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Title:Experimental analysis of flow and turbulence characteristics in oscillatory boundary layers from LDV measurements
Author(s):Mier Lopez, Jose Maria
Director of Research:Garcia, Marcelo H.
Doctoral Committee Chair(s):Garcia, Marcelo H.
Doctoral Committee Member(s):Best, James L.; Christensen, Kenneth T.; Parker, Gary
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Oscillatory flow
boundary layer
oscillatory boundary layer
transition regime
smooth bed
laser doppler velocimetry (LDV)
laser doppler anemometry
oscillatory tunnel
bed shear stress
phase difference
viscous sublayer
phase lead
phase lag
laminar regime
turbulent regime
velocity measurements
mean flow
wave Reynolds number
wave friction factor
ensemble average
Abstract:The oscillatory boundary layer represents a particular case of unsteady wall-bounded flows in which fluid particles follow a periodic sinusoidal motion. Unlike steady boundary layer flows, the flow regime and bed roughness character of oscillatory flows change in time during the oscillation, a characteristic that introduces a high degree of complexity in the analysis. Experimental work in this topic started in the 1960s followed by numerical work in the late 1980s, yet it is not completely understood, particularly in the transitional regimes. In this work, several oscillatory flow experiments were performed in the Large Oscillatory Water and Sediment Tunnel (LOWST) facility at the Ven Te Chow Hydrosystems Laboratory. A custom PVC floor was built inside the tunnel to obtain a flat and smooth bed. The range of wave Reynolds numbers tested spanned all along the transition regime of the oscillatory boundary layer between the upper limit of the laminar regime and the lower limit of the turbulent regime (3x104 < Rew < 9x105). A 3D laser Doppler velocimetry (LDV) system was used to measure instantaneous flow velocities with high spatial and temporal resolution, which allowed capturing flow features with great detail inside the boundary layer and even inside the viscous sublayer in some cases. A special set-up was built involving two LDV probes and a refraction-correcting device to be able to measure all three velocity components (u, v, w) simultaneously. From the velocity measurements, flow characteristics were obtained through the analysis of different variables including mean flow velocities, boundary layer thickness, turbulence intensities, turbulent kinetic energy, viscous and Reynolds stresses, turbulence production, eddy viscosity, quadrant analysis, bed shear stresses, shear velocity, wave friction factor and viscous sublayer thickness. In particular, the results of this work provide detailed evidence of the competition between laminar and turbulent effects taking place in the transition regime of the oscillatory boundary layer as Rew increased. A surprising behavior was observed in the phase of the peak bed shear stress, which changed dramatically with Rew: first leading about 40º ahead of the outer flow for low Rew, then lagging up to 25º behind for the transitional Rew experiments, and finally returning slightly ahead about 5º for high Rew. This finding is expected to have significant implications for the entrainment and transport of sediment near the bed. Also, investigation of the viscous sublayer revealed that the classic steady flow threshold of z+ = 5 doesn’t work well for oscillatory flows. A new method was developed to calculate the thickness of the viscous sublayer taking into account the ratio of viscous to turbulent forces near the bed. These results can be directly applied to better understand sediment transport in the ocean under the action of waves and currents. Furthermore, they will be also useful for a variety of engineering applications related to fluid mechanics including aerospace, biomedical research, engine design, turbines, industrial machinery, pumping systems, pipe transport, marine hydrokinetics, wave dynamics, and river, coastal and estuarine processes.
Issue Date:2015-04-23
Rights Information:Copyright 2015 Jose M. Mier Lopez
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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