Pattern selection in the absolutely unstable regime as a nonlinear eigenvalue problem: Taylor vortices in axial flow

A unique pattern selection in the absolutely unstable regime of a driven, nonlinear, open-flow system is analyzed: The spatiotemporal structures of rotationally symmetric vortices that propagate downstream in the annulus of the rotating Taylor-Couette system due to an externally imposed axial through-flow are investigated for two different axial boundary conditions at the in- and outlet. Unlike the stationary patterns in systems without through-flow the spatiotemporal structures of propagating vortices are independent of parameter history, initial conditions, and system's length. They do, however, depend on the axial boundary conditions, the driving rate of the inner cylinder and the through-flow rate. Our analysis of the amplitude equation shows that the pattern selection can be described by a nonlinear eigenvalue problem with the frequency being the eigenvalue. Approaching the border between absolute and convective instability the eigenvalue problem becomes effectively linear and the selection mechanism approaches that one of linear front propagation. PACS:47.54.+r,47.20.Ky,47.32.-y,47.20.FtComment: 15 pages (LateX-file), 8 figures (Postscript

Equivalences between localisations of categories provided by replacements

We give a characterisation of functors whose induced functor on the level of localisations is an equivalence and where the isomorphism inverse is induced by some kind of replacements such as projective resolutions or cofibrant replacements

Investigation of the drag reducing effect of hydrophobized sand on cylinders

Superhydrophobic surfaces show strong potential for drag reducing applications. If such a surface supports a Cassie-Baxter state with low solid surface fraction and when immersed it retains a plastron air layer, large slip can occur across its surface as well as a consequent reduction in drag. In this work we report a facile method for creating hydrophobic cylinders and hydrophobic flat surfaces with varying surface roughness able to support a Cassie-Baxter state. Cylinders of 12mm diameter were coated in hydrophobized sand with grain sizes in the ranges of 50-100, 212-300, 425-600 and 600-710 mu m to produce the varying degrees of roughness. A laser Doppler anemometer was used to measure the velocity profile of the water across their wake in a large water circulating flow chamber. The hydrophobic cylinders in the Cassie-Baxter state show drag reductions of up to 28% compared to the same sample in the Wenzel state for flows with Reynolds numbers of 10000 to 40000. These drag reduction results, in combination with confocal microscopy images of the plastron air layer and feature height, show that the thickness of the plastron and the protrusion height of the features combine to give a drag reduction or drag increase depending on the ratio of the two

Kup (Peter). A history of Sierra Leone, 1400-1787

Fage J. D. Kup (Peter). A history of Sierra Leone, 1400-1787. In: Revue belge de philologie et d'histoire, tome 40, fasc. 4, 1962. pp. 1377-1379

Kup (Peter). A history of Sierra Leone, 1400-1787

Fage J. D. Kup (Peter). A history of Sierra Leone, 1400-1787. In: Revue belge de philologie et d'histoire, tome 40, fasc. 4, 1962. pp. 1377-1379