351 research outputs found
Entropic Lattice Boltzmann Method for Moving and Deforming Geometries in Three Dimensions
Entropic lattice Boltzmann methods have been developed to alleviate intrinsic
stability issues of lattice Boltzmann models for under-resolved simulations.
Its reliability in combination with moving objects was established for various
laminar benchmark flows in two dimensions in our previous work Dorschner et al.
[11] as well as for three dimensional one-way coupled simulations of
engine-type geometries in Dorschner et al. [12] for flat moving walls. The
present contribution aims to fully exploit the advantages of entropic lattice
Boltzmann models in terms of stability and accuracy and extends the methodology
to three-dimensional cases including two-way coupling between fluid and
structure, turbulence and deformable meshes. To cover this wide range of
applications, the classical benchmark of a sedimenting sphere is chosen first
to validate the general two-way coupling algorithm. Increasing the complexity,
we subsequently consider the simulation of a plunging SD7003 airfoil at a
Reynolds number of Re = 40000 and finally, to access the model's performance
for deforming meshes, we conduct a two-way coupled simulation of a
self-propelled anguilliform swimmer. These simulations confirm the viability of
the new fluid-structure interaction lattice Boltzmann algorithm to simulate
flows of engineering relevance.Comment: submitted to Journal of Computational Physic
Flutter Instability in an Internal Flow Energy Harvester
Vibration-based flow energy harvesting enables robust, in-situ energy extraction for low-power applications, such as distributed sensor networks. Fluid-structure instabilities dictate a harvester's viability since the structural response to the flow determines its power output. Previous work on a flextensional-based flow energy harvester demonstrated that an elastic member within a converging-diverging channel is susceptible to the aeroelastic flutter. This work explores the mechanism driving flutter through experiments and simulations. A model is then developed based on channel flow-rate modulation and considering the effects of both normal and spanwise flow confinement on the instability. Linear stability analysis of the model replicates flutter onset, critical frequency, and mode shapes observed in experiments. The model suggests that flow modulation through the channel throat is the principal mechanism for the fluid-induced vibration. The generalized model presented can serve as the foundation of design parameter exploration for energy harvesters, perhaps leading to more powerful devices in the future, but also to other similar flow geometries where the flutter instability arises in an elastic member within a narrow flow passage
Thermokinetic lattice Boltzmann model of nonideal fluids
We present a kinetic model for nonideal fluids, where the local thermodynamic pressure is imposed through appropriate rescaling of the particle's velocities, accounting for both long- and short-range effects and hence full thermodynamic consistency. The model features full Galilean invariance together with mass, momentum, and energy conservation and enables simulations ranging from subcritical to supercritical flows, which is illustrated on various benchmark flows such as anomalous shock waves or shock droplet interaction
Detonation modeling with the Particles on Demand method
A kinetic model based on the Particles on Demand method is introduced for gas
phase detonation hydrodynamics in conjunction with the Lee--Tarver reaction
model. The proposed model is realized on two- and three-dimensional lattices
and is validated with a set of benchmarks. Quantitative validation is performed
with the Chapman--Jouguet theory up to a detonation wave speed of Mach 20 in
one dimension. Two-dimensional outward expanding circular detonation is tested
for isotropy of the model as well as for the asymptotic detonation wave speed.
