Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Q20: Aerodynamics: Fluid-Structure Interactions, Membranes, Flutter II |
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Chair: Yuanhang Zhu, Center for Fluid Mechanics, Brown University Room: 206 |
Monday, November 21, 2022 1:25PM - 1:38PM |
Q20.00001: Separation angles and wake features of incident flow around polygonal cylinders Esmaeel Masoudi, Lian Gan, David Sims-Williams In this study, large eddy simulation (LES) is used to investigate shear layer separation of incident flow around polygonal cylinders of side number N=5-8 at Reynolds number Re=10^{4} . In total, six equally distributed incidence angles (α) are studied on each polygon between the face and the corner orientations, thus covering the entire α spectrum. The position of separation points and the instantaneous behaviour of the separated shear layers are studied in detail. It is found that the separated shear layers are highly dynamic manifesting a flapping motion of various condition depending amplitude, which is a key factor that causes flow reattachment and results in a second separation, in addition to the first separation in some incidence angles. The strength of this flapping motion is found to be associated with shear layer penetration distance and shear layer thickness growth rate. The separation angles, which are fixed on the corners, can be calculated by the proposed empirical/analytical equations which are found to be in good agreement with available experimental results. Furthermore, a wake deflection angle is introduced and shown to be a good scaling factor for lift, drag and Strouhal number. Finally, the critical separation angle, which corresponds to the longest shear layer penetration distance, the minimum drag and maximum lift conditions is determined and estimated with analytical and empirical equations. |
Monday, November 21, 2022 1:38PM - 1:51PM |
Q20.00002: Preliminary study on the hysteresis in flutter amplitude of flexible membranes Holger Mettelsiefen, Vrishank Raghav A flexible membrane submerged in a fluid stream may exhibit self-excited oscillations, a phenomenon of interest for various engineering applications. Membrane flutter can be harnessed, for instance, to generate turbulence in channel flow to increase heat transfer or mixing. In other cases, membrane flutter is undesirable, for example, in prosthetic heart valves. Except for slender flexible membranes, the onset of flutter and the return to the steady state occur at different freestream velocities. This hysteresis has not yet been completely understood. The present study uses stereophotogrammetry to track the motion of a rectangular sheet in a low-speed wind tunnel in three dimensions with a high temporal resolution; the reaction forces at the mounting are measured simultaneously. The airspeed is gradually increased beyond the onset of flutter, and then reduced until the membrane stops moving. An airspeed-dependent modal analysis of the motion is performed and correlated with the reaction forces. The evolution of the amplitudes of the relevant modes gives new insight into the hysteresis phenomenon. |
Monday, November 21, 2022 1:51PM - 2:04PM |
Q20.00003: Vibration analysis on the original Tacoma Narrows Bridge case using large-scale fluid-structure interaction Daeun Song, Woojin Kim, Oh-Kyoung Kwon, Haecheon Choi The collapse of the Tacoma Narrows Bridge in 1940 is revisited using three-dimensional direct numerical simulation and fluid-structure interaction to study its vibration mechanism. A nonlinear model for suspension bridges (Arioli & Gazzola, 2017) is employed for the structural motion with real-scale parameters. A discrete-forcing immersed-boundary method (Kim et al. 2001) is used for the fluid flow, where Re = 10^{4} is employed instead of the actual Reynolds number (Re =3×10^{6}) to avoid enormous computational costs. Nevertheless, a total of 13.4 billion grid points is required to resolve the fluid motion. As observed on the day of the collapse (Ammann et al. 1941), the bridge first exhibits a vertical vibration with a relatively high frequency and short wavelength. Later, a large torsional vibration with a lower frequency and longer wavelength appears. The temporal variations of the vertical displacement, rotational angle and structural energy provide the detailed procedures of the vertical and torsional vibrations, and two different FSI mechanisms (lock-in with vortex shedding and aeroelastic flutter) are analyzed. |
Monday, November 21, 2022 2:04PM - 2:17PM |
Q20.00004: DMD analysis of lock-in and lock-out zones of a transversely vibrating circular cylinder in the wake of a stationary square cylinder Kumar Sourav, Colin Rodwell, Phanindra Tallapragada The complex fluid-structure interactions during vortex-induced vibrations are difficult to explain analytically because of the high-dimensional nature of the fluid. We use Dynamic Mode Decomposition (DMD) to analyze the underlying coupled fluid-structure modes and quantify their bifurcation between the observed 'lock-in' and 'lock-out' behavior for a damped transversely oscillating circular cylinder (mass ratio = 10) in the wake of a stationary square cylinder. Simulations are performed at a Reynolds number of 100, and the gap ratio (S/D) between cylinder centers vary from 2 to 5. The circular cylinder oscillates negligibly in the envelope of the wake of the upstream square cylinder until S/D = 4. The vortices shed in the gap at S/D = 5 and interact strongly with the downstream cylinder, resulting in increased oscillations. For such a tandem arrangement, the lock-in zone expands. To characterize the flow dynamics, Koopman modes computed from pressure data in the flow field are used. To describe the original flow in a lock-out situation, a number of Koopman modes must be retained, whereas the first few Koopman modes retain the total energy of the flow in the lock-in case. |
Monday, November 21, 2022 2:17PM - 2:30PM |
