Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

This article reviews stability and laminar-turbulent transition in high-speed boundary-layer flows, emphasizing qualitative features of the disturbance spectrum leading to new mechanisms of receptivity and instability. It is shown that the extension of subsonic and low-supersonic stability concepts and transition prediction methods to hypersonic speeds is not straightforward. The discussion focuses on theoretical models providing insights into the physics of instability and helping make proper decisions on transition control strategies.

Transition and Stability of High-Speed Boundary Layers

Progress in shock wave/boundary layer interactions

Conventional hypersonic wind tunnels and shock tunnels sufier from high levels of freestream ∞uctuations which are typically one to two orders of magnitude above ∞ight levels. These freestream ∞uctuations are generally dominated by acoustic noise radiated from the turbulent boundary layers on the nozzle walls. Although the efiects of the noise are often small, and so can be neglected, this noise often has a dramatic efiect on laminar-turbulent transition on models, and it can have an signiflcant efiect on other phenomena as well. Quiet-∞ow hypersonic tunnels with low noise levels comparable to ∞ight have therefore been sought for more than 50 years. The development of these tunnels is reviewed. The largest efiort was carried out at NASA Langley from the late 1960’s through the middle 1990’s, and resulted in successful quiet tunnels at Mach 3.5 and 6, although the Mach 6 tunnel was later decommissioned. The effort was then carried forward at Purdue University from 1990 through the present, leading to the nowsuccessful Boeing/AFOSR Mach-6 Quiet Tunnel.

Development of Hypersonic Quiet Tunnels

The field of aerothermoelasticity plays an important role in the analysis and optimization of airbreathing hypersonic vehicles, impacting the design of the aerodynamic, structural, control, and propulsion systems at both the component and multi-disciplinary levels. This study aims to expand the fundamental understanding of hypersonic aerothermoelasticity by performing systematic investigations into fluid-thermal-structural coupling, and also to develop frameworks, using innovative modeling strategies, for reducing the computational effort associated with aerothermoelastic analysis. Due to the fundamental nature of this work, the analysis is limited to cylindrical bending of a simply-supported, von K arm an panel. Multiple important effects are included in the analysis, namely: 1) arbitrary, nonuniform, in-plane and through-thickness temperature distributions, 2) material property degradation at elevated temperature, and 3) the effect of elastic deformation on aerodynamic heating. It is found that including elastic deformations in the aerodynamic heating computations results in non-uniform heat flux, which produces non-uniform temperature distributions and non-uniform material property degradations. This results in reduced flight time to the onset of flutter and localized regions in which the material temperature limits may be exceeded. Additionally, the trade-off between computational cost and accuracy is evaluated for aerothermoelastic analysis based on either quasi-static or time-averaged dynamic fluid-thermal-structural coupling, as well as computational fluid dynamics based reduced-order modeling of the aerodynamic heat flux. It is determined that these approaches offer the potential for significant improvements in aerothermoelastic modeling in terms of efficiency and/or accuracy.

Studies on Fluid-Thermal-Structural Coupling for Aerothermoelasticity in Hypersonic Flow

Ground Test Studies of the HIFiRE-1 Transition Experiment Part 1: Experimental Results

Ground Test Studies of the HIFiRE-1 Transition Experiment Part 2: Computational Analysis

Finite-Rate Surface Chemistry Model, I: Formulation and Reaction System Examples

Finite-Rate Surface Chemistry Model, II: Coupling to Viscous Navier-Stokes Code

Boundary-layer stability analysis is performed by computational fluid dynamics simulation of experiments conducted in theCalspan–University at BuffaloResearchCenter Large EnergyNational ShockTunnel in support of the first flight of the Hypersonic International Flight Research Experimentation program From the laminar flow solutions, disturbances are calculated using the linear parabolized stability equations method and instability is quantified by integrating the resulting disturbance growth rates Comparisons aremade between the experimentally measured transition locations and the results of the parabolized stability equations analysis The results show that for the cases tested, the e transition correlation works better than the commonly usedRe =Me engineering criterion for predicting the onset of boundary-layer transition from laminar to turbulent flow

Boundary-Layer Stability Analysis of the Hypersonic International Flight Research Transition Experiments

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