Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session HM: Aerodynamics |
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Chair: Geoffrey Spedding, University of Southern California Room: 200B |
Monday, November 23, 2009 10:30AM - 10:43AM |
HM.00001: Span efficiency of wings at moderate Reynolds number Geoffrey Spedding, John McArthur Classical aerodynamic models of lift and induced drag of finite wings work well for chord-based Reynolds numbers of 10$^{6}$ or more, where the thin, viscous boundary layers are mostly attached. Recent interest in the design of practical micro-air vehicles and in their natural counterparts (birds, bats, larger insects) brings us into a Reynolds number regime (10$^{4}$ -- 10$^{5})$ where the aerodynamic performance is strongly affected by the possibility of separation of the laminar boundary layer, and then its possible re-attachment. Now, corrections to the inviscid formulations of induced drag become substantial and difficult to estimate. Inconsistencies in the literature further confuse the issue. Here, we clarify the definitions of span efficiency and test the applicability of one-parameter correction models for moderate Reynolds numbers. Suggestions for model improvements will follow. [Preview Abstract] |
Monday, November 23, 2009 10:43AM - 10:56AM |
HM.00002: On the initial-value problem for the flow past a finite wing or blade John Russell Assume piecewise irrotational motion of an incompressible fluid in which the normal velocity distribution on the wing is given and the wake is subject to continuity of normal velocity and pressure across it. The boundary integral for the velocity potential \(\phi\) outside the wing simplifies when its inside is filled with fluid whose motion satisfies similar field equations and whose normal velocity is continuous with that outside. Calculation of \(\mathbf{u}=\nabla\phi\) for the velocity transforms a superposition of dipoles to a superposition of quadrupoles and thus to integrals that diverge in the limit as the field point approaches the boundary. Integration by parts via \textsc{Stokes} theorem yields an integral for the velocity of \textsc{Biot-Savart} type (i.e. involving dipoles). Equations for the evolution of the vortex sheet follow from equations in \S2.2 of \textsc{Saffman, P.G.} \textit{Vortex Dynamics}, Cambridge, 1992 and admit interpretations analogous to those of \textsc{Hemholtz} vortex theorems but applied to vortex ribbons instead of vortex tubes. [Preview Abstract] |
Monday, November 23, 2009 10:56AM - 11:09AM |
HM.00003: Investigation of a low Reynolds number airfoil using molecular tagging velocimetry Alan Katz, Ahmed Naguib, Manoochehr Koochesfahani Molecular tagging velocimetry (MTV) is used to study flow separation, transition, and reattachment on a SD7003 airfoil at a chord Reynolds number of 20,000. Multi-line tagging is employed to obtain high resolution one-component velocity data near the suction surface of the airfoil. The spatial resolution in the wall-normal direction normalized by the chord length is about 0.0003, nearly ten times smaller than previously reported PIV measurements on the same airfoil under similar flow conditions. Preliminary data will be presented for the locations of flow separation and reattachment for the airfoil at an angle of attack of 8 degrees, and results will be compared with existing experimental and computational data. [Preview Abstract] |
Monday, November 23, 2009 11:09AM - 11:22AM |
HM.00004: Lift production through asymmetric flapping Shreyas Jalikop, K.R. Sreenivas At present, there is a strong interest in developing Micro Air Vehicles (MAV) for applications like disaster management and aerial surveys. At these small length scales, the flight of insects and small birds suggests that unsteady aerodynamics of flapping wings can offer many advantages over fixed wing flight, such as hovering-flight, high maneuverability and high lift at large angles of attack. Various lift generating mechanims such as delayed stall, wake capture and wing rotation contribute towards our understanding of insect flight. We address the effect of \textit{asymmetric} flapping of wings on lift production. By visualising the flow around a pair of rectangular wings flapping in a water tank and numerically computing the flow using a discrete vortex method, we demonstrate that net lift can be produced by introducing an asymmetry in the upstroke-to-downstroke velocity profile of the flapping wings. The competition between generation of upstroke and downstroke tip vortices appears to hold the key to understanding this lift generation mechanism. [Preview Abstract] |
Monday, November 23, 2009 11:22AM - 11:35AM |
HM.00005: Unsteady Vortex Structures in the Wake of a Piezoelectric Flapping Wing Lucas Clemons, Hirofumi Igarashi, Hui Hu An experimental study was conducted to characterize the behavior of Unsteady Vortex Structures in the Wake of a piezoelectric flapping wing with miniaturized size (about 10mm in chord length), large flapping amplitude (up to 2.0 times of chord length) and high flapping frequency (60Hz) to explore the potential application of piezofans as the compact, gearless flapping-wings for the development of novel piezoelectric-flapping-wing-based Nano-Air-Vehicles (NAVs). The experimental investigation was performed in a low-speed wind tunnel. A digital particle image velocimetry (PIV) system was used to achieve phased-locked flow field measurements to quantify the transient behavior of the unsteady vortex structures in wake of the piezoelectric flapping wing. The effects of important parameters such as incoming flow velocity (i.e., forward flight speed), the flapping amplitude, and the incline angle of the flapping wing in relation to the incoming flow direction (i.e. the angle of attack) on the wake vortex shedding processes were examined to elucidate underlying physics in order to explore/optimize design paradigms for the development of novel piezoelectric-flapping-wing-based NAVs. [Preview Abstract] |
