Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EX: Aerodynamics III |
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Chair: Javid Bayandor, Virginia Polytechnic Institute and State University Room: Hyatt Regency Long Beach Regency D |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EX.00001: Time-dependent measurements over membrane plates at low Reynolds number James Hubner, Kyle Scott, Amory Timpe, Lawrence Ukeiley A segment of low Reynolds number aerodynamic research employs biomimetics for optimization of airfoil shapes to micro air vehicle (MAV) flight. Many of these efforts focus on thin, flexible membrane airfoils inspired by small birds, bats and insects. This design approach, mimicking low Reynolds number flyers (\textit{Re} $<$ 100,000), has led to improved aerodynamic performance, particularly the mitigation of flow disturbances through passive aerodynamic and geometric twisting. In many cases, membrane vibration exists, altering the characteristics of the separated shear layer over the wing, leading to both advantageous and disadvantageous effects. Identifying and quantifying the nature of the fluid-structure coupling and how this coupling can passively control the flow is the goal of a recently initiated research project by the authors. This talk will present the objectives of the project and initial findings of synchronized flow (hot-wire anemometry) and surface deflection (laser vibrometry) measurements over rigid plates and flexible membranes at incidence to the free stream flow. A range of flow Reynolds numbers are examined, from 10,000 to 50,000, in which vibration initiates and grow with increasing velocity. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EX.00002: Low dimensional state-space representations for classical unsteady aerodynamic models Steven L. Brunton, Clarence W. Rowley This work develops reduced order models for the unsteady aerodynamic forces on a small wing in response to agile maneuvers and gusts. In particular, the classical unsteady models of Wagner and Theodorsen are cast into a low-dimensional state-space framework. Low order state-space models are more computationally efficient than the classical formulations, and are well suited for modification with nonlinear dynamics and the application of control techniques. Reduced order models are obtained using the eigensystem realization algorithm on force data from the direct numerical simulation (DNS) of a pitching or plunging 2D flat plate at Reynolds numbers between 100 and 1000. Models are tested on rapid pitch and plunge maneuvers with a range of effective angle-of-attack. We evaluate the performance of the models based on agreement with results from DNS, in particular, the ability to reproduce lift forces over a range of pitching and plunging frequencies. Bode plots of the reduced order models, Wagner's and Theodorsen's methods, and DNS provide a concise assessment. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EX.00003: The Dynamics of Spanwise Vorticity on a Rotating Blade in Unsteady Flow Craig Wojcik, James Buchholz Spanwise flow driven by accelerations on rotating blades is known to influence the aerodynamic forces and moments in flapping flight and wind turbine aerodynamics compared with wings and airfoils that are stationary or oscillated in a planar motion. This difference is largely attributed to the resulting prolonged attachment of the leading edge vortex in the rotating case. In this experimental study, we consider the nature and dynamics of spanwise vorticity shed from the leading- and trailing edges of a spinning propeller that is yawed with respect to the free stream. Phase-averaged Digital Particle Image Velocimetry is used to interrogate the flow. The effects of tip speed ratio and yaw angle on development and strength of the shed structures will be discussed. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EX.00004: Flow Structure on Plates in Unsteady Motion T. Yilmaz, D. Rockwell The unsteady, three-dimensional flow structure is characterized along a flat plate (aspect ratio two) undergoing pitching motion to relatively high angle of attack and plunging motion associated with moderate effective angle of attack. Techniques of particle image velocimetry lead to sectional and volume representations of the phase-averaged velocity, vorticity and streamline topology. Emphasis is on a value of Reynolds number based on chord of 10,000. An inherent feature is pronounced spanwise flow in the vicinity of the leading-edge, either towards or away from the plane of symmetry of the plate, which is associated with formation of large-scale, three-dimensional vortical structures. Such structures can have substantial levels of vorticity oriented in the streamwise/surface-normal direction(s), and they are characterized in relation to the unsteady development of the tip vortices. Their continued evolution along the plate is associated with radical departures~from a quasi-two-dimensional flow pattern. This investigation is a sequel to our recent study reported in \textit{Experiments in Fluids}, Vol. 48, No. 3 (March), pp. 539-544. