189,890 research outputs found

    Density profiles and collective modes of a Bose-Einstein condensate with light-induced spin-orbit coupling

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    The phases of a Bose-Einstein condensate (BEC) with light-induced spin-orbit coupling (SOC) are studied within the mean-field approximation. The mixed BEC phase, in which the system condenses in a superposition of two plane wave states, is found to be stable for sufficiently small light-atom coupling, becoming unstable in a continuous fashion with increasing light-atom coupling. The structure of the phase diagram at fixed chemical potential for bosons with SOC is shown to imply an unusual density dependence for a trapped mixed BEC phase, with the density of one dressed spin state increasing with increasing radius, providing a unique experimental signature of this state. The collective Bogoliubov sound mode is shown to also provide a signature of the mixed BEC state, vanishing as the boundary to the regime of phase separation is approached.Comment: 9 pages, 4 figures. Published Versio

    Tame kernels and further 4-rank densites

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    There has been recent progress on computing the 4-rank of the tame kernel K2(OF)K_2(\mathcal{O}_F) for FF a quadratic number field. For certain quadratic number fields, this progress has led to "density results'' concerning the 4-rank of tame kernels. These results were first mentioned in \cite{CH} and proven in \cite{RO}. In this paper, we consider some additional quadratic number fields and obtain further density results of 4-ranks of tame kernels. Additionally, we give tables which might indicate densities in some generality.Comment: 20 page

    Experimental and numerical investigation of Helmholtz resonators and perforated liners as attenuation devices in industrial gas turbine combustors

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    This paper reports upon developments in the simulation of the passive control of combustion dynamics in industrial gas turbines using acoustic attenuation devices such as Helmholtz resonators and perforated liners. Combustion instability in gas turbine combustors may, if uncontrolled, lead to large-amplitude pressure fluctuations, with consequent serious mechanical problems in the gas turbine combustor system. Perforated combustor walls and Helmholtz resonators are two commonly used passive instability control devices. However, experimental design of the noise attenuation device is time-consuming and calls for expensive trial and error practice. Despite significant advances over recent decades, the ability of Computational Fluid Dynamics to predict the attenuation of pressure fluctuations by these instability control devices is still not well validated. In this paper, the attenuation of pressure fluctuations by a group of multi-perforated panel absorbers and Helmholtz resonators are investigated both by experiment and computational simulation. It is demonstrated that CFD can predict the noise attenuation from Helmholtz resonators with good accuracy. A porous material model is modified to represent a multi-perforated panel and this perforated wall representation approach is demonstrated to be able to accurately predict the pressure fluctuation attenuation effect of perforated panels. This work demonstrates the applicability of CFD in gas turbine combustion instability control device design