Monday, November 23, 2009 11:35AM - 11:48AM |
HM.00006: The Effect of Flexible Membrane Scalloping on the Lift and Drag of Flat Plates in Low Re Flow Melissa Conway, James Hubner Micro Air Vehicles (MAVs) have the potential to be used for surveillance and assessment of dangerous environments. Promising applications for MAVs encourage researchers to find effective designs for the most adept flying. Previous wake analysis research conducted at the University of Alabama suggested trailing-edge scalloping of the membrane reduces drag. The present study, drawing from the previous research, tests various geometries involving scalloped and nonscalloped trailing-edges on a three-component force balance. Flat plates were tested to mimic fixed MAV wings at low Reynolds number flow and varying angles-of-attack in a low-speed wind tunnel. Thin aluminum plates with repeated cell geometries were covered with a flexible latex membrane. Cell aspect ratios of 1 and 2 (80{\%} and 40{\%}, respectively, of the chordwise length) were tested along with a solid flat plate and a solid scalloped plate. The presentation will discuss results and explore the impact of changing geometries on lift, drag and aerodynamic efficiency. [Preview Abstract] |
Monday, November 23, 2009 11:48AM - 12:01PM |
HM.00007: Flow-induced oscillation of free reeds Peter Buchak, John Bush We present an investigation of the mechanism by which air flow induces oscillation in free reeds, which produce sound in several musical instruments. In this system, a thin strip of metal is clamped at one end to the top of a plate above a slot large enough for it to pass through. This geometry allows a uniform flow to induce and sustain large-amplitude oscillations of the reed. We investigate experimentally the conditions under which oscillation occurs and formulate a theoretical model, informed by the work of St. Hilaire et al. [JFM, 1971], to explain our observations. [Preview Abstract] |
Monday, November 23, 2009 12:01PM - 12:14PM |
HM.00008: The Roberto Carlos Spiral Christophe Clanet, Guillaume Dupeux, Anne le Goff, David Quere We study the motion of spinning spheres in a fluid at high Reynolds number and show that, in the ``low gravity limit,'' their trajectory is a spiral, the curvature of which increases exponentially. We show that this spiral was used by Roberto Carlos in soccer to score amazing goals and then discuss its importance in other ball games. [Preview Abstract] |
Monday, November 23, 2009 12:14PM - 12:27PM |
HM.00009: A Study of the Effects of Large Scale Gust Generation in a Small Scale Atmospheric Wind Tunnel: Application to Micro Aerial Vehicles Jason Roadman, Kamran Mohseni Modern technology operating in the atmospheric boundary layer could benefit from more accurate wind tunnel testing. While scaled atmospheric boundary layer tunnels have been well developed, tunnels replicating portions of the turbulence of the atmospheric boundary layer at full scale are a comparatively new concept. Testing at full-scale Reynolds numbers with full-scale turbulence in an ``atmospheric wind tunnel'' is sought. Many programs could utilize such a tool including that of Micro Aerial Vehicles (MAVs) and other unmanned aircraft, the wind energy industry, fuel efficient vehicles, and the study of bird and insect fight. The construction of an active ``gust generator'' for a new atmospheric tunnel is reviewed and the turbulence it generates is measured utilizing single and cross hot wires. Results from this grid are compared to atmospheric turbulence and it is shown that various gust strengths can be produced corresponding to days ranging from calm to quite gusty. An initial test is performed in the atmospheric wind tunnel whereby the effects of various turbulence conditions on transition and separation on the upper surface of a MAV wing is investigated using oil flow visualization. [Preview Abstract] |
Monday, November 23, 2009 12:27PM - 12:40PM |
HM.00010: Spiral motion of free falling disks Hongjie Zhong, Cunbiao Lee We report on experimental work on circular thin disks falling freely in quiescent water. A stereoscopic vision method was applied to measure the evolution of the six degrees of freedom. By analyzing the digital images of the disk obtained by two fixed CCD cameras, the position and orientation of the body were measured throughtout the fall. Translatory and angular motions,as well as the force and torque induced by the flow were determined. A new helical motion type was observed for moment of inertia $I^*<5\times10^{-4}$ and Reynolds number in the range of $500-3000$. This type of helical motion is distinct from the one caused by eccentricity. For the eccentricity case, the angular motion of the disk is purely precession whereas gyration and procession were both present for the low $I^*$ case. The wake structure of the zigzag motion and spiral motion were visualized in combine with PIV measurement. The instability mechanism of the wake structure, which cause the zigzag to spiral transition, is discussed. [Preview Abstract] |
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