45,057 research outputs found

    Particle image velocimetry measurements of the interaction of synthetic jets with a zero-pressure gradient laminar boundary layer

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    Copyright @ 2010 American Institute of PhysicsAn experimental investigation of the interaction between a synthetic jet actuator and a zero-pressure gradient laminar boundary layer is reported. The aim of this study is to quantify the impact of synthetic jet vortical structures; namely, hairpin vortices, stretched vortex rings and tilted vortex rings on a boundary layer, and to assess their relative potential for flow separation control. Streamwise particle image velocimetry was employed in a water flume (free stream boundary layer thickness Reynolds number of 500 and boundary layer thickness-to-jet orifice diameter ratio of 4) to obtain phase- and time-averaged boundary layer profile information of the impact of synthetic jets near the wall. The potential for flow control was assessed by analyzing near wall fluid mixing, realized by the measure of increase in wall shear stress produced by a passing vortex. Hairpin vortices (produced at a jet-to-free stream velocity ratio, VR=0.32 and dimensionless stroke length, L=1.6) and stretched vortex rings (VR=0.27; L=2.7) exhibit characteristics akin to a streamwise vortex pair with a common upwash. Conversely, tilted vortex rings (VR=0.54; L=2.7) induce a streamwise vortex pair in the near wall region with a common downwash. Wall shear stress measurements show that synthetic jets composed of stretched vortex rings offer the best combination of near wall fluid mixing, persistency, and low rms fluctuations for potential applications of flow separation control.Financial support from the Engineering and Physical Sciences Research Council (EPSRC Grant No. AF566NEZ) was used for this work

    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

    A Deep Reinforcement Learning-Based Framework for Content Caching

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    Content caching at the edge nodes is a promising technique to reduce the data traffic in next-generation wireless networks. Inspired by the success of Deep Reinforcement Learning (DRL) in solving complicated control problems, this work presents a DRL-based framework with Wolpertinger architecture for content caching at the base station. The proposed framework is aimed at maximizing the long-term cache hit rate, and it requires no knowledge of the content popularity distribution. To evaluate the proposed framework, we compare the performance with other caching algorithms, including Least Recently Used (LRU), Least Frequently Used (LFU), and First-In First-Out (FIFO) caching strategies. Meanwhile, since the Wolpertinger architecture can effectively limit the action space size, we also compare the performance with Deep Q-Network to identify the impact of dropping a portion of the actions. Our results show that the proposed framework can achieve improved short-term cache hit rate and improved and stable long-term cache hit rate in comparison with LRU, LFU, and FIFO schemes. Additionally, the performance is shown to be competitive in comparison to Deep Q-learning, while the proposed framework can provide significant savings in runtime.Comment: 6 pages, 3 figure

    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