44 research outputs found
A minimal model for acoustic forces on Brownian particles
We present a generalization of the inertial coupling (IC) [Usabiaga et al. J.
Comp. Phys. 2013] which permits the resolution of radiation forces on small
particles with arbitrary acoustic contrast factor. The IC method is based on a
Eulerian-Lagrangian approach: particles move in continuum space while the fluid
equations are solved in a regular mesh (here we use the finite volume method).
Thermal fluctuations in the fluid stress, important below the micron scale, are
also taken into account following the Landau-Lifshitz fluid description. Each
particle is described by a minimal cost resolution which consists on a single
small kernel (bell-shaped function) concomitant to the particle. The main role
of the particle kernel is to interpolate fluid properties and spread particle
forces. Here, we extend the kernel functionality to allow for an arbitrary
particle compressibility. The particle-fluid force is obtained from an imposed
no-slip constraint which enforces similar particle and kernel fluid velocities.
This coupling is instantaneous and permits to capture the fast, non-linear
effects underlying the radiation forces on particles. Acoustic forces arise
either because an excess in particle compressibility (monopolar term) or in
mass (dipolar contribution) over the fluid values. Comparison with theoretical
expressions show that the present generalization of the IC method correctly
reproduces both contributions. Due to its low computational cost, the present
method allows for simulations with many particles using a standard Graphical
Processor Unit (GPU)
Inertial Coupling Method for particles in an incompressible fluctuating fluid
We develop an inertial coupling method for modeling the dynamics of
point-like 'blob' particles immersed in an incompressible fluid, generalizing
previous work for compressible fluids. The coupling consistently includes
excess (positive or negative) inertia of the particles relative to the
displaced fluid, and accounts for thermal fluctuations in the fluid momentum
equation. The coupling between the fluid and the blob is based on a no-slip
constraint equating the particle velocity with the local average of the fluid
velocity, and conserves momentum and energy. We demonstrate that the
formulation obeys a fluctuation-dissipation balance, owing to the
non-dissipative nature of the no-slip coupling. We develop a spatio-temporal
discretization that preserves, as best as possible, these properties of the
continuum formulation. In the spatial discretization, the local averaging and
spreading operations are accomplished using compact kernels commonly used in
immersed boundary methods. We find that the special properties of these kernels
make the discrete blob a particle with surprisingly physically-consistent
volume, mass, and hydrodynamic properties. We develop a second-order
semi-implicit temporal integrator that maintains discrete
fluctuation-dissipation balance, and is not limited in stability by viscosity.
Furthermore, the temporal scheme requires only constant-coefficient Poisson and
Helmholtz linear solvers, enabling a very efficient and simple FFT-based
implementation on GPUs. We numerically investigate the performance of the
method on several standard test problems...Comment: Contains a number of corrections and an additional Figure 7 (and
associated discussion) relative to published versio
Particle hydrodynamics: from molecular to colloidal fluids
A method for particle hydrodynamics based on an hybrid Eulerian-Lagrangian approach is presented. Particles are solved in the continuum space while the fluid is solved in an Eulerian mesh, and described by finite volume fluctuating hydrodynamics. This set-up is particulary suited for micron-size devices where the Reynolds number is small but thermal fluctuations are important. The fluid-particle coupling force is obtained by imposing zero relative (particle-fluid) velocity at discrete points representing the particle sites. In this work particles are described by an only site which neglect rotation. The momentum exchanged between fluid and particle is transfered instantaneously and this brings about several benefits such as a correct treatment of inertia and proper particle velocity fluctuations uniquely driven by the fluid thermal forces. The present scheme is designed for incompressible and compressible fluids at low Mach number. This is theoretically shown by analyzing the consistency between the Eulerian and Lagrangian momentum balance.A series of tests up to moderate Reynolds number and acoustic forces under ultrasound waves are also presented.
