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Study of Turbulence and Drag Reduction for Flow Over Backswimmer Textured Surface

Takrouri, K (2021) Study of Turbulence and Drag Reduction for Flow Over Backswimmer Textured Surface. Doctoral thesis, Liverpool John Moores University.

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Great benefits of biomimetic textured surfaces have been recognised through recent research. Such studies highly impact transportation-related fuel consumption and emission of carbon-dioxide. With 10% reduction of friction drag, 4 billion GBP/year savings are expected in the ship industry alone. The shape, spacing and alignment of the textured geometries together with the Reynolds number are the main parameters to characterise turbulent flow and drag performance associated with the use of these surfaces. The present research aims at investigating the effect of a novel biomimetic texture surface on turbulent flow behaviour and drag reduction. The texture is inspired by an aerodynamically-efficient insect called the Backswimmer also known as Notonecta Glauca. Direct Numerical Simulations (DNS) has been used to examine turbulence and drag reduction for the texture geometry. An in-house code for channel and pipe flows, CHAPSim, is used for the simulations. The code is adopted for (i) treating the novel textured surface using an Immersed Boundary Method (IBM) and (ii) improving the solver scalability by implementing a hybrid parallelisation approach. Verification and validation (where possible) for smooth and textured simulations have been undertaken. The effect of using the backswimmer geometry is studied at different geometry conditions with a constant bulk velocity at Reynolds number Re = 2800. Additionally, the effect of changing the Reynolds number was studied for one geometry condition at Re = 2800, 3500, 5500 and 7400. Compared to smooth channel, the backswimmer cases showed an increase in the wall-normal and spanwise fluctuating velocities, near wall vorticity and Reynolds shear stress. Among all the backswimmer cases, the increase in the wall normal and spanwise vorticity profiles with the textured element was almost unaffected by either the changes in the geometry or the increase in Reynolds numbers. Compared against smooth channel, the backswimmer geometry case showed a decrease in the ejection events as Reynolds number increases but a significant increase in the sweep events. The flow visualisations and vortex identification were simulated by calculating low- and high- speed streaks and second-largest Eigen value of the symmetric tensor (λ₂). It was shown that the streaky structures and vortex strength within the roughness elements were dependent on the change in the geometry conditions and Reynolds number. As the Reynolds number increased, the intensity of λ₂ increased within the roughness for the smaller Reynolds numbers and above the roughness crest for the higher Reynolds numbers.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: Drag Reduction; Backswimmer; CFD; Turbulence Modelling
Subjects: Q Science > QC Physics
T Technology > TA Engineering (General). Civil engineering (General)
Divisions: Engineering
Date Deposited: 08 Jan 2021 13:09
Last Modified: 31 Aug 2022 10:35
DOI or ID number: 10.24377/LJMU.t.00014232
Supervisors: Seddighi, M, Allanson, D and Batako, A
URI: https://researchonline.ljmu.ac.uk/id/eprint/14232
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