INVESTIGATION OF THE PRIMARY MECHANISMS OF CAVITATION-INDUCED DAMAGES

空化由液体中气泡的形成、生长和内爆组成，当暴露在快速压降下时。气泡内爆的最后一步包括内部气体的快速压缩，比热交换快得多，这导致高振幅压力脉冲，对附近的固体表面造成一些损伤。这个项目的重点是这个过程的小规模机制，称为空化侵蚀。更具体地说，它旨在表征流体和材料响应之间复杂的相互作用，并澄清靠近气泡崩溃的材料所经历的冲击的主要原因。为此目的，将进行联合数值和实验工作：(i)将开发一种新颖先进的多物理场计算框架，能够预测塌陷空化气泡与附近可变形材料的动态相互作用；（ii）将进行气泡演化成像与液体中速度和温度场的测量相结合的实验，并在固体表面进行局部努力。这两种方法都将关注材料表面附近单个气泡的崩溃。该项目的完成将有可能导致可控的空化气泡破裂，这是优化各种医疗和工业过程的主要挑战。该项目还将使海洋和生物医学工程师能够教授跨学科的空化科学；让K-12年级的学生了解有趣的空化现象及其更广泛的影响。气泡-材料相互作用问题是一个具有挑战性的多物理场和多尺度问题，涉及流体力学和壁面变形之间的强耦合。动态过程高度非线性，具有激波、高速流动、液气界面大变形和拓扑变化、激波诱发断裂等特点。气泡的大小、与壁面的距离和坍塌的特征时间对壁面的影响目前是一个悬而未决的问题。更具体地说，根据这些不同的参数，必须明确微射流和激波在局部努力、弹塑性变形和潜在质量损失方面的各自影响。单个气泡崩溃的影响和崩溃的累积影响都是感兴趣的，最终确定造成损害的主要机制。在本项目中，采用数值和实验相结合的方法来研究这个问题。计算框架将结合高保真模型，以捕获冲击波在材料界面上的传播，气泡和固体材料的大变形，以及冲击引起的材料破坏。经过验证后，它将首次能够明确和定量地探索双向流固耦合，即(i)由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特性对激波主导的两相流体流动的相互影响。实验将使用高速光学和x射线成像，冷线进行高频温度测量，以及基于PVDF（聚偏氟乙烯）涂层的创新阵列传感器进行局部努力测量。

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.