Secondary currents in turbulent flows over rough walls

Lead Research Organisation: University of Southampton
Department Name: Sch of Engineering

Abstract

A majority of engineering and environmental flows occur over surfaces that exhibit spatial variations in roughness and/or topography. When a turbulent wall-flow evolves over such surfaces, it may exhibit unusual physical properties, depending on the relationship between the dominant length-scales of the surface and that of the flow. Specifically, when the dominant length-scale(s) of the surface in the cross-stream direction become(s) comparable to the dominant length-scale of the flow (such as boundary layer thickness or water-depth), then the flow also exhibits large-scale spatial heterogeneity that is locked-on to the surface heterogeneity. This flow heterogeneity is expressed in the form of localised secondary currents (SCs) that often extend across the entire depth of the flow and manifest themselves as large 'time-averaged' streamwise vortices accompanied by low- and high-speed regions. This surface-induced flow heterogeneity invalidates some of the fundamental tenets of turbulent wall-flows that were developed for flows over homogeneous surfaces. Therefore, current predictive tools that rely on these tenets can neither accurately predict nor offer insights into the complex physics of flows that contain surface-induced SCs. The significant effects of surface-induced SCs have recently been recognised in at least two disparate areas: 1) Performance of engineering systems such as in-service turbine blades, bio-fouled ship hulls and flow control; and 2) Understanding of the river flow dynamics with applications in flood management, eco-hydraulics and sediment transport. Over recent years, Southampton, Aberdeen, Glasgow and UCL have invested considerable efforts in advancing both these areas. Given the burgeoning interest in this topic, it would be timely to harness the synergies between these four leading groups to develop comprehensive understanding of turbulent flows in the presence of surface-induced SCs and establish a novel transformative framework to predict such flows.

This project will leverage the expertise, domain knowledge and infrastructure of four leading groups in the above-mentioned areas to bring about a paradigm shift in our approach to flows over heterogeneous surfaces that generate secondary currents. A comprehensive series of physical experiments (at Southampton & Aberdeen) and complementary numerical simulations (at Glasgow & UCL) will be performed to generate unprecedented data on surface-induced SCs. We will compare and contrast the behaviour of SCs across all four canonical wall-flows (boundary layers, open-channels, pipes and closed-channels) for the first time. The obtained data will underpin identification and validation of potential universalities (and differences) in drag mechanisms and momentum/energy transfer in these flows in the presence of surface-induced SCs. Synthesising the insights obtained from the data, a new framework leading to physics-informed semi-empirical and and theoretically-based numerical models will be developed to predict and optimise the influence of surface-induced SCs on turbulent wall-flows relevant to engineering/environmental applications.

Planned Impact

The design, maintenance, prediction, and control of any fluid system, either natural or manufactured, requires knowledge of the flow structure and friction at the wall/bed, which is necessary to predict flow rates, pressure gradients, flow-structure interaction, and/or fluid levels. The approaches currently used for quantifying flow structure and friction at solid boundaries are empirical and typically do not involve information on the secondary currents. This omission should be considered among the weakest elements of otherwise sophisticated design and modelling methodologies, particularly bearing in mind complex interrelations between turbulence and secondary currents. The proposed research intends to eliminate, or at least significantly reduce, this weakness by addressing the fundamentals of the problem and developing ways of incorporating the gained theoretical and physical insights into applied numerical modelling tools for the prediction of turbulent flows over rough walls.

In addition to academic beneficiaries, the outcomes of this research will lay a foundation for future societal and economic benefits to be reaped through existing networks and industrial partnerships, as well as providing further impact by training new highly-skilled researchers, public engagement, and developing a major cross-disciplinary collaboration between two disparate areas of fluids engineering (i.e., environmental and industrial flows) which until recently have been developing nearly independently.

The societal and economic benefits are relevant to: (1) central and regional agencies for waterway management, flood risk reduction, water quality control, and protection of ecosystem services (e.g. Scottish Environmental Protection Agency and similar) and international inter-governmental agencies dealing with regulations for ocean-going vessels (International Maritime Organisation); (2) ship and aircraft development firms as well as operators (e.g. BAE Systems, Shell) and anti-fouling coatings manufacturers (e.g. AkzoNobel, Hempel); (3) civil engineering and environmental consultancies developing and/or applying hydro-environmental modelling tools (e.g., Arup, Halcrow, HR Wallingford); (4) design firms dealing with fluids engineering systems and water companies responsible for the management, collection, treatment, and distribution of water; and (5) other end-users, directly and indirectly related to traditional applied fluid mechanics and hydraulic engineering and ranging from aerospace (US Air Force) to micro-fluidics applications that may take advantage of the project findings. To facilitate and accelerate this process, we will involve end-user representatives in the conceptual development and initial testing of the new approaches, and in identification of appropriate ways of their subsequent implementation in practice. Moreover, the host institutions will provide resources in the form of PhD studentships aimed at pursuing impactful projects that are natural extensions of the proposed research. Pairing the PhD students with the PDRAs will add significant value to the overall impact of the project and will be highly beneficial to the individuals involved (in terms of career development as well as training).

Finally, this project will support ongoing outreach activities which are fundamental in making science relevant to society and inspiring the next generation of scientists and engineers. We will participate in outreach at local primary and secondary schools as well as in planned events that take place in all four institutions as part of our general outreach activities.

Publications

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