【作者】 王昆；

【导师】 王航；

【作者基本信息】 四川大学 ， 水力学及河流动力学， 2022， 硕士

【摘要】 在剧烈掺气的高速水流中进行湍流强度测量十分困难。较为可行的方法之一是通过气泡速度的湍流强度间接反映水流的湍动程度。传统上通过电阻式两相流探针感知气泡在两点之间的运动并对大量样本进行相关性分析,基于一定假设条件近似得到气泡速度分布特征的方法,本文称之为全信号互相关方法(FSCC)。该方法的估值可靠性取决于测量时段内完整信号序列的整体相关性表现,在复杂流动中存在一定局限性。在FSCC方法的基础上发展出基于特定信号频率范围的三重分解相关性分析法(TDCC)和基于特定信号区间及相关性判定准则的自适应窗口相关性分析法(AWCC)。各方法在强湍流、强掺气流态中的适用性和可靠性仍存争议,对其表现进行系统性评估,对于水气两相流湍流特性研究方法的推进具有重要意义。本研究回归气泡速度测量的本质,通过人工识别水气界面测量信号中的有效信息,获得气泡在极短时间内的时均速度,以及大量气泡在给定采样时长内的速度波动情况。该方法以大量样本数据为基础,提供了最为接近原始数据的气泡湍流速度信息。以人工识别气泡方法(MBI)为基准,对现有相关性分析方法进行了分析、对比和评估。在水利工程普遍涉及的复杂水气两相流态中,选取消力池水跃和溢洪道掺气坎跌流两种掺气流动型式,分析其高含气量流动区域的速度分布及湍动特性,得到如下主要结论:(1)在弗劳德数近似等于12的水跃和Fr=6.6,8.5的掺气坎跌流中,上述四种数据分析方法得到的气泡时均流速分布基本一致,对MBI方法的主观性进行了验证。在水跃的底部剪切区、表面漩滚区和二者之间的过渡区,由AWCC方法计算得到的湍流强度与MBI方法最为接近。当选定合理的相关性判别参数时,使用AWCC方法估算湍流强度可以得到较为准确的结果。在掺气坎跌流冲击区内,由AWCC方法计算得到的湍流强度与MBI方法的最为接近。(2)综合分析表明,在水跃旋滚长度内,两相流的湍流强度未随时均流速的减小发生明显的沿程衰减。掺气坎跌流中的湍流强度在临底冲击区达到相对较大值。对弗劳德数相似、雷诺数不同的水跃和跌坎掺气流动分别进行对比,未发现明显的湍流强度差异。有关掺气水流湍流强度的比尺效应仍有待进一步研究。更多还原

【Abstract】 Estimation of turbulence intensity within a high-speed turbulent flow with intense aeration is challenging.A possible way to reflect the flow’s turbulence level is acquiring bubbles’ turbulence intensity.A traditional method is to detect bubble’s movement information between two fixed positions using intrusive dual-tip phasedetection conductivity probes,perform correlation analysis on a large number of samples,and obtain approximate bubble velocity distribution based on a series of assumptions,which is called full signal cross-correlation(FSCC)method in this thesis.The reliability of this method depends on the overall correlation performance of the complete voltage signal sequence during the measurement,and it presents some limitations in complex flows.Based on FSCC method,novel data processing methods are developed,such as the triple-decomposition cross-correlation(TDCC)technique based on specific signal frequency ranges,and the adaptive-window crosscorrelation(AWCC)technique based on specific signal interval and correlation filtering criteria.There is still argument about the applicability and reliability of these methods in strongly turbulent and intensely aerated flows.A systematic evaluation of their performance is of great significance for the advancement of research methods on turbulence characteristics of air-water two-phase flow.This study revisits the nature of bubble velocity measurement.The average bubble velocity in a very short time period and the velocity variation of a large number of bubbles in a given sampling period can be obtained by manually identifying the bubble detection information in the air-water interface measurement signal.Based on a large number of sample data,this method provides the bubbles’ velocity information closest to the ground truth.The existing correlation analysis methods are analyzed,compared and evaluated with reference to the manual bubble identification(MBI)method.Among the complex air-water two-phase flow regimes commonly involved in hydraulic engineering,two aerated flow types,i.e.,hydraulic jump in stilling basin and offset aerator flow in sloping chute,are selected to analyze the velocity distribution and velocity variation distributions within the highly-aerated flow regions,and the main conclusions are as follows:(1)In hydraulic jump(Fr ≈ 12)and offset aerator flow(Fr = 6.6 and 8.5),the four aforementioned approaches yield basically consistent bubble time-averaged velocity distributions,which validates the rationality of the MBI method.The turbulence intensity results of MBI are closest to those of the AWCC method in strong hydraulic jumps,including in the shear region,recirculation region and transition region in between.When appropriate correlation parameters are selected,the AWCC method can be used to estimate turbulence intensity accurately.The turbulence intensity results of MBI are closest to those of the AWCC method,in the impact zone of the offset aerator flow.(2)The comprehensive analysis shows that the turbulence intensity of the twophase flow does not attenuate significantly with the decrease of the time-averaged velocity within the roller length of hydraulic jump.The magnitude of turbulence intensity in offset aerator flow reaches relatively large value in the impinging region near the bottom.The aerated flows with similar Froude number and different Reynolds number are compared,and no obvious difference in turbulence intensity is found.The scale effects on the turbulence intensity of aerated flows remain to be further investigated and clarified.更多还原