我们提出了详细的观察，在单一的空化气泡崩溃时发出的冲击波，同时使用时间分辨阴影和水听器压力测量。气泡的几何形状系统地从球形变化到非常非球形，通过减小它们到自由或刚性表面的距离或通过调制抛物线飞行上的重力引起的压力梯度。非球形的崩溃产生多个冲击，显然与不同的过程，如射流的影响和不同的气泡段的个人崩溃。对于在自由表面附近塌陷的气泡，作为各向异性参数$ζ$的函数，分别测量每个冲击的能量和时间，该参数代表开尔文脉冲的无量纲等效值。对于给定的气泡变形源（自由表面、刚性表面或重力），归一化激波能量仅取决于$ζ$，而与气泡半径$R_{0}$和驱动压力$Δp$无关。基于这一发现，我们开发了一个预测框架的峰值压力和能量的冲击波从非球形气泡崩溃。结合实验数据的统计分析和理论推导，我们发现冲击波峰值压力可以用射流冲击诱导的锤击压力来估算，表示为$p_{h} = 0.45\left(ρc^{2}Δp\right)^{1/2} ζ^{-1}$ at $ζ> 10^{-3}$。同样的方法被发现来解释作为$ζ^{-2/3}$的函数的冲击能量淬火。
英文摘要：
We present detailed observations of the shock waves emitted at the collapse of single cavitation bubbles using simultaneous time-resolved shadowgraphy and hydrophone pressure measurements. The geometry of the bubbles is systematically varied from spherical to very non-spherical by decreasing their distance to a free or rigid surface or by modulating the gravity-induced pressure gradient aboard parabolic flights. The non-spherical collapse produces multiple shocks that are clearly associated with different processes, such as the jet impact and the individual collapses of the distinct bubble segments. For bubbles collapsing near a free surface, the energy and timing of each shock are measured separately as a function of the anisotropy parameter $ζ$, which represents the dimensionless equivalent of the Kelvin impulse. For a given source of bubble deformation (free surface, rigid surface or gravity), the normalized shock energy depends only on $ζ$, irrespective of the bubble radius $R_{0}$ and driving pressure $Δp$. Based on this finding, we develop a predictive framework for the peak pressure and energy of shock waves from non-spherical bubble collapses. Combining statistical analysis of the experimental data with theoretical derivations, we find that the shock peak pressures can be estimated as jet impact-induced hammer pressures, expressed as $p_{h} = 0.45\left(ρc^{2}Δp\right)^{1/2} ζ^{-1}$ at $ζ> 10^{-3}$. The same approach is found to explain the shock energy quenching as a function of $ζ^{-2/3}$.
Extreme cavitation scenarios such as water column separations in hydraulic systems during transient processes caused by large cavitation bubbles can lead to catastrophic destruction. In the present paper, we study the onset criteria and dynamics of large cavitation bubbles in a tube. A new cavitation number $Ca_2 = {l^*}^{-1} Ca_0$ is proposed to describe the maximum length $L_{\max}$ of the cavitation bubble, where $l^*$ is a non-dimensional length of the water column indicating its slenderness, and $Ca_0$ is the classic cavitation number. Combined with the onset criteria for acceleration-ind…
The transient process accompanied by extreme acceleration in the conical sections of hydraulic systems (e.g., draft tube, diffuser) can induce large cavitation bubbles both at the closed ends and in the bulk liquid. The collapses of the large cavitation bubbles can cause severe damage to the solid walls. We conduct experiments in the tubes with different conical-frustum shaped closed ends with the `tube-arrest' method and observe bubbles generated at these two locations. For the bubbles generated at the close end of the tube, we propose the onset criteria, consisting of two universal non-dimen…
Cavitation is a phenomenon in which bubbles form and collapse in liquids due to pressure or temperature changes. Even common tools like a rubber popper can be used to create cavitation at home. As a rubber popper toy slams a solid wall underwater, toroidal cavitation forms. As part of this project, we aim to explain how an elastic shell causes cavitation and to describe the bubble morphology. High-speed imaging reveals that a fast fluid flow between a snapping popper and a solid glass reduces the fluid pressure to cavitate. Cavitation occurs on the popper surface in the form of sheet cavitatio…
A Free Sphere Reverses the Rebound Direction of a Near-Wall Cavitation Bubble
School of Marine Science and Technology, Northwestern Polytechnical University（西北工业大学航海学院）；College of Shipbuilding Engineering, Harbin Engineering University（哈尔滨工程大学船舶工程学院）
Bubbles generated with lasers under confinement have been investigated for their potential use as the driving mechanism for liquid micro-jets in various microfluidic devices, such as needle free jet injectors. Here, we report on the study of bubble formation by a continuous-wave (CW) and a pulsed laser inside an open-ended microfluidic capillary. This results in a direct comparison between bubbles generated by laser sources emitting light in different time scales (ms and ns). The bubble kinetics represents an important parameter because it determines the available kinetic energy for a subseque…
Cavitation-bubble Interaction with an Initially Perturbed Free Surface
College of Shipbuilding Engineering, Harbin Engineering University（哈尔滨工程大学船舶工程学院）；Nanhai Institute of Harbin Engineering University（哈尔滨工程大学南海研究院）；National Key Lab of Ship Structural Safety, Harbin Engineering University（哈尔滨工程大学船舶结构安全国家重点实验室）
In this paper, we present a simplified theoretical model based on the method of images that predicts the direction of the microjet produced after the implosion of the cavitation bubble created in between a free interface and rigid wall. Our theoretical predictions have been verified by means of a thorough experimental study in which the distances of the pulsed-laser cavitation bubble to the wall and the free surface are varied in a systematic manner. In addition, we extend the predictions to arbitrary values of the corner angle, $π/(2n)$ with $n$ a natural number. The present analytical soluti…
Inertial cavitation in soft matter is an important phenomenon featured in a wide array of biological and engineering processes. Recent advances in experimental, theoretical, and numerical techniques have provided access into a world full of nonlinear physics, yet most of our quantitative understanding to date has been centered on a spherically symmetric description of the cavitation process. However, cavitation bubble growth and collapse rarely occur in a perfectly symmetrical fashion, particularly in soft materials. Predicting the onset of dynamically arising, non-spherical instabilities has …
Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized PFA flow tubes, submerged in an ultrasonic bath reactor, are studied at 27 kHz driving frequency. Nucleation of cavitation inside the tubes only takes place via a free interface, realized here in form of an alternating water-air slug flow. High-speed recordings show that cavitation bubbles in the water slugs often develop localized structures in form of clusters or bubble plugs, and that such structures can be seeded via a single pinch-off from the free interface. Within the structures, bubbles strongly interact and fr…
Ultrasonic irradiation of liquids, such as water-alcohol solutions, results in cavitation or the formation of small bubbles. Cavitation bubbles are generated in real solutions without the use of optical traps making our system as close to real conditions as possible. Under the action of the ultrasound, bubbles can grow, oscillate, and eventually, they collapse or decompose. We apply the mathematical method of separation of motions to interpret the acoustic effect on the bubbles. While in most situations, the spherical shape of a bubble is the most energetically profitable as it minimizes the s…
The Gilmore model is combined with the Noble-Abel-stiffened-gas (NASG) equation of state to yield a simple model to predict the expansion and collapse of spherical bubbles based on real gas thermodynamics. The NASG equation of state resolves the temperature inaccuracy associated with the commonly employed Tait equation of state for liquids and, thus, can provide a consistent description of compressible and thermal effects of the bubble content and the surrounding liquid during cavitation. After a detailed derivation of the proposed Gilmore-NASG model, the differences between the classical Gilm…
