Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R36: Aerodynamics: FSI, Membranes and Flutter |
Hide Abstracts |
Chair: Juan Alonso, Stanford University Room: Alcove A |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R36.00001: Vortex Formation, Shedding and Energy Harvesting from a Cyber-Physical Pitching Flat Plate Kyohei Onoue, Kenneth Breuer We examine the dynamics and energy harvesting capabilities of an elastically mounted flat plate undergoing large amplitude limit cycle oscillations in a uniform flow. All experiments are performed using a cyber-physical system, wherein the structural inertia, stiffness and damping are numerically simulated using a position-following feedback algorithm. The cyber-physical system also allows for implementation of nonlinear spring and damping coefficients, which control the plate dynamics and subsequent energy harvesting characteristics. Analysis of the plate kinematics and the fluid flow over the plate and in the wake (measured using PIV) are used to understand the interplay between structural motion and vortex formation at the sharp leading and trailing edges of the plate. By varying the structural properties of the system we systematically analyze the formation, strength, stability and separation of the leading edge vortex, as well as the dependence on kinematic parameters and Reynolds number. Connections to previous results on vortex formation time and bluff body aerodynamics are discussed. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R36.00002: Catenaries in Drag Brato Chakrabarti, James Hanna Dynamical equilibria of towed cables and sedimenting filaments have been the targets of much numerical work; here, we provide \emph{analytical} expressions for the configurations of a translating and axially moving string subjected to a uniform body force and local, linear, anisotropic drag forces. Generically, these configurations comprise a five-parameter family of planar shapes determined by the ratio of tangential (axial) and normal drag coefficients, the angle between the translational velocity and the body force, the relative magnitudes of translational and axial drag forces with respect to the body force, and a scaling parameter. This five-parameter family of shapes is, in fact, a degenerate six-parameter family of equilibria in which inertial forces rescale the tension in the string without affecting its shape. Each configuration is represented by a first order dynamical system for the tangential angle of the body. Limiting cases include the dynamic catenaries with or without drag, and purely sedimenting or towed strings. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R36.00003: Cambering effects on Rapidly-Prototyped, Highly-Flexible Membrane Wings David Pepley, Andrew Wrist, Paul Hubner Much of the inspiration for micro air vehicle (MAV) design comes from animals, likes bats, which use membrane wings for flying and gliding at low Reynolds numbers. Previous research has shown that membrane wings are more aerodynamically efficient than rigid wings. This is a result of both time-average cambering of the membrane and dynamic interaction with the shear layer. In most of the previous research, the membrane was attached to a flat (uncambered) frame. Traditional airfoil theory suggests that the cambering of wings improves aerodynamic efficiency and endurance. This research analyzed the effects of cambering the frames on wing efficiency and endurance. Six different cambered membrane wings with an aspect ratio of two, each with two cells with an aspect ratio of one, were 3-D printed using an Objet30 Pro and tested in a low-speed wind tunnel at 10 m/s (Re $=$ 50,000). A NACA 4504 profile was used as a baseline with the frame thickness, percent camber, and maximum camber location being altered for comparison. The lift, drag, and pitching moment of the cambered and flat wings were recorded using a load cell. Results showed that cambering the frame of membrane wings increases aerodynamic and endurance efficiency at low Re. The effects of altering the camber, increasing the batten thickness, and changing the max camber location on aerodynamic and endurance efficiency were also examined. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R36.00004: Aerodynamic Performance of Electro-Active Membrane Wings Ioan-Alexandru Barbu, Roeland De Kat, Bharathram Ganapathisubramani Electro-active polymers offer due to their multivariate compliant nature a great potential for integrating the lift producing system and the control system into one. This work presents the first step in describing both the mechanical and aerodynamic performance of such materials and focuses on both understanding their behaviour in aerodynamic applications and on analysing their aerodynamic performance. Photogrammetry and load measurements are conducted in a wind tunnel for both silicone-based and acrylic-based membranes at zero prestrain supported in a perimeter reinforced frame in electrically passive, active and pulsing conditions. A wide range of fixed voltages and pulsing frequencies are considered. Due to their hyper-viscoelastic nature, both short and long term hysteresis analysis are conducted in terms of aerodynamic performance. Along with these tests, analyses of the effects of the percentage electrode area and silicone content on aerodynamic performance are conducted. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R36.00005: Effects of morphology on the flapping dynamics of inverted flags Boyu Fan, Julia Cosse, John Sader, Morteza Gharib The behavior of inverted flags has received recent attention in the study of the interaction of flexible bodies with fluid flows. It has implications in a variety of natural phenomena, such as the fluttering of leaves in the wind. As opposed to a conventional flag, defined by a fixed leading edge and a free trailing edge, an inverted flag has a free leading edge and a fixed trailing edge. The reversed flow orientation of inverted flags has led to a surprising observation. Over a narrow range of wind speeds, they exhibit a large-amplitude flapping motion that is not present in their conventional counterparts. Our study experimentally investigates the effects of flag morphology on the flapping behavior of inverted flags. Different flags ranging from rectangles to triangles are studied in a wind tunnel to assess the underlying parameters that govern their dynamics. We observe a significant shift in the limit-cycle flapping mode that is a function of flag shape parameters. