7,615 research outputs found
An experimental investigation of the recirculation zone formed downstream of a forward facing step
An experimental investigation of the recirculation zone formed downstream of a forward facing step immersed in a turbulent boundary layer has been undertaken using particle image velocimetry. Bluff body flow is observed with the fixed separation point located at the leading edge of the step. The recirculation region dimensions are characterised over a range of Reynolds numbers (1400–19 000), with Reh based on the step height and the free stream velocity. Turbulent perturbations are produced in the free shear layer which develops between the recirculating flow close to the step and the free stream flow. Contour maps of amplification factor, streamwise perturbation velocity and Reynolds stresses are constructed, providing insight into optimal placement of structures within such topographical features. The mechanisms affecting the reattachment distance, namely the turbulent mixing within the boundary layer and the velocity deficit in the boundary layer, are discussed
Characterisation of a horizontal axis wind turbine’s tip and root vortices
The vortical near wake of a model horizontal
axis wind turbine has been investigated experimentally in a
water channel. The objective of this work is to study vortex
interaction and stability of the helical vortex filaments
within a horizontal axis wind turbine wake. The experimental model is a geometrically scaled version of the
Tjæreborg wind turbine, which existed in western Denmark
in the late 1980s. Here, the turbine was tested in both the
upwind and downwind configurations. Qualitative flow
visualisations using hydrogen bubble, particle streakline
and planar laser-induced fluorescence techniques were
combined with quantitative data measurements taken using
planar particle image velocimetry. Vortices were identified
using velocity gradient tensor invariants. Parameters that
describe the helical vortex wake, such as the helicoidal
pitch, and vortex circulation, were determined for three tip
speed ratios. Particular attention is given here to the root
vortex, which has been investigated minimally to date.
Signatures of the coherent tip vortices are seen throughout
the measurement domain; however, the signature of the
root vortex is only evident much closer to the rotor plane,
irrespective of the turbine configuration. It is postulated
that the root vortex diffuses rapidly due to the effects of the
turbine support geometries
On-line Ramsey numbers of paths and cycles
Consider a game played on the edge set of the infinite clique by two players,
Builder and Painter. In each round, Builder chooses an edge and Painter colours
it red or blue. Builder wins by creating either a red copy of or a blue
copy of for some fixed graphs and . The minimum number of rounds
within which Builder can win, assuming both players play perfectly, is the
on-line Ramsey number . In this paper, we consider the case
where is a path . We prove that for all , and determine
) up to an additive constant for all .
We also prove some general lower bounds for on-line Ramsey numbers of the form
.Comment: Preprin
Complementarity in Wormhole Chromodynamics
The electric charge of a wormhole mouth and the magnetic flux ``linked'' by
the wormhole are non-commuting observables, and so cannot be simultaneously
diagonalized. We use this observation to resolve some puzzles in wormhole
electrodynamics and chromodynamics. Specifically, we analyze the color electric
field that results when a colored object traverses a wormhole, and we discuss
the measurement of the wormhole charge and flux using Aharonov-Bohm
interference effects. We suggest that wormhole mouths may obey conventional
quantum statistics, contrary to a recent proposal by Strominger.Comment: 12 pages and 2 figures, phyzzx, CALT-68-188
Vortex-induced vibration of a square-section cylinder with incidence angle variation
Vortex-induced vibration (VIV) occurs when vortex shedding from a body results in fluctuating forces that, in turn, cause the body to vibrate. This can result in undesired large-amplitude vibrations leading to structural damage or catastrophic failure. While much has been done on the VIV of a circular cylinder less has been done on other canonical bluff bodies, such as rectangular cylinders. The present experimental work studied the VIV of a square cross-section cylinder in a water channel, with three different incidence angles (α 15 , 30 and 45 ). The influence of geometry on the body’s oscillation amplitude and frequency response, and its wake structure over a range of reduced velocity were investigated. The oscillations were measured at a low mass damping ratio of m ζ 0.013 , which was comparable to the circular cylinder system with m ζ 0.013 studied by Khalak & Williamson (1997)1. The comparison showed that the incidence angle change had a significant impact on amplitude response. For α 15 the maximum non-dimensional amplitude was A 1.11, 10% larger than the circular case, as shown in figure 1 (a). Asymmetric amplitudes with respect to the cylinder’s equilibrium position in still water were observed in α 15 and 30 cases, due to the one-sided nature of the mean lift force. Compared with the circular cyinder, the square cylinder locked on to the structural natural frequency in water over a smaller reduced velocity regime. An oscillation frequency drop was found in the α 15 case, during which the cylinder experienced its largest amplitude response. The wake structure for each case in the different flow regimes was determined using particle image velocimetry and will also be presented
The interaction between flow-induced vibration mechanisms of a square cylinder with varying angles of attack
This study examines the influence of angle of attack of a square section cylinder on the cylinder’s flow-induced vibration, where the direction of the vibration is transverse to the oncoming flow. Our experiments, which traversed the velocity–angle of attack parameter space in considerable breadth and depth, show that a low-mass ratio body can undergo combinations of both vortex-induced vibration and galloping. When the body has an angle of attack that makes it symmetric to the flow, such as when it assumes the square or diamond orientation, the two mechanisms remain independent. However, when symmetry is lost we find a mixed mode response with a new branch of vortex-induced oscillations that exceeds the amplitudes resulting from the two phenomena independently. The oscillations of this higher branch have amplitudes larger than the ‘upper branch’ of vortex-induced vibrations and at half the frequency. For velocities above this resonant region, the frequency splits into two diverging branches. Analysis of the amplitude response reveals that the transition between galloping and vortex-induced vibrations occurs over a narrow range of angle of incidence. Despite the rich set of states found in the parameter space the vortex shedding modes remain very similar to those found previously in vortex-induced vibration
Security of quantum key distribution with imperfect devices
We prove the security of the Bennett-Brassard (BB84) quantum key distribution
protocol in the case where the source and detector are under the limited
control of an adversary. Our proof applies when both the source and the
detector have small basis-dependent flaws, as is typical in practical
implementations of the protocol. We derive a general lower bound on the
asymptotic key generation rate for weakly basis-dependent eavesdropping
attacks, and also estimate the rate in some special cases: sources that emit
weak coherent states with random phases, detectors with basis-dependent
efficiency, and misaligned sources and detectors.Comment: 22 pages. (v3): Minor changes. (v2): Extensively revised and
expanded. New results include a security proof for generic small flaws in the
source and the detecto
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