Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Siddiqui, Kamran

Abstract

The contributions of drag to energy consumption in the transportation sector are significant and often unavoidable. Biomimetic surfaces are promising as passive drag reduction mechanisms. Among them, fish scale arrays are beneficial in the laminar and transitional flow regimes but lack fundamental understanding. This research addressed this need and investigated the underlying flow mechanisms over fish scale arrays. Experimental measurements revealed the presence of flow recirculation, streamwise velocity streaks, spanwise velocity fluctuations, and wall normal vorticity streaks, all of which play a role in the near wall flow behaviour. Numerical simulations revealed the superior friction drag reduction capabilities of the diamond scale shape. The findings highlight how the surface variations contribute to the formation of flow behaviours which influence the skin friction and contribute to delaying the transition to turbulence. The improved understanding of underlying processes from this study will aid the optimization of scale shape to reduce drag.

Summary for Lay Audience

The interaction between solid surfaces and surrounding fluids is common in our everyday lives. Whether it be in a car or on a plane, the interaction between the vehicle and the surrounding air generates drag forces. These drag forces lead to excessive energy consumption and consequently greenhouse gas emissions, especially in the transportation sector. The shear drag associated with the near wall boundary layer can often account for between 20-50% of the drag force depending on the mode of transportation. Several approaches to friction drag reduction have been tested with varying results. Biomimetic surface modifications such as shark skin inspired riblets and hydrophobic microstructures have shown promising drag reduction capabilities in a turbulent flow. Biomimetic fish scale arrays are another technique which has shown promising results in the laminar and transitional flow regimes. However, the lack of understanding of how the fish scale arrays interact with the fluid raises questions about how they function as a drag reduction mechanism. Thus, a deeper understanding of how fish scale arrays interact with the surrounding flow is needed to understand the underlying friction drag reduction mechanisms. This deeper understanding will help inform the design of structured surfaces which target drag reduction in commercial applications.

The current study uses both experimental and numerical techniques to evaluate and understand the unique flow patterns in the near wall boundary layer over biomimetic fish scale arrays. Experimental measurements provide an in-depth analysis of the near wall flow behaviour and highlight four mechanisms which are generated due to the unique scale pattern. Flow recirculation, streamwise velocity streaks, spanwise velocity fluctuations, and wall normal vorticity streaks are all found to play an important role in changing the friction forces the surface experiences and delaying the transition from laminar to turbulent flow. Numerical simulations explored the impact of scale shape, highlighting the superior capabilities of the diamond scale shape for improved drag reduction. This work provides new contributions in terms of the understanding of the mechanisms driving the flow behaviour over fish scales and potential techniques for optimization of the drag reduction behaviour.

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