Coherent Structures in Complex Sheared Flows

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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.