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EX.00005: Unsteady Aerodynamics on a Pitching Plunging Flat Plate Adam Hart, Lawrence Ukeiley Biology has shown that natural fliers utilize unsteady flow mechanisms to enhance their lift characteristics in low Reynolds number flight regimes. This study will investigate the interaction between the leading edge vortices (LEVs) and tip vortices over a low aspect ratio flat plate being subjected to a pitch-plunge kinematic motion. Previous studies have shown the creation of stable vortices off the leading edge at the three quarter span location between times 0.25 and 0.50 in the kinematic motion. This study furthers previous knowledge by mapping the flow field around these vortex cores. This will allow for an understanding into the interaction of the LEV with tip vortices and how the LEVs convect downstream. Specifically we look to validate the interactions between these vortex systems leading to enhanced lift as has been demonstrated in very low Reynolds number numerical simulations. A combination of two dimensional and stereo Particle Image Velocimetery (PIV) is used to measure the flow field around the flat plate at various spanwise and chordwise locations. The PIV measurements are triggered by the dynamic motion rig allowing for phase averaging at key locations throughout the motion cycle. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EX.00006: Stability Characteristics of Low Reynolds Number, Low Aspect Ratio Wings Matthew Shields, Kamran Mohseni The recent interest in Micro Aerial Vehicles (MAVs) has led to the development of many different aircraft; however, little progress has been made in understanding the physics of MAV flow. MAVs aerodynamics is affected by low Reynolds number flow and low aspect ratios. As a result nonlinear effects due to tip vortices are quite important. We have developed a new experimental setup for measuring stability derivatives in a small wind tunnel. Using a four degree of freedom actuation system, a model can be placed in the test section and maneuvered in such a way to isolate the flow components responsible for creating stability derivatives. Accurate measurements of the aerodynamic loading can then be used to compute these values. Initial testing was conducted primarily on a series of flat plates of different aspect ratios. In addition, the CU MAV was tested as a specific case study. Test results indicate that some of the cross coupled stability derivatives, ignored for larger aircrafts, are on the same order of magnitude as standard derivatives and thus can not be ignored in the derivation of the linear equations of motion for a micro aerial vehicle. As a result, a more general set of equations of motion are derived based upon experimentally obtained stability derivatives. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EX.00007: Surface Visualization of Low Re Flow over Low Aspect Ratio Flat Plate Wings Blair Farley, James Hubner Unmanned air vehicles are an increasingly important focus of the military. Micro air vehicles (MAVs) are a subset of these for which the size is limited to 15 cm. MAVs generally fly at low speeds, 5-20 m/s, and Reynolds numbers less than 200,000. For designs to be compact while maintaining high lift, low aspect ratio wings are generally employed, creating larger tip vortices. This study investigates the extent of flow separation and reattachment as well as the effect of the tip vortex over the lee-side of low-aspect ratio wings. Surface visualization methods, such as powder tracking, are discussed, and the results for the flow over Zimmerman and rectangular rigid, flat plate wings of aspect ratio 1.27, 3, and 5 will be presented for Re near 50,000. