Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D18: Aerodynamics: Flutter & Dynamic StallAerodynamics
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Chair: Justin Jaworski, Lehigh University Room: 607 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D18.00001: Bending mode flutter in a transonic linear cascade Raghuraman Govardhan, Prahallada Jutur Vibration related issues like flutter pose a serious challenge to aircraft engine designers. The phenomenon has gained relevance for modern engines that employ thin and long fan blade rows to satisfy the growing need for compact and powerful engines. The tip regions of such blade rows operate with transonic relative flow velocities, and are susceptible to bending mode flutter. In such cases, the flow field around individual blades of the cascade is dominated by shock motions generated by the blade motions. In the present work, a new transonic linear cascade facility with the ability to oscillate a blade at realistic reduced frequencies has been developed. The facility operates at a Mach number of 1.3, with the central blade being oscillated in heave corresponding to the bending mode of the rotor. The susceptibility of the blade to undergo flutter at different reduced frequencies is quantified by the cycle-averaged power transfer to the blade calculated using the measured unsteady load on the oscillating blade. These measurements show fluid excitation (flutter) at low reduced frequencies and fluid damping (no flutter) at higher reduced frequencies. Simultaneous measurements of the unsteady shock motions are done with high speed shadowgraphy to elucidate the differences in shock motions between the excitation and damping cases. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D18.00002: Computational Modeling and Analysis of Aeroelastic Wing Flutter Karthik Menon, Joseph Katz, Rajat Mittal Aeroelastic flutter is ubiquitous in aeronautics; of particular relevance here is the flutter of aircraft wings, helicopter rotor blades, flexible wing MAVs and UAVs, and long-endurance aerial systems such as airships and solar powered air-vehicles. Here, we attempt to understand some fundamental aspects of this problem via immersed boundary method based numerical simulations of canonical bodies. We report findings on the effect of body geometry on the dynamics of flutter involving coupled pitch-heave oscillations. We also explore flow-induced flutter of airfoils in pre and post-stall configurations, including the effect of stiffness and pitch axis location. Finally, a novel force decomposition method is used to provide some insight into the flutter dynamics and associated unsteady flow physics. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D18.00003: Non-circulatory fluid forces on porous bodies with application to panel flutter Rozhin Hajian, Justin Jaworski The non-circulatory fluid forces acting on an oscillating porous panel or airfoil in uniform incompressible flow are derived from linearized potential theory. The fundamental integral equation for Holder-continuous porosity distributions is formulated and solved numerically for the special cases of non-porous and uniformly-porous panels with prescribed structural deformations. The new unsteady aerodynamic forces are then applied to aeroelastic stability predictions for porous panels or liners. Results from this analysis aim to form the basis of a complete unsteady aerodynamic theory for porous airfoils and their acoustic emissions based upon the unique attributes of natural fliers and swimmers. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D18.00004: Dynamic stall reattachment revisited Karen Mulleners Dynamic stall on pitching airfoils is an important practical problem that affects for example rotary wing aircraft and wind turbines. It also comprises a number of interesting fundamental fluid dynamical phenomena such as unsteady flow separation, vortex formation and shedding, unsteady flow reattachment, and dynamic hysteresis. Following up on past efforts focussing on the separation development, we now revisited the flow reattachment or stall recovery process. Experimental time-resolved velocity field and surface pressure data for a two-dimensional sinusoidally pitching airfoil with various reduced frequencies was analysed using different Eulerian, Lagrangian, and modal decomposition methods. This complementary analysis resulted in the identification of the chain of events that play a role in the flow reattachment process, a detailed description of that role, and characterisation of the individual events by the governing time-scales and flow features. