Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L14: Aerodynamics: Fluid-Structure Interaction II |
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Chair: Behrouz Karami, George Washington University Room: 202 |
Monday, November 23, 2015 4:05PM - 4:18PM |
L14.00001: Flag flapping in a channel Silas Alben, Kourosh Shoele, Rajat Mittal, Sourabh Jha, Ari Glezer We study the flapping of a flag in an inviscid channel flow. We focus especially on how quantities vary with channel spacing. As the channel walls move inwards towards the flag, heavier flags become more unstable, while light flags' stability is less affected. We use a vortex sheet model to compute large-amplitude flapping, and find that the flag undergoes a series of jumps to higher flapping modes as the channel walls are moved towards the flag. Meanwhile, the drag on the flag and the energy lost to the wake first rise as the walls become closer, then drop sharply as the flag moves to a higher flapping mode. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L14.00002: Fluidic harvesters in free stream turbulence undergoing flow-induced vibrations or flutter Joan Gomez, Vahid Azadeh Ranjbar, Oleg Goushcha, Yiannis Andreopoulos, Niell Elvin In the present experimental work we investigated the performance of fluidic harvesters consisting of cylindrical body mounted of the tip of a flexible beam in the presence of nearly homogeneous and isotropic turbulence. Circular, semi-circular and square shapes have been tested. It was found that turbulence interferes with resonance conditions between the flow and the structure in the case of vortex induced vibrations and has absolutely no effect in flutter dominated case. As a result, turbulence increases the power output of non-linear harvesters subjected to vortex induces vibration and it has no effect in harvester under flutter conditions. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L14.00003: Experimental investigation of the effects of high-frequency electroactive morphing on the shear-layer Johannes Scheller, Karl-Joseph Rizzo, Gurvan Jodin, Eric Duhayon, Jean-Fran\c{c}ois Rouchon, Julian Hunt, Marianna Braza Time-resolved PIV measurements are conducted at a Reynolds number of $270.000$ downstream of the trailing edge of a NACA4412 airfoil equipped with trailing-edge piezoelectric tab actuators to investigate the high-frequency low-amplitude actuation's effect on the shear-layer. A comparison of the time-averaged Reynolds stress tensor components at different actuation frequency reveals a significant impact of the actuation on the shear-layer dynamics. A proper orthogonal decomposition analysis is conducted in order to investigate the actuation's impact on the vortex breakdown. It will be shown that a specific low-amplitude actuation frequency enables a reduction of the predominant shear-layer frequencies. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L14.00004: Self-sustained oscillations of a sinusoidally-deformed plate Diego F Muriel, Edwin A Cowen Motivated by energy harvesting, the oscillatory motion of a deformed elastic material with aspect ratio Length/Width=2, immerse in an incompressible flow is studied experimentally. To induce the wave-like deformation a polycarbonate sheet is placed under longitudinal compression with external forcing provided by equispaced tension lines anchored in a frame. No additional constrains are placed in the material. Based on quantitative image-based edge detection, ADV, and PIV measurements, we document the existence of three natural states of motion. Bellow a critical velocity, a stable state presents a sinusoidal-like deformation with weak small perturbations. Above a critical velocity, instability appears in the form of a traveling wave with predictable dominant frequency accompanied by higher-order harmonics. As the flow velocity increases the instability converges faster to its limit cycle in the phase plane (e.g., vertical velocity and position), until the stable oscillatory mode transitions to chaos showing a broad energy spectrum and unstable limit cycle. The underlying objective is to induce the onset of the instability at lower critical velocities for higher bending rigidities, promoting possible energy extraction and increasing the range at which stable oscillations appear. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L14.00005: Streamwise vortex-induced and galloping-like vibrations of a rotating cylinder Remi Bourguet, David Lo Jacono The flow-induced vibrations of an elastically mounted circular cylinder, free to oscillate in the direction parallel to the current and subjected to a forced rotation about its axis, are investigated numerically at a Reynolds number equal to 100. The cylinder is found to oscillate up to a rotation rate close to 2 (first vibration region), then the body and the flow are steady until a rotation rate close to 2.7, where a second vibration region begins. Each vibration region is characterized by a specific regime of response. In the first region, the oscillation amplitude follows a bell-shaped evolution as a function of the reduced velocity (inverse of the natural frequency) and the vibration develops under a condition of wake-body synchronization: such behavior resembles the vortex-induced vibrations previously described in the absence of rotation. In the second region, the vibration amplitude increases unboundedly with the reduced velocity and may become very large, higher than 2.5 body diameters in the present parameter space. Such galloping-like responses were not observed when the body was restrained to oscillate in the cross-flow direction. They cannot be predicted through quasi-steady analysis and it is found that body oscillation and flow unsteadiness remain synchronized. