223,159 research outputs found

    Hierarchical Learning Approach for One-shot Action Imitation in Humanoid Robots

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    Long Waves in Ocean and Coastal Waters

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    Water waves occurring in the ocean have a wide spectrum of wavelength and period, ranging from capillary waves of 1 cm or shorter wavelength to long waves with wavelength being large compared to ocean depth, anywhere from tens to thousands of kilometers. Of the various long-wavegenic sources, distant body forces can act as the continuous ponderomotive force for the tides. Hurricanes and storms in the sea can develop a sea state, with the waves being worked on by winds and eventually cascading down to swells after a long distance of travel away from their birthplace. Large tsunamis can be ascribed to a rapidly occurring tectonic displacement of the ocean floor (usually near the coast of the Pacific Ocean) over a large horizontal dimension (of hundreds to over a thousand square kilometers) during strong earthquakes, causing vertical displacements to ocean floor of tens of meters. Other generation mechanisms include underwater subsidence or land avalanche in the ocean and submarine volcanic eruption. Gigantic rockfalls and long-period seismic waves can also produce gravity waves in lakes, reservoirs, and rivers. Generation, propagation, and evolution of such long waves in the ocean and their effects in coastal waters and harbors is a subject of increasing importance in civil, coastal, and environmental engineering and science. Of the various long wave phenomena, tsunami appears to stand out in possessing a broad variation of wave characteristics and scaling parameters on the one hand, and, on the other, in having the capacity of inflicting a disastrous effect on the target area. In taking tsunamis as a representative case for the study of long waves in the ocean, it can be said that large tsunamis are generated with a great source of potential energy (as high as 10^15-10^16J ), though the detailed source motion of a specific tsunami is generally difficult to determine. The large size of source region implies that the "new born" waves would be initially long and the energy contained in the large wave-number part (k, nondimensionalized with respect to the local ocean depth, h) would be unimportant. Soon after leaving the source region, the low wave-number components of the source spectrum are further dispersed effectively by the factor sech kh into the even lower wave-number parts. Tsunamis thus evolve into a train of long waves, with wavelength continually increasing from about 50 km to as high as 250 km, but with a quite small amplitude, typically of 1/2 m or smaller, as they travel across the Pacific Ocean at a speed of 650 km/h-760 km/h. There is experimental evidence indicating that tsunamis continually, though slowly, evolve due to dispersion while propagating in the open ocean; this property has been observed by Van Dorn (16) from the data taken at Wake Island of the March 9, 1957 Aleutian tsunami. One of our primary interests is, of course, the evolution of tsumanis in coastal waters and their terminal effects. Large tsunamis can have their wave height amplified many fold in climbing up the continental slope and propagating into shallower water, producing devastating waves (up to 20 m or higher on record) upon arriving at a beach. The terminal amplification can be crucially affected by three-dimensional configurations of the coastal environment enroute to beach. These factors dictate the transmission, reflection, rate of growth, and trapping of tsunamis in their terminal stage. After the first hit on target, a tsunami is partly reflected to travel once over across the Pacific Ocean, with some degree of attenuation -- a process which is still unclear, but is generally known to be small. Based on observations, Munk (13) suggests the figure of the "decay time" (intensity reducing to 1/e) being about 112 day, and the "reverberation time" (intensity falling off to 10^-6) about a week, while the reflection frequency (across the Pacific) is around 1.7/day. To fix idea, the pertinent physical characteristics and their scaling parameters of a tsunami through its life span of evolution can be described qualitatively in Table I. From the aforementioned estimate we note that the dispersion parameter, h/[lambda], and the amplitude parameter, a/h, are both small in general. However, their competitive roles as rated by the Ursell number Ur, can increase from some small values in the deep ocean, typically of order 10^-2 for large tsunamis, by a factor of 10^3 upon arriving in near-shore waters. This indicates that the effects of nonlinearity (amplitude dispersion) are practically nonexistent in the deep ocean, but gradually become more important and can no longer be neglected when the Ursell number increases to order unity or greater during the terminal stage in which the coastal effects manifest. The small values of the dimensionless wave number, kh = 2[pi]h/[lamda] being in the range of 0.6-0.03 during travel in open ocean, suggests that a slight dispersive effect is still present and this can lead to an accumulated effect in predicting the phase position over very large distances of travel. The overall evolution of tsunamis, as only crudely characterized in Table 1, depends in fact on many factors such as the features of source motion, nonlinear and dispersive effects on propagation in one and two dimensions, the three-dimensional configuration of the coastal region, the direction of incidence, converging or diverging passage of the waves, local reflection and adsorption, density stratification in water, etc. While these aspects of physical behavior are akin to tsunamis, they are also relevant to the consideration of other long wave phenomena. With an intent to provide a sound basis for general applications to long wave phenomena in nature, this paper presents (in the section on three-dimensional long-wave models) a basic long-wave equation which is of the Boussinesq class with special reference to tsunami propagation in two horizontal dimensions through water having spatial and temporal variations in depth. Under certain particular conditions (such as the propagation in one space dimension, or primarily one space dimensional of long waves in water of constant depth) this equation reduces to the Korteweg-de Vries equation or the nonlinear Schrodinger equation. In these special cases we have seen the impressive developments in recent studies of the "soliton-bearing" nonlinear partial differential equations by means of such methods as the variational modulation, the inverse scattering analysis, and modern differential geometry (12,14,17). While extensions of these methods to more general cases will require further major developments, the present analysis and survey will concentrate on the three-dimensional (with propagation in two horizontal dimensions) effects under various conditions by examining the validity of different wave models (based on neglecting the effects of nonlinearity, dispersion, or reflection) in different circumstances. From the example of self focusing of weakly-nonlinear waves (given in the section on converging cylindrical long waves), the effects of nonlinearity, dispersion, and reflection will be seen all to play such a major role that the present basic equation cannot be further modified without suffering from a significant loss of accuracy

