Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G16: Air Vehicle Aerodynamics |
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Chair: Thomas Corke, University of Notre Dame Room: 319 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G16.00001: High Speed Focused Schlieren Flow Visualization of Shock Induced Dynamic Stall Patrick Bowles, Dustin Coleman, Thomas Corke, Flint Thomas, Mark Wasikowski Focused Schlieren images of the leading edge flowfield ($0 \leq x/c \leq 0.10$) of a modern helicopter rotor-blade airfoil are presented in order to document the development of shock waves during compressible light dynamic stall at moderate subsonic freestream Mach numbers ($M_{\infty}$ $\in$ [0.2,0.6]). The focused Schlieren system was designed to provide a ``macro'' image of the near (suction) surface boundary layer and shock features with a marginal depth of focus, effectively eliminating density distortions associated with the windtunnel (plexiglass) walls or the (polycarbonate) rotating airfoil endplates. A high-speed camera operating at up to 4\,kFPS was used to capture the dynamic behavior of the shock, leading-edge separated shear layer and dynamic-stall vortex during the pitching cycle. These images were then correlated with static pressure time series on the airfoil surface. The effect of leading-edge roughness was then investigated. Under certain conditions, this was shown to alter the shock development and shock streamwise propagation. The effect that the shock formation had on the pitch-moment stability for these different conditions is then presented. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G16.00002: Boundary Layer Trip Effects On Incipient Dynamic Stall Flutter Thomas Corke, Patrick Bowles, Dustin Coleman, Flint Thomas, Mark Wasikowski Experimental results of compressible light dynamic stall on a modern helicopter rotor-blade airfoil are presented at free-stream Mach numbers ranging from 0.2 to 0.6, Reynolds numbers up to $3.5 \times 10^6$, and reduced frequencies corresponding to relevant helicopter rotor speeds ($k \leq 0.05$ for $M_{\infty} \geq 0.4$). Diagnostic tools include high frequency pressure transducers to study the temporal behavior of the pressure field and integrated loads, as well as macro focused Schlieren images of the leading edge flow field (0 $\leq$ $x/c$ $\leq$ 0.10). The study concentrates on the air load's net aerodynamic work on the airfoil to identify combinations of collective and cyclic input that cause negative or reduced torsional damping. Free (un-tripped) and forced (tripped) laminar to turbulent transition were explored in order to constrain the mechanism for dynamic stall onset. Leading edge roughness resulted in a suppression of the suction peak and strong dynamic vortex compared to the baseline that resulted in reduced torsional damping at high subsonic free-stream Mach numbers ($M_{\infty} \geq 0.5$). Comparisons to predictions from a Leishman-Beddoes model for dynamic stall are also presented. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G16.00003: Velocity measurements in the near wake of a ship superstructure Cody Brownell, Luksa Luznik, Murray Snyder, Hyung Suk Kang, Colin Wilkinson Velocity measurements in a ship airwake are obtained \textit{in situ} aboard a 108 ft naval training vessel. Three-component sonic anemometers are placed at the bow of the ship, for reference wind measurement, and at numerous locations above a flight deck at the stern of the ship. The mean flow structure resembles that of a 3D backward-facing step, with a recirculation region covering much of the forward flight deck, and significant downwash over the remainder of the surface. Reynolds stresses and two-point velocity correlations are presented, and placed in the context of shipborne helicopter operations. The influence of an atmospheric boundary layer, often unavailable in ship airwake measurements from a wind tunnel, is discussed. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G16.00004: Energy Exchange during Plunge/Surge Motions of a 2D Wing Wesley Kerstens, Jeesoon Choi, Tim Colonius, David Williams The rate of energy transfer between an NACA-0006 wing and an unsteady flow is examined at pre-stall and post-stall conditions using numerical simulations and wind tunnel experiments. The plunge and surge motions simulate the fluctuating vertical (w$_{z})$ and longitudinal (w$_{x})$ velocity components of a wind gust. In a steady flow the wing loses energy to the flow through the drag power term, but in an unsteady flow the wing may gain energy from the fluctuating lift power and fluctuating drag power terms. The net energy transfer averaged over the period of oscillation depends on the phase angle between the plunge and surge motions. The largest increase of energy occurs when w$_{x}$ and w$_{z}$ are in-phase. When the fluctuations are large enough, then it is possible for the net energy gain to be positive. The numerical simulations conducted at Reynolds numbers near the critical value for vortex shedding show qualitative agreement with the experiments. The simulations highlight the role of vortex shedding in determining the optimal frequency and phase for energy extraction from the gust. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G16.00005: Facility Dependent Issues Influencing the Experimental Characterization of a Laminar Separation Bubble David A. Olson, Ahmed M. Naguib, Manoochehr M. Koochesfahani There is a documented discrepancy in the characteristics of the laminar separation bubble on the steady SD7003 airfoil obtained from various experimental and computational studies. The influence of added freestream turbulence, among other factors, is studied for a range of Reynolds number (2$\times 10^4$ -- 6$\times 10^4$), and angle of attack (2$^{\circ}$ -- 12$^{\circ}$). Both the baseline cases and those with added grid-generated turbulence utilize single-component Molecular Tagging Velocimetry with a cross-stream measurement spatial resolution of 52$\mu$m. The addition of freestream turbulence is shown to decrease the size of the separation bubble by both delaying separation and triggering an earlier reattachment. The measured separation location is shown to be also sensitive to the uncertainty of wall location and near-wall spatial resolution. