Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session Q25: Aerodynamics: Fluid Structure Interactions, Membranes, Flutter I |
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Chair: Daniel Huang, Caltech Room: North 225 AB |
Tuesday, November 23, 2021 8:00AM - 8:13AM |
Q25.00001: Dynamics of finite-length compliant cylinders in cross-flow Joel D Hartenberger, Jin-hyeong Yoo Aquatic environments are filled with organisms that sense and interact with flow using finite-length, streamlining structures such as seal vibrissae (ie whiskers) and algae filaments. The compliance of these structures plays a key role in how they interact with surrounding flow and can lead to widely varying dynamics. Prior studies have investigated the advantages and disadvantages of specific systems but further research is required to develop a framework for general understanding of compliant streamers. Experiments investigating finite-length compliant cylinders in turbulent cross-flow were performed in the 12-inch variable pressure water tunnel at the Naval Surface Warfare Center Carderock Division. Cylinders (D = 1 cm, L ~ 10 cm) were fabricated in a range of material compliance and their structural response characterized using Dynamic Mechanical Analysis (DMA). The hydrodynamic behavior of each cylinder was determined from time-resolved measurements of the drag and side force it experienced across Reynolds numbers ranging from High-speed videography captured the structural response of each cylinder concurrently with force measurements. Details of the flow field are captured for select conditions in separate trials by particle image velocimetry (PIV). |
Tuesday, November 23, 2021 8:13AM - 8:26AM |
Q25.00002: Fluid damping scaling of elastically mounted pitching wings in quiescent water Yuanhang Zhu, Varghese Mathai, Kenneth Breuer Fluid damping plays an important role in shaping damped oscillations of aeroelastic systems. In this study, we experimentally characterize the nonlinear fluid damping associated with vortices shed from the rounded leading edge and the sharp trailing edge of a rigid but elastically mounted pitching wing in the absence of a free-stream flow. We simulate the dynamics of the elastic mount using a cyber-physical system. We perturb the wing and measure the fluid damping coefficient from damped oscillations over a large range of pitching frequencies, pitching amplitudes, pivot locations and leading-/trailing-edge sweep angles. A universal fluid damping scaling based on the Morison equation is proposed and validated. Within the small-amplitude limit, the scaled non-dimensional fluid damping is found to increase linearly with the pitching amplitude, with a constant slope corresponding to the unsteady drag coefficient. This slope decreases as the pitching amplitude increases, presumably because the shed vortices no longer follow the rotating wing. Flow fields obtained using particle image velocimetry (PIV) are used to explain the nonlinear behavior of the fluid damping. |
Tuesday, November 23, 2021 8:26AM - 8:39AM |
Q25.00003: Synchronization between a structure and surrounding flow during aeroelastic flutter Sombuddha Bagchi, Vishnu R Unni, Abhishek Saha Aeroelastic flutter caused by the mutual interaction between surrounding flow and lift-generating structures of an aircraft has been studied for decades due to perils to life and property. Even though extensive research has been performed on this phenomenon, the complete mechanistic understanding of temporal and spatial synchronization between flow and structure, which leads to such dangerous large amplitude oscillations, continues to evade us. Analysis of the synchronization mechanism between the flow and structure is essential to understand the underlying physics and to design control strategies. In this experimental study, we use a multivariate complex network-based method corresponding to time series of fluctuations in strain (on a structure) and velocity fluctuations in a surrounding turbulent flow. A weighted cross recurrence network is used to analyze the interdependencies and coupling between them. Additionally, we calculate the correlation of probability of recurrence between velocity and strain to highlight the emergence of spatial and temporal synchronization as the system transitions towards flutter. Finally, we calculate the phase differences between these two variables in the region adjacent to the structure using the Hilbert transform to provide a causal relationship and the state of synchronization between the structure and the flow. |
Tuesday, November 23, 2021 8:39AM - 8:52AM |
