Exploring the dynamics of fluid-structure interaction: From single structures to complex environmental systems

Cheng, Shyuan (Jeffrey)

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https://hdl.handle.net/2142/125746 Copy

Description

Title

Exploring the dynamics of fluid-structure interaction: From single structures to complex environmental systems

Author(s)

Cheng, Shyuan (Jeffrey)

Issue Date

2024-06-24

Director of Research (if dissertation) or Advisor (if thesis)

Chamorro, Leonardo P.

Doctoral Committee Chair(s)

Chamorro, Leonardo P.

Committee Member(s)

• Best, James L.
• Ansell, Phillip J.
• Feng, Jie

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Fluid-structure interaction
• Turbulence
• Experimental fluid dynamics

Abstract

Fluid-structure interaction (FSI) is a ubiquitous and significant phenomenon in engineering systems. Over recent decades, considerable efforts have been devoted to understanding how structures behave under various flow conditions. However, there is still much to learn about systems where distinct features, such as porosity and geometry, interact with turbulent flows. Quantitatively describing nonlinear, multiscale interactions between structural dynamics and flow remains a crucial challenge in scientific and engineering domains. Identifying the primary mechanisms influencing flow and structural dynamics could substantially improve the design, reliability, and life span of many engineering systems. This thesis aims to contribute to understanding FSI phenomena associated with single structures and collections of structures. It explores the influence of geometry, stiffness, porosity, and their interplay with turbulence. Specifically, the FSI scenarios studied include the passive pitching dynamics of rigid plates in stratified turbulence, the unsteady dynamics of flexible plates with a single perforation, a case involving a collection of highly flexible plates, and an FSI application in wind energy, which investigates the performance of model wind turbines under passive tower oscillations. These findings are briefly summarized below. The first part is devoted to single structures. The initial case involves a rigid structure able to rotate freely. In this case, the distinct pitching of a rigid plate exposed to stratified turbulence from the wake of tapered cylinders was experimentally studied for various cylinder taper ratios, R_t. The tapered cylinders generated stratified turbulence containing spatially varied, energetic vortices. Vertically oriented von-Kármán vortices, which varied in frequency and strength along the vertical span of the plate, were imposed on the incoming flow, resulting in the oscillation of a rigid splitter plate around a vertical axis located a quarter chord length from the leading edge. Results show that two distinct modes dominate the plate oscillations. One of them, f_p, corresponds to the mean flow-induced oscillation frequency, while the other, f_v, is determined by the distribution of the vortex-shedding cells from the non-uniform cylinders. These frequencies exhibited distinct trends with the distance from the cylinders and the distribution of the incoming coherent motions determined by the taper ratio. In particular, f_v increased proportionally with R_t, whereas f_p was inversely proportional to R_t in the near wake. These characteristic frequencies were constant in the far field, with values dependent on R_t. The focus then shifted to the dynamics of flexible structures. Although this phenomenon has been extensively explored due to its significant relevance in engineering, the role of a single, minor perforation on the drag, oscillations, wake, and dynamics in general and the impact of the perforation within the plate remains obscure. To tackle this problem, laboratory experiments were conducted to quantify the effects of single square perforations and their location on the drag and reconfiguration of flexible plates. These plates were tested under uniform flows with minimal turbulence, featuring square cross-section perforations resulting in a low porosity ratio of approximately 1/25. Baseline cases included rigid plates with and without perforations and flexible plates without perforations. The presence of perforations led to distinct jets in the wake, significantly altering the aerodynamic forces and plate deformation. The incoming flow and the perforation location influenced the velocity and position of the central jet relative to the downwind distance. Normalized center jet velocity profiles, using an effective velocity and adjusted perforation half-width, highlighted their dependence on these variables. A simple first-order model was derived to predict the drag changes across various perforated plates under diverse incoming velocities. Following the study of single structures, the next step involved investigating the characteristics of a collection of highly flexible structures with environmental relevance. Specifically, the impact of a seagrass-like canopy on turbulence and transport phenomena was studied in a refractive-index-matching facility to capture the interaction between inner and outer canopy flows. The canopy mimicked the natural seagrass's dynamic behavior and morphological properties. The study focused on fully submerged canopy flows under subcritical conditions with Froude numbers (F_r<0.26) and covered a range of Reynolds numbers (R_e∈[3.4×10^4,1.1×10^5]) and Cauchy numbers (C_a∈[120,1200]), with additional rigid cases conducted for comparison. The results revealed that blade deflection and coordinated waving motion redistributed Reynolds stresses above and below the canopy top. The flexible in-canopy turbulence lacked periodic stem wake vortex shedding, as seen in rigid canopies, but shedding from blade tips. Spectral proper orthogonal decomposition (SPOD) unveiled Kelvin-Helmholtz-type vortices as dominant flow structures associated with seagrass waving motion, impacting local flow exchange in both canopy types. Blade deflections created a barrier-like effect, hindering large-scale turbulence transport and reducing vortex penetration into the canopy. Using conditionally averaged quadrant analysis during blade motion helped identify a transition from sweep-dominated to ejection-dominated behavior in the surrogate seagrass. Finally, attention was placed on uncovering some unique phenomena associated with the highly complex dynamics of floating wind turbines. The interplay of wind, the water's free surface, and the structural platform pose multiple challenges. This problem was approached using a simplified representation by examining a model wind turbine subjected to passive oscillations. Specifically, laboratory experiments were conducted to quantify the effects of small-amplitude, passive oscillations on the structure's unsteady motions, wake statistics, and mean power output, along with associated fluctuations across yawing angles β∈[0°,30°] for every ∆β=5°. An additional fixed turbine case was included to aid insight. Our findings highlight that the pitch motion dominated the turbine dynamics; interestingly, this is especially relevant for small but non-zero yaw of β≈5°. Despite minor mean velocity differences in the wake observed between fixed and oscillating turbines at a given yaw, turbulence levels were substantially modulated by the turbine motions. A formulation was derived for the turbine oscillation spectrum that shows good agreement with measurements; it serves as a foundation to include various input modulation types. It was also noted a monotonic decrease in power output with increasing βfor the fixed and oscillating turbines, more pronounced for β>15°. Passive oscillations produced higher power for a given yaw, and a simple yaw correction of the power output structure showed reasonable agreement with the experimental measurements.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125746

Copyright and License Information

Copyright 2024 Shyuan Cheng

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