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Mechanistic Study of Turbulence Boundary-Layer Flows at Wavy Surfaces
Lian Shen, Sc.D.
Abstract
The mechanism of turbulence at wavy boundaries is important to many applications. Examples include turbulence transport of momentum, mass, and heat at gas-liquid interfaces in many industrial processes involving multi-fluids flows, biomimetics of fish swimming using body waving motion for drag reduction and efficient propulsion, and environmental flows involving undulatory terrains and water surface waves. In this work, a systematic, simulation-based study was carried out to understand the fundamental mechanism of turbulence boundary-layer flows at wavy surfaces. Direct numerical simulation was first performed for flows over monochromatic waves to elucidate the details of the flow structure; large-eddy simulation with a new dynamic wave-surface roughness model was then performed for flows over irregular waves with broadband spectra to quantify the form drag and mechanical energy flux at the surface. Extensive comparison of the simulation results with measurement data was performed, and good agreement has been obtained. Substantial new understanding of the flow physics has been gained in terms of vortex dynamics, Reynolds stress variation, and kinetic energy budget. As an application, atmospheric boundary layers over ocean waves with wind turbine model were simulated for the study of offshore wind energy. It is found that wind-waves at different development stages possess different sea-surface roughness and have an appreciable effect on wind profile and the energy extraction rate of the turbines. In the presence of swells, which are large waves generated by distant storms, swell-to-wind momentum and energy transfer can substantially affect the wind field to cause fluctuations in and change the mean of wind power.
Biographic sketch:
Dr. Lian Shen earned his bachelor’s degree in mechanics from University of Science and Technology of China in 1993 and his doctoral degree in fluid mechanics from Massachusetts Institute of Technology in 2001. He currently holds the position of Benjamin Mayhugh Associate Professor in Department of Mechanical Engineering at University of Minnesota. Dr. Shen’s research interests include turbulence simulation and modeling, boundary-layer flows, free-surface and multi-fluids flows, and fluid mechanics in energy and environment applications. Dr. Shen has published extensively in top journals in fluid mechanics, including 14 papers in Journal of Fluid Mechanics. In addition, Dr. Shen has received a number of awards including the Martin A. Abkowitz Fellowship, the Office of Naval Research Young Investigator Award, and the T. Francis Ogilvie Lectureship.