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Abstract

Drag reduction techniques have been the crux of aerodynamic research for many decades due to their potential to reduce the use of fossil fuels and the resulting air pollution. One such strategy is to delay the laminar-turbulent transition employing active or passive ow control methods to maintain the laminar flow to a maximum extent which directly reduces the skin friction drag on the streamlined bodies. In this aspect, two biomimetic geometries inspired by fish scale arrays and the leadingedge serrations have been investigated in this work. Both geometries modify the base flow to be more stable and less susceptible to fundamental fluid instability, namely the Tollmien-Schlichting wave and the cross flow vortices. Biomimetic fish scale arrays produce periodic and spatially varying velocity modulation known as streaks. The backward-facing step like flow in the central region and the zig-zag motion in the overlapping region of the scale arrays is the main factor to generate the streaky flow. When this streaky base flow interacts with the Tollmien Schlichting wave the growth rate of the unstable wave is attenuated to delay the transition. The extent of the transition delay depends on the amplitude of the streaks and for the maximum streak amplitude, the streaky base flow can completely cancel the unstable Tollmien-Schlichting wave. This model can be customized on aerofoils where the transition is promoted by the Tollmien-Schlichting wave and to design a natural laminar flow aerofoil. Biomimetic leading edge serrations act like three-dimensional cascade blades which are very different from the conventional vortex generators. The serrations turn the incoming flow towards the inboard portions of the wing and this flow turning was proved using flow visualisation study on a flat plate in a water tunnel. Large Eddy Simulations on a serrated aerofoil also indicate the inward turning of the flow to delay the instabilities on a swept wing. Based on the current findings, noise reduction is also hypothesized and therefore this special geometry should be further investigated for consideration in the development of silent flight.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology > Engineering

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