1,144 research outputs found
Simulating Brownian suspensions with fluctuating hydrodynamics
Fluctuating hydrodynamics has been successfully combined with several
computational methods to rapidly compute the correlated random velocities of
Brownian particles. In the overdamped limit where both particle and fluid
inertia are ignored, one must also account for a Brownian drift term in order
to successfully update the particle positions. In this paper, we present an
efficient computational method for the dynamic simulation of Brownian
suspensions with fluctuating hydrodynamics that handles both computations and
provides a similar approximation as Stokesian Dynamics for dilute and
semidilute suspensions. This advancement relies on combining the fluctuating
force-coupling method (FCM) with a new midpoint time-integration scheme we
refer to as the drifter-corrector (DC). The DC resolves the drift term for
fluctuating hydrodynamics-based methods at a minimal computational cost when
constraints are imposed on the fluid flow to obtain the stresslet corrections
to the particle hydrodynamic interactions. With the DC, this constraint need
only be imposed once per time step, reducing the simulation cost to nearly that
of a completely deterministic simulation. By performing a series of
simulations, we show that the DC with fluctuating FCM is an effective and
versatile approach as it reproduces both the equilibrium distribution and the
evolution of particulate suspensions in periodic as well as bounded domains. In
addition, we demonstrate that fluctuating FCM coupled with the DC provides an
efficient and accurate method for large-scale dynamic simulation of colloidal
dispersions and the study of processes such as colloidal gelation
Spatiotemporal and Wavenumber Resolved Bicoherence at the Low to High Confinement Transition in the TJ-II Stellarator
Plasma turbulence is studied using Doppler reflectometry at the TJ-II
stellarator. By scanning the tilt angle of the probing beam, different values
of the perpendicular wave numbers are probed at the reflection layer. In this
way, the interaction between zonal flows and turbulence is reported with (a)
spatial, (b) temporal, and (c) wavenumber resolution for the first time in any
magnetic confinement fusion device.
We report measurements of the bicoherence across the Low to High (L--H)
confinement transition at TJ-II. We examine both fast transitions and slow
transitions characterized by an intermediate (I) phase. The bicoherence,
understood to reflect the non-linear coupling between the perpendicular
velocity (zonal flow) and turbulence amplitude, is significantly enhanced in a
time window of several tens of ms around the time of the L--H transition. It is
found to peak at a specific radial position (slightly inward from the radial
electric field shear layer in H mode), and is associated with a specific
perpendicular wave number ( cm, ). In all cases, the bicoherence is due to the interaction between
high frequencies ( MHz) and a rather low frequency (
kHz), as expected for a zonal flow.Comment: 11 pages, 3 figure
Force calculation on walls and embedded particles in multiparticle collision dynamics simulations
Colloidal solutions posses a wide range of time and length scales, so that it
is unfeasible to keep track of all of them within a single simulation. As a
consequence some form of coarse-graining must be applied. In this work we use
the Multi-Particle Collision Dynamics scheme. We describe a particular
implementation of no-slip boundary conditions upon a solid surface, capable of
providing correct force s on the solid bypassing the calculation of the
velocity profile or the stre ss tensor in the fluid near the surface. As an
application we measure the friction on a spherical particle, when it is placed
in a bulk fluid and when it is confined in a slit. We show that the
implementation of the no-slip boundary conditions leads to an enhanced Ensko g
friction, which can be understood analytically. Because of the long-range
nature of hydrodynamic interactions, the Stokes friction obtained from the
simulations is sensitive of the simulation box size. We address this topic for
the slit geometry, showing that that the dependence on the system size differs
very much from what is expected in a 3D system, where periodic boundary
conditions are used in all directions.Comment: To appear in Physical Review
Dynamics of non-equilibrium membrane bud formation
The dynamical response of a lipid membrane to a local perturbation of its
molecular symmetry is investigated theoretically. A density asymmetry between
the two membrane leaflets is predominantly released by in-plane lipid diffusion
or membrane curvature, depending upon the spatial extent of the perturbation.
It may result in the formation of non-equilibrium structures (buds), for which
a dynamical size selection is observed. A preferred size in the micrometer
range is predicted, as a signature of the crossover between membrane and
solvent dominated dynamical membrane response.Comment: 7 pages 3 figure
Jet propulsion without inertia
A body immersed in a highly viscous fluid can locomote by drawing in and
expelling fluid through pores at its surface. We consider this mechanism of jet
propulsion without inertia in the case of spheroidal bodies, and derive both
the swimming velocity and the hydrodynamic efficiency. Elementary examples are
presented, and exact axisymmetric solutions for spherical, prolate spheroidal,
and oblate spheroidal body shapes are provided. In each case, entirely and
partially porous (i.e. jetting) surfaces are considered, and the optimal
jetting flow profiles at the surface for maximizing the hydrodynamic efficiency
are determined computationally. The maximal efficiency which may be achieved by
a sphere using such jet propulsion is 12.5%, a significant improvement upon
traditional flagella-based means of locomotion at zero Reynolds number. Unlike
other swimming mechanisms which rely on the presentation of a small cross
section in the direction of motion, the efficiency of a jetting body at low
Reynolds number increases as the body becomes more oblate, and limits to
approximately 162% in the case of a flat plate swimming along its axis of
symmetry. Our results are discussed in the light of slime extrusion mechanisms
occurring in many cyanobacteria
Low-Reynolds number swimming in gels
Many microorganisms swim through gels, materials with nonzero zero-frequency
elastic shear modulus, such as mucus. Biological gels are typically
heterogeneous, containing both a structural scaffold (network) and a fluid
solvent. We analyze the swimming of an infinite sheet undergoing transverse
traveling wave deformations in the "two-fluid" model of a gel, which treats the
network and solvent as two coupled elastic and viscous continuum phases. We
show that geometric nonlinearities must be incorporated to obtain physically
meaningful results. We identify a transition between regimes where the network
deforms to follow solvent flows and where the network is stationary. Swimming
speeds can be enhanced relative to Newtonian fluids when the network is
stationary. Compressibility effects can also enhance swimming velocities.
Finally, microscopic details of sheet-network interactions influence the
boundary conditions between the sheet and network. The nature of these boundary
conditions significantly impacts swimming speeds.Comment: 6 pages, 5 figures, submitted to EP
A Simplest Swimmer at Low Reynolds Number: Three Linked Spheres
We propose a very simple one-dimensional swimmer consisting of three spheres
that are linked by rigid rods whose lengths can change between two values. With
a periodic motion in a non-reciprocal fashion, which breaks the time-reversal
symmetry as well as the translational symmetry, we show that the model device
can swim at low Reynolds number. This model system could be used in
constructing molecular-size machines
The symmetry of mobility laws for viscous flow along arbitrarily patterned surfaces
Generalizations of the no-slip boundary condition to allow for slip at a patterned fluid-solid boundary introduce a surface mobility tensor, which relates the shear traction vector tangent to the mean surface to an apparent surface velocity vector. For steady, low-Reynolds-number fluid motions over planar surfaces perturbed by arbitrary periodic height and Navier slip fluctuations, we prove that the resulting mobility tensor is always symmetric, which had previously been conjectured. We describe generalizations of the results to three other families of geometries, which typically have unsteady flow.National Science Foundation (U.S.) (NSF MSPRF program)National Institutes of Health (U.S.) (NSF Grant No. CBET-0961081
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