[论文解读] Long-Pulse Laser-Induced Cavitation: A Race Between Advection and Phase Transition

Xuning Zhao, Wentao Ma+4 more|arXiv (Cornell University)|Aug 23, 2023

Ultrasound and Cavitation PhenomenaMaterials Science被引用 3

一句话总结

本研究提出了一种计算模型，耦合可压缩多相流、激光辐射与相变过程，以解释长脉冲激光诱导蒸气泡的形状演化。其核心机制在于输运与持续汽化之间的竞争：当汽化速度快于输运时，形成细长的圆锥形气泡；当输运占主导时，则形成球形梨状气泡——该结论得到模拟结果支持，模拟结果与实验观测的气泡形态高度吻合，并揭示了汽化持续时间的显著差异：梨状气泡为<1 μs，而细长气泡则超过50 μs。

ABSTRACT

Vapor bubbles generated by long-pulsed laser often have complex non-spherical shapes that reflect some characteristics (e.g., direction, width) of the laser beam. The transition between two commonly observed shapes - namely, a rounded pear-like shape and an elongated conical shape - is studied using a new computational model that combines compressible multiphase fluid dynamics with laser radiation and phase transition. Two laboratory experiments are simulated, in which Holmium:YAG and Thulium fiber lasers are used separately to generate bubbles of different shapes. In both cases, the bubble morphology predicted by the simulation agrees reasonably well with the experimental measurement. The simulated laser radiance, temperature, velocity, and pressure fields are analyzed to explain bubble dynamics and energy transmission. It is found that due to the lasting energy input (i.e. long-pulsed laser), the vapor bubble's dynamics is driven not only by advection, but also by the continuation of vaporization. Notably, vaporization lasts less than 1 microsecond in the case of the pear-shaped bubble, versus more than 50 microseconds for the elongated bubble. It is hypothesized that the bubble's shape is the result of a competition. When the speed of advection is higher than that of vaporization, the bubble tends to grow spherically. Otherwise, it elongates along the laser beam direction. To clarify and test this hypothesis, the two speeds are defined analytically using a simplified model, then estimated for the experiments using simulation results. The results support the hypothesis. They also suggest that a higher laser absorption coefficient and a narrower beam facilitate bubble elongation.

研究动机与目标

• 理解长脉冲激光诱导空化中非球形蒸气泡形态背后的物理机制。
• 解决激光参数与气泡形状（如梨状与圆锥形）之间尚未明确的因果关联。
• 探究相变过程是否在长脉冲激光辐照期间持续存在，从而挑战瞬时汽化的假设。
• 开发一种可预测的框架，通过调节吸收系数和光束宽度等激光参数来控制气泡形状。

提出的方法

• 开发了一种计算模型，整合可压缩多相流体力学、激光能量沉积与相变过程。
• 利用实测的激光功率分布作为输入，对钬：钇铝石榴石（Ho:YAG）和铥光纤激光器的两种实验设置进行了模拟。
• 采用精细的计算网格，分辨率覆盖激光光纤直径并超过240个单元，以实现高空间分辨率。
• 使用简化的分析模型定义并估算通过输运和汽化驱动的气泡生长速度。
• 追踪激光辐射强度、温度、速度与压力场，以分析能量传输与气泡动力学。
• 将模拟结果与实验气泡形态进行对比，以验证模型并检验输运-汽化竞争假说。

实验结果

研究问题

• RQ1 在长脉冲激光空化中，哪些物理过程控制着梨状与细长圆锥形气泡之间的转变？
• RQ2 在长脉冲激光辐照期间，相变（汽化）是否持续显著时间，还是瞬时完成？
• RQ3 输运与汽化速度的相对大小如何决定气泡形态？
• RQ4 哪些激光参数（如吸收系数、光束宽度）会影响一种生长机制对另一种的主导作用？
• RQ5 能否基于输运与持续汽化之间的竞争关系，实现对气泡形状的预测与控制？

主要发现

• 模拟结果准确再现了使用Ho:YAG和铥光纤激光器在实验中观测到的梨状与细长气泡形态。
• 梨状气泡的汽化持续时间小于1 μs，而细长气泡则超过50 μs，表明后者存在持续的相变过程。
• 在细长气泡情况下，汽化驱动的气泡生长速度约为梨状气泡情况的两个数量级。
• 输运速度在两种情况下基本保持恒定，表明气泡形状的差异主要源于汽化速度的差异。
• 基于模拟的速度估算验证了气泡形状源于输运与汽化之间竞争的假说。
• 预测较高的激光吸收系数和更窄的光束宽度将增强汽化速度，促进细长气泡的形成。