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G16.00006: Experimental Investigation of Tip Vortex Control Using a Half-Delta Shaped Tip Strake Jennifer Pereira Due to their undesirable lift-induced drag effects, the control of tip vortices remains a challenge for the aviation industry. Naturally, most types of control devices involve tip vortex alleviation and thus focus on altering the flow at the wingtip. One such device is a half-delta shaped strake that when added to the wingtip could not only reduce induced drag by improving end effects but also increase lift through the delta wing LEV lift. To investigate and better understand this concept a rectangular planform NACA0012 wing was fitted with a 65$^{\circ} $ sweep half-delta shaped tip strake and force measurements were obtained at a chord Reynolds number of 2.7$\times$10$^5$. This data was complimented with seven-hole probe flowfield measurements over the tip and in the near field. Results were compared to the rectangular planform wing (baseline wing), a sharp 65$^{\circ}$ sweep half-delta and full delta wing for direct comparison and to better understand the flow physics involved. The effect of both streamwise location, angle of attack and strake setting were examined. It was found that the strake acts much like a delta wing albeit with a strengthened LEV that prematurely breaks down. The broken down LEV then results in a tip vortex which is much more diffused than its baseline counterpart. In order to quantify this effect, the Maskell model was used to calculate the induced drag of both the baseline wing and the strake at various angles of attack and strake settings. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G16.00007: Drag reduction in large wind turbines through riblets: evaluation of different geometries Roger Arndt, Leonardo Chamorro, Fotis Sotiropoulos Achieving skin friction drag reduction by use of riblets has been a topic of intensive research throughout the last several decades. The majority of the effort on this topic has been based on both numerical (mainly DNS) and experimental (wind tunnel and fluid channel) approaches. Yet, despite these valuable endeavors, the fundamental mechanisms that induce the drag reduction are still not well established. In this study, wind tunnel experiments were performed to quantify the drag reduction for a wind turbine airfoil caused by different V-grooved riblet configurations. A full-scale 2.5MW Clipper wind turbine airfoil section (of 1 meter chord length, typical of the 88{\%} blade span) was placed in the freestream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. The drag forces the airfoil experienced were measured for different riblet configurations and at different angles of attack, all with a constant Reynolds number of Re=2.2 millions (based on the airfoil chord length). Layouts of both complete and partial airfoil coverage, of riblets, were considered in the study. Force sensors were used to measure Lift and Drag but more accurate Drag forces were obtained through wake surveys using a pitot static probe. The measurements will be used to help develop and test the performance of near-wall boundary conditions in the context of RANS and hybrid RANS/LES models. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G16.00008: Aerodynamic Behavior at One Revolution Angle of Attack of Two-Dimensional Wings Yong Oun Han, Eun Ha Lee, Jeong Hyun Kim, Yong Oh Shin In order to investigate aerodynamic behaviors at extreme angles of attack beyond the normal static stall angle and in the reversed flow, lift and drag have been measured at one revolution angles of attack by rotating the wing around the 1/4 chord with use of a dynamic balance in the low speed wind tunnel. Three different geometries of wing section; a flat plate, a symmetric airfoil, NACA0018, and a cambered airfoil, Goe222, were selected for these experiments. It was turned out that the lift coefficient maintains substantially even beyond the traditional stall AoA of the wing. Drag coefficients of these wings showed sinusoidal profiles, and polar plots of Cl versus Cd provided distinctive behaviors unseen in the calculation by the classical wing theory. Application of the cyclic aerodynamic characteristics to a vertical axis wind turbine and wake characteristics around the critical angle will be displayed. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G16.00009: Electro-Luminescence based Pressure-Sensitive Paint System for Unsteady Flow Field Measurements Yoshimi Iijima, Hirotaka Sakaue Electro-luminescence (EL) based pressure-sensitive paint (PSP) system is developed for capturing unsteady flow fields. It has advantages in uniform distribution in the illumination without remotely apply the illumination source from the testing object. The resultant system can be applied directly onto a testing object surface. It consists of an inorganic EL and a PSP. The EL emits blue illumination uniformly applied onto the PSP layer. Because of a sheet illumination, the EL gives uniform distribution, while a point illumination gives a spot in illumination. The PSP is developed to provide a fast response to a change in pressure. It uses a porous particle and a polymer to create porous-polymer PSP. The response time characterization of the developed system is included in the presentation. The developed system is applied to an unsteady flow field, such as a sound field in a resonance tube. [Preview Abstract] |
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