Mach reflection angles are verified in setups consisting of interacting strong
bow shocks emanating from detonation. Spherical detonation is computed to show
viability of the proposed model for three dimensional simulations.Comment: Submitted to Physics of Fluids. 11 pages, 10 figure
Particles-on-Demand for Kinetic Theory
A novel formulation of fluid dynamics as a kinetic theory with tailored,
on-demand constructed particles removes any restrictions on Mach number and
temperature as compared to its predecessors, the lattice Boltzmann methods and
their modifications. In the new kinetic theory, discrete particles are
determined by a rigorous limit process which avoids ad hoc assumptions about
their velocities. Classical benchmarks for incompressible and compressible
flows demonstrate that the proposed discrete-particles kinetic theory opens up
an unprecedented wide domain of applications for computational fluid dynamics
Multi-scale semi-Lagrangian lattice Boltzmann method
We present a multi-scale lattice Boltzmann scheme, which adaptively refines
particles' velocity space. Different velocity sets, i.e., higher- and
lower-order lattices, are consistently and efficiently coupled, allowing us to
use the higher-order lattice only when and where needed. This includes regions
of either high Mach number or high Knudsen number. The coupling procedure of
different lattices consists of either projection of the moments of the
higher-order lattice onto the lower-order lattice or lifting of the lower-order
lattice to the higher-order velocity space. Both lifting and projection are
local operations, which enable a flexible adaptive velocity set. The proposed
scheme can be formulated both in a static and an optimal, co-moving reference
frame, in the spirit of the recently introduced Particles on Demand method. The
multi-scale scheme is first validated through a convected athermal vortex and
also studied in a jet flow setup. The performance of the proposed scheme is
further investigated through the shock structure problem and a high Knudsen
Couette flow, typical examples of highly non-equilibrium flows in which the
order of the velocity set plays a decisive role. The results demonstrate that
the proposed multi-scale scheme can operate accurately, with flexibility in
terms of the underlying models and with reduced computational requirements
Particles on Demand for Kinetic Theory
A novel formulation of fluid dynamics as a kinetic theory with tailored, on-demand constructed particles removes restrictions on flow speed and temperature as compared to its predecessors, the lattice Boltzmann methods and their modifications. In the new kinetic theory, discrete particles are determined by a rigorous limit process which avoids ad hoc assumptions about their velocities. Classical benchmarks for incompressible and compressible flows demonstrate that the proposed discrete-particles kinetic theory opens up an unprecedented wide domain of applications for computational fluid dynamics
Spectroscopic diagnostic for the mineralogy of large dust grains
We examine the thermal infrared spectra of large dust grains of different
chemical composition and mineralogy. Strong resonances in the optical
properties result in detectable spectral structure even when the grain is much
larger than the wavelength at which it radiates. We apply this to the thermal
infrared spectra of compact amorphous and crystalline silicates. The weak
resonances of amorphous silicates at 9.7 and 18 micron virtually disappear for
grains larger than about 10 micron. In contrast, the strong resonances of
crystalline silicates produce emission dips in the infrared spectra of large
grains; these emission dips are shifted in wavelength compared to the emission
peaks commonly seen in small crystalline silicate grains. We discuss the effect
of a fluffy or compact grain structure on the infrared emission spectra of
large grains, and apply our theory to the dust shell surrounding Vega.Comment: Submitted to A&A Letter
2-Dust : a Dust Radiative Transfer Code for an Axisymmetric System
We have developed a general purpose dust radiative transfer code for an
axisymmetric system, 2-Dust, motivated by the recent increasing availability of
high-resolution images of circumstellar dust shells at various wavelengths.
This code solves the equation of radiative transfer following the principle of
long characteristic in a 2-D polar grid while considering a 3-D radiation field
at each grid point. A solution is sought through an iterative scheme in which
self-consistency of the solution is achieved by requiring a global luminosity
constancy throughout the shell. The dust opacities are calculated through Mie
theory from the given size distribution and optical properties of the dust
grains. The main focus of the code is to obtain insights on (1) the global
energetics of dust grains in the shell (2) the 2-D projected morphologies that
are strongly dependent on the mixed effects of the axisymmetric dust
distribution and inclination angle of the shell. Here, test models are
presented with discussion of the results. The code can be supplied with a
user-defined density distribution function, and thus, is applicable to a
variety of dusty astronomical objects possessing the axisymmetric geometry.Comment: To be published in ApJ, April 2003 issue; 13 pages, 4 tables, 17
figures, 5-page appendix (no figures for the main text included in this
preprint). For the complete preprint and code distribution, contact the
author
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