Viscous flows in rotating pipes with internal cavities and related vortex breakdowns

The problem of predicting vortex breakdown, which is characterized by a drastic change in core size and trajectory, is still basically unsolved. In the present paper, viscous flow in a long rotating pipe is analyzed, as a model for the type of breakdown observed in wake vortices. The full Navier-Stokes equations are shown to simplify to a set of unlinked partial differential equations. These equations allow for two separate solutions, for both full pipe flows (the classic rotating Hagen-Poiseuille flow) and cavity flows, in which a central area, with no net mass flux or viscous dissipation, is produced. Conditions permitting switching from one type of flow to the other, with no change in the mass flux or velocities at the pipe surface, are found. It is then argued that these conditions indicate when vortex breakdown will occur

Gas flows through shallow T-junctions and parallel microchannel networks

We apply a recent extension of the Hele-Shaw scheme to analyze steady compressible viscous flows through micro T-junctions. The linearity of the problem in terms of an appropriately defined quadratic form of the pressure facilitates the definition of the viscous resistance of the configuration, relating the gas mass-flow rate to entrance and exit conditions. Furthermore, under rather mild restrictions, the performance of complex microchannel networks may be estimated through superposition of the contributions of multiple basic junction elements. This procedure is applied to an optimization model problem of a parallel microchannel network. The analysis and results are readily adaptable to incompressible flows

Intermittent Gliding in the Hunting Flight of the Kestrel, Falco tinnunculus L.

The hunting flight of the kestrel (Falco tinnunculus) consists of short bouts of flight at wind speed against the wind with the eyes in a fixed position relative to the ground, and of short flights from one such position to the next. High speed films taken with a camera in a fixed position of a hunting kestrel of known weight and dimensions, allow estimates to be made of the amount of energy required for this behaviour. A theoretical model shows how a bird could economise by alternating flapping flight with short gliding bouts, without changing the position of the eyes above the ground, by mere displacement of the centre of gravity relative to the head. High speed film data confirm predictions from this model.

Parametric down-conversion in photonic crystal waveguides

Photonic crystals create dramatic new possibilities for nonlinear optics. Line defects are shown to support modes suitable for the production of pairs of photons by the material's second order nonlinearity even if the phase-matching conditions cannot be satisfied in the bulk. These structures offer the flexibility to achieve specific dispersion characteristics and potentially very high brightness. In this work, two phase matching schemes are identified and analyzed regarding their dispersive properties.Comment: Due to a miscommunication the unrevised version of this paper was published instead of this one. v2 has quite some modifications and corrections. 7 pages, 4 figure

Comment on "Exclusion of time in the theorem of Bell" by K. Hess and W. Philipp

A recent Letter by Hess and Philipp claims that Bell's theorem neglects the possibility of time-like dependence in local hidden variables, hence is not conclusive. Moreover the authors claim that they have constructed, in an earlier paper, a local realistic model of the EPR correlations. However, they themselves have neglected the experimenter's freedom to choose settings, while on the other hand, Bell's theorem can be formulated to cope with time-like dependence. This in itself proves that their toy model cannot satisfy local realism, but we also indicate where their proof of its local realistic nature fails.Comment: Latex needs epl.cl

Comparing the states of many quantum systems

We investigate how to determine whether the states of a set of quantum systems are identical or not. This paper treats both error-free comparison, and comparison where errors in the result are allowed. Error-free comparison means that we aim to obtain definite answers, which are known to be correct, as often as possible. In general, we will have to accept also inconclusive results, giving no information. To obtain a definite answer that the states of the systems are not identical is always possible, whereas, in the situation considered here, a definite answer that they are identical will not be possible. The optimal universal error-free comparison strategy is a projection onto the totally symmetric and the different non-symmetric subspaces, invariant under permutations and unitary transformations. We also show how to construct optimal comparison strategies when allowing for some errors in the result, minimising either the error probability, or the average cost of making an error. We point out that it is possible to realise universal error-free comparison strategies using only linear elements and particle detectors, albeit with less than ideal efficiency. Also minimum-error and minimum-cost strategies may sometimes be realised in this way. This is of great significance for practical applications of quantum comparison.Comment: 13 pages, 2 figures. Corrected a misprint on p. 7 and added a few references. Accepted for publication in J Mod Op