The linear stability of the flow between concentric cylinders, with the inner cylinder rotating at a constant angular velocity
and the outer cylinder with an angular velocity varying harmonically about a zero mean, is addressed. The bifurcations of
the base state are analyzed using Floquet theory, paying particular attention to non-axisymmetric bifurcations which are dominant
in significant regions of parameter space. In these regions the spiral modes of the unforced system become parametrically
excited and dominant. This is typical behavior of parametrically forced extended systems, where some modes are stabilized,
but others are simultaneously excited. The flow structure of the bifurcated states are examined in detail, paying particular
attention to the dynamic implications of their symmetries, and in particular how and when subsequent period doublings are
inhibited.
Received 5 November 2001 and accepted 29 March 2002 Published online: 2 October 2002
RID="*"
ID="*" This work was supported by NSF Grants INT-9732637 and CTS-9908599 (U.S.A.) and MCYT Grants PB97-0685 and BFM2001-2350
(Spain).
Communicated by H.J.S. Fernando 相似文献
Perfluoropolyether (PFPE)‐based thermoplastic fluoropolymers are synthesized by A2 + B2 step‐growth polymerization between PFPE‐diyne and fluorinated diazides. This versatile method allows synthesizing PFPE‐based materials with tunable physicochemical properties depending on the exact nature of the fluorinated segment of the diazide precursor. Semicrystalline or amorphous materials endowed with high thermostability (≈300 °C under air) and low glass transition temperature (≈−100 °C) are obtained, as confirmed by differential scanning calorimetry, thermogravimetry, and rheometry. Step‐growth polymerizations can be copper‐catalyzed but also thermally activated in some cases, thus avoiding the presence of copper residues in the final materials. This strategy opens up new opportunities to easily access PFPE‐based materials on an industrial scale. Furthermore, a plethora of developments can be envisioned (e.g., by adding a third trifunctional component to the formulations for the synthesis of PFPE‐based elastomers).
This paper is dedicated to a study of different extensions of the classical knapsack problem to the case when different elements
of the problem formulation are subject to a degree of uncertainty described by random variables. This brings the knapsack
problem into the realm of stochastic programming. Two different model formulations are proposed, based on the introduction
of probability constraints. The first one is a static quadratic knapsack with a probability constraint on the capacity of
the knapsack. The second one is a two-stage quadratic knapsack model, with recourse, where we introduce a probability constraint
on the capacity of the knapsack in the second stage. As far as we know, this is the first time such a constraint has been
used in a two-stage model. The solution techniques are based on the semidefinite relaxations. This allows for solving large
instances, for which exact methods cannot be used. Numerical experiments on a set of randomly generated instances are discussed
below. 相似文献
Dynamic nuclear polarization (DNP) has made it possible to record 2D double-quantum-filtered (DQF) solid-state NMR (ssNMR) spectra of a signal peptide bound to a lipid-reconstituted SecYEG translocon complex. The small quantity of peptide in the sample (~40 nmol) normally prohibits multidimensional ssNMR experiments. Such small amounts are not the exception, because for samples involving membrane proteins, most of the limited sample space is occupied by lipids. As a consequence, a conventional 2D DQF ssNMR spectrum with the sample used here would require many weeks if not months of measurement time. With the help of DNP, however, we were able to acquire such a 2D spectrum within 20 h. This development opens up new possibilities for membrane protein studies, particularly in the exploitation of high-resolution spectroscopy and the assignment of individual amino acid signals, in this case for a signal peptide bound to the translocon complex. 相似文献
This paper deals with pairs of binary relations defined on the same finite basis and which the 3-element restrictions are isomorphic and those of 5-element restrictions are isomorphic or anti-isomorphic. To each of these pairs, we associate an equivalence relation which yields a decomposition of these relations into classes that we will characterize. As application, we get the treshold of half-reconstruction for tournaments. 相似文献
Among the most frequent protein binding sites served by Mg(II), we identify those which have higher affinity towards Al(III). We also estimate the free energies of metal exchange for all these binding sites taking into account solvent effects explicitly. The obtained results show that thermodynamically favored Mg(II)/Al(III) exchange reactions take place at a number of these metal binding sites, which could possibly be related to some dysfunctions of Mg(II)-dependent biological processes. Additionally, they shed light on the molecular basis of the toxicity of Al(III) in living organisms. 相似文献
A special case of Mahler's conjecture on the volume-product of symmetric convex bodies in n -dimensional Euclidean space is treated here. This is the case of polytopes with at most 2n+2 vertices (or facets). Mahler's conjecture is proved in this case for n≤ 8 and the minimal bodies are characterized.
<lsiheader>
<onlinepub>7 August, 1998
<editor>Editors-in-Chief: &lsilt;a href=../edboard.html#chiefs&lsigt;Jacob E. Goodman, Richard Pollack&lsilt;/a&lsigt;
<pdfname>20n2p163.pdf
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Received May 28, 1996, and in revised form November 7, 1996. 相似文献