2,793 research outputs found
Three-dimensional flow structures and vorticity control in fish-like swimming
We employ a three-dimensional, nonlinear inviscid numerical method, in conjunction
with experimental data from live fish and from a fish-like robotic mechanism, to
establish the three-dimensional features of the flow around a fish-like body swimming
in a straight line, and to identify the principal mechanisms of vorticity control
employed in fish-like swimming. The computations contain no structural model for
the fish and hence no recoil correction. First, we show the near-body flow structure
produced by the travelling-wave undulations of the bodies of a tuna and a giant
danio. As revealed in cross-sectional planes, for tuna the flow contains dominant
features resembling the flow around a two-dimensional oscillating plate over most
of the length of the fish body. For the giant danio, on the other hand, a mixed
longitudinal-transverse structure appears along the hind part of the body. We also
investigate the interaction of the body-generated vortices with the oscillating caudal
fin and with tail-generated vorticity. Two distinct vorticity interaction modes are
identified: the first mode results in high thrust and is generated by constructive
pairing of body-generated vorticity with same-sign tail-generated vorticity, resulting
in the formation of a strong thrust wake; the second corresponds to high propulsive
efficiency and is generated by destructive pairing of body-generated vorticity with
opposite-sign tail-generated vorticity, resulting in the formation of a weak thrust
wake
Mechanics of nonlinear short-wave generation by a moored near-surface buoy
We consider the nonlinear interaction problem of surface waves with a tethered
near-surface buoy. Our objective is to investigate mechanisms for nonlinear short
surface wave generation in this complete coupled wave-buoy-cable dynamical system.
We develop an effective numerical simulation capability coupling an efficient and
high-resolution high-order spectral method for the nonlinear wave-buoy interaction
problem with a robust implicit finite-difference method for the cable-buoy dynamics.
The numerical scheme accounts for nonlinear wave-wave and wave-body interactions
up to an arbitrary high order in the wave steepness and is able to treat extreme motions
of the cable including conditions of negative cable tension. Systematic simulations
show that beyond a small threshold value of the incident wave amplitude, the buoy
performs chaotic motions, characterized by the snapping of the cable. The root
cause of the chaotic response is the interplay between the snapping of the cable
and the generation of surface waves, which provides a source of strong (radiation)
damping. As a result of this interaction, the chaotic buoy motion switches between
two competing modes of snapping response: one with larger average peak amplitude
and lower characteristic frequency, and the other with smaller amplitude and higher
frequency. The generated high-harmonic/short surface waves are greatly amplified
once the chaotic motion sets in. Analyses of the radiated wave spectra show significant
energy at higher frequencies which is orders of magnitude larger than can be expected
from nonlinear generation under regular motion
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Modelling and experimental evidence of the cathode erosion in a plasma spray torch
The lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan-Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values
Localized boundary-domain singular integral equations based on harmonic parametrix for divergence-form elliptic PDEs with variable matrix coefficients
This is the post-print version of the Article. The official publised version can be accessed from the links below. Copyright @ 2013 Springer BaselEmploying the localized integral potentials associated with the Laplace operator, the Dirichlet, Neumann and Robin boundary value problems for general variable-coefficient divergence-form second-order elliptic partial differential equations are reduced to some systems of localized boundary-domain singular integral equations. Equivalence of the integral equations systems to the original boundary value problems is proved. It is established that the corresponding localized boundary-domain integral operators belong to the Boutet de Monvel algebra of pseudo-differential operators. Applying the Vishik-Eskin theory based on the factorization method, the Fredholm properties and invertibility of the operators are proved in appropriate Sobolev spaces.This research was supported by the grant EP/H020497/1: "Mathematical Analysis of Localized Boundary-Domain Integral Equations for Variable-Coefficient Boundary Value Problems" from the EPSRC, UK
Pleiotrophin is a key switch of human monocytes and bone marrow stem cells into endotherlial-like cells for angiogenesis in tumor growth
Perturbative Formulation and Non-adiabatic Corrections in Adiabatic Quantum Computing Schemes
Adiabatic limit is the presumption of the adiabatic geometric quantum
computation and of the adiabatic quantum algorithm. But in reality, the
variation speed of the Hamiltonian is finite. Here we develop a general
formulation of adiabatic quantum computing, which accurately describes the
evolution of the quantum state in a perturbative way, in which the adiabatic
limit is the zeroth-order approximation. As an application of this formulation,
non-adiabatic correction or error is estimated for several physical
implementations of the adiabatic geometric gates. A quantum computing process
consisting of many adiabatic gate operations is considered, for which the total
non-adiabatic error is found to be about the sum of those of all the gates.
