6,398 research outputs found
A non-hybrid method for the PDF equations of turbulent flows on unstructured grids
In probability density function (PDF) methods of turbulent flows, the joint
PDF of several flow variables is computed by numerically integrating a system
of stochastic differential equations for Lagrangian particles. A set of
parallel algorithms is proposed to provide an efficient solution of the PDF
transport equation, modeling the joint PDF of turbulent velocity, frequency and
concentration of a passive scalar in geometrically complex configurations. An
unstructured Eulerian grid is employed to extract Eulerian statistics, to solve
for quantities represented at fixed locations of the domain (e.g. the mean
pressure) and to track particles. All three aspects regarding the grid make use
of the finite element method (FEM) employing the simplest linear FEM shape
functions. To model the small-scale mixing of the transported scalar, the
interaction by exchange with the conditional mean model is adopted. An adaptive
algorithm that computes the velocity-conditioned scalar mean is proposed that
homogenizes the statistical error over the sample space with no assumption on
the shape of the underlying velocity PDF. Compared to other hybrid
particle-in-cell approaches for the PDF equations, the current methodology is
consistent without the need for consistency conditions. The algorithm is tested
by computing the dispersion of passive scalars released from concentrated
sources in two different turbulent flows: the fully developed turbulent channel
flow and a street canyon (or cavity) flow. Algorithmic details on estimating
conditional and unconditional statistics, particle tracking and particle-number
control are presented in detail. Relevant aspects of performance and
parallelism on cache-based shared memory machines are discussed.Comment: Accepted in Journal of Computational Physics, Feb. 20, 200
A Survey of Ocean Simulation and Rendering Techniques in Computer Graphics
This paper presents a survey of ocean simulation and rendering methods in
computer graphics. To model and animate the ocean's surface, these methods
mainly rely on two main approaches: on the one hand, those which approximate
ocean dynamics with parametric, spectral or hybrid models and use empirical
laws from oceanographic research. We will see that this type of methods
essentially allows the simulation of ocean scenes in the deep water domain,
without breaking waves. On the other hand, physically-based methods use
Navier-Stokes Equations (NSE) to represent breaking waves and more generally
ocean surface near the shore. We also describe ocean rendering methods in
computer graphics, with a special interest in the simulation of phenomena such
as foam and spray, and light's interaction with the ocean surface
Joint PDF modelling of turbulent flow and dispersion in an urban street canyon
The joint probability density function (PDF) of turbulent velocity and
concentration of a passive scalar in an urban street canyon is computed using a
newly developed particle-in-cell Monte Carlo method. Compared to moment
closures, the PDF methodology provides the full one-point one-time PDF of the
underlying fields containing all higher moments and correlations. The
small-scale mixing of the scalar released from a concentrated source at the
street level is modelled by the interaction by exchange with the conditional
mean (IECM) model, with a micro-mixing time scale designed for geometrically
complex settings. The boundary layer along no-slip walls (building sides and
tops) is fully resolved using an elliptic relaxation technique, which captures
the high anisotropy and inhomogeneity of the Reynolds stress tensor in these
regions. A less computationally intensive technique based on wall functions to
represent boundary layers and its effect on the solution are also explored. The
calculated statistics are compared to experimental data and large-eddy
simulation. The present work can be considered as the first example of
computation of the full joint PDF of velocity and a transported passive scalar
in an urban setting. The methodology proves successful in providing high level
statistical information on the turbulence and pollutant concentration fields in
complex urban scenarios.Comment: Accepted in Boundary-Layer Meteorology, Feb. 19, 200
Variational approach to low-frequency kinetic-MHD in the current coupling scheme
Hybrid kinetic-MHD models describe the interaction of an MHD bulk fluid with
an ensemble of hot particles, which is described by a kinetic equation. When
the Vlasov description is adopted for the energetic particles, different
Vlasov-MHD models have been shown to lack an exact energy balance, which was
recently recovered by the introduction of non-inertial force terms in the
kinetic equation. These force terms arise from fundamental approaches based on
Hamiltonian and variational methods. In this work we apply Hamilton's
variational principle to formulate new current-coupling kinetic-MHD models in
the low-frequency approximation (i.e. large Larmor frequency limit). More
particularly, we formulate current-coupling hybrid schemes, in which energetic
particle dynamics are expressed in either guiding-center or gyrocenter
coordinates.Comment: v3.0. 30 page
A Moving Frame Algorithm for High Mach Number Hydrodynamics
We present a new approach to Eulerian computational fluid dynamics that is
designed to work at high Mach numbers encountered in astrophysical hydrodynamic
simulations. The Eulerian fluid conservation equations are solved in an
adaptive frame moving with the fluid where Mach numbers are minimized. The
moving frame approach uses a velocity decomposition technique to define local
kinetic variables while storing the bulk kinetic components in a smoothed
background velocity field that is associated with the grid velocity.
Gravitationally induced accelerations are added to the grid, thereby minimizing
the spurious heating problem encountered in cold gas flows. Separately tracking
local and bulk flow components allows thermodynamic variables to be accurately
calculated in both subsonic and supersonic regions. A main feature of the
algorithm, that is not possible in previous Eulerian implementations, is the
ability to resolve shocks and prevent spurious heating where both the preshock
and postshock Mach numbers are high. The hybrid algorithm combines the high
resolution shock capturing ability of the second-order accurate Eulerian TVD
scheme with a low-diffusion Lagrangian advection scheme. We have implemented a
cosmological code where the hydrodynamic evolution of the baryons is captured
using the moving frame algorithm while the gravitational evolution of the
collisionless dark matter is tracked using a particle-mesh N-body algorithm.
The MACH code is highly suited for simulating the evolution of the IGM where
accurate thermodynamic evolution is needed for studies of the Lyman alpha
forest, the Sunyaev-Zeldovich effect, and the X-ray background. Hydrodynamic
and cosmological tests are described and results presented. The current code is
fast, memory-friendly, and parallelized for shared-memory machines.Comment: 19 pages, 5 figure
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