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 M30: Aerodynamics: Unsteady Airfoils and Wings |
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Chair: Firas Siala, Oregon State University Room: 2016 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M30.00001: Buffeting of NACA 0012 airfoil at high angle of attack Tong Zhou, Earl Dowell Buffeting is a fluid instability caused by flow separation or shock wave oscillations in the flow around a bluff body. Typically there is a dominant frequency of these flow oscillations called Strouhal or buffeting frequency. In prior work several researchers at Duke University have noted the analogy between the classic Von Karman Vortex Street behind a bluff body and the flow oscillations that occur for flow around a NACA 0012 airfoil at sufficiently large angle of attack. Lock-in is found for certain combinations of airfoil oscillation (pitching motion) frequencies and amplitudes when the frequency of the airfoil motion is sufficiently close to the buffeting frequency. The goal of this paper is to explore the flow around a static and an oscillating airfoil at high angle of attack by developing a method for computing buffet response. Simulation results are compared with experimental data. Conditions for the onset of buffeting and lock-in of a NACA 0012 airfoil at high angle of attack are determined. Effects of several parameters on lift coefficient and flow response frequency are studied including Reynolds number, angle of attack and blockage ratio of the airfoil size to the wind tunnel dimensions. Also more detailed flow field characteristics are determined. For a static airfoil, a universal Strouhal number scaling has been found for angles of attack from 30$^{\circ}$ to 90$^{\circ}$, where the flow around airfoil is fully separated. For an oscillating airfoil, conditions for lock-in are discussed. Differences between the lock-in case and the unlocked case are also studied. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M30.00002: Spanwise gradients in flow speed and leading edge vortex attachment on low Reynolds number wings Thierry Jardin, Laurent David It is now accepted that the aerodynamic performance of low aspect ratio revolving wings, such as insect wings or maple seed membranes, largely relies on sustained leading edge vortex attachment. However, the mechanisms responsible for this sustained attachment are still poorly understood. Here, we compute the Navier-Stokes solution of the flow around a finite wing (i) subjected to a uniform oncoming flow, (ii) subjected to a spanwise varying oncoming flow and (iii) revolving about its root. Therefore, we are able to isolate the mechanisms associated with the spanwise gradient of the local wing speed from those associated with centrifugal and Coriolis effects. We show that over flapping amplitudes typical of insect flight the spanwise gradient of the local wing speed may suffice in maintaining leading edge vortex attachment. We correlate this result with the development of spanwise flow and we evaluate the sensitivity of such a mechanism to the Reynolds number. It is noted, however, that leading edge vortex attachment through the spanwise gradient of the local wing speed does not promote large lift, which ultimately arises from centrifugal and Coriolis effects. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M30.00003: Exploring Unsteady Sail Propulsion in Olympic Class Sailboats Riley Schutt, C.H.K. Williamson Unsteady sailing techniques, defined as ``flicking,'' ``roll-tacking'' and ``roll-gybing'' are used by athletes to propel their boats on an Olympic race course faster than using the wind alone. Body weight movements induce unsteady sail motion, increasing driving force and enhancing maneuvering performance. In this research, we explore the dynamics of an Olympic class Laser sailboat equipped with a GPS, IMU, wind sensor, and camera array. The velocity heading of a sailing boat is oriented at an apparent wind angle to the flow. In contrast to classic flapping propulsion, the heaving of the sail section (induced by the sailor's body movement) is not perpendicular to the sail's motion through the air. This leads to an ``exotic heave,'' with components parallel and perpendicular to the incident flow. The characteristic motion is recreated in a towing tank where the vortex structures generated by a representative 2-D sail section are observed, along with a measurement of thrust and lift forces. When combined with turning maneuvers, these heaving sail motions can lead to significant increases in velocity made good, a critical variable used when assessing racing performance. