arXiv subjects

Nicholas A. Conzelmann

Publications and source records attributed to Nicholas A. Conzelmann.

4 recordsLinked to original sources

arXiv2022

Fingering instabilities in binary granular systems

Fingering instabilities akin to the Rayleigh-Taylor (RT) instability in fluids have been observed in a binary granular system consisting of dense and small particles layered on top of lighter and larger particles, when the system is subjected to vertical vibration and fluidizing gas flow. Using observations from experiments and numerical modelling we explore whether the theory developed to describe the Rayleigh-Taylor (RT) instability in fluids is also applicable to binary granular systems. Our results confirm the applicability of the classic RT instability theory for binary granular systems demonstrating that several key features are observed in both types of systems, viz: (i) The characteristic wavenumber of the instability is constant with time, (ii) the amplitude of the characteristic wavenumber initially grows exponentially and (iii) the dispersion relation between the wavenumbers k of the interface instability and the growth rates n(k) of their amplitudes holds in both fluid-fluid and binary granular systems. Our results also demonstrate that inter-particle friction is essential for the RT instability to occur in granular media. For zero particle friction the interface instability bears a greater resembles to the Richtmyer-Meshkov instability. We further define a yield criterion Y for the interface by treating the granular medium as a viscoplastic material; only for Y > 15 fingering occurs. Interestingly, previous work has shown that instabilities in the Earth's lower mantle, another viscoplastic material, also occur for similar values of Y.

cond-mat.soft↗

arXiv2021

On the rising and sinking of granular bubbles and droplets

Recently, the existence of so-called granular bubbles and droplets has been demonstrated experimentally. Granular bubbles and droplets are clusters of particles that respectively rise and sink if submerged in an aerated and vibrated bed of another granular material of different size and/or density. However, currently there is no model that explains the coherent motion of these clusters and predicts the transition between a rising and sinking motion. Here, we propose an analytical model predicting accurately the neutral buoyancy limit of a granular bubble/droplet. This model allows the compilation of a regime map identifying five distinct regimes of granular bubble/droplet motion.

cond-mat.soft↗

arXiv2021

The sinking dynamics and splitting of a granular droplet

Recent experimental results have shown that vibro-fluidized, binary granular materials exhibit Rayleigh-Taylor-like instabilities that manifest themselves in rising plumes, rising bubbles and the sinking and splitting of granular droplets. This work explores the physics behind the splitting of a granular droplet that is composed of smaller and denser particles in a bed of larger and lighter particles. During its sinking motion, a granular droplet undergoes a series of binary splits resembling the fragmentation of a liquid droplet falling in a miscible fluid. However, different physical mechanisms cause a granular droplet to split. By applying particle-image-velocimetry and numerical simulations, we demonstrate that the droplet of high-density particles causes the formation of an immobilized zone underneath the droplet. This zone obstructs the downwards motion of the droplet and causes the droplet to spread and ultimately to split. The resulting fragments sink at inclined trajectories around the immobilized zone until another splitting event is initiated. The occurrence of consecutive splitting events is explained by the re-formation of an immobilized zone underneath the droplet fragments. Our investigations identified three requirements for a granular droplet to split: 1) frictional inter-particle contacts, 2) a higher density of the particles composing the granular droplet compared to the bulk particles and 3) and a minimal granular droplet diameter.

cond-mat.soft↗

arXiv2020

The Link Between Packing Morphology and the Distribution of Contact Forces and Stresses in Packings of Highly Non-Convex Particles

An external load on a particle packing is distributed internally through a heterogeneous network of particle contacts. This contact force distribution determines the stability of the particle packing and the resulting structure. Here, we investigate the homogeneity of the contact force distribution in packings of highly non-convex particles both in two-dimensional (2D) and three-dimensional (3D) packings. A newly developed discrete element method is used to model packings of non-convex particles of varying sphericity. Our results establish that in 3D packings the distribution of the contact forces in the normal direction becomes increasingly heterogeneous with decreasing particle sphericity. However, in 2D packings the contact force distribution is independent of particle sphericity, indicating that results obtained in 2D packings cannot be extrapolated readily to 3D packings. Radial distribution functions (RDFs) show that the crystallinity in 3D packings decreases with decreasing particle sphericity. We link the decreasing homogeneity of the contact force distributions to the decreasing crystallinity of 3D packings. These findings are complementary to the previously observed link between the heterogeneity of the contact force distribution and a decreasing packing crystallinity due to an increasing polydispersity of spherical particles.

cond-mat.soft↗