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 M13: Aerodynamics: Membranes and Flutter |
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Chair: Kourosh Shoele, Johns Hopkins University Room: 201 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M13.00001: Energy Harvesting for Micropower Applications by Flow-Induced Flutter of an Inverted Piezoelectric Flag Kourosh Shoele, Rajat Mittal Piezoelectric flexible flags can be used to continuously generate energy for small-scale sensor used in a wide variety of applications ranging from measurement/monitoring of environmental conditions (outdoors or indoors) to \emph{in-situ} tracking of wild animals. Here, we study the energy harvesting performance as well as the flow-structure interaction of an inverted piezoelectric flag. We use a coupled fluid-structure-electric solver to examine the dynamic response of the inverted flag as well as the associated vortical characteristics with different inertia and bending stiffness. Simulations indicate that large amplitude vibrations can be achieved over a large range of parameters over which lock-on between the flag flutter and the intrinsic wake shedding occurs. The effects of initial inclination of the flag to the prevailing flow as well as Reynolds number of the flow are explored, and the effect of piezoelectric material parameters on the energy harvesting performance of this flutter state is examined in detail. The maximum energy efficiency occurs when there is a match between the intrinsic timescales of flutter and the piezoelectric circuit. The simulations are used to formulate a scaling law that could be used to predict the energy harvesting performance of such devices. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M13.00002: A novel immersed boundary method applied to the inverted flag problem Andres Goza, Timothy Colonius This work uses a 2-D immersed boundary method to study the inverted flag problem, in which a deformable flag is pinned at the trailing edge with its leading edge free to flap. Compared with the canonical flag problem, the inverted configuration more readily undergoes large flapping behavior for a wide range of mass ratios, making it promising in the field of flow-energy harvesting. A previous study identified several flapping modes as a function of dimensionless mass and stiffness ratios (Kim, D. et. al, J. Fluid Mech, 2013). The present work investigates the role of vortex formation and wake-flag interaction on the different flapping regimes. Simulations are performed using a 2-D immersed boundary method that accurately computes surface stresses imposed on the body by the fluid boundary conditions. Unlike many immersed boundary methods that reconstruct surface stresses from the velocity field, the current method only uses information from the immersed surface, leading to a smaller algebraic system for the fluid-solid coupling. The large flapping behavior of the inverted flag problem highlights the method's ability to simulate flows around bodies undergoing large deformations. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M13.00003: Theoretical and Experimental Comparison of Aerodynamic Characteristics for Flexible Membrane Wings with Cambered Frames Andrew Wrist, James Hubner Flexible membrane wings of the MAV (micro air vehicle) scale can experience improved lift/drag ratios, delays in stall, and decreased time-averaged flow separation when compared to rigid wings. Previous research examined the effect of frame camber on the time-averaged shapes of membrane wings and observed that increasing frame camber results in increased aero-induced membrane camber. This study involves a more in-depth DIC (Digital Image Correlation) analysis of the previous research to increase the understanding of the time-averaged shapes for membrane wings with cambered frames and offers a theoretical comparison to the experimental results. The author performed a theoretical lifting-line analysis based on the time-averaged shape for the membrane wings to calculate lift, induced drag, and circulation. The calculations include the effects of geometric twist, aspect ratio, and effective angle-of-attack. The wings, with an aspect ratio of 2, were fabricated with silicone rubber membranes and 3D printed cambered frames differing in percent camber, maximum camber location, and thickness. The DIC images were acquired in The University of Alabama's MAV wind tunnel as tests were performed at 10 m/s (Re $=$ 50,000). The analysis will be discussed in the presentation. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M13.00004: Kite propulsion: single and multi-kite stability Emmanuel du Pontavice, Yves Parlier, David Qu\'er\'e, Christophe Clanet Kite propulsion is one way to harvest wind energy. The typical force is 1 kilo Newton per square meter, which means that with kites in the range 100 to 1000 square meters, one is able to propel ships from the trawler to the tanker. The stability of the kite with no active control is however an issue that needs to be addressed in order to develop viable systems. Under certain conditions, kites tend to engage into large oscillations and eventually crash. Through wind tunnel experiments and basic mechanic modeling, we try to understand (and avoid) this instability. In order to increase the traction of kite propulsion devices, one needs to increase their surface. One way is to superpose a large number of kites. It appears that these chains of kites are much more stable than single kites. A simple physical model is developed to understand this behavior. