Thesis
Experimental characterization of surface-piercing turbulent wake flows
University of Iowa
Master of Science (MS), University of Iowa
Spring 2026
DOI: 10.25820/etd.008425
Abstract
Surface-piercing bluff body wakes govern the surface signature, drag, and flow control strategies of naval vessels, and drive wake-induced erosion of structures such as bridge piers. The deformable free surface produces anisotropic mass and momentum transfer that remains poorly characterized experimentally, particularly for sharp trailing edge geometries within the subcritical Froude number regime. High-fidelity experimental data is needed to compute and validate the eddy viscosity and diffusivity tensors via the macroscopic forcing method (MFM) for the closure of the Reynolds-averaged Navier-Stokes. In this work, the turbulent wakes of two 2D surface-piercing models, a triangular wedge and a slender extended wedge, were characterized over $Re_L=O(10^4-10^5)$ and $Fr_L=0.1-1$. Complemented by fluorescent dye visualization, planar PIV was used to measure the shear layer, color-encoded dual-plane PIV to synchronously resolve the near-surface and sub-surface wake, and stereoscopic PIV to obtain a cross-stream measurement.
At low Froude numbers, the interface behaved as a rigid free-slip wall, stabilizing the wake such that the periodic shedding component accounted for 40-50\% of the near-surface streamwise fluctuations. As the Froude number increased, the deformable interface progressively delayed and suppressed K\'{a}rm\'{a}n shedding, producing a 38\% elongation of the near-surface recirculation region for the triangle, attenuation of the dominant peak in the transverse-velocity power spectra, a near half-cycle phase lag between the near-surface and sub-surface shedding, and a loss of coherence in swirling strength. Stereo PIV directly resolved a mean counter-rotating streamwise vortex pair near the interface that is responsible for the suppression. A term-by-term analysis of the mean streamwise vorticity transport equation identifies surface-normal tilting as the dominant production mechanism of the streamwise vorticity. In contrast to prior results for surface-piercing circular cylinders, the fixed separation point concentrates the horizontal shear and the competing surface-parallel tilting term does not cancel the surface-normal tilting.
Details
- Title: Subtitle
- Experimental characterization of surface-piercing turbulent wake flows
- Creators
- David Butler
- Contributors
- Cong Wang (Advisor)Austin Krebill (Committee Member)Frederick Stern (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Spring 2026
- DOI
- 10.25820/etd.008425
- Publisher
- University of Iowa
- Number of pages
- xiii, 71 pages
- Copyright
- Copyright 2026 David Butler
- Language
- English
- Date submitted
- 04/21/2026
- Description illustrations
- Illustrations, graphs, charts, tables
- Description bibliographic
- Includes bibliographical references (pages 54-56).
- Public Abstract (ETD)
The turbulent wakes left behind by ships, bridge piers, and other partially submerged structures influence engineering applications ranging from naval vessel safety to the erosion of riverbeds and the exchange of gases between the ocean and atmosphere. Despite their practical relevance, the region where the flow meets the air-water interface is difficult to study experimentally because of the complex interactions of waves, air bubbles, and strong three-dimensional turbulence. Improved measurements are also needed to enhance the models used within computer simulations of such flows. This work examines the wakes behind two surface-piercing, wedge-shaped objects in a water channel across a range of flow speeds. Laser-based imaging was used near the air-water interface and in the sub-surface to track small particles placed in the water, thereby recovering the evolution of the flow velocity in both space and time. Fluorescent dyes were also released into the flow to make the behavior directly visible.
At slow speeds, the water surface behaved almost like a rigid lid and the wake closely resembled what has been observed deep underwater. As the flow speed increased, the surface deformed into waves and a depression behind the body, and the familiar alternating pattern of shed vortices became delayed and weakened near the surface. Careful comparison between the shallow and deep measurements revealed that a pair of horizontal vortices forms near the interface that is primarily responsible for the change in the wake pattern. The results confirm the effect of an air-water interface on the turbulent wake flow and provide benchmark data for validation of simulations used to design safer and more efficient vessels and structures.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9985177273002771
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