AI 中文总结
研究壁面湍流中相干结构因果关系，发现因果假设不成立，仅约一半结构比非相干湍流因果性更强。通过分析表明其因果性取决于局部流环境，如 Q2、Q4 事件各有不同主导指标，强调解释相干结构需考虑其与周围湍流相互作用。
AI 中文摘要
强烈切向雷诺应力的紧凑结构（Q 事件）是壁面湍流的著名组成部分，且已被证明具有相干性，因为它们大致支配自身演化。因此常假定它们在因果关系上也很重要，即能解释流动的非相干部分的演化。本文表明因果关系假设通常不成立。只有约一半的流动结构在因果关系上比等量的非相干湍流更显著。为解释这种变异性，进行了基于特征的分析和条件平均。对于附着壁面的 Q2 事件和脱离壁面的结构，因果增强的事件其应变率和展向涡度升高。对于附着壁面的 Q4 事件，主要指标是壁法向和流向涡度分量；增强的因果重要性与强壁法向涡度相关。这些发现表明象限事件在因果分析中不能被视为动态均匀的类别。它们的因果重要性很大程度上取决于局部流动环境，强调了根据其与周围湍流的相互作用来解释相干结构的必要性。
英文摘要
Compact structures of intense tangential Reynolds stress (Q events) are well-known components of wall-bounded turbulence, and have been shown to be coherent because they approximately govern their own evolution. It has therefore often been assumed that they also are causally significant, in the sense that they explain the evolution of the incoherent component of the flow, which could thus be modeled as a superposition of structures. Since strong events typically only fill a small percentage of the total flow volume, this is also cited as a reason for considering structures as targets for efficient engineering flow control. This paper shows that the causality assumption does not hold in general. Only about half of the structures identified in the flow are causally more significant than an equivalent volume of incoherent turbulence. To explain this variability, feature-based analysis and conditional averaging are performed. For wall-attached Q2 events and wall-detached structures, causally enhanced events are characterized by elevated strain rate and spanwise vorticity. These signatures are traced to intense upstream strain regions generated by the interaction of Q4-like motions impinging on Q2 events. For wall-attached Q4 events, the dominant indicators are instead the wall-normal and streamwise vorticity components; enhanced causal significance is associated with strong wall-normal vorticity. These findings show that quadrant events cannot be treated as a dynamically homogeneous class in causal analyses. Their causal significance depends strongly on the local flow environment, emphasizing the need to interpret coherent structures in terms of their interactions with the surrounding turbulence.
Comments29 pages, 18 figures