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M30.00004: Three-dimensional flow fields and forces on revolving flat plates Mustafa Percin, Bas W. van Oudheusden The evolution of three-dimensional flow structures of revolving low-aspect-ratio plates in the Reynolds number range of 10,000 to 20,000 was studied, combining Tomographic Particle Image Velocimetry with force measurements. Two motion kinematics were considered: (1) a revolving surge motion where the wing accelerates to a terminal velocity with a constant acceleration at a fixed angle of attack and then remains to revolve at a constant rate; (2) a revolving pitch motion which is initiated by a constant acceleration from rest to a terminal velocity at zero angle of attack, followed by a pitch-up motion at a constant pitch rate and revolution at a constant rate. In the experiments, the terminal velocity, acceleration, angle of attack and pitch rate were varied to study their effect on the resultant flow fields and forces. In general, a vortex system that consists of a leading edge vortex, a tip vortex and a trailing edge vortex is observed. The vortex system bursts into substructures as the motion progresses, which does not lead to a decrease in the forces. The evolution of spanwise flow and the effects of centrifugal acceleration and spanwise pressure gradient are discussed. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M30.00005: Formation Flight: Modes of Interaction of a Streamwise Vortex with a Wing Chris McKenna, Matthew Bross, Donald Rockwell Aircraft flying together in an echelon or V formation experience aerodynamic advantages. Impingement of the tip vortex of the leader (upstream) wing on the follower wing can yield an increase of lift to drag ratio. This enhancement is known to be sensitive to the location of vortex impingement on the follower wing. Particle image velocimetry is employed to determine patterns of velocity and vorticity in successive crossflow planes, which characterize the streamwise evolution of the vortex structure along the chord of the follower wing and into its wake. Different modes of vortex-follower wing interaction are created by varying the spanwise location of the leader wing. These modes are defined by differences in the development of, and interaction between, the incident tip vortex from the leader wing and the tip vortex along the follower wing. Modes of development/interaction of the tip vortices include bifurcation, attenuation, and mutual induction. The bifurcation and attenuation modes decrease the strength of the follower tip vortex. In contrast, the mutual induction mode increases the strength of the follower tip vortex. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M30.00006: Vortex shedding and aerodynamic performance of an airfoil with multi-scale trailing edge modifications Jovan Nedic, J. Christos Vassilicos An experimental investigation was conducted into the aerodynamic performance and nature of the vortex shedding generated by truncated and non-flat serrated trailing edges of a NACA 0012 wing section. The truncated trailing edge generates a significant amount of vortex shedding, whilst increasing both the maximum lift and drag coefficients, resulting in an overall reduction in the maximum lift-to-drag ratio (L/D) compared to a plain NACA0012 wing section. By decreasing the chevron angle ($\phi$) of the non-flat trailing edge serrations (i.e. by making them sharper), the energy of the vortex shedding significantly decreases and L/D increase compared to a plain NACA0012 wing section. Fractal/multi-scale patterns were also investigated with a view to further improve performance. It was found that the energy of the vortex shedding increases with increasing fractal iteration if the chevron is broad ($\phi \approx 65^{\circ}$), but decreases for sharper chevrons ($\phi = 45^{\circ}$). It is believed that if $\phi$ is too big, the multi-scale trailing edges are too far away from each other to interact and break down the vortex shedding mechanism. Fractal/multi-scale trailing edges are also able to improve aerodynamic performance compared to the NACA 0012 wing section. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M30.00007: Identification of Scaling Parameters for Rotor-Induced Sediment Mobilization Gino Perrotta Flow imaging and particle imaging velocimetry experiments were conducted in a water tank to investigate the effects of rotor wake and sediment characteristics on rotor-induced sediment mobilization during hover in ground effect. The two-phase flow was separated into carrier phase and dispersed phase. The carrier phase was studied using PIV to acquire time-resolved planar velocity measurements for a field of view within the rotor wake. The rotor-induced flow was confirmed to be dominated by blade tip vortices and was thus characterized in terms of the vortex properties. Vortices were identified using a nonlocal function and were fit to the Lamb-Oseen vortex velocity profile to evaluate size and strength. The rotor-induced flow was also characterized in terms of wall jet velocity and turbulent kinetic energy. The dispersed phase was separated from the carrier phase using image filtering procedures and was quantified by identifying mobilized sediment particles visible in the field of view. Candidate scaling parameters were created by combining rotor-induced sediment mobilization system characteristics. These candidate parameters were inspected for correlation with sediment mobilization. Three new scaling parameters are proposed and evaluated. