Coherent Structures in Complex Sheared Flows

在工程和工业应用中，湍流是大多数流体流动的普遍特征。在有壁流动中，流体颗粒在流动中的随机运动对混合过程和与固体表面的相互作用有重要影响。湍流一般可以分解为相干和非相干两部分。流动中多尺度相干（或湍流）结构的存在对流动动力学有重要影响。它们的作用是重新分配动能，并负责混合层中质量、热量和动量的传输。因此，关注这些结构可以被认为是表征和理解湍流的适当和富有成效的方法。假设两相流中流体-空气相互作用对相干结构有显著影响，特别是在靠近自由表面的位置。此外，流动尺寸决定了二次流的强度和波动程度，也影响了结构的演化和分布。尽管研究人员对湍流进行了广泛的研究，但内部湍流机制以及上述效应对湍流特性的影响仍然存在很大的争议。湍流具有强烈的三维性，这一事实为在中高雷诺数下使用定量技术识别相干结构带来了挑战。这是由于传统测量设备的固有局限性和高成本，与数值方法相反，它们不能提供速度、压力和涡度的定量等值面。随着仪器和增强的计算工具的可用性，有可能开始一个研究计划来解决上述问题并了解它们对湍流结构的影响。本研究将考虑一个创新和先进的计算研究计划，以应对实验方法的局限性。我们实验室中可用的工具将用于运行一些实验以进行验证。为了实现当前的研究目标，我们选择了不同的流场进行研究：光滑和粗糙的明渠流场、流过钝体的流场、流过向前和向后台阶的流场。在所有情况下，将使用不同展弦比的两相流模型来研究流动参数对相干结构的影响。本研究的主要目标是：(1)提高我们对不同流场中湍流结构的认识；(2)增强对自由表面附近结构的形成和动力学的了解；(3)研究二次流对这些结构的影响。该研究将为工程应用提供有价值的工具。在5年的周期内，将培训10名HQP（5名理学硕士，5名理学学士）。

Turbulence is a ubiquitous feature of most fluid flows in engineering and industrial applications. The random motion of the fluid particles within the flow provides a major influence on the mixing process and the interaction with the solid surfaces in wall-bounded flows. Turbulent flows can generally be decomposed into coherent and incoherent parts. The existence of the multi-scale coherent (or turbulent) structures in the flow has an important effect on flow dynamics. They act to redistribute the kinetic energy and they are responsible for the transport of mass, heat, and momentum in the mixing layers. Thus, focusing on these structures can be considered as an appropriate and fruitful approach to characterize and understand turbulent flows. It is postulated that the fluid-air interaction in a two-phase flow has a significant effect on the coherent structures, particularly at locations close to the free-surface. Furthermore, the flow dimensions determine the strength and the fluctuation degree of the secondary currents, which also affect the evolution and distribution of the structures. Although researchers have broadly investigated turbulent flows, the internal turbulence mechanism and the influence of the aforementioned effects on the turbulence characteristics are still highly debated. The fact that turbulent flows are strongly three-dimensional creates challenges for identifying the coherent structures using quantitative techniques at moderate and high Reynolds numbers. This is attributed to the inherent limitations and high cost of conventional measurement devices which, contrary to a numerical approach, do not offer quantitative isosurfaces of velocity, pressure, and vorticity With the availability of instrumentation and enhanced computational tools, it is possible to begin a research program to address the above issues and understand their effects on the turbulent structures. An innovative and advanced computational research program will be considered in this research to cope with the limitations in the experimental methods. Available tools in our labs will be used to run a few experiments for validation purposes. To achieve the goals of the current research, different flow fields have been chosen for the study: smooth and rough open-channel flow, flow past a bluff body, and flow over forward and backward-facing steps. In all cases, a two-phase flow model with various aspect ratios will be used to investigate the effect of the flow parameters on the coherent structures. The key objectives of the present research are: (i) to improve our understanding related to the turbulent structures in different flow fields, (ii) provide an enhanced picture of the formation and dynamics of the structures near the free-surface and (iii) investigate the effect of the secondary current on these structures. The study will provide valuable tools that can be used in engineering applications. Over the 5-year cycle, 10 HQP (5 MSc, 5 BSc) will be trained.