Purpose

The study aims to display the bubbles' evolution in the shear layer and their relationship with the pressure fluctuations. Furthermore, the coherent structures of the first six modes are extracted, in order to provide insight into their temporal and spatial evolution and determine the relationship between cavitating bubbles and coherent structures.

Design/methodology/approach

In the present study, numerical simulations of submerged jet cavitating flow were carried out at a cavitation inception condition inside an axisymmetric cavity using the large eddy simulation (LES) turbulence model and the Schnerr–Sauer (S–S) cavitation model. Based on snapshots produced by the numerical simulation, dynamic mode decomposition (DMD) was performed to extract the three-dimensional coherent structures of the first six modes in the shear layer.

Findings

The cavitating bubbles in the shear layer are deformed to elongated ellipsoid shapes by shear forces. The significant pressure fluctuations are induced by the collapse of the biggest bubble in the group. The first mode illustrates the mean characteristics of the flow field. The flow in the peripheral region of the shear layer is mainly dominated by large-scale coherent structures revealed by the second and third modes, while different small-scale coherent structures are contained in the central region. The cavitating bubbles are associated with small size coherent structures as the sixth or higher modes.

Practical implications

This work demonstrates the feasibility of LES for high Reynolds number shear layer flow. The dynamic mode decomposition method is a novel method to extract coherent structures and obtain their dynamic information that will help us to optimize and control the flow.

Originality/value

(1) This paper first displays the three-dimensional coherent structures and their characteristics in the shear layer of confined jet flow. (2) The relationship of bubbles shape and pressure fluctuations is illustrated. (3) The visualization of coherent structures benefits the understanding of the mixing process and cavitation inception in jet shear layers.

中文翻译：

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受限水下射流剪切层中空化气泡和相干结构的时空演化

目的

该研究旨在显示剪切层中气泡的演变及其与压力波动的关系。此外，提取了前六种模式的相干结构，以深入了解它们的时间和空间演变，并确定空化气泡与相干结构之间的关系。

设计/方法/方法

在本研究中，使用大涡模拟 (LES) 湍流模型和 Schnerr-Sauer (S-S) 空化模型在轴对称腔内的空化开始条件下对浸没射流空化流进行数值模拟。基于数值模拟产生的快照，进行动态模式分解（DMD）以提取剪切层中前六种模式的三维相干结构。

发现

剪切层中的空化气泡在剪切力的作用下变形为细长的椭圆体形状。显着的压力波动是由组中最大气泡的破裂引起的。第一种模式说明了流场的平均特性。剪切层外围区域的流动主要以第二和第三模式揭示的大尺度相干结构为主，而中心区域包含不同的小尺度相干结构。空化气泡与作为第六或更高模式的小尺寸相干结构相关联。

实际影响

这项工作证明了 LES 用于高雷诺数剪切层流的可行性。动态模态分解方法是一种提取相干结构并获取其动态信息的新方法，有助于我们优化和控制流动。

原创性/价值

(1) 本文首先展示了受限射流剪切层中的三维相干结构及其特征。(2) 说明气泡形状与压力波动的关系。(3) 相干结构的可视化有助于理解射流剪切层中的混合过程和空化开始。