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L14.00006: One- versus two-degree-of-freedom vortex-induced vibrations of a circular cylinder at Re=3900 Simon Gsell, R\'emi Bourguet, Marianna Braza The response of an elastically-mounted circular cylinder, immersed in a current and free to move either in the streamwise or cross-flow direction, or in both directions, is predicted by means of direct numerical simulation. The Reynolds number based on the inflow velocity and the cylinder diameter is kept equal to 3900. Each configuration is studied over a range of the reduced velocity (inverse of the oscillator natural frequency) encompassing the entire region of lock-in, i.e. where body motion and flow unsteadiness are synchronized. The impact of an additional degree of freedom on the body response and fluid loading is analyzed. Particular attention is paid to the synchronization between the streamwise and cross-flow oscillations, their frequency ratio and phase difference, and to the frequency content of the fluid forces, including the occurrence of large higher harmonic contributions. The reciprocal influence of flow patterns and body motion is investigated on the basis of three-dimensional visualizations of the wake; the formation and modulation of the shear layer vortices is explored in this context. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L14.00007: Flow-Induced Vibration of a Reed in a Channel: Effect of Reed Shape on Convective Heat Transfer with Application to Electronic Cooling Aaron Rips, Kourosh Shoele, Ari Glezer, Rajat Mittal Flow-induced vibration of a reed (a thin plate or flag) in a channel can improve heat transfer efficiency in forced convection applications, allowing for more heat transfer for the same fan power. Such systems have wide ranging applications in electronic and power cooling. We investigate the effect of 3D reed shape on heat transfer enhancement. To study 3D effects, we first use 2D fluid-structure interaction (FSI) simulations of an optimized reed (in terms of mass and stiffness) to generate a prescribed reed motion. We then apply that motion to a pseudo 3D reed (i.e. infinitely stiff in the spanwise direction) and study the heat transfer enhancement in a 3D channel. This method allows us to explore a large parameter space exhaustively, and using this method, we examine the effect of several parameters, such as reed planform and spanwise gap, on the heat transfer enhancements for forced convection in a channel. Simulations indicate that these geometrical feature have a significant effect on the vortex dynamics in the wake as well as the heat transfer efficiency. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L14.00008: Generalized ''thick'' strip modelling for vortex-induced vibration of long flexible cylinders Yan Bao, Rafael Palocios, Spencer Sherwin We propose a generalized strip modelling method that is computationally efficient for the VIV prediction of long flexible cylinders in three-dimensional incompressible flow. In order to overcome the shortcomings of conventional strip theory-based 2D models, the fluid domain is divided into ``thick'' strips, which are sufficiently thick to locally resolve the small scale turbulence effects and three dimensionality of the flow around the cylinder. An attractive feature of the model is that we independently construct a three-dimensional scale resolving model for individual strips, which have local spanwise scale along the cylinder's axial direction and are only coupled through the structural model of the cylinder. Therefore, this model is able to cover the situations of fully resolved 3D model and 2D strip theory model. The connection between these strips is achieved through the calculation of a tensioned beam equation, which is used to represent the dynamics of the flexible body. In the limit, however, a single ``thick'' strip would request the full 3D domain. A parallel Fourier spectral/\textit{hp} element method is employed to solve the 3D flow dynamics in the strip-domain, and then the VIV response prediction is achieved through the strip-structure interactions. