The inception cavitating flows over an axisymmetric body with a blunt head-form

The inception cavitating flows around a blunt body are studied based on flow visualizations and velocity field measurements. The main purpose of the present work is to study the incipient cavity evolution and the interplay between the inception cavitation and the local turbulent flows. A high-speed video camera is used to visualize the cavitating flow structures, and the particle image velocimetry(PIV) technique is used to measure the velocity field, the vorticity, and the Reynolds stresses under non-cavitating and inception cavitating flow conditions. It is found that the appearance of visible cavities is preceded by the formation of a cluster of micro-bubbles not attached to the body surface and in a hairpin-shaped vortex structure. During its evolution, the cavity moves downstream with a lower speed. The effect of the incipient cavity is significant on the local vortical structures but slight on the timeaveraged velocity distribution. The mean Reynolds stress distributions in the turbulent shear flow can be substantially altered by the incipient cavities. The presence of the incipient cavities can lead to the production of turbulent fluctuations.     

Bed morphology, flow structure, and sediment transport at the outlet of Lake Huron and in the upper St. Clair River

An integrated multibeam echo sounder and acoustic Doppler current profiler field survey was conducted in July 2008 to investigate the morphodynamics of the St. Clair River at the outlet of Lake Huron. The principal morphological features of the upper St. Clair River included flow-transverse bedforms that appear weakly mobile, erosive bedforms in cohesive muds, thin non-cohesive veneers of weakly mobile sediment that cover an underlying cohesive (till or glacio-lacustrine) surface, and vegetation that covers the bed. The flow was characterized by acceleration as the banks constrict from Lake Huron into the St. Clair River, an approximately 1500-m long region of flow separation downstream from the Blue Water Bridge, and secondary flow connected to: i) channel curvature; ii) forcing of the flow by local bed topography, and iii) flow wakes in the lee side of ship wrecks. Nearshore, sand-sized, sediment from Lake Huron was capable of being transported into, and principally along, the banks of the upper St. Clair River by the measured flow. A comparison of bathymetric surveys conducted in 2007 and 2008 identifies that the gravel bed does undergo slow downstream movement, but that this movement does not appear to be generated by the mean flow, and could possibly be caused by ship-propeller-induced turbulence. The study results suggest that the measured mean flow and dredging within the channel have not produced major scour of the upper St. Clair River and that the recent fall in the level of Lake Huron is unlikely to have been caused by these mechanisms.     

小湾水电站泄洪洞抗冲耐磨混凝土施工综述

小湾水电站泄洪洞在泄洪时具有高水头、高流速、泄量大的特点,对混凝土冲刷力强,容易产生空蚀破坏.通过合理选择浇筑措施、优选配合比、有效的温度控制以及合理的养护,目前,泄洪洞各方面质量均满足要求,且未发现裂缝.泄洪洞抗冲耐磨混凝土施工及质量控制的成功,为减小泄洪洞过流面空蚀和抗冲耐磨提供了保障.     

亭子口水电站跌坎消力池减压模型试验研究

采用1:40减压模型,通过测试流态、水下噪声和脉动压力等水力参数,对亭子口水电站闸墩尾部跌坎式底流消力池的空化空蚀特性进行了研究。成果表明：闸墩尾部区域存在强度为发展阶段的剪切流空化,采用8.00 m高度的跌坎消力池后,消力池底板空蚀破坏的可能性较小;但消力池边墙与闸墩内壁侧扩距离较小时,边墙空蚀破坏的可能性较大;消力池边墙与闸墩内壁侧扩距离增至4.20 m时,边墙空蚀破坏的可能性显著降低。     

EFFECTS OF ENTRAINED AIR MANNER ON CAVITATION DAMAGE

Early in 1953 the experiments by Peterka proved that air entrainment has effects on decreasing cavitation damage.This technology has been widely used in the release works of high dams since the inception of air entrainment in the Grand Goulee Dam in 1960.Behavior,mechanism and application of air entrainment for cavitation damage control have been investigated for over half century.However,severe cavitation damage happened due to complex mechanism of air entrainment.The effects of air entrainment are related...     

