38 research outputs found
Numerical simulation of moving rigid body in rarefied gases
In this paper we present a numerical scheme to simulate a moving rigid body
with arbitrary shape suspended in a rarefied gas. The rarefied gas is simulated
by solving the Boltzmann equation using a DSMC particle method. The motion of
the rigid body is governed by the Newton-Euler equations, where the force and
the torque on the rigid body is computed from the momentum transfer of the gas
molecules colliding with the body. On the other hand, the motion of the rigid
body influences the gas flow in its surroundings. We validate the numerical
results by testing the Einstein relation for Brownian motion of the suspended
particle. The translational as well as the rotational degrees of freedom are
taken into account. It is shown that the numerically computed translational and
rotational diffusion coefficients converge to the theoretical values.Comment: 16 pages, 8 figure
Brownian dynamics of rigid particles in an incompressible fluctuating fluid by a meshfree method
A meshfree Lagrangian method for the fluctuating hydrodynamic equations
(FHEs) with fluid-structure interactions is presented. Brownian motion of the
particle is investigated by direct numerical simulation of the fluctuating
hydrodynamic equations. In this framework a bidirectional coupling has been
introduced between the fluctuating fluid and the solid object. The force
governing the motion of the solid object is solely due to the surrounding fluid
particles. Since a meshfree formulation is used, the method can be extended to
many real applications involving complex fluid flows. A three-dimensional
implementation is presented. In particular, we observe the short and long-time
behaviour of the velocity autocorrelation function (VACF) of Brownian particles
and compare it with the analytical expression. Moreover, the Stokes-Einstein
relation is reproduced to ensure the correct long-time behaviour of Brownian
dynamics.Comment: 24 pages, 2 figure
Minimum L-distance estimators for non-normalized parametric models
We propose and investigate a new estimation method for the parameters of models consisting of smooth density functions on the positive half axis. The procedure is based on a recently introduced characterization result for the respective probability distributions, and is to be classified as a minimum distance estimator, incorporating as a distance function the L‐norm. Throughout, we deal rigorously with issues of existence and measurability of these implicitly defined estimators. Moreover, we provide consistency results in a common asymptotic setting, and compare our new method with classical estimators for the exponential, the Rayleigh and the Burr Type XII distribution in Monte Carlo simulation studies. We also assess the performance of different estimators for non‐normalized models in the context of an exponential‐polynomial family
Minimum -distance estimators for non-normalized parametric models
We propose and investigate a new estimation method for the parameters of
models consisting of smooth density functions on the positive half axis. The
procedure is based on a recently introduced characterization result for the
respective probability distributions, and is to be classified as a minimum
distance estimator, incorporating as a distance function the -norm.
Throughout, we deal rigorously with issues of existence and measurability of
these implicitly defined estimators. Moreover, we provide consistency results
in a common asymptotic setting, and compare our new method with classical
estimators for the exponential-, the Rayleigh-, and the Burr Type XII
distribution in Monte Carlo simulation studies. We also assess the performance
of different estimators for non-normalized models in the context of an
exponential-polynomial family.Comment: 27 pages, 8 table
Momentum and Mass Fluxes in a Gas Confined between Periodically Structured Surfaces at Different Temperatures
It is well known that in a gas-filled duct or channel along which a
temperature gradient is applied, a thermal creep flow is created. Here we show
that a mass and momentum flux can also be induced in a gas confined between two
parallel structured surfaces at different temperatures, i.e.
\textit{orthogonal} to the temperature gradient. We use both analytical and
numerical methods to compute the resulting fluxes. The momentum flux assumes
its maximum value in the free-molecular flow regime, the (normalized) mass flux
in the transition flow regime. The discovered phenomena could find applications
in novel methods for energy-conversion and thermal pumping of gases.Comment: 6 pages, 5 figures, updated fig.5, updated text for the numerical
metho
Thermophoresis of Janus particles at large Knudsen numbers
The force and torque on a Janus sphere moving in a rarefied gas with a
thermal gradient are calculated. The regime of large Knudsen number is
considered, with the momenta of impinging gas molecules either obtained from a
Chapman-Enskog distribution or from a binary Maxwellian distribution between
two opposing parallel plates at different temperature. The reflection
properties at the surface of the Janus particle are characterized by
accommodation coefficients having constant but dissimilar values on each
hemisphere. It is shown that the Janus particle preferentially orients such
that the hemisphere with a larger accommodation coefficient points towards the
lower temperature. The thermophoretic velocity of the particle is computed, and
the influence of the thermophoretic motion on the magnitude of the torque
responsible for the particle orientation is studied. The analytical
calculations are supported by Direct Simulation Monte Carlo results, extending
the scope of the study towards smaller Knudsen numbers. The results shed light
on the efficiency of oriented deposition of nanoparticles from the gas phase
onto a cold surface
Photoluminescence and photoluminescence excitation studies of lateral size effects in Zn_{1-x}Mn_xSe/ZnSe quantum disc samples of different radii
Quantum disc structures (with diameters of 200 nm and 100 nm) were prepared
from a Zn_{0.72}Mn_{0.28}Se/ZnSe single quantum well structure by electron beam
lithography followed by an etching procedure which combined dry and wet etching
techniques. The quantum disc structures and the parent structure were studied
by photoluminescence and photoluminescence excitation spectroscopy. For the
light-hole excitons in the quantum well region, shifts of the energy positions
are observed following fabrication of the discs, confirming that strain
relaxation occurs in the pillars. The light-hole exciton lines also sharpen
following disc fabrication: this is due to an interplay between strain effects
(related to dislocations) and the lateral size of the discs. A further
consequence of the small lateral sizes of the discs is that the intensity of
the donor-bound exciton emission from the disc is found to decrease with the
disc radius. These size-related effects occur before the disc radius is reduced
to dimensions necessary for lateral quantum confinement to occur but will
remain important when the discs are made small enough to be considered as
quantum dots.Comment: LaTeX2e, 13 pages, 6 figures (epsfig