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Title:Experimental investigation of the impact of the permeable layer thickness on the surface flow above and the threshold for incipient motion of submerged objects
Author(s):Wu, Heng
Director of Research:Garcia, Marcelo H.
Doctoral Committee Chair(s):Garcia, Marcelo H.
Doctoral Committee Member(s):Landry, Blake J.; Parker, Gary; Tinoco, Rafael O.; Valocchi, Albert J.
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Porous media, PIV, initiation of motion
Abstract:Previous work regarding flows over a permeable bed has confirmed that the flow characteristics deviate from those observed in impermeable rough boundary layers. However, the effect of the permeable bed thickness is not fully understood. Current models for threshold of incipient motion implicitly assume an impermeable rough bed when evaluating the fluid forces exerted on submerged objects. Natural river beds are permeable and the impact of the presence of bed permeability or the thickness of the permeable bed on the threshold for incipient motion still remain unclear. The main research questions of the systematic experimental study in this dissertation are to better understand the following: (1) How does the thickness of the permeable bed modifies the turbulent boundary layer above, specifically, the mean velocity profiles, turbulence intensities, and flow resistance? (2) How does the presence of bed permeability as well as the thickness of the permeable bed affect the threshold for incipient motion of submerged spheres and cylinders? The experiments involved two facilities: the Illinois Hyporheic Flow Facility (IHFF) and the Woods Hole Oceanographic Institute (WHOI) flume. The WHOI flume was used to address the first research question. In this set of experiments, double-averaged (temporal and spatial) velocity fields obtained from Particle Image Velocimetry (PIV) showed that the mean velocity profiles for different thickness of the permeable layer collapse to the fully-rough velocity profile when scaling the flow velocity with shear velocity and the wall-normal distance with the diameter of the marbles in the bed. The location of the peak in turbulence intensities migrates closer to the bed as the thickness of the permeable bed increases. The friction factor increases with the Reynolds number based on momentum thickness of the boundary layer and was found to be slightly larger for thicker permeable beds. Both flumes, IHFF and WHOI, were used to address the second research question. In these experiments, the threshold of incipient motion was characterized with the Shields parameter, which was estimated with upstream flow velocity measurements. A comparison of the threshold for incipient motion of objects resting on permeable and impermeable beds having the same flow-bed geometries showed larger Shields parameter required to initiate motion on a permeable bed than on an impermeable one. The increment in the critical Shields parameter was found to be associated with a decrease of the lift force exerted on the objects when resting on a permeable bed. Bed permeability of the bed in the experiments evaluating the effect of permeable bed thickness, was an order of magnitude smaller than the ones comparing permeable and impermeable beds. As a consequence, the critical Shields parameter for submerged spheres and cylinders increases with the thickness of the permeable bed but the effect was limited compared to the case with larger bed permeability.
Issue Date:2019-04-18
Rights Information:Copyright 2019 Heng Wu
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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