1,217 research outputs found
Unsteady aerodynamic simulation of multiple bodies in relative motion: A prototype method
A prototype method for time-accurate simulation of multiple aerodynamic bodies in relative motion is presented. The method is general and features unsteady chimera domain decomposition techniques and an implicit approximately factored finite-difference procedure to solve the time-dependent thin-layer Navier-Stokes equations. The method is applied to a set of two- and three- dimensional test problems to establish spatial and temporal accuracy, quantify computational efficiency, and begin to test overall code robustness
Conformal approach to cylindrical DLA
We extend the conformal mapping approach elaborated for the radial Diffusion
Limited Aggregation model (DLA) to the cylindrical geometry. We introduce in
particular a complex function which allows to grow a cylindrical cluster using
as intermediate step a radial aggregate. The grown aggregate exhibits the same
self-affine features of the original cylindrical DLA. The specific choice of
the transformation allows us to study the relationship between the radial and
the cylindrical geometry. In particular the cylindrical aggregate can be seen
as a radial aggregate with particles of size increasing with the radius. On the
other hand the radial aggregate can be seen as a cylindrical aggregate with
particles of size decreasing with the height. This framework, which shifts the
point of view from the geometry to the size of the particles, can open the way
to more quantitative studies on the relationship between radial and cylindrical
DLA.Comment: 16 pages, 8 figure
A new stellar mixing process operating below shell convection zones following off-center ignition
During most stages of stellar evolution the nuclear burning of lighter to
heavier elements results in a radial composition profile which is stabilizing
against buoyant acceleration, with light material residing above heavier
material. However, under some circumstances, such as off-center ignition, the
composition profile resulting from nuclear burning can be destabilizing, and
characterized by an outwardly increasing mean molecular weight. The potential
for instabilities under these circumstances, and the consequences that they may
have on stellar structural evolution, remain largely unexplored. In this paper
we study the development and evolution of instabilities associated with
unstable composition gradients in regions which are initially stable according
to linear Schwarzschild and Ledoux criteria. In particular, we explore the
mixing taking place under various conditions with multi-dimensional
hydrodynamic convection models based on stellar evolutionary calculations of
the core helium flash in a 1.25 \Msun star, the core carbon flash in a
9.3\,\Msun star, and of oxygen shell burning in a star with a mass of
23\,\Msun. The results of our simulations reveal a mixing process associated
with regions having outwardly increasing mean molecular weight that reside
below convection zones. The mixing is not due to overshooting from the
convection zone, nor is it due directly to thermohaline mixing which operates
on a timescale several orders of magnitude larger than the simulated flows.
Instead, the mixing appears to be due to the presence of a wave field induced
in the stable layers residing beneath the convection zone which enhances the
mixing rate by many orders of magnitude and allows a thermohaline type mixing
process to operate on a dynamical, rather than thermal, timescale. We discuss
our results in terms of related laboratory phenomena and associated theoretical
developments.Comment: accepted for publication in Astrophysical Journal, 9 pages, 8 figure
Unsteady Simulation of the Viscous Flow About a V-22 Rotor and Wing in Hover
Results of an unsteady thin-layer Navier-Stokes simulation of a 0.658-scale V-22 rotor and wing configuration in hover are presented. All geometric components of the flapped-wing and rotor test rig, including rotor blades, are accurately modeled. Rotor motion and rotor/airframe interference effects are simulated directly using moving body overset grid methods. Tiltrotor hover aerodynamics are visualized via unsteady particle trace images. Wing download predictive ability is demonstrated. Simulation results are compared with experimental data
On the Spatial and Temporal Accuracy of Overset Grid Methods for Moving Body Problems
A study of numerical attributes peculiar to an overset grid approach to unsteady aerodynamics prediction is presented. Attention is focused on the effect of spatial error associated with interpolation of intergrid boundary conditions and temporal error associated with explicit update of intergrid boundary points on overall solution accuracy. A set of numerical experiments are used to verify whether or not the use of simple interpolation for intergrid boundary conditions degrades the formal accuracy of a conventional second-order flow solver, and to quantify the error associated with explicit updating of intergrid boundary points. Test conditions correspond to the transonic regime. The validity of the numerical results presented here are established by comparison with existing numerical results of documented accuracy, and by direct comparison with experimental results
Grid related issues for static and dynamic geometry problems using systems of overset structured grids
Grid related issues of the Chimera overset grid method are discussed in the context of a method of solution and analysis of unsteady three-dimensional viscous flows. The state of maturity of the various pieces of support software required to use the approach is considered. Current limitations of the approach are identified
Global Flowfield About the V-22 Tiltrotor Aircraft
This final report includes five publications that resulted from the studies of the global flowfield about the V-22 Tiltrotor Aircraft. The first of the five is 'The Chimera Method of Simulation for Unsteady Three-Dimensional Viscous Flow', as presented in 'Computational Fluid Dynamics Review 1995.' The remaining papers, all presented at AIAA conferences, are 'Unsteady Simulation of the Viscous Flow About a V-22 Rotor and Wing in Hover', 'An Efficient Means of Adaptive Refinement Within Systems of Overset Grids', 'On the Spatial and Temporal Accuracy of Overset Grid Methods for MOving Body Problems', and 'Moving Body Overset Grid Methods for Complete Aircraft Tiltrotor Simulations.
Domain connectivity among systems of overset grids
This progress report describes research performed to achieve algorithmic improvements in the use of overset grids for the computation of unsteady flowfields about geometrically complex and moving component configurations. A hole-map computational method is detailed that reduces computational demand and corresponding demand on human resources. Results indicate that the method is both efficient and flexible for use in complex simulation cases. Tests were performed on shuttle orbiter/external tank, wing/store, and V-22 tilt-rotor configurations. The number of simulated points and the corresponding time on a SGI 4D-210 workstation are presented
An Efficient Means of Adaptive Refinement Within Systems of Overset Grids
An efficient means of adaptive refinement within systems of overset grids is presented. Problem domains are segregated into near-body and off-body fields. Near-body fields are discretized via overlapping body-fitted grids that extend only a short distance from body surfaces. Off-body fields are discretized via systems of overlapping uniform Cartesian grids of varying levels of refinement. a novel off-body grid generation and management scheme provides the mechanism for carrying out adaptive refinement of off-body flow dynamics and solid body motion. The scheme allows for very efficient use of memory resources, and flow solvers and domain connectivity routines that can exploit the structure inherent to uniform Cartesian grids
Anisotropic anomalous diffusion modulated by log-periodic oscillations
We introduce finite ramified self-affine substrates in two dimensions with a
set of appropriate hopping rates between nearest-neighbor sites, where the
diffusion of a single random walk presents an anomalous {\it anisotropic}
behavior modulated by log-periodic oscillations. The anisotropy is revealed by
two different random walk exponents, and , in the {\it x} and
{\it y} direction, respectively. The values of these exponents, as well as the
period of the oscillation, are analytically obtained and confirmed by Monte
Carlo simulations.Comment: 7 pages, 7 figure
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