Ultra-broadband acoustic ventilation barrier based on multi-cavity resonators

The numerical simulations and experimental results of an ultra-broadband acoustic ventilation barrier composed of periodic unit cells are reported in this paper.Based on multiple mechanisms,including sound absorption by eigenmodes of the unit cell and sound reflection by a plate structure on upper surface of the unit cell,a single-layer ventilation barrier with broadband sound reduction is designed,and its working bandwidth can reach about 1560 Hz.The experimental results accord well with the simulation results.Furthermore,two types of three-layer ventilation barriers are designed and demonstrated by using the unit cells with different values of a(the length of the hollow square region)and w(the width of the channel between the adjacent cavities),and the bandwidths of both ventilation barriers can increase to 3160 Hz and 3230 Hz,respectively.The designed barrier structures have the advantages of ultra-broadband sound reduction and ventilation,which paves the way to designing high-performance ventilation barriers for the applications in environmental protection and architectural acoustics.

Thermoacoustic-reflected focusing lens based on acoustic Bessel-like beam with phase manipulation

We report the realization of broadband reflected acoustic focusing lenses based on thermoacoustic phased arrays of Bessel-like beams, in which the units of phase manipulation are composed of three rigid insulated boundaries and a thermal insulation film in air with different temperatures. Based on these units, we realize a reflected focusing lens which can focus reflected acoustic energy on a line, and its fractional bandwidth can reach about 0.29. In addition, we discuss the influences of the base angle of Bessel-like beam, the number of basic unit, and the variation of unit temperature on focusing performances in details. Furthermore, the reflected focusing lens for the cylindrical acoustic wave based on the Bessel-like beam is also demonstrated. The proposed focusing lens has the advantages of a broad working bandwidth, large focus size,and high robustness, which may provide possibilities for the design and application of acoustic lenses.

Evaluation of vortex evolution and energy loss within the impeller of a side channel pump

Side channel pumps can provide a huge pressure boost at very low specific speeds,making them useful in various industrial operations.Due to its very complex flow pattern,the high hydraulic head is often accompanied by lower efficiency and higher flow losses.In addition,the shape of the impeller has a direct influence on the flow pattern of the pump as well as the energy conversion that takes place,and this has a substantial bearing on the overall performance of the pump.Thus,to gain a deeper comprehension of the flow behavior and deep-seated reasons for performance optimization in side channel pumps with different geometry parameters.Extensive research has been conducted on the vortex formations and entropy production that occur inside the flow channels of the impeller.Three alternative impeller configurations were systematically created for in-depth research,each including a different suction angle for the blades.According to the data,the head of the side channel pump rises together with an increase in the suction angle,and this rule is exhibited in the whole working condition,and the most effective blade suction angle isθ=30°,which is the greatest of the available options.The vortex area and intensity have an obvious decrease when the suction angle increases to 30°,which is the main reason for the performance optimization.This paper firstly introduces the wall friction dissipation into the research on the side channel pump.Compared with turbulence dissipation and direct dissipation,the wall friction dissipation is far less than turbulence dissipation but far higher than direct dissipation that should not be neglected,although it also increases with the suction angle elevation.As the primary dissipation,the tendency of turbulence dissipation is upward with the performance increase,which is opposite to the common vane pump.The reason for this phenomenon is probably due to the deviation of the primary and secondary flow pattern in the side channel pump.As a result,this will assist to increase the performance and operational dependability of side channel pumps,which will ultimately lead to an expansion of the applications for these pumps.

Influence of pump noise on the health of fish in a large pumping station

The axial flow pump,characteristic of a large flow,is widely used in pumping stations of hydraulic engineering.However,the internal noise of the axial-flow pump of a high sound pressure level is notable,with a great influence on the fishing livelihood.At present,the studies of the impact of the noise on fish are mainly focused on the explosion,the offshore wind power plant,the ship and other fields,with little attention on the noise generated by the pumping station.This paper applies the combination of the computational fluid dynamics and the Actran to study the sound pressure level distribution in the high-noise area inside the axial-flow pump and the noise distribution in the downstream areas of the pumping station.Extensive comparisons are made of the hearing threshold and the hearing damage threshold of fish,and it is revealed that the noise inside the axial-flow pump exceeds the hearing damage threshold of fish,which could lead to the hearing damage of over-pumped fish.Further,the noises emitted from the pumping station to the downstream are beyond the fish hearing threshold and will also have a significant impact on the fish around the pumping station.This study can provide a reference for studying the impact of the noise generated by various large-scale water transfer projects on fish.

Large eddy simulation of cloud cavitation and wake vortex cavitation around a trailing-truncated hydrofoil

The cavitation has received considerable attention for decades because of its negative influence on the performance and the safety of the hydraulic machinery.In this study,a large eddy simulation is carried out to numerically investigate the unsteady cavitating flow around a trailing-truncated NACA 0009 hydrofoil for determining the underlying physical mechanisms.Two types of cavitation morphologies are identified:The large-scale bubble cluster and the von Kármán vortex cavity,named as the cloud cavitation and the wake vortex cavitation,respectively.It is shown that the velocity profiles obtained over the hydrofoil suction surface are in good agreement with the experimental data,indicating the accuracy of the current simulation.The dynamic evolution of the sheet/cloud cavity is also well reproduced,covering the sheet cavity breakup,the sheet/cloud transformation,and the collapse of the cloudy bubble cluster.The wake-vortex cavitation is caused by the blunt geometry at the hydrofoil trailing edge,where pairs of vortex cavities are induced.Both the cloud and vortex cavities significantly affect the lift oscillation,which makes it difficult to decompose the components.The fundamental shedding mechanisms of the wake vortex cavitation are discussed based on the finite-time Lyapunov exponent field.Specifically,the suction-side bubble grows and squeezes the giant pressure bubble away from the trailing edge.After the pressure bubble detaches,a new counterclockwise vortex or a new bubble appears at the pressure side,thus lifting the ridge towards the suction trailing edge and generating a strong vortex eye that pinches off the trailing portion of the suction-side bubble.

A decomposition method of vortex identification and its application in side channel pumps

This research utilizes theΩvortex identification method to address the turbulent flows in a single-stage side channel pump,to comprehensively characterize the three types of dynamic vortex structures classified based on directions.Premised on the Galilean invariance,the work employs coordinate rotation and transformation.Thus,the indistinguishable 3-D vortex can be simplified to 2-D vortex on typical research planes.When juxtaposing the overall performance,it was revealed that a diversity of areas with high values yielded enhanced reflection of the vortex intensity,as measured by velocity distribution.The axial vortex structure with high intensity exists at the outer radius under all conditions largely.While the longitudinal vortex usually shows high intensity between the middle and outer radius.Simultaneously,the radial vortex is more likely to be at the inner radius near the suction face.Finally,this paper introduces a specific valueξ,which represents the ratio of decomposition to the total of the manifestation of the fluid rotational pattern.From the fluctuation and mean value,it can be realized that the development of the specific vortex in three directions at different positions.For example,the specific valueξ2 refers to the typical longitudinal vortex as dynamic vortex are almost from 20%to 50%,which illustrates that the longitudinal vortex only occupies a minor percentage in the total vortex.This phenomenon is one of the main reasons for the low efficiency.The present work could provide some suggestions and references for in-depth studies in fluid engineering with intense swirling flows.