23 research outputs found
Resolving the fine-scale structure in turbulent Rayleigh-Benard convection
We present high-resolution direct numerical simulation studies of turbulent
Rayleigh-Benard convection in a closed cylindrical cell with an aspect ratio of
one. The focus of our analysis is on the finest scales of convective
turbulence, in particular the statistics of the kinetic energy and thermal
dissipation rates in the bulk and the whole cell. The fluctuations of the
energy dissipation field can directly be translated into a fluctuating local
dissipation scale which is found to develop ever finer fluctuations with
increasing Rayleigh number. The range of these scales as well as the
probability of high-amplitude dissipation events decreases with increasing
Prandtl number. In addition, we examine the joint statistics of the two
dissipation fields and the consequences of high-amplitude events. We also have
investigated the convergence properties of our spectral element method and have
found that both dissipation fields are very sensitive to insufficient
resolution. We demonstrate that global transport properties, such as the
Nusselt number, and the energy balances are partly insensitive to insufficient
resolution and yield correct results even when the dissipation fields are
under-resolved. Our present numerical framework is also compared with
high-resolution simulations which use a finite difference method. For most of
the compared quantities the agreement is found to be satisfactory.Comment: 33 pages, 24 figure
Reactive Rayleigh-Taylor Turbulence
The Rayleigh-Taylor (RT) instability develops and leads to turbulence when a
heavy fluid falls under the action of gravity through a light one. We consider
this phenomenon accompanied by a reactive transformation between the fluids,
and study with Direct Numerical Simulations (DNS) how the reaction (flame)
affects the turbulent mixing in the Boussinesq approximation. We discuss "slow"
reactions where the characteristic reaction time exceeds the temporal scale of
the RT instability. In the early turbulent stage, effects of the flame are
distributed over a maturing mixing zone, whose development is weakly influenced
by the reaction. At later times, the fully mixed zone transforms into a
conglomerate of pure-fluid patches of sizes proportional to the mixing zone
width. In this "stirred flame'' regime, temperature fluctuations are consumed
by reactions in the regions separating the pure-fluid patches. This DNS-based
qualitative description is followed by a phenomenology suggesting that thin
turbulent flame is of a single-fractal character, and thus distribution of the
temperature field is strongly intermittent.Comment: 14 pages, 4 figure
On the miscible Rayleigh-Taylor instability: two and three dimensions
We investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3
dimensions using direct numerical simulations, where the working fluid is
assumed incompressible under the Boussinesq approximation. We first consider
the case of randomly perturbed interfaces. With a variety of diagnostics, we
develop a physical picture for the detailed temporal development of the mixed
layer: We identify three distinct evolutionary phases in the development of the
mixed layer, which can be related to detailed variations in the growth of the
mixing zone. Our analysis provides an explanation for the observed differences
between two and three-dimensional RT instability; the analysis also leads us to
concentrate on the RT models which (1) work equally well for both laminar and
turbulent flows, and (2) do not depend on turbulent scaling within the mixing
layer between fluids. These candidate RT models are based on point sources
within bubbles (or plumes) and interaction with each other (or the background
flow). With this motivation, we examine the evolution of single plumes, and
relate our numerical results (of single plumes) to a simple analytical model
for plume evolution.Comment: 31 pages, 27 figures, to appear in November issue of JFM, 2001. For
better figures: http://astro.uchicago.edu/~young/ps/jfmtry08.ps.
Investigation of the Global Instability of the Rotating-disk Boundary Layer
The development of the flow over a rotating disk is investigated by direct numerical simulations using both the linearized and fully
nonlinear incompressible Navier–Stokes equations. These simulations allow investigation of the transition to turbulence of the
realistic spatially-developing boundary layer. The current research aims to elucidate further the global linear stability properties
of the flow, and relate these to local analysis and discussions in literature. An investigation of the nonlinear upstream (inward)
influence is conducted by simulating a small azimuthal section of the disk (1/68). The simulations are initially perturbed by
an impulse disturbance where, after the initial transient behaviour, both the linear and nonlinear simulations show a temporally
growing upstream mode. This upstream global mode originates in the linear case close to the end of the domain, excited by
an absolute instability at this downstream position. In the nonlinear case, it instead originates where the linear region ends and
nonlinear harmonics enter the flow field, also where an absolute instability can be found. This upstream global mode can be
shown to match a theoretical mode from local linear theory involved in the absolute instability at either the end of the domain
(linear case) or where nonlinear harmonics enter the field (nonlinear case). The linear simulation grows continuously in time
whereas the nonlinear simulation saturates and the transition to turbulence moves slowly upstream towards smaller radial positions
asymptotically approaching a global upstream mode with zero temporal growth rate, which is estimated at a nondimensional radius
of 582.Swedish Research Counci
Dynamic Load Balancing for Compressible Multiphase Turbulence
CMT-nek is a new scientific application for performing high fidelity
predictive simulations of particle laden explosively dispersed turbulent flows.
CMT-nek involves detailed simulations, is compute intensive and is targeted to
be deployed on exascale platforms. The moving particles are the main source of
load imbalance as the application is executed on parallel processors. In a
demonstration problem, all the particles are initially in a closed container
until a detonation occurs and the particles move apart. If all processors get
an equal share of the fluid domain, then only some of the processors get
sections of the domain that are initially laden with particles, leading to
disparate load on the processors. In order to eliminate load imbalance in
different processors and to speedup the makespan, we present different load
balancing algorithms for CMT-nek on large scale multi-core platforms consisting
of hundred of thousands of cores. The detailed process of the load balancing
algorithms are presented. The performance of the different load balancing
algorithms are compared and the associated overheads are analyzed. Evaluations
on the application with and without load balancing are conducted and these show
that with load balancing, simulation time becomes faster by a factor of up to
.Comment: This paper has been accepted by ACM International Conference on
Supercomputing (ICS) 201
Multirate Timestepping for the Incompressible Navier-Stokes Equations in Overlapping Grids
We develop a multirate timestepper for semi-implicit solutions of the
unsteady incompressible Navier-Stokes equations (INSE) based on a
recently-developed multidomain spectral element method (SEM). For {\em
incompressible} flows, multirate timestepping (MTS) is particularly challenging
because of the tight coupling implied by the incompressibility constraint,
which manifests as an elliptic subproblem for the pressure at each timestep.
The novelty of our approach stems from the development of a stable overlapping
Schwarz method applied directly to the Navier-Stokes equations, rather than to
the convective, viscous, and pressure substeps that are at the heart of most
INSE solvers. Our MTS approach is based on a predictor-corrector (PC) strategy
that preserves the temporal convergence of the underlying semi-implicit
timestepper. We present numerical results demonstrating that this approach
scales to an arbitrary number of overlapping grids, accurately models complex
turbulent flow phenomenon, and improves computational efficiency in comparison
to singlerate timestepping-based calculations.Comment: 40 pages, 13 figure