    A nonlinear unsteady flexible wing theory

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    This paper extends a previous study by Wu (Adv. Appl. Mech. 2001; 38:291-353) to continue developing a fully non-linear theory for calculation of unsteady flow generated by a two-dimensional flexible lifting surface moving in arbitrary manner through an incompressible and inviscid fluid for modelling bird/insect flight and fish swimming. The original physical concept elucidated by von Kármán and Sears (J. Aeronau Sci. 1938; 5:379-390) in describing the complete vortex system of a wing and its wake in non-uniform motion for their linear theory is adapted and applied to a fully non-linear consideration. The new theory employs a joint Eulerian and Lagrangian description of the lifting-surface movement to facilitate the formulation. The present investigation presents further analysis for addressing arbitrary variations in wing shape and trajectory to achieve a non-linear integral equation akin to Wagner's (Z. Angew. Math. Mech. 1925; 5:17-35) linear version for accurate computation of the entire system of vorticity distribution

    Lifetime Difference and Endpoint effect in the Inclusive Bottom Hadron Decays

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    The lifetime differences of bottom hadrons are known to be properly explained within the framework of heavy quark effective field theory(HQEFT) of QCD via the inverse expansion of the dressed heavy quark mass. In general, the spectrum around the endpoint region is not well behaved due to the invalidity of 1/mQ1/m_Q expansion near the endpoint. The curve fitting method is adopted to treat the endpoint behavior. It turns out that the endpoint effects are truly small and the explanation on the lifetime differences in the HQEFT of QCD is then well justified. The inclusion of the endpoint effects makes the prediction on the lifetime differences and the extraction on the CKM matrix element Vcb|V_{cb}| more reliable.Comment: 11 pages, Revtex, 10 figures, 6 tables, published versio

    Tunneling-induced restoration of classical degeneracy in quantum kagome ice

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    Quantum effect is expected to dictate the behavior of physical systems at low temperature. For quantum magnets with geometrical frustration, quantum fluctuation usually lifts the macroscopic classical degeneracy, and exotic quantum states emerge. However, how different types of quantum processes entangle wave functions in a constrained Hilbert space is not well understood. Here, we study the topological entanglement entropy and the thermal entropy of a quantum ice model on a geometrically frustrated kagome lattice. We find that the system does not show a Z(2) topological order down to extremely low temperature, yet continues to behave like a classical kagome ice with finite residual entropy. Our theoretical analysis indicates an intricate competition of off-diagonal and diagonal quantum processes leading to the quasidegeneracy of states and effectively, the classical degeneracy is restored

    Lepton flavor-changing Scalar Interactions and Muon g2g-2

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    A systematic investigation on muon anomalous magnetic moment and related lepton flavor-violating process such as \m\to e\g, \t\to e\g and \t\to \m\g is made at two loop level in the models with flavor-changing scalar interactions. The two loop diagrams with double scalar exchanges are studied and their contributions are found to be compatible with the ones from Barr-Zee diagram. By comparing with the latest data, the allowed ranges for the relevant Yukawa couplings YijY_{ij} in lepton sector are obtained. The results show a hierarchical structure of Y_{\m e, \t e} \ll Y_{\m \t} \simeq Y_{\m\m} in the physical basis if Δaμ\Delta a_{\mu} is found to be >50×1011>50\times 10^{-11}. It deviates from the widely used ansatz in which the off diagonal elements are proportional to the square root of the products of related fermion masses. An alternative Yukawa coupling matrix in the lepton sector is suggested to understand the current data. With such a reasonable Yukawa coupling ansatz, the decay rate of \t\to \m\g is found to be near the current experiment upper bound.Comment: 15 pages, Revtex, 9 figures, published version in EPJ