24,576 research outputs found
Direct simulation of a turbulent oscillating boundary layer
The turbulent boundary layer driven by a freestream velocity that varies sinusoidally in time around a zero mean is considered. The flow has a rich behavior including strong pressure gradients, inflection points, and reversal. A theory for the velocity and stress profiles at high Reynolds number is formulated. Well-resolved direct Navier-Stokes simulations are conducted over a narrow range of Reynolds numbers, and the results are compared with the theoretical predictions. The flow is also computed over a wide range of Reynolds numbers using a new algebraic turbulence model; the results are compared with the direct simulations and the theory
Improving Performance of Iterative Methods by Lossy Checkponting
Iterative methods are commonly used approaches to solve large, sparse linear
systems, which are fundamental operations for many modern scientific
simulations. When the large-scale iterative methods are running with a large
number of ranks in parallel, they have to checkpoint the dynamic variables
periodically in case of unavoidable fail-stop errors, requiring fast I/O
systems and large storage space. To this end, significantly reducing the
checkpointing overhead is critical to improving the overall performance of
iterative methods. Our contribution is fourfold. (1) We propose a novel lossy
checkpointing scheme that can significantly improve the checkpointing
performance of iterative methods by leveraging lossy compressors. (2) We
formulate a lossy checkpointing performance model and derive theoretically an
upper bound for the extra number of iterations caused by the distortion of data
in lossy checkpoints, in order to guarantee the performance improvement under
the lossy checkpointing scheme. (3) We analyze the impact of lossy
checkpointing (i.e., extra number of iterations caused by lossy checkpointing
files) for multiple types of iterative methods. (4)We evaluate the lossy
checkpointing scheme with optimal checkpointing intervals on a high-performance
computing environment with 2,048 cores, using a well-known scientific
computation package PETSc and a state-of-the-art checkpoint/restart toolkit.
Experiments show that our optimized lossy checkpointing scheme can
significantly reduce the fault tolerance overhead for iterative methods by
23%~70% compared with traditional checkpointing and 20%~58% compared with
lossless-compressed checkpointing, in the presence of system failures.Comment: 14 pages, 10 figures, HPDC'1
A study to define an in-flight dynamics measurement and data applications program for space shuttle payloads
Data measurement and interpretation techniques were defined for application to the first few space shuttle flights, so that the dynamic environment could be sufficiently well established to be used to reduce the cost of future payloads through more efficient design and environmental test techniques. It was concluded that: (1) initial payloads must be given comprehensive instrumentation coverage to obtain detailed definition of acoustics, vibration, and interface loads, (2) analytical models of selected initial payloads must be developed and verified by modal surveys and flight measurements, (3) acoustic tests should be performed on initial payloads to establish realistic test criteria for components and experiments in order to minimize unrealistic failures and retest requirements, (4) permanent data banks should be set up to establish statistical confidence in the data to be used, (5) a more unified design/test specification philosophy is needed, (6) additional work is needed to establish a practical testing technique for simulation of vehicle transients
Perturbative Effective Theory in an Oscillator Basis?
The effective interaction/operator problem in nuclear physics is believed to
be highly nonperturbative, requiring extended high-momentum spaces for accurate
solution. We trace this to difficulties that arise at both short and long
distances when the included space is defined in terms of a basis of harmonic
oscillator Slater determinants. We show, in the simplest case of the deuteron,
that both difficulties can be circumvented, yielding highly perturbative
results in the potential even for modest (~6hw) included spaces.Comment: 10 pages, 4 figure
Viking dynamics experience with application to future payload design
Analytical and test techniques are discussed. Areas in which hindsight indicated erroneous, redundant, or unnecessarily severe design and test specifications are identified. Recommendations are made for improvements in the dynamic design and criteria philosophy, aimed at reducing costs for payloads
Building Blocks in the Economics of Mandates
The paper constructs an asymmetric information model to investigate the efficiency and equity cases for government mandated benefits. A mandate can improve workers' insurance, and may also redistribute in favour of more "deserving" workers. The risk is that it may also reduce output. The more diverse are free market contracts – separating the various worker types – the more likely it is that such output effects will on balance serve to reduce welfare. It is shown that adverse effects can be reduced by restricting mandates to larger firms. An alternative to a mandate is direct government provision. We demonstrate that direct government provision has the advantage over mandates of preserving separations.asymmetric information, labour mandates, compensation packages
Extrapolation Method for the No-Core Shell Model
Nuclear many-body calculations are computationally demanding. An estimate of
their accuracy is often hampered by the limited amount of computational
resources even on present-day supercomputers. We provide an extrapolation
method based on perturbation theory, so that the binding energy of a large
basis-space calculation can be estimated without diagonalizing the Hamiltonian
in this space. The extrapolation method is tested for 3H and 6Li nuclei. It
will extend our computational abilities significantly and allow for reliable
error estimates.Comment: 8 pages, 7 figures, PRC accepte
- …