This is a useful constraint on the computational power. The formalism is also
briefly applied to the adiabatic quantum algorithm.Comment: 5 pages, revtex. some references adde
Are Prices Sticky in Large Developing Economies? An Empirical Comparison of China and India
This paper compares the role of macroeconomic and sector-specific factors in price movements for China and India, taking into account the features unique to developing economies. We find that fluctuations in the aggregated prices in China are more persistent than the underlying disaggregated prices. Compared to China, prices in India respond more promptly to macroeconomic and monetary policy shocks. We also show that the urban CPI in China responds more sharply than rural CPI when facing sector-specific shocks, while the opposite is true for India
Are Prices Sticky in Large Developing Economies? An Empirical Comparison of China and India
This paper compares the role of macroeconomic and sector-specific factors in price movements for China and India, taking into account the features unique to developing economies. We find that fluctuations in the aggregated prices in China are more persistent than the underlying disaggregated prices. Compared to China, prices in India respond more promptly to macroeconomic and monetary policy shocks. We also show that the urban CPI in China responds more sharply than rural CPI when facing sector-specific shocks, while the opposite is true for India
Connecting Berry's phase and the pumped charge in a Cooper pair pump
The properties of the tunnelling-charging Hamiltonian of a Cooper pair pump
are well understood in the regime of weak and intermediate Josephson coupling,
i.e. when . It is also known that
Berry's phase is related to the pumped charge induced by the adiabatical
variation of the eigenstates. We show explicitly that pumped charge in Cooper
pair pump can be understood as a partial derivative of Berry's phase with
respect to the phase difference across the array. The phase fluctuations
always present in real experiments can also be taken into account, although
only approximately. Thus the measurement of the pumped current gives reliable,
yet indirect, information on Berry's phase. As closing remarks, we give the
differential relation between Berry's phase and the pumped charge, and state
that the mathematical results are valid for any observable expressible as a
partial derivative of the Hamiltonian.Comment: 5 pages, 5 figures, RevTeX, Presentation has been clarifie
A microsatellite marker for yellow rust resistance in wheat
Bulk segregant analysis (BSA) was used to identify molecular markers associated with yellow rust disease resistance in wheat (Triticum aestivum L.). DNAs isolated from the selected yellow rust tolerant and susceptible F-2 individuals derived from a cross between yellow rust resistant and susceptible wheat genotypes were used to established a "tolerant" and a "susceptible" DNA pool. The BSA was then performed on these DNA pools using 230 markers that were previously mapped onto the individual wheat chromosomes. One of the SSR markers (Xgwm382) located on chromosome group 2 (A, B, D genomes) was present in the resistant parent and the resistant bulk but not in the susceptible parent and the susceptible bulk, suggesting that this marker is linked to a yellow rust resistance gene. The presence of Xgwm382 was also tested in 108 additional wheat genotypes differing in yellow rust resistance. This analysis showed that 81% of the wheat genotypes known to be yellow rust resistant had the Xgwm382 marker, further suggesting that the presence of this marker correlates with yellow rust resistance in diverse wheat germplasm. Therefore, Xgwm382 could be useful for marker assisted selection of yellow rust resistances genotypes in wheat breeding programs
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