1 INTRODUCTIO
Swimming Efficiently by Wrapping
Single flagellated bacteria are ubiquitous in nature. They exhibit various
swimming modes using their flagella to explore complex surroundings such as
soil and porous polymer networks. Some single-flagellated bacteria swim with
two distinct modes, one with its flagellum extended away from its body and
another with its flagellum wrapped around it. The wrapped mode has been
observed when the bacteria swim under tight confinements or in highly viscous
polymeric melts. In this study we investigate the hydrodynamics of these two
modes inside a circular pipe. We find that the wrap mode is slower than the
extended mode in bulk but more efficient under strong confinement due to a
hydrodynamic increased of its flagellum translation-rotation coupling.Comment: 10 pages, 5 figure
Hydrodynamics of Suspensions of Passive and Active Rigid Particles: A Rigid Multiblob Approach
We develop a rigid multiblob method for numerically solving the mobility
problem for suspensions of passive and active rigid particles of complex shape
in Stokes flow in unconfined, partially confined, and fully confined
geometries. As in a number of existing methods, we discretize rigid bodies
using a collection of minimally-resolved spherical blobs constrained to move as
a rigid body, to arrive at a potentially large linear system of equations for
the unknown Lagrange multipliers and rigid-body motions. Here we develop a
block-diagonal preconditioner for this linear system and show that a standard
Krylov solver converges in a modest number of iterations that is essentially
independent of the number of particles. For unbounded suspensions and
suspensions sedimented against a single no-slip boundary, we rely on existing
analytical expressions for the Rotne-Prager tensor combined with a fast
multipole method or a direct summation on a Graphical Processing Unit to obtain
an simple yet efficient and scalable implementation. For fully confined
domains, such as periodic suspensions or suspensions confined in slit and
square channels, we extend a recently-developed rigid-body immersed boundary
method to suspensions of freely-moving passive or active rigid particles at
zero Reynolds number. We demonstrate that the iterative solver for the coupled
fluid and rigid body equations converges in a bounded number of iterations
regardless of the system size. We optimize a number of parameters in the
iterative solvers and apply our method to a variety of benchmark problems to
carefully assess the accuracy of the rigid multiblob approach as a function of
the resolution. We also model the dynamics of colloidal particles studied in
recent experiments, such as passive boomerangs in a slit channel, as well as a
pair of non-Brownian active nanorods sedimented against a wall.Comment: Under revision in CAMCOS, Nov 201
On the formation and morphology of coherent particulate structures in non-isothermal enclosures subjected to rotating g-jitters
The strategy undertaken in the author's earlier work [M. Lappa, "The patterning behaviour and accumulation of spherical particles in a vibrated non-isothermal liquid," Phys. Fluids 26(9), 093301 (2014) and M. Lappa, "On the multiplicity and symmetry of particle attractors in confined non-isothermal fluids subjected to inclined vibrations," Int. J. Multiphase Flow 93, 71-83 (2017)] based on the use of polarized (purely translational) vibrations for achieving the segregation or accumulation of solid particles in specific regions of an initially dilute dispersion is further pursued by allowing the direction of vibrations to change in time with respect to the applied temperature difference. In particular, the potential of the considered approach in separating the particles from the liquid is explored under the assumption that the angular velocity by which the vibrations axis rotates about a fixed axis is of the same order of magnitude or smaller (one or two orders of magnitude) than the frequency of shaking. A new family of particle coherent structures is identified in the physical space, which can be distinguished from the companion category of particle attractors for fixed vibration direction due to its increased symmetry properties. It is shown how the average nonlinear effects produced by the rotation of the vibration axis, together with those induced by the finite size of the enclosure, accumulate over time leading to the observed fascinating variety of symmetrical patterns
Applications of computational geometry to the molecular simulation of interfaces
The identification of the interfacial molecules in fluid-fluid equilibrium is
a long-standing problem in the area of simulation. We here propose a new point
of view, making use of concepts taken from the field of computational geometry,
where the definition of the "shape" of a set of point is a well-known problem.
In particular, we employ the -shape construction which, applied to the
positions of the molecules, selects a shape and identifies its boundary points,
which we will take to define our interfacial molecules. A single parameter
needs to be fixed (the "" of the -shape), and several proposals
are examined, all leading to very similar choices. Results of this methodology
are evaluated against previous proposals, and seen to be reasonable.Comment: 22 pages, 8 figure
Neural correlates of phonological, orthographic and semantic reading processing in dyslexia
Available online 10 August 2018Developmental dyslexia is one of the most prevalent learning disabilities, thought to be associated with dysfunction in the neural systems underlying typical reading acquisition. Neuroimaging research has shown that readers with dyslexia exhibit regional hypoactivation in left hemisphere reading nodes, relative to control counterparts. This evidence, however, comes from studies that have focused only on isolated aspects of reading. The present study aims to characterize left hemisphere regional hypoactivation in readers with dyslexia for the main processes involved in successful reading: phonological, orthographic and semantic. Forty-one participants performed a demanding reading task during MRI scanning. Results showed that readers with dyslexia exhibited hypoactivation associated with phonological processing in parietal regions; with orthographic processing in parietal regions, Broca's area, ventral occipitotemporal cortex and thalamus; and with semantic processing in angular gyrus and hippocampus. Stronger functional connectivity was observed for readers with dyslexia than for control readers 1) between the thalamus and the inferior parietal cortex/ventral occipitotemporal cortex during pseudoword reading; and, 2) between the hippocampus and the pars opercularis during word reading. These findings constitute the strongest evidence to date for the interplay between regional hypoactivation and functional connectivity in the main processes supporting reading in dyslexia.Supported by grants (RYC-2014-15440, PSI2015-65696) from the Spanish Ministry of Economy and Competitiveness (MINECO), a grant (PI2016-12) from the Basque Government and a grant (Exp. 65/15) from the Programa Red guipuzcoana de Ciencia, Tecnología e Innovación from the Diputación Foral de Gipuzkoa (P.M.P-A.); a predoctoral grant from the Department of Education, Universities and Research from the Basque Government (M.O.); grant (PSI2015-64174P) from the MINECO (F.C.); grants (PSI2015-67353-R) from the MINECO and (ERC-2011-ADG-295362) from the European Research Council (M.C.). BCBL acknowledges funding from Ayuda Centro de Excelencia Severo OchoaSEV-2015-0490 from the MINECO