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We consider the optimal service control of a multiclass M/G/1 queueing system in which customers are served nonpreemptively and the system cost rate is additive across classes and increasing convex in the numbers present in each class. Following Whittle's approach to a class of restless bandit problems, we develop a Langrangian relaxation of the service control problem which serves to motivate the development of a class of index heuristics. The index for a particular customer class is characterised as a fair charge for service of that class. The paper develops these indices and reports an extensive numerical investigation which exhibits strong performance of the index heuristics for both discounted and average costs. 相似文献
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A method has been developed for the simultaneous extraction and determination of five of the most commonly used anticoccidial drugs (nicarbazin, lasalocid, monensin, narasin and salinomycin) from livers and eggs by LC-MS-MS. Results show good repeatability, with mean spiked recoveries for nicarbazin, lasalocid, monensin, narasin and salinomycin in poultry livers in the average range of 92-118%, and 86-110% in eggs. The detection limit is at 1 ng ml(-1) for all the named compounds and a quantitation level of 2.5 ng g(-1) has been achieved. A high throughput of samples is achievable using this method which allows the analysis of up to 40 samples by one analyst in a day. 相似文献
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The low-dimensional model derived for the wall region of a turbulent boundary layer (Aubry et al., 1988) is applied to a drag-reduced flow. In agreement with some experimental results, drag reduction is modeled by thickening the wall region, which is achieved by applying stretching transformations to the original flow. By application of a Galerkin projection, a set of ordinary differential equations (ODEs) is obtained whose structure is identical to the set corresponding to the unmodified flow. The coefficients of the ODEs are modified in a nontrivial way. The bifurcation diagrams plotted for different values of the stretching parameter are different in detail but the structure is globally the same. In particular, the intermittent behavior which Aubry et al. identified with the cyclic bursting events experimentally observed is still present. The scenario by which intermittency appears through a subcritical Hopf bifurcation in which a heteroclinic cycle is created and disappears through a bifurcation to traveling waves is identical. These results hold for values of the stretching between 1 and 2.65, the value at which the top of the buffer layer reaches the centerline of the pipe. This is in agreement with experimental results for flows whose drag is reduced but which still display intermittency. The bifurcations occur in the stretched flow at increased levels of dissipation (relative to the unstretched flow), consistent with theoretical pictures of drag reduction, in which the increase of scale is due to stabilization by an increase of dissipation in the turbulent part of the flow. Moreover, this method is a systematic way to perturb the coefficients of the ODEs of Aubry et al. (1988). Under this kind of perturbation, the behavior of the solution (in the part of the bifurcation diagram physically relevant) is found to be extremely robust. 相似文献
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Gal Berkooz Philip Holmes J. L. Lumley 《Theoretical and Computational Fluid Dynamics》1993,4(6):255-269
In this paper we establish some rigorous connections between the dynamics of coherent structures in the wall region of the turbulent boundary layer and the low-dimensional models of the type studied by Aubry et al. (1988). An important first step is to determine what sort of connection is feasible. We choose to study the energy budget of the models in comparison with the energy budget of the real flow. This is done by comparing the respective kinetic energy equations. In the process we reexamine some of the assumptions and approximations of Aubry et al. (1988) and perform order of magnitude analyses to determine when they hold. We find that, for the models developed in that paper, involving modes which do not vary in the streamwise direction, the energy production lies within positive, experimentally determined, bounds. Moreover, the dissipation due to neglected modes may be reproduced correctly with an order 1 value of the Heisenberg parameter , as assumed by Aubry et al.
This work was supported by AFOSR F49620-92-J-0287. 相似文献
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A generalization of the eddy viscosity concept permits an extension of the Heisenberg model for spectral energy transfer.
The results are applied to the isotropic homogeneous turbulent flow of an incompressible second order fluid.
This work was supported by the U.S. Office of Naval Research, Fluid Mechanics Branch, under Contract Nonr 656(33) and is taken
from the Ph.D. Dissertation of the first author in the department of Aerospace Engineering, March 1966. 相似文献
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Flow past a rough wall is examined. Calculations are made to find the roughness-induced mean velocity which is expressed in
an integral form in terms of the spectral density of the roughness and an influence function. Values of the influence function
are tabulated using the known values of the modified Hankel functions of order 1/3 and their integrals. The first order change
in lower critical Reynolds number due to the roughness-induced change in profile is calculated; the stability of the profile
is increased due to the presence of roughness.
This work was supported by U.S. Naval Ordnance Systems Command under Contract NOw 65-0123-d through the Garfield Thomas Water
Tunnel, Ordnance Research Laboratory. 相似文献
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A brief, superficial survey of some very personal nominations for highpoints of the last hundred years in turbulence. Some
conclusions can be dimly seen. This field does not appear to have a pyramidal structure, like the best of physics. We have
very few great hypotheses. Most of our experiments are exploratory experiments. What does this mean?
We believe it means that, even after 100 years, turbulence studies are still in their infancy. We are naturalists, observing
butterflies in the wild. We are still discovering how turbulence behaves, in many respects. We do have a crude, practical,
working understanding of many turbulence phenomena but certainly nothing approaching a comprehensive theory, and nothing that
will provide predictions of an accuracy demanded by designers.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献