Coherent Structures in Complex Sheared Flows

湍流是工程和工业应用中大多数流体流动的普遍特征。流体颗粒在流动中的随机运动对混合过程以及与壁面边界流动中的固体表面的相互作用产生了重大影响。湍流一般可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，集中在这些结构可以被认为是一个适当的和富有成效的方法来表征和理解湍流。 据推测，在两相流中的流体-空气相互作用的相干结构，特别是在靠近自由表面的位置上具有显着的效果。此外，水流尺度决定了二次流的强度和起伏程度，从而影响了结构的演化和分布。虽然研究人员对湍流流动进行了广泛的研究，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争议。湍流是强三维的这一事实为在中等和高雷诺数下使用定量技术识别相干结构带来了挑战。这是由于固有的局限性和传统的测量设备，相反的数值方法，不提供定量的速度，压力和涡度的等值面的成本高。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，以解决上述问题，并了解其对湍流结构的影响。 一个创新的和先进的计算研究计划将被认为是在这项研究中，以科普在实验方法的局限性。我们实验室中的可用工具将用于运行一些实验以进行验证。为了实现当前研究的目标，不同的流场已被选择用于研究：光滑和粗糙的明渠流，流过海崖体，以及流过向前和向后的台阶。在所有情况下，将使用具有各种纵横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：（i）提高我们的理解有关的湍流结构在不同的流场，（ii）提供一个增强的图片的形成和动态的结构附近的自由表面和（iii）调查这些结构的二次流的效果。这项研究将提供有价值的工具，可用于工程应用。在5年周期内，将培训10名HQP（5名MSc，5名BSC）。

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.