetti 1993) and attenuated via friction (Royer et al 2009;Mitrano et al 2013) during interparticle contact. In the presence of a carrier phase, viscous damping by the fluid results in clustering of non-dissipative particles (Wylie & Koch 2000).…”