38,413 research outputs found

    The near wall effect of synthetic jets in a boundary layer

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    Copyright @ 2007 Elsevier Inc. All rights reserved.An experimental investigation to analyse the qualitative near wall effect of synthetic jets in a laminar boundary layer has been undertaken for the purpose of identifying the types of vortical structures likely to have delayed separation on a 2D circular cylinder model described in this paper. In the first instance, dye visualisation of the synthetic jet was facilitated in conjunction with a stereoscopic imaging system to provide a unique quasi three-dimensional identification of the vortical structures. Secondly, the impact of synthetic jet structures along the wall was analysed using a thermochromic liquid crystal-based convective heat transfer sensing system in which, liquid crystals change colour in response to the thermal footprints of a passing flow structure. Of the different vortical structures produced as a result of varying actuator operating and freestream conditions, the footprints of hairpin vortices and stretched vortex rings revealed a marked similarity with the oil flow pattern of a vortex pair interacting with the separation line on the cylinder hence suggesting that either of these structures was responsible in delaying separation. Conditions were established for the formation of the different synthetic jet structures in non-dimensional parameter space

    The KpΣ0π0K^-p\to \Sigma^0\pi^0 reaction at low energies in a chiral quark model

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    A chiral quark-model approach is extended to the study of the KˉN\bar{K}N scattering at low energies. The process of KpΣ0π0K^-p\to \Sigma^0\pi^0 at PK800P_K\lesssim 800 MeV/c (i.e. the center mass energy W1.7W\lesssim 1.7 GeV) is investigated. This approach is successful in describing the differential cross sections and total cross section with the roles of the low-lying Λ\Lambda resonances in n=1n=1 shells clarified. The Λ(1405)S01\Lambda(1405)S_{01} dominates the reactions over the energy region considered here. Around PK400P_K\simeq 400 MeV/c, the Λ(1520)D03\Lambda(1520)D_{03} is responsible for a strong resonant peak in the cross section. The Λ(1670)S01\Lambda(1670)S_{01} has obvious contributions around PK=750P_K=750 MeV/c, while the contribution of Λ(1690)D03\Lambda(1690)D_{03} is less important in this energy region. The non-resonant background contributions, i.e. uu-channel and tt-channel, also play important roles in the explanation of the angular distributions due to amplitude interferences.Comment: 18 pages and 7 figure

    Collective Neutrino Oscillations

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    We review the rich phenomena associated with neutrino flavor transformation in the presence of neutrino self-coupling. Our exposition centers on three collective neutrino oscillation scenarios: a simple bipolar neutrino system that initially consists of mono-energetic electron neutrinos and antineutrinos; a homogeneous and isotropic neutrino gas with multiple neutrino/antineutrino species and continuous energy spectra; and a generic neutrino gas in an anisotropic environment. We use each of these scenarios to illustrate key facets of collective neutrino oscillations. We discuss the implications of collective neutrino flavor oscillations for core collapse supernova physics and for the prospects of obtaining fundamental neutrino properties, e.g., the neutrino mass hierarchy and θ13\theta_{13} from a future observed supernova neutrino signal.Comment: Submitted to Annual Review of Nuclear and Particle Scienc

    Simple Picture for Neutrino Flavor Transformation in Supernovae

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    We can understand many recently-discovered features of flavor evolution in dense, self-coupled supernova neutrino and antineutrino systems with a simple, physical scheme consisting of two quasi-static solutions. One solution closely resembles the conventional, adiabatic single neutrino Mikheyev-Smirnov-Wolfenstein (MSW) mechanism, in that neutrinos and antineutrinos remain in mass eigenstates as they evolve in flavor space. The other solution is analogous to the regular precession of a gyroscopic pendulum in flavor space, and has been discussed extensively in recent works. Results of recent numerical studies are best explained with combinations of these solutions in the following general scenario: (1) Near the neutrino sphere, the MSW-like many-body solution obtains. (2) Depending on neutrino vacuum mixing parameters, luminosities, energy spectra, and the matter density profile, collective flavor transformation in the nutation mode develops and drives neutrinos away from the MSW-like evolution and toward regular precession. (3) Neutrino and antineutrino flavors roughly evolve according to the regular precession solution until neutrino densities are low. In the late stage of the precession solution, a stepwise swapping develops in the energy spectra of νe\nu_e and νμ/ντ\nu_\mu/\nu_\tau. We also discuss some subtle points regards adiabaticity in flavor transformation in dense neutrino systems.Comment: 11 pages, 2 figure, retex4 format. Split fig.1 into two figures. Minor corrections. Version accepted by PR