Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session E24: Aerodynamics I |
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Chair: Mohamed Gad-el-Hak, Virginia Commonwealth University Room: 319 |
Sunday, November 24, 2013 4:45PM - 4:58PM |
E24.00001: Flow Structure on a Rotating Wing: Effect of Rossby Number Maxwell Wolfinger, Donald Rockwell The flow structure on a rotating wing is determined via stereoscopic particle image velocimetry. Sectional and three-dimensional, volumetric reconstructions define the flow patterns as a function of Rossby number \textit{Ro}. An aspect ratio \textit{AR} $=$ 1 rectangular, flat plate is rotated at a geometric angle of attack $\alpha = $ 45$^{\circ}$. The flow structure is determined at various angles of rotation, in order to characterize both the initial development and the fully evolved state of the flow structure. The Rossby number \textit{Ro} $= r_{g}/C$ is varied via alteration of the radius of gyration $r_{g}$ of the wing, to give values from \textit{Ro} $=$ 1.2 to \textit{Ro} $=$ 5.1. Large changes of the flow structure are represented by images of of spanwise vorticity, Q-criterion; spanwise velocity; and downwash velocity. At the lowest Rossby number \textit{Ro} $=$ 1.2, a vortex is attached to the leading edge of the wing; it is present along most of the span. At higher Rossby numbers \textit{Ro} $=$ 2.1 and \textit{Ro} $=$ 5.1, this leading-edge vortex becomes less organized and deflects away from the surface of the wing. At a Rossby number \textit{Ro} $=$ 5.1 the structure of the flow in the vicinity of the leading edge resembles a separated shear layer. The nature of other elements of the three-dimensional flow, such as the root and tip vortices and the downwash velocity, are closely related to the degree of coherence of the leading-edge vortex. [Preview Abstract] |
Sunday, November 24, 2013 4:58PM - 5:11PM |
E24.00002: Vortex Interaction on Low Aspect Ratio Membrane Wings Rye M. Waldman, Kenneth S. Breuer Inspired by the flight of bats and by recent interest in Micro Air Vehicles, we present measurements on the steady and unsteady behavior of low aspect ratio membrane wings. We conduct wind tunnel experiments with coupled force, kinematic, and flow field measurements, both on the wing and in the near wake. Membrane wings interact strongly with the vortices shed from the leading- and trailing-edges and the wing tips, and the details of the membrane support play an important role in the fluid-structure interaction. Membranes that are supported at the wing tip exhibit less membrane flutter, more coherent tip vortices, and enhanced lift. The interior wake can exhibit organized spanwise vortex shedding, and shows little influence from the tip vortex. In contrast, membranes with an unsupported wing tip show exaggerated static deformation, significant membrane fluttering and a diffuse, unsteady tip vortex. The unsteady tip vortex modifies the behavior of the interior wake, disrupting the wake coherence. [Preview Abstract] |
Sunday, November 24, 2013 5:11PM - 5:24PM |
E24.00003: Unique stability modes of low aspect ratio wings Kamran Mohseni, Matt Shields The unique aerodynamic regime of low aspect ratio (LAR) wings is strongly affected by the phenomenon of roll stall. In this study, it is shown that roll stall induces inherently aerodynamic stability modes on a flat plate wing with an aspect ratio of unity. These modes are seen to create divergent oscillations in the lateral state variables even for minor perturbations from equilibrium flight. Furthermore, the nature of the response is fundamentally altered in the presence of angle of attack variations; if the frequency of the angle of attack oscillations is close to the natural frequency of the lateral response, the bank angle $\phi$ is seen to drift away from equilibrium in a manner not well modeled by a linear stability analysis. This newly considered mode, inherent to LAR wings, is referred to as the roll resonance mode due to its dependence on the frequencies of lateral and longitudinal motion. A linear time invariant model is shown to accurately represent the initial condition response of the pure lateral mode, and a linear time variant model in which the roll stability derivative is updated at every time step captures the divergent response of roll resonance. Understanding these modes is critical for implementation of improved control laws for Micro Aerial Vehicles. [Preview Abstract] |
Sunday, November 24, 2013 5:24PM - 5:37PM |
E24.00004: Vorticity Confinement Applied to Turbulent Wing Tip Vortices for Wake-Integral Drag Prediction Kristopher Pierson, Alex Povitsky In the current study the vorticity confinement (VC) approach was applied to tip vortices shed by edges of stationary wings in order to predict induced drag by far-field integration in Trefftz plane. The VC parameter was evaluated first by application to convection of vortices in 2-D uniform flow and then to tip vortices shed in 3-D simulation of finite-aspect ratio rectangular wing in subsonic flight. Dependence of VC parameter on the flight Mach number and the angle of attack was evaluated. The aerodynamic drag results with application of VC to prevent numerical diffusion are much closer to analytic lifting line theory compared to integration over surface of wing while the viscous profile drag is more accurately evaluated by surface integration. To apply VC to viscous and turbulent flows, it is shown that VC does not affect the physical rate of dissipation of vortices in viscous/turbulent flows at time scales corresponding to convection of vortices from the wing to Trefftz plane of integration. To account for turbulent effects on tip vortices, VC was applied in combination with Spalart-Allmaras, k-$\varepsilon $, and six Reynolds stresses models of turbulence. The results are compared to experiments to validate the physical dissipation of tip vortex. [Preview Abstract] |
Sunday, November 24, 2013 5:37PM - 5:50PM |
E24.00005: Lattice Boltzmann simulations of deformation and efficiency of a chord-wise flexible wing in a free stream flow Dewei Qi, Guowei He Flapping of a chord-wise flexible wing in a steady free stream is studied by using a lattice Boltzmann flexible particle method (LBFPM) in a three-dimensional space at a chord based Reynolds number of 100. The flexibility and wing mass ratios are systematically varied, and their effects on aerodynamic forces and power efficiency are explored. It is demonstrated that large the deformation is controlled by the wing and fluid flow inertia (``added mass) and the wing flexibility. A dynamic balance between the inertia and the appropriate level of the flexibility allows the flexible motion in phase with the driving base and have a much larger rotational velocity or rotational momentum than a rigid wing, resulting in a better performance. [Preview Abstract] |
Sunday, November 24, 2013 5:50PM - 6:03PM |
E24.00006: Force Element Theory for Finite Wings at Low Reynolds numbers Chin-Chou Chu, Jian-Jhih Lee, Cheng-Ta Hsieh, Chien-Cheng Chang This paper is aimed to examine various contributions to the forces on an impulsively started finite plate from the perspective of a force-element representation. The wing plate has an aspect ratio (AR) between 1 and 3, and is placed at low and high angles of attack ($\alpha = $5$^{\circ}$, 10$^{\circ}$, 15$^{\circ}$, 30$^{\circ}$, 45$^{\circ}$, and 60$^{\circ}$), while the Reynolds number Re is varied between 100 or 300. The force theory enables us to quantify the contributions to the forces exerted on the plate in terms of all the fluid elements with nonzero vorticity, such as in the tip vortices (TiVs), leading- and trailing-edge vortices (LEV and TEV) as well on the plate surface. The present vorticity force analysis (VFA) was made parallel to the pressure force analysis (PFA) by examining the sectional force contributions along the wing span, but can further extend to include the outer regions (of TiVs). The interplay between the LEV and the TiVs by assessing the relative importance of the transverse as well as the longitudinal vorticity components at various time stages leads to insightful physical explanations of the force mechanisms. [Preview Abstract] |
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