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L14.00009: Large-eddy simulations of a flexible cylinder in axial flow Behrouz Karami, Elias Balaras, Philippe Bardet A slender cylinder immersed in axial flow shows different behavior for different flow and material properties. Several studies have pointed to the importance of the dimensionless velocity, $\mathcal{U}=(\rho A/EI)^{0.5}U_oD$, relating the fluid and structural inertia. However, it is not clear how this behavior changes for different Reynolds numbers and flow regimes, while keeping $\mathcal{U}$ constant. In this study a slender cylinder immersed in axial flow is considered as an one-dimensional beam. The fluid-structure interaction is simulated using an immersed-boundary method for a series of Re numbers. A non-linear Euler-Bernouli hypothesis is utilized to account for the deflection and rotation of the cylinder. It is observed that for small dimensionless velocities the cylinder oscillates with small amplitude around its axis. Increasing $\mathcal{U}$ results in buckling of the cylinder. For higher $\mathcal{U}$ beam looses its quasi steady buckled state and flutters. It is investigated that how this behavior changes for different Re and different flow regimes (laminar vs turbulent boundary layers). Overall buckling occurs at higher $\mathcal{U}$ at laminar flow conditions. The results are in agreement both qualitatively and quantitatively with experiments in the literature. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L14.00010: Aeroelastic Flutter Behavior of Cantilever within a Nozzle-Diffuser Geometry Luis Phillipe Tosi, Tim Colonius, Stewart Sherrit, Hyeong Jae Lee Aeroelastic flutter arises when the motion of a structure and its surrounding flowing fluid are coupled in a constructive manner, causing large amplitudes of vibration in the immersed solid. A cantilevered beam in axial flow within a nozzle-diffuser geometry exhibits interesting resonance behavior that presents good prospects for internal flow energy harvesting. Different modes can be excited as a function of throat velocity, nozzle geometry, fluid and cantilever material parameters. This work explores the relationship between the aeroelastic flutter instability boundaries and relevant non-dimensional parameters via experiments. Results suggest that for a linear expansion diffuser geometry, a non-dimensional stiffness, non-dimensional mass, and non-dimensional throat size are the critical parameters in mapping the instability. This map can serve as a guide to future work concerning possible electrical output and failure prediction in energy harvesters. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L14.00011: Flapping Instability of Two Tandem Flexible Foils in Uniform Axial Flow Pardha Saradhi Gurugubelli, Rajeev Kumar Jaiman, Cassey Chua We present a numerical analysis on the stability and coupled dynamics of two tandem flexible foils clamped at their leading edges in a uniform axial flow. The flexible foils considered for this study correspond to the fixed-point stable regime of the single flexible foil where the flexible foil aligns itself in the flow direction with no significant trailing edge oscillations. A high-order nonlinear coupled solver based on the variational formulation has been considered for analyzing the effects of gap between the foils on the stability and coupled behaviour of both the upstream and downstream foils. As a function of decreasing gap, it is observed that the tandem foil configuration is more prone to flapping instability than its single flexible foil counterpart. The evolution of the instability for the downstream foil shows two distinct dynamical scenarios: (i) only the downstream foil exhibits flapping motion and (ii) both the upstream and the downstream foils perform flapping. With the aid of a rigid foil in the upstream of a flexible foil, we further present a detailed analysis on the effects of the upstream wake and vortex shedding on the stability and flapping dynamics of the downstream foil. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L14.00012: Small-Scale Vortical Motions induced by Aeroelastically Fluttering Reed for Enhanced Heat Transfer in a Rectangular Channel Sourabh Jha, Pablo Hidalgo, Ari Glezer Small-scale vortical motions effected by an aeroelastically fluttering thin reed cantilevered across the span of a rectangular channel are exploited for heat transfer enhancement at transitional Reynolds numbers. The reed's concave/convex surface undulations lead to the time-periodic formation, advection, and shedding of vorticity concentrations that scale with the motion amplitude. The reed motion is captured using phase-locked imaging and its interactions with the core flow and surface boundary layers are investigated using high-resolution PIV. Phase-averaged distributions of the reed's mechanical energy demonstrate variations of the vibration modes across the channel. The reed's impact on the surface is accompanied by transitory vorticity shedding coupled with a local increase in the turbulent kinetic energy that results in a strong increase in heat transfer. The reciprocal interactions between the reed dynamics and the channel flow are captured using cross stream velocity distributions along the channel ($L$/$W=$50) that link the kinetic energy shape factor to the rise in heat transfer (e.g., \textit{Nu}) relative to the base flow. It is shown that the reed-induced heat transfer increases with \textit{Re} and results in significant improvement in the global coefficient of performance. [Preview Abstract] |
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