Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session A09: FSI: Sheets / Membranes |
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Chair: Shai B. Elbaz, Technion - Israel Institute of Technology Room: 213 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A09.00001: Experimental and computational studies of flexible membrane aerodynamics Rodrigo Padilla, Conal Thie, Vibhav Durgesh, Tao Xing Fluid-structure interaction (FSI) problems involve the interaction of flexible bodies or membranes like flexible airfoils, parachutes, and sails, with fluid flows. A detailed understanding of the flow dynamics and structure behavior is critical to resolving FSI problems. The objective of the current study is to quantify the impact of critical dimensionless parameters (Re, aspect ratio, dimensionless rigidity) on flow field, vortex shedding, surface pressure distribution, and membrane response, for a canonical rectangular flexible membrane with varying aspect ratios. A collaborative experimental and computational investigation was performed, and the membrane was fabricated using 3D printers. The experiments for this investigation were performed in a subsonic wind tunnel facility and the flow field was measured using Particle Image Velocimetry (PIV). Complementary computational fluid dynamics (CFD) studies were performed were first validated using the experimental data and then used to perform a parametric FSI. The results from this investigation showed that flow behavior is impacted by dimensionless parameters, this was reflected in a computational study that successfully captured the observed FSI behavior for the studied cases. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A09.00002: ABSTRACT WITHDRAWN |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A09.00003: Dynamics and instabilities of an arbitrarily clamped elastic sheet in potential flow Shai B. Elbaz, Nethanel Chen, Amir D. Gat Shape-morphing airfoils have attracted much attention in recent years. They offer substantial drag reduction by comparison to conventional airfoils and a premise of superior aerodynamic performance. In the current work, we model a shape-morphing airfoil as two, rear and front, Euler-Bernoulli beams connected to a rigid support. The setup is contained within a uniform potential flow field and the aerodynamic loads are modelled by thin airfoil theory. The aim is to study the dynamics and stability of such soft shape-morphing configurations and specifically the modes of interaction between the front and rear airfoil segments. Initially we present several steady-state solutions which are validated by numerical calculations based on commercially available software. We then examine stability and transient dynamics by assuming small deflections and applying multiple-scale analysis to obtain a stability condition. The condition is attained via the compatibility equations of the orthogonal spatial modes of the first-order correction. The results yield the maximal stable speed as a function of elastic damping, fluid density and location of clamping. The results show that the interaction between the front and rear segments is the dominant mechanism for instability for various discrete locations of clamping. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A09.00004: Deformations, forces and flow field around a compliant membrane disk Asimanshu Das, Varghese Mathai, Kenneth Breuer Highly compliant membranes exhibit large scale deformations and can alter the nature of the flow with which they interact. We focus on understanding the kinematics and dynamics of circular compliant membranes with varying shear modulus, G, placed head-on in a uniform flow. The experiments were carried out in a closed-loop low-speed wind tunnel with Reynolds number in the range of $10^4 -10^5$. We measure the average and fluctuating properties of the compliant structure, including its deformation and drag forces. The membrane deforms into parachute-like shapes depending on the value of a dimensionless number - the Aeroelastic number, Ae - which measures the relative importance of elastic and aerodynamic stresses. With decreasing Ae (either higher speed, or a softer membrane), the membrane evolves from a flat circular disk-like shape to a nearly hemispherical shape. The drag coefficients vary from $1.1 - 1.4$. We provide comparisons with the flow-induced forces on rigid shells of similar shape. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A09.00005: Lift and Thrust Measurements on a Flapping Membrane Foil Gali Alon Tzezana, Varghese Mathai, Kenneth Breuer Flapping compliant membrane wings demonstrate increased lift and thrust when compared to rigid wings, particularly when flapping close to the natural frequency of the wing. However, if the wing is too compliant, the forces are decreased. Here, we performed experiments to measure the hydrodynamic forces acting on a heaving compliant membrane in a water flume facility, at a Reynolds number of 20,000-60,000. By varying the membrane's elastic modulus, thickness and prestretch, we modify the natural frequency of the membrane. We observe linear and nonlinear behaviors of the forces and membrane deformations with respect to the Strouhal number and natural frequency. For cases with small deformations, we compare the experimental results with a linear, small amplitude potential flow based model. Finally, we discuss the flow structure and wake patterns obtained from Particle Image Velocimetry (2-D PIV) measurements, and their relation to propulsive performance. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A09.00006: Large-amplitude membrane dynamics in inviscid flow Christiana Mavroyiakoumou, Silas Alben We study the dynamics of thin membranes---extensible sheets with negligible bending stiffness---initially aligned with a uniform inviscid background flow. This is a benchmark fluid-structure interaction that has previously been studied mainly in the small-deflection limit, where the flat state may be unstable. Related work includes the shape-morphing of airfoils and bat wings. We study the initial instability and large-amplitude dynamics with respect to three key parameters: membrane mass density, stretching rigidity, and pretension. When both membrane ends are fixed, the membranes become unstable by a divergence instability and converge to steady deflected shapes. With the leading edge fixed and trailing edge free, divergence and/or flutter occurs, and a variety of periodic and aperiodic oscillations are found. With both edges free, the membrane may also translate transverse to the flow, with steady, periodic, or aperiodic trajectories. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A09.00007: Efficiency Enhancements in Hydrokinetic Energy Harvesting Achieved Using a Compliant Membrane Hydrofoil Varghese Mathai, Gali Alon Tzezana, Kenneth Breuer Hydrokinetic energy harvesting using an oscillating hydrofoil has recently received increased attention as an alternative to conventional rotary turbines. One of the challenges in the development of commercially viable flapping foil technology is to attain cycle efficiencies comparable to those of rotary turbines. Here we experimentally study the energy harvesting performance of a compliant membrane hydrofoil undergoing heaving and pitching oscillations in a uniform flow. Membrane foils with different properties: elastic modulus, thickness and prestretch were fabricated and tested in a water flume facility. The Reynolds number based on the chord length and free stream velocity was Re $ = 5 \times 10^{4}$, and the reduced frequency ${f}^*\in [0.1, 0.2]$. When compared to a rigid symmetric hydrofoil, the membrane foil is able to dynamically adapt its shape and camber during each oscillation cycle, yielding up to 50$\%$ higher lift forces and 30$\%$ improvement in cycle efficiency. The flow structure and wake patterns obtained from Particle Image Velocimetry (2-D PIV) measurements will also be discussed. [Preview Abstract] |
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