数据分析方法在气液相变复杂流动中的应用

Application of Data Analysis Methods to Complex Flows with Gas-Liquid Phase Change

空化作为一种重要的复杂水动力学问题,是当液体局部压力降低至饱和蒸气压时,发生的由液相转变为气相的现象。空化现象具有明显的三维流动特征与剧烈的非定常特性,在水力机械、船舶推进器、水利工程中广泛存在,且通常会带来不利的影响,长期以来一直是水动力学领域研究的重点与难点。为探究非定常空化流场结构,采用图像粒子测速PIV-LIF的实验方法,利用高速相机从实验段侧拍摄空化流动,通过荧光示踪粒子跟随液体流动,从而测得空化速度场。利用高分辨率的速度场和灰度快照数据,进行动力学模态分解(dynamic mode decomposition,DMD),开展非定常空化流场结构分析。DMD方法可以有效地将流场分解成若干个模态,并识别出主导模态进行分析。基于DMD方法对空化的流场进行模态分解,具有模态频率和增长率单一的特点,在对空化周期性流动的分析中有较大优势。通过分析占较高能量的主要模态,并进行模态重构,发现这些模态均含有一定空泡运动特征,与流场中回射流与伴生涡结构相关,空泡的主要演变频率与涡脱落频率一致。各个模态的流场特征结果表明:空化流第1阶模态对应频率为0,代表平均流场;第2阶模态对应频率约为空泡脱落频率,揭示了空泡在发生处前缘周期性地生长与脱落;第3阶模态对应频率约为第2阶模态的2倍,揭示了2个大尺度旋涡在水翼后方存在融合行为;第4阶模态对应频率约为第2阶模态的3倍,具有更高的频率,表征流场中存在一些小尺度旋涡的融合行为。最后对不同空化数下的空化流场进行了模态分解分析,发现脱落空泡的旋涡结构随着空化数的减小而增大,第2阶模态频率(case 1:302.66 Hz,case 2:105.35 Hz,case 3:69.32 Hz)随着空化数(case 1:1.49,case 2:1.4,case 3:1.31)的减小而减小。

Cavitation is a complex hydrodynamic phenomenon that occurs when the local pressure in a liquid drops below the saturation vapor pressure,resulting in the phase transition of the liquid into vapor.This process was characterized by intricate three-dimensional flow structures and highly unsteady,violent dynamics.For applications such as hydraulic machinery,ship propulsion,and hydraulic engineering,cavitation often induces detrimental effects,and has significant impact on performance and reliability in engineering systems.In order to investigate the structure of non-constant cavitation flow field,this paper adopted the experimental method of image particle velocimetry PIV-LIF,which used a high-speed camera to photograph the cavitation flow from the side of the experimental section and followed the liquid flow by fluorescent tracer particles,so as to measure the cavitation velocity field.Using the high-resolution velocity field and grayscale snapshot data,the dynamic mode decomposition(DMD)was carried out to analyze the structure of the non-constant cavitation flow field.The DMD method can effectively decompose the flow field into several modes and identify the dominant mode for analysis.Based on the DMD method,the decomposition of the cavitated flow field has the characteristics of a single mode frequency and growth rate,which is more advantageous in the analysis of cavitated periodic flow.By analyzing the main modes that account for higher energy and performing modal reconstruction,it is found that all these modes contain certain characteristics of vacuole motion,which are related to the structure of the re-entrant jet and the accompanying vortex in the flow field,and the main evolution frequency of the vacuole is consistent with the vortex shedding frequency.The results of the flow field characteristics of each mode show that the first-order mode of the cavitation flow corresponds to a frequency of 0,which represents the average flow field;the second-order mode corresponds to a frequency of about the vacuole shedding frequency,which reveals that the vacuole grows and sheds periodically at the leading edge of the occurrence;the third-order mode corresponds to a frequency about twice that of the second-order mode,which reveals the fusion behavior of two large-scale vortices behind the hydrofoil;the fourth-order mode corresponds to a frequency about three times higher than the second-order mode,which characterizes the fusion behavior of some small-scale vortices in the flow field.Finally,the modal decomposition analysis of the cavitation flow field with different cavitation numbers was carried out,and it is found that the vortex structure of the shedding vacuole increases with decreasing cavitation number,and the second-order mode frequency decreases with decreasing cavitation number.