功率超声在金属熔体成形中的作用效应及其可视化研究进展

主要介绍功率超声在金属熔体成形领域中的作用,详述了超声在铸造成形、辅助焊接成形中细化晶粒、除气除杂、强化焊缝、减小残余应力、促进界面润湿等方面的应用和作用机理。针对超声空化气泡的高速摄影及金属熔体中晶体生长的同步辐射X射线成像观察的研究进展,提出将高速摄影与同步辐射X射线成像结合起来,通过原位观察方法研究超声波与液态金属媒质的相互作用机理,为进一步促进超声在金属熔体成形中的应用提供理论指导。     

Development and progress in acoustic phase-gradient metamaterials for wavefront modulation

Acoustic metamaterials (AMs) for sound wave manipulation have attracted significant attention due to their fascinating functionalities, such as anomalous reflection/refraction, acoustic cloaking, sound absorption, acoustic imaging, etc. The acoustic phase-gradient metamaterials possess the capability of wavefront manipulation, thus, are fundamental to designing these fascinating functionalities. The underlying mechanism is controlling the acoustic responses (the phase and/or amplitude) of the units by varying the parameters so that one can redirect the wavefront in the desired manner. In this article, we review the state-of-the-art on development of phase-gradient metamaterials for wavefront manipulation. The governing principles of the phase-gradient metamaterials for wave control in static and moving media are first introduced. Then, according to the unit type, the phase-gradient metamaterials are roughly classified into three categories: the locally resonant structures, the space-coiling structures and the material-filling structures. Afterwards, three representative functionalities of the gradient metamaterials are reviewed, including acoustic cloaking, sound absorption/isolation and acoustic lens. Finally, the limitations of present metamaterials and possible future directions for development are concluded.     

Langasite based surface acoustic wave sensors for high temperature chemical detection in harsh environment: Design of the transducers and packaging

This work presents the design and the thermal behavior characterization of an innovative self-test portable surface acoustic wave platform for chemical detection under high temperature. Before the forthcoming deposition of the sensitive coating, the thermal behavior of the bare LGS acoustic platform has been focused on. The system includes a (0°, 140°, 25°) crystallographic cut langasite (LGS) piezoelectric substrate, a ceramic heater, and a platform with RF connections for remote measurements. The packaging consists in a hermetic stainless steel cell, which enables safe gas detection. Its thermal behavior was successfully investigated in the temperature range 25-500 °C thanks to the integrated heater, without using an external furnace. Finite element modeling aided the development of this platform structure by predicting the thermal behavior of each of its parts and their cross-influences. The structure of the platform was specifically designed so that 500 °C could be reached on the LGS acoustic device while the temperature on the PCB connections should not exceed 50 °C. Then, the temperature-dependence on the waves generated by the acoustic transducers has been investigated through numerical modeling by resolving the wave propagation equations with several sets of LGS constants. Corresponding simulations showed good agreement with experiments, Thermal cycling up to 350 °C highlighted satisfactory hardiness and response-reproducibility of the system towards thermal stress, after a first burn effect.     

A new robust double-talk detector based on the Stockwell transform for acoustic echo cancellation

Despite great developments in the field of acoustic echo cancellation (AEC), the presence of double-talk remains difficult problem. The main role of double-talk detection (DTD) is to control adaptation of the filter coefficients by halting their update in double-talk situations. In this paper, we propose a new method of DTD based on a time–frequency analysis that uses the Stockwell transform (ST).The ST is a time–frequency spectral localization method that combines the characteristics of the short-time Fourier transform and the wavelet transform. This method provides better time–frequency resolution, especially for non-stationary signals. In the experimental tests, the normalized least mean squares (NLMS) algorithm is used to update the filter coefficients along with speech signals taken from the TIMIT database. The obtained results show better performance compared to existing methods in terms of misalignment convergence and speech intelligibility enhancement.     

P(VDF-TrFE) copolymer acoustic emission sensors

Acoustic emission (AE) sensors have been fabricated using polarized polyvinylidene fluoride/trifluoroethylene (P(VDF-TrFE)) piezoelectric copolymer films. The acoustic and electromechanical properties of the copolymers have been determined using the ultrasonic immersion technique and the resonance technique, respectively. The P(VDF-TrFE) AE sensors have been calibrated according to the ASTM standard and evaluated for potential application in the detection of AE in glass fiber reinforced polypropylene (GFPP). A ceramic AE sensor also has been fabricated using lead zirconate titanate (PZT) 7A piezoelectric ceramic and its sensitivity and performance are reported as well. The copolymer sensors do not show resonance peaks of the ceramic sensor and have adequate sensitivity. They can reproduce AE signals accurately without giving artifacts and have potential use in commercial AE systems.