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R36.00006: Investigation of an Ablative Body under Different Flow Configurations Michael Allard, Christopher M. White, Ryan Crocker, Yves Dubief The erosion of a bluff body low temperature ablator (para-dichlorobenzene) is investigated in various flow configurations. CCD image sequencing is used to quantify the time evolution of the geometrical shape of the ablating body and to compute local surface recession velocity and acceleration. The results for the different flow configurations are compared to evaluate the effect of local flow conditions on the recession rate. Geometrical self-similarity of the ablating body is explored, and the self-similar form is compared to that predicted by the fluid erosion model recently published by Moore \emph{et al.} (Phys. Fluids (2013) 25:116602). [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R36.00007: A Multi-Phase Based Fluid-Structure-Microfluidic interaction sensor for Aerodynamic Shear Stress Christopher Hughes, Diganta Dutta, Yashar Bashirzadeh, Kareem Ahmed, Shizhi Qian A novel innovative microfluidic shear stress sensor is developed for measuring shear stress through multi-phase fluid-structure-microfluidic interaction. The device is composed of a microfluidic cavity filled with an electrolyte liquid. Inside the cavity, two electrodes make electrochemical velocimetry measurements of the induced convection. The cavity is sealed with a flexible superhydrophobic membrane. The membrane will dynamically stretch and flex as a result of direct shear cross-flow interaction with the seal structure, forming instability wave modes and inducing fluid motion within the microfluidic cavity. The shear stress on the membrane is measured by sensing the induced convection generated by membrane deflections. The advantages of the sensor over current MEMS based shear stress sensor technology are: a simplified design with no moving parts, optimum relationship between size and sensitivity, no gaps such as those created by micromachining sensors in MEMS processes. We present the findings of a feasibility study of the proposed sensor including wind-tunnel tests, microPIV measurements, electrochemical velocimetry, and simulation data results. The study investigates the sensor in the supersonic and subsonic flow regimes. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R36.00008: Investigating the Improved Aerodynamic Efficiency of Cambered Frames on Membrane MAV Wings Andrew Wrist, Zheng Zhang, Paul Hubner Previous research has demonstrated that membrane wings with cambered frames are more aerodynamically efficient than those with flat frames, despite passive dynamic membrane cambering for both. To help understand this aerodynamic benefit, this study compares the time-averaged membrane shape as well as membrane vibration frequency and amplitude for a group of wings with cambered frames. The frames were 3D printed with a hardened polymer material, and a silicon rubber membrane was attached to the top surface. The frame aspect ratio is two, comprised of two cells each with a cell aspect ratio of one. The rigid leading edge extended 20{\%} of the chord, and the trailing edge was scalloped at 25{\%}. Camber ranged from 2-6{\%}, camber location from 40-60{\%}, and airfoil thickness from 4-6{\%}. Tests were performed in the University of Alabama's MAV wind tunnel at 10 m/s (Re $=$ 50,000). High speed imaging results of the deformation and vibration will be discussed in context to airfoil and wing theory. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R36.00009: Flow induced vibrations in arrays of irregularly spaced cylinders Gordon Taub, S\'ebastien Michelin Historically the main industrial applications of cylinder arrays in cross flows favored regular arrangements of cylinders. For this reason, most past studies of Flow Induced Vibrations (FIV) in large cylinder arrays have focused on such arrangements. Recently there has been some interest in generating renewable energy using FIV of bluff bodies. In such applications it will likely be beneficial to enhance, rather than suppress FIV. It is not known a priori if regular or irregularly spaced arrays are most adequate for this type of application. In this study, wind tunnel experiments were conducted on one regularly spaced array and four different irregularly spaced arrays of cylinders in a cross flow. Each arrangement of cylinders was examined under eight different orientations to a cross flow ranging between 10 m/s and 17 m/s. The average amplitude of vibration of the cylinders was found to highly depend on arrangement and orientation. The typical amplitude of vibration of the rods in the irregular arrangements were found to be an order of magnitude larger than that of the regular array. A simple model was proposed in order to predict if a given arrangement was likely to produce large oscillations, and the validity of the model was examined. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R36.00010: An experimental study of flow around submerged grass vegetation Julia Lee, Shreyas Mandre, Ravi Singh Mixing of fluids through submerged vegetation caused by tidal currents facilitate various environmental and ecological transport processes. This fluid-vegetation interaction is believed to result from a Kelvin-Helmholtz instability from an inflection point in the flow profile. Recent studies suggest that flow in presence of grass can also become unstable due to shear instability of flow above the grass. We devise a two-dimensional lab scale analog of the fluid-vegetation interaction using ABS plastic filaments immersed in a soap film. We employ PIV of the surrounding flow to gain an understanding of the role of instabilities in the flow. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R36.00011: Flow-Induced Vibration of Flexible Hydrofoils in Incompressible, Turbulent Flows Eun Jung Chae, Deniz Tolga Akcabay, Yin Lu Young Flexible lifting bodies can be used to enhance the energy-efficiency and maneuverability of propulsion devices compared to their rigid counterparts. To take advantage of advances in materials and active/passive control techniques, an improved understanding of the fluid-structure interaction physics is needed. This numerical study focuses on flexible hydrofoil in incompressible, turbulent flows. The spanwise bending and twisting of a rectangular, cantilevered hydrofoil was modeled as 2DOF equations of motion coupled with the unsteady RANS equation. The results, which have been validated with experimental measurements, showed that the natural frequencies are lower in water compared to those in air due to the added mass effect, and the natural frequencies vary slightly with speed and angle of attack due to hydrodynamic bend-twist coupling and viscous effects. Lock-in of the vortex shedding frequencies with the natural frequencies was observed, along with modification of the wake patterns due to hydrodynamic bend-twist coupling. The hydrodynamic damping was found to be much greater than structural damping, and depends on the relative velocity, angle of attack, as well as structural stiffness and density, and can lead to destabilizing condition of structure in particular cases. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700