Q25.00004: Application of Proper orthogonal decomposition to study the FSI behavior of a flag in a laminar jet Rodrigo Padilla, Vibhav Durgesh, Tao Xing The flutter in the flag causes large-scale deformations of the flag with a close-coupling between flag structure and fluid flow around the flag. The flutter shape of the flag has an impact on the aerodynamic performance of the flag. This study aims to use Proper Orthogonal Decomposition (POD) to quantify the FSI flow features and their impact on aerodynamic behavior. For this study, the flag was placed at the outlet of a laminar jet facility, and the flag samples were fabricated of a flexible cellulose acetate membrane with known material properties. Particle Image Velocimetry (PIV) is used to measure the velocity flow field around the flag, and a high precision load cell is used to measure the aerodynamics load experienced by the flag. POD provided a modal description of the flow behavior and allowed to quantify the impact of the flow structures on the aerodynamic performance of the flag. The results showed a unique correlation between the flow structure and aerodynamic load experienced by the flag. |
Tuesday, November 23, 2021 8:52AM - 9:05AM |
Q25.00005: Flapping flags in grid-induced turbulence Stefano Olivieri, Francesco Viola, Andrea Mazzino, Marco Edoardo Rosti A fully-resolved direct-numerical-simulation approach is developed to study the flapping motion of a flexible flag forced by a turbulent incoming flow at moderate Reynolds number. Turbulence is generated by a passive grid at the inlet of the numerical domain and the turbulence level impacting the flag can be controlled by varying the distance from the grid. Our computational framework is based on the immersed boundary method and a spring-network model for dealing with deformable bodies of arbitrary geometry, and is implemented with a GPU-accelerated parallelisation increasing the computational efficiency. First, we characterize the turbulent flow comparing with well-known results for decaying turbulence and experimental measurements. Then, we revisit the flag-in-the-wind problem by exploring the effect of turbulence on the main features of self-sustained flapping. We show that, whilst the latter mechanism is still manifesting, the amplitude and frequency of the oscillation are remarkably altered. Moreover, the fingerprint of turbulent fluctuations is qualitatively detected by spectral analysis. Besides their relevance for fundamental understanding, these findings have potential impact for applications, e.g., aeroelastic energy harvesting or flow control. |
Tuesday, November 23, 2021 9:05AM - 9:18AM |
Q25.00006: Contact behavior of a fluttering flag under gravity effect Minseop Lee, Cheolgyun Jung, Daegyoum Kim Thin structures with elasticity found in nature, such as flags and leaves, are under the influence of gravity in the process of interacting with uniform flow. In addition, a situation in which an elastic sheet collides with another rigid body may occur in the course of motion. In this study, the stability and post-critical behaviors of a flag, which interacts with uniform flow under gravity and is in contact with a nearby rigid wall, are experimentally investigated to elucidate the effects of gravity and contact on flutter dynamics and find design principles for the application to triboelectric energy harvesting. By varying the free-stream velocity and the distance between the clamped leading edge of the flag and the rigid wall, different modes are classified into stable, flutter, partial contact, and full-contact. In terms of instability and contact process of the flag, the distance between the clamped leading edge of the flag and the rigid wall is a critical parameter. Therefore, we introduce a new dimensionless velocity considering this parameter and confirm that the transition of the mode is determined by this new dimensionless velocity. The new dimensionless velocity also characterizes the dynamic behaviors such as the amplitude, reduced frequency, and dimensionless bending energy of the flag. |
Tuesday, November 23, 2021 9:18AM - 9:31AM |
