1,474 research outputs found
Hermite regularization of the Lattice Boltzmann Method for open source computational aeroacoustics
The lattice Boltzmann method (LBM) is emerging as a powerful engineering tool
for aeroacoustic computations. However, the LBM has been shown to present
accuracy and stability issues in the medium-low Mach number range, that is of
interest for aeroacoustic applications. Several solutions have been proposed
but often are too computationally expensive, do not retain the simplicity and
the advantages typical of the LBM, or are not described well enough to be
usable by the community due to proprietary software policies. We propose to use
an original regularized collision operator, based on the expansion in Hermite
polynomials, that greatly improves the accuracy and stability of the LBM
without altering significantly its algorithm. The regularized LBM can be easily
coupled with both non-reflective boundary conditions and a multi-level grid
strategy, essential ingredients for aeroacoustic simulations. Excellent
agreement was found between our approach and both experimental and numerical
data on two different benchmarks: the laminar, unsteady flow past a 2D cylinder
and the 3D turbulent jet. Finally, most of the aeroacoustic computations with
LBM have been done with commercial softwares, while here the entire theoretical
framework is implemented on top of an open source library (Palabos).Comment: 34 pages, 12 figures, The Journal of the Acoustical Society of
America (in press
A lattice Boltzmann model for natural convection in cavities
We study a multiple relaxation time lattice Boltzmann model for natural convection with moment–based boundary conditions. The unknown primary variables of the algorithm at a boundary are found by imposing conditions directly upon hydrodynamic moments, which are then translated into conditions for the discrete velocity distribution functions. The method is formulated so that it is consistent with the second–order implementation of the discrete velocity Boltzmann equations for fluid flow and temperature. Natural convection in square cavities is studied for Rayleigh numbers ranging from 103 to 106. An excellent agreement with benchmark data is observed and the flow fields are shown to converge with second order accuracy
Explicit finite-difference and direct-simulation-MonteCarlo method for the dynamics of mixed Bose-condensate and cold-atom clouds
We present a new numerical method for studying the dynamics of quantum fluids
composed of a Bose-Einstein condensate and a cloud of bosonic or fermionic
atoms in a mean-field approximation. It combines an explicit time-marching
algorithm, previously developed for Bose-Einstein condensates in a harmonic or
optical-lattice potential, with a particle-in-cell MonteCarlo approach to the
equation of motion for the one-body Wigner distribution function in the
cold-atom cloud. The method is tested against known analytical results on the
free expansion of a fermion cloud from a cylindrical harmonic trap and is
validated by examining how the expansion of the fermionic cloud is affected by
the simultaneous expansion of a condensate. We then present wholly original
calculations on a condensate and a thermal cloud inside a harmonic well and a
superposed optical lattice, by addressing the free expansion of the two
components and their oscillations under an applied harmonic force. These
results are discussed in the light of relevant theories and experiments.Comment: 33 pages, 13 figures, 1 tabl
Large Eddy Simulation of Turbulent Flows Using the Lattice Boltzmann Method
Turbulent flow is a complex fluid phenomenon because of its disordered and chaotic flow patterns. Analysis of such flows presents practical significance and is widely performed using either experiments or simulations. The numerical simulation, or computational fluid dynamics (CFD) is one powerful technique; traditionally, it is based on the Navier-Stokes equations. A novel numerical approach called the lattice Boltzmann method (LBM) has developed quickly over the past decades, and this method is based on an entirely different mechanism. The current thesis seeks to present an investigation of turbulent flows that was performed using the LBM
Spectral Ewald Acceleration of Stokesian Dynamics for polydisperse suspensions
In this work we develop the Spectral Ewald Accelerated Stokesian Dynamics
(SEASD), a novel computational method for dynamic simulations of polydisperse
colloidal suspensions with full hydrodynamic interactions. SEASD is based on
the framework of Stokesian Dynamics (SD) with extension to compressible
solvents, and uses the Spectral Ewald (SE) method [Lindbo & Tornberg, J.
Comput. Phys. 229 (2010) 8994] for the wave-space mobility computation. To meet
the performance requirement of dynamic simulations, we use Graphic Processing
Units (GPU) to evaluate the suspension mobility, and achieve an order of
magnitude speedup compared to a CPU implementation. For further speedup, we
develop a novel far-field block-diagonal preconditioner to reduce the far-field
evaluations in the iterative solver, and SEASD-nf, a polydisperse extension of
the mean-field Brownian approximation of Banchio & Brady [J. Chem. Phys. 118
(2003) 10323]. We extensively discuss implementation and parameter selection
strategies in SEASD, and demonstrate the spectral accuracy in the mobility
evaluation and the overall computation scaling. We
present three computational examples to further validate SEASD and SEASD-nf in
monodisperse and bidisperse suspensions: the short-time transport properties,
the equilibrium osmotic pressure and viscoelastic moduli, and the steady shear
Brownian rheology. Our validation results show that the agreement between SEASD
and SEASD-nf is satisfactory over a wide range of parameters, and also provide
significant insight into the dynamics of polydisperse colloidal suspensions.Comment: 39 pages, 21 figure
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