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M13.00005: Coupling between a flag and a spring-mass oscillator Emmanuel Virot, Xavier Amandolese, Pascal Hemon We address the coupling between a flexible flag and a spring-mass oscillator in the context of energy harvesting. We report a coupling by frequency lock-in leading to resonance conditions over a large range of wind velocities. Large amplitudes of rotation of the flagpole are reported, up to 75 degrees peak-to-peak. We propose to characterize the strength of this coupling with a dimensionless rigidity, which can be considered as the ratio of the flag bending rigidity to the stiffness of the oscillator. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M13.00006: Interaction between two side-by-side inverted flags Cecilia Huertas-Cerdeira, Boyu Fan, Antoine Barizien, Morteza Gharib The inverted flag instability occurs when an elastic plate that is free at its leading edge and clamped at its trailing edge is subjected to an axial wind. The oscillating motion that follows has received recent attention. However, previous studies have focused on the dynamics of a single flag even though these are rarely found isolated in natural phenomena, such as the fluttering of leaves in the wind. The interaction between two side-by-side inverted flags has been investigated, analyzing the effects of the distance between flags and the wind speed. Both in-phase and anti-phase coupling have been observed for different ranges of these parameters. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M13.00007: Coupled-flutter of two slender flags Mougel J{\'e}r{\^o}me, Michelin S{\'e}bastien, Doar{\'e} Olivier A flag in axial flow is subject to flutter instability that leads to large-amplitude flapping of the structure. When two flags are placed parallel to each other, they interact hydrodynamically leading to coupled dynamics of the system. The understanding of the possible dynamical regimes is crucial in the recent context of energy harvesting using piezoelectric fags. In this study, we consider coupled-flutter of two slender flags. Based on an extension of the famous model by Lighthill commonly called Large Amplitude Elongated Body Theory to the two-flags case, both linear and large-amplitude dynamics of such a coupled system will be presented. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M13.00008: Charge Capacity of Piezoelectric Membrane Wings Matthew Grybas, J. Paul Hubner Micro air vehicles (MAVs) have small wings often fabricated with flexible frames and membranes. These membranes flex and vibrate. Piezoelectric films have the ability to convert induced stress or strain into electrical energy. Thus, it is of interest to investigate if piezoelectric films can be used as a structural member of an MAV wing and generate both lift and energy through passive vibrations. Both a shaker test and a wind tunnel test have been conducted to characterize and assess energy production and aerodynamic characteristics including lift, drag and efficiency. The piezoelectric film has been successful as a lifting surface and produces a measurable charge. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M13.00009: Non-Linear Aerodynamic Coupling of Piezoelectric Harvesters in Grid Turbulence Amir Danesh-Yazdi, Oleg Goushcha, Niell Elvin, Yiannis Andreopoulos Experimental and analytical results relating to the extraction of fluidic energy from decaying homogeneous and isotropic turbulence using two side-by-side cantilever beams with attached piezoelectric patches are reported. Turbulence carries mechanical energy distributed over a range of temporal and spatial scales and the resulting interaction of these scales with the immersed piezoelectric beams creates a strain field in the beam which generates electric charge. Experiments are carried out in a large scale wind tunnel in which a turbulence-generating grid is used to excite the piezoelectric cantilever beams for different gap widths between the beams at various distances from the grids and for different flow velocities. We observe that the aerodynamic coupling decays exponentially with increasing gap width between the beams. More importantly, however, it is observed that the aerodynamic coupling due to the presence of a second beam greatly improves the energy harvesting process, so much so that when the aerodynamic coupling between the beams is strong, the average power generated per beam increases by up to 20 times, potentially allowing for significant power extraction from a random, non-resonant phenomenon such as turbulence. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M13.00010: Stability and scalability of piezoelectric flag Xiaolin Wang, Silas Alben, Chenyang Li, Yin Lu Young Piezoelectric material (PZT) has drawn enormous attention in the past decades due to its ability to convert mechanical deformation energy into electrical potential energy, and vice versa, and has been applied to energy harvesting and vibration control. In this work, we consider the effect of PZT on the stability of a flexible flag using the inviscid vortex-sheet model. We find that the critical flutter speed is increased due to the extra damping effect of the PZT, and can also be altered by tuning the output inductance-resistance circuit. Optimal resistance and inductance are found to either maximize or minimize the flutter speed. The former application is useful for the vibration control while the latter is important for energy harvesting. We also discuss the scalability of above system to the actual application in air and water. [Preview Abstract] |
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