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D18.00005: The Influence of Second Harmonic Phase and Amplitude Variation in Cyclically Pitching Wings Ethan Culler, John Farnsworth From wind tunnel testing of a cyber-physical wing model, it has been found that the pitch trajectory for stall flutter is described by an array of higher harmonic frequencies with decaying energy content. These frequencies distort the stall flutter motion from that of a pure sinusoidal oscillation in pitch and can have a significant effect on the resulting force production. In order to understand how these higher harmonic frequencies contribute to the overall pitching moment characteristics of a wing in stall flutter, a rigid finite span wing model, with aspect ratio four, was pitched in the wind tunnel. The prescribed motion of the pitch cycle was varied by changing the amplitude ratio and phase of the second harmonic of the oscillation frequency. The second harmonic represents the second highest energy mode in the pitching cycle spectra. Pitching moment and planar particle image velocimetry data was collected. From these pitching trajectories, a significant dependence of pitching moment on both the phase and amplitude of the prescribed waveforms was found. Specifically, for the same amplitude ratio, variations in the phase produced changes of approximately 30 percent in the phase averaged pitching moment. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D18.00006: The convective behavior of a dynamic stall vortex at low Reynolds number John Hrynuk Dynamic stall is a fundamental flow phenomenon that is commonly observed in biological flight and rotorcraft. Under certain conditions a leading edge vortex forms generating large but temporary lift forces. A common assumption for the convective behavior of the dynamic stall vortex (DSV) is that it convects downstream after forming and diffuses while convecting away. However, experiments on a NACA 0012 full span wing at Reynolds number of 12,000 showed the dynamic stall vortex undergoing a behavior that did not resemble diffusion as it convected downstream. Instead, the DSV was observed to compress in a specific region downstream of the wing. A comparison between the DSV behavior and convection of bluff body shedding, which occurred after the DSV was no longer present will be shown. A better understanding of this vortex breakdown method for the dynamic stall vortex at low Reynolds number may help improve vortex methods and CFD for studying dynamic stall. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D18.00007: Dynamic stall characterization using modal analysis of phase-averaged pressure distributions Tanner Harms, Pourya Nikoueeyan, Jonathan Naughton Dynamic stall characterization by means of surface pressure measurements can simplify the time and cost associated with experimental investigation of unsteady airfoil aerodynamics. A unique test capability has been developed at University of Wyoming over the past few years that allows for time and cost efficient measurement of dynamic stall. A variety of rotorcraft and wind turbine airfoils have been tested under a variety of pitch oscillation conditions resulting in a range of dynamic stall behavior. Formation, development and separation of different flow structures are responsible for the complex aerodynamic loading behavior experienced during dynamic stall. These structures have unique signatures on the pressure distribution over the airfoil. This work investigates the statistical behavior of phase-averaged pressure distribution for different types of dynamic stall by means of modal analysis. The use of different modes to identify specific flow structures is being investigated. The use of these modes for different types of dynamic stall can provide a new approach for understanding and categorizing these flows. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D18.00008: Analysis of the cycle-to-cycle pressure distribution variations in dynamic stall Tanner Harms, Pourya Nikoueeyan, Jonathan Naughton Dynamic stall is an unsteady flow phenomenon observed on blades and wings that, despite decades of focused study, remains a challenging problem for rotorcraft and wind turbine applications. Traditionally, dynamic stall has been studied on pitch-oscillating airfoils by measuring the unsteady pressure distribution that is phase-averaged, by which the typical flow pattern may be observed and quantified. In cases where light to deep dynamic stall are observed, pressure distributions with high levels of variance are present in regions of separation. It was recently observed that, under certain conditions, this scatter may be the result of a two-state flow solution – as if there were a bifurcation in the unsteady pressure distribution behavior on the suction side of the airfoil. This is significant since phase-averaged dynamic stall data are often used to tune dynamic stall models and for validation of simulations of dynamic stall. In order to better understand this phenomenon, statistical analysis of the pressure data using probability density functions (PDFs) and other statistical approaches has been carried out for the SC 1094R8, DU97-W-300, and NACA 0015 airfoil geometries. [Preview Abstract] |
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