Q25.00007: Aeroelastic mode selection mechanism for flexible membranes interacting with separated flow GuoJun Li, Rajeev K Jaiman, Boo Cheong Khoo Flexible membrane immersed in separated flow can experience self-excited vibrations and induce particular vortex shedding patterns via flow-excited instability. To examine how the specific aeroelastic characteristics are selected in the coupled system, a three-dimensional membrane at moderate to high angles of attack in unsteady flow is simulated by a high-fidelity aeroelastic framework. With the aid of a global Fourier mode decomposition technique, we extract the correlated aeroelastic modes (in frequency ranking) from the coupled system. Using the aeroelastic mode decomposition, we show that the dominant structural mode exhibits a chordwise second and spanwise first mode at different angles of attack. Frequency synchronization between the dominant membrane vibration and the vortex shedding process is reported from the mode frequency spectrum. We propose an approximate analytical formula to estimate the structural natural frequency and show the aeroelastic mode selection process is primarily driven by the frequency lock-in between the structural natural frequency and the dominant vortex shedding frequency. By comparing the flow features among a rigid wing, a rigid cambered wing and the flexible membrane, we find that the non-periodic aeroelastic responses at higher angles of attack are associated with the bluff-body vortex shedding instability. |
Tuesday, November 23, 2021 9:31AM - 9:44AM |
Q25.00008: Mode split prediction for a rotating disk coupled with a flexible stator through water Lucas Berthet, Philippe Blais, Bernd Nennemann, Christine Monette, Frederick P Gosselin High-head turbine runners are subject to multiple sources of excitations. Coupled with the added mass of water, rotation induces a mode split in natural frequencies of runners, where co-rotative and counter-rotative waves travel at different relative speeds through the runner. Disks, by displaying a similar behavior, are used as a simpler model. Mode split is characterized for a rotating disk in water but, in high-head turbines, the runner and the flexible upper cover are coupled through the axial gap fluid. In this project, we develop an analytical model of coupled stationary and rotating disks to analyze the effect of their interaction on the mode split. First, we apply the potential flow theory, considering water as irrotationnal, incompressible, and inviscid. We assume the same mode shapes in water as in air for the disks. We then derive the potential flows that respect the no penetration boundary conditions. One after the other, each disk is considered flexible while the other one is rigid. We finally couple the two obtained fluid flows through the structural equations of motion. The derived model displays less than 3% error with experimental data for large axial gaps and a thick rotor. |
Tuesday, November 23, 2021 9:44AM - 9:57AM |
Q25.00009: Experimental investigation of flexible rotors in water Ahmed Eldemerdash, Thomas Leweke Rotor blades often exhibit some degree of flexibility that can potentially be used to increase the mechanical strength of the rotor or expand its operational range. Our interest is to explore experimentally the fundamental fluid-structure interactions of a flexible rotor in water. We consider a single-bladed small-scale rotor of 88 mm radius and 20 mm chord. The rotor blade is rectangular, without twist or taper, and has a flat plate profile. The effect of varying the pitch angle, the freestream velocity, and the rotation frequency is analyzed through optical measurements of the flapwise bending and twist. The flow field is obtained through Particle Image Velocimetry. Interesting behaviors are observed, including extreme bending states and the reversal of bending direction for high pitch angles. In addition, for negative pitch angles, large-amplitude, low-frequency bending fluctuations are observed, accompanied by large-scale recirculation zones. These zones are formed and shed behind the rotor, resembling the dynamics of the Vortex Ring State known from helicopter aerodynamics. The blade deformation depends mainly on the tip speed ratio at low pitch and on the tip Reynolds number at high pitch. Comparisons with a rigid blade and two-bladed flexible rotors will also be presented. |
Tuesday, November 23, 2021 9:57AM - 10:10AM |
Q25.00010: Fluid structure interactions of a rotor with flexible blades in water Tristan Aurégan, Benjamin Thiria, Sylvain Courrech du Pont Adding flexibility to rotating structures has been shown to improve their efficiency and reliability as well as their compliance to external flow perturbations (V. Cognet et al. 2017, 2020). Here, we investigate experimentally the problem of a propulsive rotor submerged in water with flexible blades in both spanwise and chordwise directions : when subjected to an incoming flow, the blades bend upstream and twist. We report measurements of both forces and associated blade deformations and give predicting scaling laws. A simulation of the full system using blade element momentum theory shows good agreement with the experiments. This simulation also reveals the existence of regions in the parameter space where the efficiency of flexible blades outperforms that of rigid ones. |
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