Q20.00005: The dynamics of vortex induced vibrations starting from rest Nikhilesh Tumuluru Ramesh, Serhiy Yarusevych, Christopher R Morton The present study investigates the structural and wake dynamics of a 1-DOF elastically mounted rigid cylinder released from rest in a freestream. The mass-damping (m*ζ=0.025), and Reynolds number (Re=4400) are fixed, and the reduced velocity is varied from U*=4 to 11. The results show that the system response can be subdivided into three stages. The first stage comprises small cylinder oscillations (A*<0.05 D) that are desynchronized from the Strouhal shedding. The second stage involves cycle-to-cycle increases in oscillation amplitude until the system attains the third stage: steady-state oscillations. Both the duration of the first stage, as well as the rate of amplitude changes in the second stage are related to the difference in Strouhal shedding frequency (f_{vs}), and natural frequency (f_{n}). For around |f_{n}-f_{vs}|/f_{vs}<0.2, the system bypasses the first stage and exhibits rapid changes in amplitude response in the second stage. For |f_{n}-f_{vs}|/f_{vs}>0.2, the first stage comprises multiple shedding cycles, and the second stage amplitude response changes are comparably slower. Planar particle image velocity at the midspan is used to explore the wake dynamics at each stage. |
Monday, November 21, 2022 2:30PM - 2:43PM |
Q20.00006: On the wake-body synchronization in flexible cylindrical cantilevers for sensing flow Shayan Heydari, Neelesh A Patankar, Mitra Hartmann, Rajeev K Jaiman Many animals navigate and explore their environment using complex, three-dimensional (3D) fluid flow information. Insects, crustaceans, rodents, and pinnipeds have specialized sensors that enable them to detect and localize flow sources and, in some cases, to track fluid currents and wakes. Notably, across all these species, the fluid-detecting sensors – hairs, antennae, and whiskers – take the form of slender, flexible cantilevers. These cantilevered sensors operate by undergoing self-sustained oscillations generally at low Reynolds number (Re) flows, often well below the critical Reynolds number of vortex-shedding (Re_{cr}). In this work, we conduct high-fidelity 3D numerical experiments to examine the wake-body synchronization of slender, flexible cylindrical cantilevers in air and water flow for Re<Re_{cr}. We pinpoint the origin of the underlying mechanism for cantilevers' self-sustained vibrations in this Re regime and outline how the synchronization/lock-in phenomenon, a universal concept in nonlinear physical systems, helps establish a general understanding of the coupled feedback mechanisms in fluid-structure systems. All the results presented in this work have significant implications for studying biologically-based flow sensing and engineered fluid sensors. |
Monday, November 21, 2022 2:43PM - 2:56PM |
Q20.00007: Flow-induced vibrations of a cantilevered flexible vertical plate. Avinash K Pandey, Gaurav Sharma, Rajneesh Bhardwaj We study the flow-induced vibrations of a flexible vertical plate cantilevered at its bottom at Re=100. We model our system by solving the 2D incompressible Navier-Stokes equations with the Continuity equation for the fluid and use the Saint-Venant Kirchhoff material model for the plate. The dynamic response of the plate is analysed at various reduced velocities obtained by varying its bending stiffness at a constant density ratio of 100 (equivalent to a mass ratio of 10). We plot the output frequency contours and variation in amplitude against reduced velocity to examine the plate lock-in characteristics with its first and second-mode natural frequencies. The plate is modelled as an Euler Bernoulli beam for calculating the approximate values of its modal frequencies. We find a plate response similar to vortex-induced vibrations (VIV), where the plate locks in and de-synchronizes with its natural frequencies as the reduced velocity changes. We identify four regions: (i) lock-in with the first mode, (ii) de-synchronization, (iii) lock-in with the second mode, and (iv) de-synchronization. The trajectories of a probe point (at the tip of the plate) are also plotted to qualitatively demonstrate the mean position and angular displacement variations with reduced velocity. |
Monday, November 21, 2022 2:56PM - 3:09PM Author not Attending |
Q20.00008: Influence of wake interference on flapping dynamics of an inverted flexible foil Aarshana R Parekh, Rajeev K Jaiman Due to a mutual interaction with a uniform flow, a flexible foil with its trailing edge clamped, also known as the inverted foil, exhibits a wide range of complex self-induced flapping regimes, such as large amplitude flapping (LAF), deformed, and flipped flapping. When placed in tandem with a bluff body in the upstream region, the foil’s proximity and wake flow patterns could impact the flapping dynamics. To study the coupled wake-body interaction and the self-induced response of an inverted flexible foil when placed in the wake of a circular cylinder, we carry out high-fidelity numerical experiments for varying gap distances. Our goal is to investigate the mechanism leading to the foil's self-sustained flapping response in the wake flow. In this study, we particularly attempt to answer the following questions: (i) How does proximity and wake interference influence the foil's flapping response at various separation distances, (ii) what is the intrinsic relationship between the wake vortices and different flapping modes, (iii) what is the optimal range of bending stiffness to sustain LAF response in wake flow compared to uniform flow, and (iv) how does the mass ratio and Reynolds number influence the LAF response in this arrangement? |
Monday, November 21, 2022 3:09PM - 3:22PM |
Q20.00009: Effect of frontbody and afterbody on flow-induced vibration of a cylinder Gaurav Sharma, Rajneesh Bhardwaj The effect of frontbody and afterbody on Flow-Induced Vibration (FIV) of an elastically mounted rigid cylinder has been a subject of major interest for the past decade. We present a numerical study on the FIV of the cylinder at Re=100, and the fluid-structure interaction solver is based on a sharp interface immersed boundary method (Sharma et al. J. Fluids Strut, 2022). The cylinders with the following cross-section are considered: circular, inverted D, C, D and inverted C. D and inverted C cylinders exhibit a combined VIV-galloping response, and we discuss the effect of reducing frontbody on these two cylinders. We explain the initiation of lock-in for these cylinders prior to Strouhal frequency approaching the structural natural frequency by a "modified Strouhal number" and discuss its origin using a quasi-steady galloping analysis. |
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