Weilin Xu

Generation and control of acoustic cavitation structure.

The generation and control of acoustic cavitation structure are a prerequisite for application of cavitation in the field of ultrasonic sonochemistry and ultrasonic cleaning. The generation and control of several typical acoustic cavitation structures (conical bubble structure, smoker, acoustic Lichtenberg figure, tailing bubble structure, jet-induced bubble structures) in a 20-50 kHz ultrasonic field are investigated. Cavitation bubbles tend to move along the direction of pressure drop in the region in front of radiating surface, which are the premise and the foundation of some strong acoustic cavitation structure formation. The nuclei source of above-mentioned acoustic cavitation structures is analyzed. The relationship and mutual transformation of these acoustic cavitation structures are discussed.

Acoustic cavitation structures produced by artificial implants of nuclei

High-density controllable bubble structures are produced in the vicinity of radiating surface by artificially implant nuclei. Two kinds of typical cavitation structures produced by artificially implant nuclei are investigated. The focusing action and the physical origin of jet-induced cone-like bubble structure are analyzed. The sonochemical activity of cavitation structures is measured by using the standard method of potassium iodide dosimetry. The controllability of cavitation bubble cluster in the acoustic field is also discussed in this work.

Surface tension and quasi-emulsion of cavitation bubble cloud.

A quasi-emulsion phenomenon of cavitation structure in a thin liquid layer (the thin liquid layer is trapped between a radiating surface and a hard reflector) is investigated experimentally with high-speed photography. The transformation from cloud-in-water (c/w) emulsion to water-in-cloud (w/c) emulsion is related to the increase of cavitation bubble cloud. The acoustic field in the thin liquid layer is analyzed. It is found that the liquid region has higher acoustic pressure than the cloud region. The bubbles are pushed from liquid region to cloud region by the primary Bjerknes forces. The rate of change of CSF increased with the increase of CSF. The cavitation bubbles on the surface of cavitation cloud are attracted by the cavitation bubbles inside the cloud due to secondary Bjerknes forces. The existence of surface tension on the interface of liquid region and cloud region is proved. The formation mechanism of disc-shaped liquid region and cloud region are analysed by surface tension and incompressibility of cavitation bubble cloud.

Evolution of Pressure and Cavitation on Side Walls Affected by Lateral Divergence Angle and Opening of Radial Gate

High-speed diffuse flow is a commonly complex process in tunnels with full-section aerators and lateral expansion walls. The transformation of pressure is prone to produce cavitation on the lateral expansion walls downstream of an aerator. The expansion threatens the stability of the structure and can cause damage. In this paper, the diffuse flows downstream of a radial sluice and full-section aerator were studied by combining a realizable k-epsilon turbulent model with a mixture multiphase model. Hydraulic characteristics such as pressure distribution and cavitation were investigated. The relationship among pressure, cavitation index, lateral divergence angle , and Froude number F were proposed. Empirical formulas were presented to calculate and evaluate the pressure and cavitation index on side walls. The calculated results agree well with the physical model. With decrease in divergence angle, pressure increases gradually so that the flow cavitation index increases as well. Profiles of gate openings demonstrated gradual improvement in pressure and cavitation index with an increase in the magnitude of the gate opening. The studies showed that the degree of gate opening has a great effect on the pressure and flow cavitation index of side walls. The lateral expansion and partial gate-opening increased the risk of cavitation erosion on side walls downstream of the radial sluice. Research results provided a reference for optimizing the design of similar engineering.

Experimental Investigation of Pressure Load Exerted on a Downstream Dam by Dam-Break Flow

The failure of a natural or an artificial dam often leads to catastrophic flooding downstream. Studies have been conducted to investigate the mechanisms of single dam failure and the propagation of dam-break flood wave. However, little attention has been paid to the cascade failure of two or more sequential dams. In this article, consequences of a dam-break flood wave down a channel exerting pressure load on a downstream dam with different backwater effects were studied experimentally. Three typical flood patterns were observed for different downstream reservoir water depths. High precision pressure sensors were employed to measure the pressure load at different heights on the dam. Changes of the maximum pressure load were measured as the bed slope and the upstream and downstream reservoir water depths varied. An empirical formula was obtained by linear regression to predict the maximum pressure load. Additionally, a sinusoidal attenuation function was introduced to describe the time evolution of pressure load exerted on the downstream dam, and its parameters were determined using the measurement data in different cases.

Characteristics of the Velocity Distribution in a Hydraulic Jump Stilling Basin with Five Parallel Offset Jets in a Twin-Layer Configuration

The flow structure of vertical and horizontal planes in a stilling basin with five parallel offset jets in a twin-layer configuration for four offset ratios (S1/h=1,2,3,4) has been investigated. The velocity distribution was measured, processed, and subsequently used to characterize the flow structure. The flow pattern in the stilling basin exhibits a mixing flow pattern of jets and hydraulic jumps. As the downstream water depth increases in the stilling basin, five different flow patterns appear successively: free jets, hydraulic jumps, submerged jets and hydraulic jumps, mixed submerged jets and critical jumps, and mixed submerged jets and submerged jumps. The results indicate that the velocity distributions in the vertical and horizontal planes of the lower-layer orifices and in the potential core region of the upper-layer orifices are similar to the classical jet; in other regions, the velocity distribution is similar to the hydraulic jump. In addition, the energy dissipation ratio increases with the increasing Froude number, with a value of greater than 70% at F>8.0. The energy dissipation ratio in the stilling basin with five parallel offset jets in a twin-layer configuration is, on average, 5% larger than that of the SBJ basin and 13% larger than that of the classical jump basin.

Experimental investigations on the collapse of cavity cluster in high power ultrasound fields

Applications of high power ultrasound span many industrial sectors, from healthcare, through sonochemistry, to ultrasonic cleaning. These applications depend on the dynamics of cavity cluster. This study was conducted to investigate the collapse of cavity cluster in high power ultrasound fields. The cloud cavitation was produced by a 20 kHz ultrasonic horn used for sonochemistry. The dynamics of cavity cluster below the ultrasonic horn was recorded with high-speed photography at framing rate of 100,000 fps. PIV (particle image velocimetry) techniques were also used to get a velocity field of acoustic streaming. The motion of cavitation bubbles was strongly influenced by the acoustic streaming. They moved from the perisphere downward towards the axis and convergent at a point about 2.5 mm below the horn (where small bubbles incorporated into big bubbles) and then move downward along the axial direction. However, bubbles were found stray from the acoustic streaming upwards towards the center of the cavity cluster when the cavitation bubbles collapse. The growth and collapse of incipient cavitation bubbles in the cavity cluster in two acoustic cycles were recorded with high-speed photography and the variation of the diameter of the bubbles with time was investigated. The primary Bjerknes force and jets directed toward the center of the cloud produced during the bubbles collapse may be responsible for the motion of bubbles towards the center of cavity cluster. The first collapse of incipient bubbles is influenced by the maximum diameter of cavitation bubbles, while the subsequent rebound and collapse make the cluster very complicated. Asynchrony of the collapse of cavitation bubbles may be responsible for the propagation of shock wave in the cavity cluster.