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EX.00008: Unsteady force measurement of SD7003 foil under pitch-up, hold and pitch-down motion at Re = 1x10$^{4 }$ for Micro Aerial Vehicle applications Sutthiphong Srigrarom, Wee Sern Chai The unsteady force applied on the SD7003 foil under pitch-up, hold and pitch-down motion was studied. This canonical pitch-up, hold and pitch down motion pattern resembles the transient lift creation during perching of the micro aerial vehicle in flapping flight. The 2D SD7003 foil with pivot point at $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ chord was tested in water tunnel at Reynolds of 1 x 10$^{4}$. Three pitch-up rates corresponds to pitch rate, $\Omega $+ = 0.2, 1.4 and 2.8 (reduced frequency, k = 0.62, 4.33, and 8.65) were tested. This is to investigate the effect of rapid pitch and magnitude of the leading-edge vortex (LEV) on the non-linear lift. The faster pitch-up rate results in the stronger lead-edge vortex and deeper subsequent dynamic stall. The non-circular lift due to acceleration effects are captured and shown in $\Omega $+ = 1.4 case. The effect of the hold time after pitch-up motion was also examined. For the t$_{hold}$/t+ = 1.00 (t+ $\equiv $ c/U$_{\infty })$, the LEV created during ramp-up motion remains over the foil to provide vortex lift at longer period, resulting in larger average lift over the cycle in comparison to the t$_{hold}$/t+ = 0.05. In addition, the spike in C$_{L}$ during high-Frequency low-Re ramp and return are captured when k = 0.62. The dominant frequency is found to be O(10) Hz observed from power spectral density. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EX.00009: Vortex Formation vs. Aerodynamic Force Coefficient Variations for Rapidly-Pitching Flat Plates Michael Ol, Kenneth Granlund We consider a flat plate in constant-rate (linear) pitch, from angle of attack $\alpha $ = 0\r{ } to 90\r{ }, at a Reynolds number of 20,000. The motion is via a three-component electric rig fitted atop a water tunnel. Various smoothing transients at motion initiation and cessation are applied. Pitch pivot point is at the plate chordwise locations x/c = 0, 0.25, 0.5 and 0.75. Pitch rates range from K = ${c\dot {\theta }} \mathord{\left/ {\vphantom {{c\dot {\theta }} {2U_\infty }}} \right. \kern-\nulldelimiterspace} {2U_\infty }$ of 0.0025 through 1.0. Plate geometries include nominally 2D and aspect ratio 2.0; both have round edges. Lift, drag and pitching moment were measured directly with a force balance, while flowfield data included dye injection and PIV. For reduced frequencies K $<$ 0.05, lift for the 2D plate followed the usual relation of 2$\pi \alpha $, with stall delay in proportion to pitch rate, and stall behavior increasingly smoother as pitch rate increases. At higher K, acceleration or noncirculatory effects are manifest, with a rise in lift at low $\alpha $, and a rise in drag at high $\alpha $. Noncirculatory and circulatory effects are additive and the noncirculatory portion is well predicted by potential-flow methods. K $>$ 0.02 evinces the formation of a leading edge vortex. Peak in lift correlates to the angle of attack where the leading edge vortex reaches maximum circulation and begins to shed. Lift and drag are seen to obey a scaling with pitch rate, for K $>$ 0.03. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EX.00010: Lift generation of an impulsively started flat wing from the perspective of a diagnostic vorticity force theory Cheng-Ta Hsieh, Jian-Jhih Lee, Chih-Yu Kao, Chien-Cheng Chang, Chin-Chou Chu In this study, we consider various force contributions to an impulsively started wing plate from the perspective of a diagnostic vorticity force theory. A wing plate has the aspect ratio between 1 and 3 and is placed at different angles of attack ($\alpha$) 30$^{\circ}$, 45$^{\circ}$ and 60$^{\circ}$, while the Reynolds number is varied between 100 and 300. The force theory enables us to examine forces exerted on the wing plate credited to the individual flow structures, such as the leading-edge vortex (LEV), trailing edge vortex (TEV), tip vortices as well the contribution from the surface vorticity. Given a force direction (in particular lift and drag), each fluid element with nonzero vorticity in the flow is associated with a vorticity force element which gives the intensity contributed by that element. The contour plots of the force elements help us visualize where the fluid elements play more significant roles than elements elsewhere. The relative importance of the various contributions to the hydrodynamic forces is analyzed in terms of the aspect ratio, angle of attack and the Reynolds number. [Preview Abstract] |
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