64 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)
USHER: an algorithm for particle insertion in dense fluids
The insertion of solvent particles in molecular dynamics simulations of
complex fluids is required in many situations involving open systems, but this
challenging task has been scarcely explored in the literature. We propose a
simple and fast algorithm (USHER) that inserts the new solvent particles at
locations where the potential energy has the desired prespecified value. For
instance, this value may be set equal to the system's excess energy per
particle, in such way that the inserted particles are energetically
indistinguishable from the other particles present. During the search for the
insertion site, the USHER algorithm uses a steepest descent iterator with a
displacement whose magnitude is adapted to the local features of the energy
landscape. The only adjustable parameter in the algorithm is the maximum
displacement and we show that its optimal value can be extracted from an
analysis of the structure of the potential energy landscape. We present
insertion tests in periodic and non-periodic systems filled with a
Lennard-Jones fluid whose density ranges from moderate values to high values.Comment: 10 pages (Latex), 8 figures (postscript); J. Chem. Phys. (in press)
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Multiscale modelling of liquids with molecular specificity
The separation between molecular and mesoscopic length and time scales poses
a severe limit to molecular simulations of mesoscale phenomena. We describe a
hybrid multiscale computational technique which address this problem by keeping
the full molecular nature of the system where it is of interest and
coarse-graining it elsewhere. This is made possible by coupling molecular
dynamics with a mesoscopic description of realistic liquids based on Landau's
fluctuating hydrodynamics. We show that our scheme correctly couples
hydrodynamics and that fluctuations, at both the molecular and continuum
levels, are thermodynamically consistent. Hybrid simulations of sound waves in
bulk water and reflected by a lipid monolayer are presented as illustrations of
the scheme
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
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
Determination of the chemical potential using energy-biased sampling
An energy-biased method to evaluate ensemble averages requiring test-particle
insertion is presented. The method is based on biasing the sampling within the
subdomains of the test-particle configurational space with energies smaller
than a given value freely assigned. These energy-wells are located via unbiased
random insertion over the whole configurational space and are sampled using the
so called Hit&Run algorithm, which uniformly samples compact regions of any
shape immersed in a space of arbitrary dimensions. Because the bias is defined
in terms of the energy landscape it can be exactly corrected to obtain the
unbiased distribution. The test-particle energy distribution is then combined
with the Bennett relation for the evaluation of the chemical potential. We
apply this protocol to a system with relatively small probability of low-energy
test-particle insertion, liquid argon at high density and low temperature, and
show that the energy-biased Bennett method is around five times more efficient
than the standard Bennett method. A similar performance gain is observed in the
reconstruction of the energy distribution.Comment: 10 pages, 4 figure
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