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Geometry Mediated Drag Reduction Using Wrinkled & Textured Surfaces

Event Type: 
Date and Time: 
Friday, May 19, 2017 - 16:15
CTR Conference Room 103
Event Sponsor: 
Parviz Moin, Director of Center for Turbulence Research
Shabnam Raayai-Ardakani, PhD Candidate, Department of Mechanical Engineering, Massachusetts Institute of Technology

Previous investigations have suggested that surfaces textured with riblets can reduce the frictional drag force in high Reynolds number laminar and turbulent flow regimes. Shark skin and synthetic manufactured “shark tape” are widely known as examples of such passive drag reduction mechanisms. Inspired by the ribs on the denticles of fast swimming shark species, riblets of different shapes have been studied under laminar and turbulent flow conditions to understand their drag reduction mechanism and to offer guides for designing optimized low-friction bio-inspired surfaces, often with confounding results that show a net frictional drag increase instead of drag reduction. In this talk, Raayai-Ardakani seeks to identify and understand the different physical mechanisms that contribute to the viscous skin friction on textured and wrinkled substrates. First, Raayai-Ardakani uses a rescaling of the Navier-Stokes equations together with conformal mapping to establish a simplified theory for laminar boundary layer flow over V-groove riblets and explore the self-similarity of the velocity profiles as a function of a newly rescaled form of the local Reynolds number and the aspect ratio of the riblets. Then, Raayai-Ardakani uses numerical simulation of boundary layers over sinusoidal riblet surfaces to investigate the use of wrinkled surfaces as a form of riblet to explore more complex textures than the asymptotic theory can describe. Finally, Raayai-Ardakani presents the results of combined computational and experimental exploration of the effects of textured rotors on torque reduction in steady flow between concentric cylinders, widely known as Taylor-Couette Flow. Using 3D printed periodical-textured rotors and a custom designed and built Taylor-Couette cell which can be mounted on a stress-controlled rheometer, Raayai-Ardakani measures the torque on the inner rotor as a function of the Reynolds number of the flow, the aspect ratio of the riblets and the relative size of the riblets (with respect to the gap of the Taylor-Couette cell). Experimental data showing up to 20% torque reduction are then compared with numerical simulations of steady and unsteady axisymmetric Taylor-Couette flow over textured rotors to highlight how the kinematics near the periodic riblets are changed and give rise to a torque reduction as the aspect ratio and riblet wavelength are progressively varied. 

Shabnam Raayai-Ardakani is a PhD candidate at the Department of Mechanical Engineering at MIT, working at the Hatsopoulos Microfluidics Laboratory (HML) with Professor Gareth McKinley and is interested in understanding the mechanics of surface textures (especially wrinkled surfaces) and their effect on drag force experienced by the surfaces. She has obtained her Master’s degree in Mechanical Engineering from MIT in 2013 and worked with Professor Mary Boyce on mechanics of graded wrinkling. She received her Bachelor’s degree in Mechanical Engineering from Sharif University of Technology in 2011.