5,693 research outputs found
Emergent vortices in populations of colloidal rollers
Coherent vortical motion has been reported in a wide variety of populations
including living organisms (bacteria, fishes, human crowds) and synthetic
active matter (shaken grains, mixtures of biopolymers), yet a unified
description of the formation and structure of this pattern remains lacking.
Here we report the self-organization of motile colloids into a macroscopic
steadily rotating vortex. Combining physical experiments and numerical
simulations, we elucidate this collective behavior. We demonstrate that the
emergent-vortex structure lives on the verge of a phase separation, and single
out the very constituents responsible for this state of polar active matter.
Building on this observation, we establish a continuum theory and lay out a
strong foundation for the description of vortical collective motion in a broad
class of motile populations constrained by geometrical boundaries
Cavitation inception of a van der Waals fluid at a sack-wall obstacle
Cavitation in a liquid moving past a constraint is numerically investigated
by means of a free-energy lattice Boltzmann simulation based on the van der
Waals equation of state. The fluid is streamed past an obstacle and, depending
on the pressure drop between inlet and outlet, vapor formation underneath the
corner of the sack-wall is observed. The circumstances of cavitation formation
are investigated and it is found that the local bulk pressure and mean stress
are insufficient to explain the phenomenon. Results obtained in this study
strongly suggest that the viscous stress, interfacial contributions to the
local pressure, and the Laplace pressure are relevant to the opening of a vapor
cavity. This can be described by a generalization of Joseph's criterion that
includes these contributions. A macroscopic investigation measuring mass flow
rate behavior and discharge coefficient was also performed. As theoretically
predicted, mass flow rate increases linearly with the square root of the
pressure drop. However, when cavitation occurs, the mass flow growth rate is
reduced and eventually it collapses into a choked flow state. In the cavitating
regime, as theoretically predicted and experimentally verified, the discharge
coefficient grows with the Nurick cavitation number
Numerical Simulation of Non-Homogeneous Viscous Debris-Flows Based on the Smoothed Particle Hydrodynamics (SPH) Method
Non-homogeneous viscous debris flows are characterized by high density, impact force and destructiveness, and the complexity of the materials they are made of. This has always made these flows challenging to simulate numerically, and to reproduce experimentally debris flow processes. In this study, the formation-movement process of non-homogeneous debris flow under three different soil configurations was simulated numerically by modifying the formulation of collision, friction, and yield stresses for the existing Smoothed Particle Hydrodynamics (SPH) method. The results obtained by applying this modification to the SPH model clearly demonstrated that the configuration where fine and coarse particles are fully mixed, with no specific layering, produces more fluctuations and instability of the debris flow. The kinetic and potential energies of the fluctuating particles calculated for each scenario have been shown to be affected by the water content by focusing on small local areas. Therefore, this study provides a better understanding and new insights regarding intermittent debris flows, and explains the impact of the water content on their formation and movement processes
A Multiscale Method for Two-Component, Two-Phase Flow with a Neural Network Surrogate
Understanding the dynamics of phase boundaries in fluids requires
quantitative knowledge about the microscale processes at the interface. We
consider the sharp-interface motion of compressible two-component flow, and
propose a heterogeneous multiscale method (HMM) to describe the flow fields
accurately. The multiscale approach combines a hyperbolic system of balance
laws on the continuum scale with molecular-dynamics simulations on the
microscale level. Notably, the multiscale approach is necessary to compute the
interface dynamics because there is -- at present -- no closed continuum-scale
model. The basic HMM relies on a moving-mesh finite-volume method, and has been
introduced recently for compressible one-component flow with phase transitions
in [Magiera and Rohde, JCP. 469 (2022)]. To overcome the numerical complexity
of the molecular-dynamics microscale model a deep neural network is employed as
an efficient surrogate model. The entire approach is finally applied to
simulate droplet dynamics for argon-methane mixtures in several
space-dimensions. Up to our knowledge such compressible two-phase dynamics
accounting for microscale phase-change transfer rates have not yet been
computed
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