27 research outputs found
Capillary-gravity wave resistance in ordinary and magnetic fluids
Wave resistance is the drag force associated to the emission of waves by a
moving disturbance at a fluid free surface. In the case of capillary-gravity
waves it undergoes a transition from zero to a finite value as the speed of the
disturbance is increased. For the first time an experiment is designed in order
to obtain the wave resistance as a function of speed. The effect of viscosity
is explored, and a magnetic fluid is used to extend the available range of
critical speeds. The threshold values are in good agreement with the proposed
theory. Contrary to the theoretical model, however, the measured wave
resistance reveals a non monotonic speed dependence after the threshold.Comment: 12 pages, 4 figures, 1 table, submitted to Physical Review Letter
Capillary-Gravity Waves on Depth-Dependent Currents: Consequences for the Wave Resistance
We study theoretically the capillary-gravity waves created at the water-air
interface by a small two-dimensional perturbation when a depth-dependent
current is initially present in the fluid. Assuming linear wave theory, we
derive a general expression of the wave resistance experienced by the
perturbation as a function of the current profile in the case of an inviscid
fluid. We then analyze and discuss in details the behavior of the wave
resistance in the particular case of a linear current, a valid approximation
for some wind generated currents.Comment: Submitted to EP
Capillary-gravity waves: The effect of viscosity on the wave resistance
The effect of viscosity on the wave resistance experienced by a 2d
perturbation moving at uniform velocity over the free surface of a fluid is
investigated. The analysis is based on Rayleigh's linearized theory of
capillary-gravity waves. It is shown in particular that the wave resistance
remains bounded as the velocity of the perturbation approches the minimun phase
speed, unlike what is predicted by the inviscid theory.Comment: Europhysics Letters, in pres
Onset of Wave Drag due to Generation of Capillary-Gravity Waves by a Moving Object as a Critical Phenomenon
The onset of the {\em wave resistance}, via generation of capillary gravity
waves, of a small object moving with velocity , is investigated
experimentally. Due to the existence of a minimum phase velocity for
surface waves, the problem is similar to the generation of rotons in superfluid
helium near their minimum. In both cases waves or rotons are produced at
due to {\em Cherenkov radiation}. We find that the transition to the
wave drag state is continuous: in the vicinity of the bifurcation the wave
resistance force is proportional to for various fluids.Comment: 4 pages, 7 figure
Enhanced Angular Momentum Transport in Accretion Disks
The status of our current understanding of angular momentum transport in
accretion disks is reviewed. The last decade has seen a dramatic increase both
in the recognition of key physical processes and in our ability to carry
through direct numerical simulations of turbulent flow. Magnetic fields have at
once powerful and subtle influences on the behavior of (sufficiently) ionized
gas, rendering them directly unstable to free energy gradients. Outwardly
decreasing angular velocity profiles are unstable. The breakdown of Keplerian
rotation into MHD turbulence may be studied in some numerical detail, and key
transport coefficients may be evaluated. Chandra observations of the Galactic
Center support the existence of low luminosity accretion, which may ultimately
prove amenable to global three-dimensional numerical simulation.Comment: 43 pages, 2 figures, to appear v.43 A.R.A.A. October 200
Utility of whole-genome sequencing during an investigation of multiple foodborne outbreaks of Shigella sonnei
In April 2018, Public Health England was notified of cases of Shigella sonnei who had eaten food from three different catering outlets in England. The outbreaks were initially investigated as separate events, but whole-genome sequencing (WGS) showed they were caused by the same strain. The investigation included analyses of epidemiological data, the food chain and microbiological examination of food samples. WGS was used to determine the phylogenetic relatedness and antimicrobial resistance profile of the outbreak strain. Ultimately, 33 cases were linked to this outbreak; the majority had eaten food from seven outlets specialising in Indian or Middle Eastern cuisine. Five outlets were linked to two or more cases, all of which used fresh coriander although a shared supplier was not identified. An investigation at one of the venues recorded that 86% of cases reported eating dishes with coriander as an ingredient or garnish. Four cases were admitted to hospital and one had evidence of treatment failure with ciprofloxacin. Phylogenetic analysis showed that the outbreak strain was part of a wider multidrug-resistant clade associated with travel to Pakistan. Poor hygiene practices during cultivation, distribution or preparation of fresh produce are likely contributing factors
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie