Theoretical analysis of perching and hovering maneuvers

Unsteady aerodynamic phenomena are encountered in a large number of modern aerospace and non-aerospace applications. Leading edge vortices (LEVs) are of particular interest because of their large impact on the forces and performance. In rotorcraft applications, they cause large vibrations and torsional loads (dynamic stall), affecting the performance adversely. In insect flight however, they contribute positively by enabling high-lift flight. Identifying the conditions that result in LEV formation and modeling their effects on the flow is an important ongoing challenge. Perching (airfoil decelerates to rest) and hovering (zero freestream velocity) maneuvers are of special interest. In earlier work by the authors, a Leading Edge Suction Parameter (LESP) was developed to predict LEV formation for airfoils undergoing arbitrary variation in pitch and plunge at a constant freestream velocity. In this research, the LESP criterion is extended to situations where the freestream velocity is varying or zero. A point-vortex model based on this criterion is developed and results from the model are compared against those from a computational fluid dynamics (CFD) method. Abstractions of perching and hovering maneuvers are used to validate the low-order model's performance in highly unsteady vortex-dominated flows, where the time-varying freestream/translational velocity is small in magnitude compared to the other contributions to the velocity experienced by the leading edge region of the airfoil. Time instants of LEV formation, flow topologies and force coefficient histories for the various motion kinematics from the low-order model and CFD are obtained and compared. The LESP criterion is seen to be successful in predicting the start of LEV formation and the point-vortex method is effective in modeling the flow development and forces on the airfoil. Typical run-times for the low-order method are between 30-40 seconds, making it a potentially convenient tool for control/design applications

Effects of Reynolds Number and Flapping Kinematics on Hovering Aerodynamics

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76695/1/AIAA-2007-129-236.pd

Computational Fluid-Structure Interaction of a Deformable Flapping Wing for Micro Air Vehicle Applications

Motivated by micro air vehicle applications, a fluid-structure coupling procedure between a Navier- Stokes solver and a three-dimensional FEM beam solver is presented along with selected results highlighting some of the aerodynamics implications. The fluid model includes laminar, the k -ε turbulence closure, and a filter-based k -ε closure. The structural model is based on an asymptotic approximation to the equations of elasticity. Using the slenderness as the small parameter, the equations are decomposed into two independent variational problems, corresponding to (i) crosssectional, small-deformation and (ii) longitudinal, large deformation analyses. A model example problem corresponding to a NACA0012 wing of aspect ratio 3 in pure heave motion is presented and the results compared against available experiment data. Quantitative comparisons with experiment are done for the rigid wing and the implications of wing flexibility on aerodynamics are presented in a qualitative sense. It was observed that phase lag of the wing tip displacement relative to the flapping motion becomes more pronounced as the fluid density increases. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.Published versio

Membrane wing-based micro air vehicles

Micro air vehicles (MAVs) with a wingspan of 15 cm or shorter, and flight speed around 10 m/s have attracted substantial interest in recent years. There are several prominent features of MAVflight: (i) low Reynolds number (10 4-10 5), resulting in degraded aerodynamic performance, (ii) small physical dimensions, resulting in certain favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance, and (iii) low flight speed, resulting in order one effect of the flight environment and intrinsically unsteady flight characteristics. Flexible wings utilizing membrane materials are employed by natural flyers such as bats and insects. Compared to a rigid wing, a membrane wing can better adapt to the stall and has the potential for morphing to achieve enhanced agility and storage consideration. We will discuss the aerodynamics of both rigid and membrane wings under the MAV flight condition. To understand membrane wing performance, the fluid and structure interaction is of critical importance. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble, and tip vortex, as well as structural dynamics in response to the surrounding flow field are discussed. Based on the computational capabilities for treating moving boundary problems, an automated wing shape optimization technique is also developed. Salient features of the flexible-wing-based MAV, including the vehicle concept, flexible wing design, novel fabrication methods, aerodynamic assessment, and flight data analysis are highlighted Copyright ©2005 by ASME

Membrane wing aerodynamics for micro air vehicles

The aerodynamic performance of a wing deteriorates considerably as the Reynolds number decreases from 106 to 104. In particular, flow separation can result in substantial change in effective airfoil shape and cause reduced aerodynamic performance. Lately, there has been growing interest in developing suitable techniques for sustained and robust flight of micro air vehicles (MAVs) with a wingspan of 15 cm or smaller, flight speed around 10 m/s, and a corresponding Reynolds number of 104-105. This paper reviews the aerodynamics of membrane and corresponding rigid wings under the MAV flight conditions. The membrane wing is observed to yield desirable characteristics in delaying stall as well as adapting to the unsteady flight environment, which is intrinsic to the designated flight speed. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble and tip vortex are reviewed. Structural dynamics in response to the surrounding flow field is presented to highlight the multiple time-scale phenomena. Based on the computational capabilities for treating moving boundary problems, wing shape optimization can be conducted in automated manners. To enhance the lift, the effect of endplates is evaluated. The proper orthogonal decomposition method is also discussed as an economic tool to describe the flow structure around a wing and to facilitate flow and vehicle control. © 2003 Elsevier Ltd. All rights reserved

Effect of tip vortex on wing aerodynamics of micro air vehicles

Tip vortex induces downwash movement, which reduces the effective angle of attack of a wing. For a low-aspect-ratio, low-Reynolds-number wing, such as that employed by the micro air vehicle (MAV), the induced drag by the tip vortex substantially affects its aerodynamic performance. In this paper we use the endplate concept to help probe the tip-vortex effects on the MAV aerodynamic characteristics. The investigation is facilitated by solving the Navier-Stokes equations around a rigid wing with a root-chord Reynolds number of 9 × 10 4. It is confirmed that with modest angles of attack the endplate can improve the lift-to-drag ratio by reducing the drag. However, as the angle of attack becomes substantial, the wing tip vortex is stronger and the endplate loses its effectiveness. Detailed fluid flow structures are presented to offer insight into the physics responsible for the observed phenomena