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M30.00008: Use of passively actuated flaps for enhanced lift for pitching and heaving airfoils Firas Siala, Cameron Planck, James Liburdy The enhanced lift and reduced drag obtained by applying passively actuated leading and trailing flaps to a low aspect ratio flat wing during heaving and pitching at moderate Reynolds numbers ($10^4$) is demonstrated. Direct force measurements are obtained during the cyclic motion and are synchronized with the tracking of the motion of the passive flaps. The flaps are controlled using torsion springs and their natural frequency is found to play a dominant role in determining the lift enhancement. Results are shown for a range of heaving and pitching conditions of amplitude and frequency, with the pitching phase offset ninety degrees from the heaving. Flow visualization is used to document the transient wake conditions. The lift and drag forces are shown to be enhanced near the peak effective angle of attack during the cycling motion resulting in a net mean lift increase. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M30.00009: Force and vortical flow development on pitching wings at high rates Luis Bernal, Huai-Te Yu, Michael OL, Kenneth Granlund Recent experimental results of pitching flat plate wings are presented. High pitch-rate perching maneuvers are frequently used by birds for feeding and landing. Insects use very fast rotation rates at the end of each flapping stroke, which results in high thrust and precise flight. These wing motions are also of interest for engineered micro air vehicles to achieve semi-autonomous landing by unskilled operators. The wing motion considered is a constant rotation rate pitch motion from 0 to 45 degrees of an aspect-ratio-4 flat-plate wing. The goal is to gain a better understanding of force generation mechanisms and their relationship to two- and three-dimensional vortical flow structures. Leading edge, trailing edge, and tip vortices form with large separated flow regions over the wing, however comparison with linear potential flow theory gives good agreement. The evolution of the leading edge vortex is delayed for pivot axes locations downstream of the leading edge. Large forces at the end of the motion slowly return to the steady state value over more than 30 convective times. The flow in the near wake shows a brief period of vortex shedding and strong three dimensional effects. Two different three-dimensional flow features are observed: A rapid development of three-dimensionality in the core of the leading and trailing edge vortices and a swirl motion in the near wake. However the impact of these three-dimensional flow features on force development is small. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M30.00010: Unsteady Airloads on Airfoils in Reverse Flow Andrew Lind, Anya Jones This work gives insight into the influence of airfoil characteristics on unsteady airloads for rotor applications where local airfoil sections may operate at high and/or reverse flow angles of attack. Two-dimensional wind tunnel experiments have been performed on four airfoil sections to investigate the effects of thickness, camber, and trailing edge shape on unsteady airloads (lift, pressure drag, and pitching moment). These model rotor blades were tested through 360 deg of incidence for $10^4\leq Re\leq 10^6$. Unsteady pressure transducers were mounted on the airfoil surface to measure the high frequency, dynamic pressure variations. The temporal evolution of chordwise pressure distributions and resulting airloads is quantified for each airfoil in each of the three unsteady wake regimes present in reverse flow. Specifically, the influence of the formation, growth, and shedding of vortices on the surface pressure distribution is quantified and compared between airfoils with a sharp geometric trailing edge and those with a blunt geometric trailing edge. These findings are integral to mitigation of rotor blade vibrations for applications where airfoil sections are subjected to reverse flow, such as high-speed helicopters and tidal turbines. [Preview Abstract] |
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