Low Reynolds number aerodynamics is important to a number of natural and manmade flyers. Birds, bats, and insects have been investigated by biologists for years, and active study in the aerospace engineering community, motivated by interest in micro air vehicles (MAVs), has been increasing rapidly. The primary focus of this book is the aerodynamics associated with fixed and flapping wings. The book considers both biological flyers and MAVs, including a summary of the scaling laws that relate the aerodynamics and flight characteristics to a flyer's sizing on the basis of simple geometric and dynamics analyses, structural flexibility, laminar-turbulent transition, airfoil shapes, and unsteady flapping-wing aerodynamics. The interplay between flapping kinematics and key dimensionless parameters such as the Reynolds number, Strouhal number, and reduced frequency is highlighted. The various unsteady lift-enhancement mechanisms are also addressed.
Micro air vehicles (MAVs) with a wingspan of 15cm or shorter, and flight speed around 10m∕s have attracted substantial interest in recent years. There are several prominent features of MAV flight: (i) low Reynolds number (104-105), 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.
For flight vehicles operated at the low Reynolds number regime, such as birds, bats, insects, as well as small man-made vehicles, flapping and fixed wings are employed in various ways to generate aerodynamic forces. For flapping wings, the unsteady fluid physics, interacting with wing kinematics and shapes determine the lift generation. For fixed wings, laminarturbulent transition, three dimensional flows around low aspect ratio vehicles, and coupling between flexible wing structures and surrounding fluid flows are of major interest. In the present paper we discuss recent progress in understanding the low Reynolds number unsteady fluid dynamics associated with flapping wings, including leading-edge vortices, pitching-up rotation and wake-capturing mechanisms. For fixed wings, recent efforts in fluid-structure interaction and laminar-turbulent transition are highlighted. I.
The interaction between aerodynamics and structural flexibility in a low Reynolds number environment is of considerable interest to biological and micro air vehicles. In this study, coupled fluid-structure computations of the Navier-Stokes fluid flow and a flexible airfoil in low Reynolds number environments are conducted to probe the aerodynamic implications. While a flexible airfoil deforms in response to the aerodynamic loading, it exhibits an equivalent pitching motion, which modifies the effective angle of attack, causing noticeable differences in lift and thrust generation. Within the range of the flexibility considered, the flow fields are similar in all cases. Even at Re=100, in the plunging motion, the force acting on airfoil is dominated by pressure and the viscous force is of little impact on the overall lift and thrust generation. Detailed airfoil shape is secondary compared to the equivalent angle of attack.
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