首页>
外文OA文献
>Shock waves in bubbly cavitating flows. Part I. Shock waves in cloud cavitation. Part II. Bubbly cavitating flows through a converging-diverging nozzle
【2h】
Shock waves in bubbly cavitating flows. Part I. Shock waves in cloud cavitation. Part II. Bubbly cavitating flows through a converging-diverging nozzle
Two problems are considered in this thesis: the nonlinear dynamics of a cloud of cavitation bubbles, and bubbly cavitating flows in a converging-diverging nozzle. The focus of the first problem is to explore the characteristics of the growth and collapse of a spherical cloud of bubbles. The prototypical problem solved considers a finite cloud of nuclei that is exposed to a decrease in the ambient pressure which causes the cloud to cavitate. A subsequent pressure recovery then causes the cloud to collapse. This is typical of the transient behaviour exhibited by a bubble cloud as it passes a body or the blade of a ship propeller. The simulations employ the fully nonlinear, non-barotropic, homogeneous two-phase flow equations coupled with the Rayleigh-Plesset equation for the dynamics of individual bubbles. A Lagrangian integral method is developed to solve this set of equations. The computational results confirm the idea put forward by Morch and his co-workers (Morch [1980], [1981], [1982]; Hanson et al. [1981]) who speculated that the collapse of the cloud involved the formation of a shock wave on the surface of the cloud and that inward propagation and geometric focusing of this shock would lead to very large localized pressure pulses. The effects of varying the bubble population density, the cavitation number, and the ratio of the cloud size to the bubble size are examined. The theoretical results are shown to provide a satisfactory explanation for dynamic structures and acoustic signature observed in recently conducted experiments of cloud cavitation at California Institute of Technology (Reisman and Brennen [1996]; Brennen et al. [1996]). It is concluded that the formation and focusing of bubbly shock waves are responsible for the severe noise and damage potential in cloud cavitation.ududThe second problem investigates the nonlinear behavior of a bubbly cavitating flow, both steady and unsteady, through a converging-diverging nozzle. Two different flow regimes are found from steady state solutions: quasi-steady and quasi-unsteady. The former is characterized by the large spatial fluctuations in the downstream of the flow. Bifurcation occurs as the flow transitions from one regime to the other. An analytical expression for the critical bubble size at bifurcation is obtained. Finally, unsteady solutions in a period of consecutive times are presented. These solutions are characterized by the downstream spatial fluctuations coupled with large pressure pulses changing in both magnitude and location with time. The characteristics of these pulses are similar to the shock pulses of Part I and are produced by the local violent collapse of the bubbles in the flow.ud
展开▼
机译：本文考虑了两个问题：空化气泡云的非线性动力学和会聚-发散喷嘴中的气泡空化流。第一个问题的重点是探索球形气泡云的生长和破裂的特征。解决的原型问题考虑了有限的原子核云，该云暴露于环境压力的降低之下，这导致该云汽化。随后的压力恢复会导致云崩溃。这是气泡云通过船体或螺旋桨叶片时所表现出的瞬态行为的典型表现。该模拟使用完全非线性，非正压，均质的两相流动方程式与Rayleigh-Plesset方程式相结合来求解单个气泡的动力学。拉格朗日积分法被开发来解决这组方程。计算结果证实了Morch及其同事提出的想法（Morch [1980]，[1981]，[1982]； Hanson等人[1981]），他们推测云的崩溃牵涉到云的形成。云表面的冲击波以及这种冲击的向内传播和几何聚焦会导致很大的局部压力脉冲。研究了改变气泡密度，空化数以及云尺寸与气泡尺寸之比的影响。理论结果表明，对于最近在加利福尼亚理工学院进行的云空化实验中观察到的动态结构和声学特征提供了令人满意的解释（Reisman和Brennen [1996]； Brennen等[1996]）。结论是，气泡冲击波的形成和聚焦是造成云空化过程中严重噪声和破坏潜能的原因。 ud ud第二个问题是通过收敛，研究气泡空化流的稳态和非稳态非线性行为。发散喷嘴。从稳态解中发现了两种不同的流态：准稳态和准非稳态。前者的特征是流的下游空间波动大。当流量从一种状态过渡到另一种状态时，会发生分叉。获得了分叉处的临界气泡尺寸的解析表达式。最后，给出了连续时间内的非定常解。这些解决方案的特征是下游空间波动，加上大的压力脉冲，其大小和位置都随时间变化。这些脉冲的特性类似于第I部分的冲击脉冲，并且是由气泡在流中的局部剧烈崩塌而产生的。 ud
展开▼
