INVESTIGATION OF THE PRIMARY MECHANISMS OF CAVITATION-INDUCED DAMAGES

空化是指液体中的气泡在接触到快速压降时的形成、增长和内爆。气泡内爆的最后一步是内部气体的快速压缩，比热交换快得多，这会导致高幅度的压力脉冲，从而对附近的固体表面造成一些损害。这个项目的重点是这一过程的小范围机制，称为气蚀。更具体地说，它旨在描述流体和材料响应之间的复杂相互作用，并澄清位于气泡破裂附近的材料所经历的冲击的主要原因。为此，将开展联合的数值和实验工作：(1)将开发一种新的和先进的多物理计算框架，能够预测坍塌的空化气泡与附近的可变形材料的动态相互作用；(2)将气泡演化的成像与液体中的速度和温度场测量相结合的实验，以及在固体表面的局部努力。这两种方法都将专注于材料表面附近单个气泡的破裂。该项目的完成可能会导致可控的空化气泡坍塌，这对各种医疗和工业过程的优化是一个重大挑战。该项目还将能够向海洋和生物医学工程师教授空化的跨学科科学；并使K-12学生了解有趣的空化现象及其更广泛的影响。近固体表面空化气泡坍塌的气泡-材料相互作用问题是一个涉及流体动力学和壁面变形强耦合的具有挑战性的多物理、多尺度问题。动力学过程是一个高度非线性的过程，具有激波、高速流动、液气界面大变形和拓扑变化、激波诱导破裂等特点。气泡的大小、到壁面的距离和崩塌的特征时间对壁面影响的影响目前是一个悬而未决的问题。更具体地说，必须根据这些不同的参数，在局部作用力、弹性或塑性变形以及潜在质量损失方面阐明微射流和冲击波各自的影响。单个气泡破裂的影响和破裂的累积影响都是令人感兴趣的，以最终确定造成损害的主要机制。在本项目中，我们采用数值和实验相结合的方法来研究这一问题。计算框架将包括高保真模型，以捕捉冲击波在材料界面上的传播，气泡和固体材料的大变形，以及冲击导致的材料破坏。经过验证，它将首次明确和定量地探索双向流固耦合，即(I)气泡破裂引起的脉动高速、压力和温度引起的固体材料的应力、变形和破坏；(Ii)固体材料的声学和弹性性质对以激波为主的两相流体流动的相互影响。这些实验将使用高速光学和X射线成像，用于高频温度测量的冷线，以及用于局部作用力测量的基于PVDF(聚偏氟乙烯)涂层的创新阵列传感器。

Cavitation consists of the formation, growth, and implosion of bubbles in a liquid when exposed to rapid pressure drop. The final step of the bubble implosion consists of a rapid compression of the internal gases, much faster than the thermal exchanges, which results in high amplitude pressure pulses that cause some damage on nearby solid surfaces. This project focuses on the small-scale mechanisms of this process, called cavitation erosion. More specifically, it is intended to characterize the complex interaction between the fluid and the material response, and to clarify the primary causes of the impacts experienced by a material located close to bubble collapses. For that purpose, joint numerical and experimental works will be performed: (i) A novel and advanced multiphysics computational framework capable of predicting the dynamic interaction of collapsing cavitation bubbles with a nearby deformable material will be developed, (ii) Experiments combining the imaging of the bubble evolution with measurements of the velocity and temperature fields in the liquid, and local efforts on the solid surface will be conducted. Both approaches will focus on the collapse of a single bubble near the material surface. Completion of this project will potentially lead to controllable cavitation bubble collapse, which is a major challenge for the optimization of various medical and industrial processes. The project will also enable teaching the cross-disciplinary science of cavitation to both ocean and biomedical engineers; and to engage K-12 students to learn about the interesting phenomenon of cavitation and its broader impacts. The bubble-material interaction problem related to the collapse of cavitation bubbles close to a solid surface is a challenging multiphysics and multiscale problem involving a strong coupling between the fluid dynamics and the wall deformation. The dynamic process is highly nonlinear, featuring shock waves, high speed flows, large deformation and topological change of liquid-gas interface, and shock-induced fracture. The effects of the bubble size, distance to the wall and characteristic time of the collapse on the effects on the wall are currently an open question. More specifically, the respective impacts of the microjet and the shock waves, according to these different parameters, in terms of local efforts, elastic or plastic deformation, and potential mass loss have to be clarified. Both the effects of a single bubble collapse and the cumulative effects of the collapses are of interest, to eventually determine the primary mechanisms that are responsible for the damages. In the present project, this problem is investigated by a joint numerical and experimental approach. The computational framework will incorporate high-fidelity models to capture the propagation of shock waves across material interfaces, large deformation of bubbles and solid materials, and shock-induced material failure. After validation, it will enable, for the first time, to explicitly and quantitatively explore the two-way fluid-solid coupling, that is, both (i) the stress, deformation, and failure of the solid material induced by the pulsatile high velocities, pressures, and temperatures resulting from bubble collapse; and (ii) the reciprocal impact of the acoustic and elastic properties of the solid material to the shock-dominated two-phase fluid flow. The experiments will use high speed optical and X-ray imaging, cold wires for high frequency temperature measurements, and innovative array sensor based on PVDF (polyvinylidene fluoride) coating for local effort measurement.