Vortex evolution of the flow around an underwater vehicle in density-stratified flow
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摘要: 在真实海洋环境中，海水存在密度分层。当水下航行器在密度分层界面附近航行时，其尾流会破坏初始密度分层界面，形成分层尾迹。分层尾迹是非声目标特征之一，分层尾迹探测已成为水下航行体探测的新兴技术。为建立分层环境下水下航行体尾迹探测的理论基础和方法，应先探明水下航行体绕流涡系在密度分层介质中的发展演化特性。本文以缩比SUBOFF模型为研究对象，开展了实验测试和大涡模拟，得到了水下航行体绕流涡系在密度分层介质中的精细流场，大涡模拟结果、实验测试结果与相关文献测试结果基本吻合。研究发现：由于模型尾部分离涡存在强非定常性，尾流中盐水与淡水的掺混表现出多尺度特征；较强的孤立涡结构在掺混分层流体的过程中，逐渐演化为规则结构并脱离分层主界面，分层界面在一定宽度范围内整体上呈现出离散特征。研究结果表明，基于密度梯度的尾迹探测手段可以对水下航行体尾流场进行有效探测。Abstract: Density stratification is a natural existing phenomenon observed in marine environ-ments. When underwater vehicles navigate near density stratification interface, the wake flow disrupts the initial stratification interface, resulting in a stratified wake. As one of the non-acoustic target features, stratified wake becomes a typical signal for underwater vehicle detection. To establish theoretic basis for wake flow detection in stratified environments, it is imperative to understand the characteristics of vortex generation and evolution around underwater vehicles in the density-stratified flows. This work focuses on the fine flow field around a SUBOFF scale model with full-appendage, by experiment and Large Eddy Simulation (LES) method, to figure out the characteristics of vortex evolution around the SUBOFF scale model in density-stratified flow. It is found that the mixing of saline water and fresh water in the wake displays multi-scale features, due to the strong unsteadiness of the separated vortices in the wake flow. The isolated and intensified vortex tends to evolve into a relatively regular configuration in the process of stratified fluid mixing, and causes this part of the fluid to detach from the primary interface, leading to a stratified interface with discrete characteristics in a given lateral range in the wake flow. Therefore, wake detection based on density gradients of the wake flow of an underwater target might be feasible.
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图 1 中纬度海洋的典型密度分布
Figure 1. Representative density profile for the ocean at mid-latitudes
图 7 中横截面的流向速度结果对比
Figure 7. Comparison of axial velocities in the mid-transverse section
图 9 y = 0截面的时均密度分布与涡量分布
Figure 9. Time-averaged distribution of density and vorticity in the mid-longitudinal section
图 10 SUBOFF模型绕流涡结构（Q = 500 s−2等值面）
Figure 10. Three-dimensional vortex around SUBOFF （isosurface of Q = 500 s−2）
图 11 指挥台及平行中体周围涡结构（Q = 500 s−2等值面）
Figure 11. Three-dimensional vortex near the sail and the midbody of SUBOFF （Isosurface of Q = 500 s−2）
图 12 x = 0.5L处中纵截面上下侧湍动能对比
Figure 12. Comparison of turbulent kinetic energy on the upper and lower sides of the mid-longitudinal section at x = 0.5L
图 13 尾部收缩段涡结构（Q = 500 s−2等值面）
Figure 13. Hairpin vortex near the stern of SUBOFF （Isosurface of Q = 500 s−2）
图 14 t = 5.6 s时刻y = 0截面瞬时密度分布和涡量分布
Figure 14. Instantaneous distribution of density and vorticity in the longitudinal section at t = 5.6 s
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