This paper considers an idealized subclass of surface reflectivities;namely, a simple superposition of ideal diffuse and ideal specular, restricted to point light sources. The paper derives a model of diffuse and spec...
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Magnetic resonance imaging and spectroscopy techniques are now being used to detect changes in blood flow, volume and oxygenation level associated with brain function. The authors describe a prototype system, called a...
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The aims of this paper are threefold: to increase the level of awareness within the shock-capturing community of the fact that many Godunov-type methods contain subtle flaws that can cause spurious solutions to be com...
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The aims of this paper are threefold: to increase the level of awareness within the shock-capturing community of the fact that many Godunov-type methods contain subtle flaws that can cause spurious solutions to be computed;to identify one mechanism that might thwart attempts to produce very-high-resolution simulations;and to proffer a simple strategy for overcoming the specific failings of individual Riemann solvers.
Convergence difficulties were encountered in our recent efforts toward a combined aerodynamic-structural optimization of the High Speed Civil Transport (HS-CT). The underlying causes of the convergence problems were t...
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An alternative approach, based on a multiple scale analysis, is presented in order to reconcile the traditional WKB approach to the modeling of interplanetary fluctuations in a mildly inhomogeneous large-scale flow wi...
An alternative approach, based on a multiple scale analysis, is presented in order to reconcile the traditional WKB approach to the modeling of interplanetary fluctuations in a mildly inhomogeneous large-scale flow with a more recently developed transport theory. This enables us to compare directly, at a formal level, the inherent structure of the two models. In the case of noninteracting, incompressible (Alfven) waves, the principle difference between the two models is the presence of leading-order couplings (called ''mixing effects'') in the non-WKB turbulence model which are absent in a WKB development. Within the context of linearized MHD, two cases have been identified for which the leading order non-WKB ''mixing term'' does not vanish at zero wavelength. For these cases the WKB expansion is divergent, whereas the multiple-scale theory is well behaved. We have thus established that the WKB results are contained within the multiple-scale theory, but leading order mixing effects, which are Likely to have important observational consequences, can never be recovered in the WKB style expansion. Properties of the higher-order terms in each expansion are also discussed, leading to the conclusion that the non-WKB hierarchy may be applicable even when the scale separation parameter is not small.
The role of streamwise-spanwise vorticity interactions that occur in turbulent shear flows on flame/vortex interactions is examined by means of asymptotic analysis and numerical simulation in the limit of small Mach n...
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The role of streamwise-spanwise vorticity interactions that occur in turbulent shear flows on flame/vortex interactions is examined by means of asymptotic analysis and numerical simulation in the limit of small Mach number. An idealized model is employed to describe the interaction process. The model consists of a one-step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a streamwise vortex embedded in a shear flow. It is found that the interaction of the streamwise vortex with shear gives rise to small-scale velocity oscillations which increase in magnitude with shear strength. These oscillations give rise to regions of strong temperature gradients via viscous heating, which can lead to multiple ignition points and substantially decrease ignition times. The evolution in time of the temperature and mass-fraction fields is followed, and emphasis is placed on the ignition time and structure as a function of vortex and shear strength.
This paper describes a parallel implementation of the direct simulation Monte Carlo (DSMC) method. Runtime library support is used for scheduling and execution of communication between nodes, and domain decomposition ...
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A new algorithm for identifying vortices in complex flows is presented. The scheme uses both the vorticity and pressure fields. A skeleton line along the center of a vortex is produced by a two-step predictor-correcto...
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ISBN:
(纸本)9780780325210
A new algorithm for identifying vortices in complex flows is presented. The scheme uses both the vorticity and pressure fields. A skeleton line along the center of a vortex is produced by a two-step predictor-corrector scheme. The technique uses the vector field to move in the direction of the skeleton line and the scalar field to correct the location in the plane perpendicular to the skeleton line. With an economical description of the vortex tube's cross-section, the skeleton compresses the representation of the flow by a factor of 4000 or more. We show how the reconstructed geometry of vortex tubes can be enhanced to help visualize helical motion.< >
Lightweight threads are becoming increasingly useful for supporting parallelism and asynchronous control structures in applications and language implementations. However, lightweight thread packages for distributed me...
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ISBN:
(纸本)9780818666056
Lightweight threads are becoming increasingly useful for supporting parallelism and asynchronous control structures in applications and language implementations. However, lightweight thread packages for distributed memory systems have received little attention. We introduce the design of a runtime interface, called Chant, that supports communicating threads in a distributed memory environment. In particular, Chant is layered atop standard message passing and lightweight thread libraries, and supports efficient point-to-point and remote service request communication primitives. We examine the design issues of Chant, the efficiency of its point-to-point communication layer, and the evaluation of scheduling policies to poll for the presence of incoming messages.< >
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