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Dissertation
Turbulent flow patterns around variably submerged boulders in mountain streams and implications to bedload transport
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2014
DOI: 10.17077/etd.005711
Abstract
The mean and turbulent flow fields around a single, spherical boulder mounted atop a flat rough bed were experimentally investigated under different relative submergence, i.e., ratio of the approach flow to the boulder diameter, H/dc, conditions. The effects of the boulder on its surrounding mean flow patterns and turbulent statistics were investigated first. Particular emphasis was given on determining the spatial distribution of the bed shear stress in the vicinity of the boulder. The topology of the vortex structures that develop around a fully-, a barely- and a partially-submerged boulder were identified. The corresponding Reynolds numbers based on the flow depth, H, and the bulk flow velocity, Ubulk, were 7.22×105, 2.10×105, and 1.44×105, respectively. State-of-the art experiments were conducted and the flow field around the boulder was interrogated using Digital Particle Image Velocimetry (DPIV). The mean flow for the fully-submerged boulder was characterized by flow separation at the boulder crest and a recirculation region extending 1.4dc downstream of the boulder. For the barely- and partially-submerged boulders the recirculation region was smaller and non-existent, respectively, and intense downwelling immediately downstream of the boulder was present. For all relative submergence conditions, the boulder increased the magnitude of the Turbulent Kinetic Energy and Reynolds shear stress by 2 to 3 times. The variability of the bed shear stress (or equivalently of the friction velocity, u*) was examined using 5 methods. The Boundary Characteristics Method (BCM) most accurately captured the u* distribution for the fully submerged boulder, as it accounted for the additional form drag generated by the boulder. The u* distribution around the barely and partially submerged boulders was best attributed via the simplified TKE method, which considered the turbulent flow patterns around the boulder. The wake of the fully submerged boulder was found to be dominated by arch vortices, which tilted in the downstream direction under the influence of the strong mean flow shear and a jet near the junction of the boulder with the bed. The arch structures also dominated the boulder wake for the barely submerged boulder. In this case, however, the arch vortices were tilted upstream under the influence of the self-induction due to the Biot-Savart Law and the presence of the jet. In the shallow wake of a partially submerged boulder, the key feature was a Kármán vortex street resulting from the transverse shear layers shed from the boulder flanks. For all relative submergence conditions a weak horseshoe vortex and a pair of trailing vortices were observed, highlighting the robustness of these features. The findings of this study provide a kernel of knowledge on the hydrodynamics around a single boulder that is the stepping stone for investigating flow and sediment transport dynamics within a boulder array.
Details
- Title: Subtitle
- Turbulent flow patterns around variably submerged boulders in mountain streams and implications to bedload transport
- Creators
- Achilleas Georgios Tsakiris
- Contributors
- Thanos Papanicolaou (Advisor)Jacob A. Odgaard (Committee Member)James H J Buchholz (Committee Member)William E. Eichinger (Committee Member)Christopher G. Wilson (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Autumn 2014
- Publisher
- University of Iowa
- DOI
- 10.17077/etd.005711
- Number of pages
- xviii, 231 pages
- Copyright
- Copyright 2014 Achilleas Georgios Tsakiris
- Comment
- This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: https://www.lib.uiowa.edu/sc/contact/
- Language
- English
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (pages 225-231).
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9984036791802771
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