Airfoil

About: Airfoil is a research topic. Over the lifetime, 24696 publications have been published within this topic receiving 337709 citations. The topic is also known as: aerofoil & wing section.

On unsteady surface forces, and sound produced by the normal chopping of a rectilinear vortex

Abstract: An investigation is made of the sound produced when a rectilinear vortex is cut at right angles to its axis by a non-lifting airfoil of symmetric section. The motions are at sufficiently low Mach number that the wavelength of the sound is large relative to the chord of the airfoil. In these circumstances the airfoil experiences no fluctuating lift during the interaction, and the radiation may be ascribed to an acoustic source of dipole type whose strength is equal to the unsteady drag. It is argued that previous analyses of the related problem of ‘unsteady thickness noise’ have ignored certain terms whose inclusion greatly reduces the predicted intensity of the radiation. A general formula for the surface forces (derived in an appendix) is applied to deduce that the dipole strength is proportional to the square of the circulation of the vortex, and depends on the spanwise acceleration of the vortex induced by images in the airfoil. Numerical results are presented for typical airfoil sections, and a comparison is made with the unsteady lifting noise generated when the axis of the vortex is inclined at a small angle to the normal to the median plane of the airfoil.

Experiments on airfoils at low reynolds numbers

Abstract: Lift and drag measurements were taken on 34 airfoils at low Reynolds numbers in an attempt to develop a consistent database for use in design studies that require accurate low Reynolds number airfoil data. Prom these data emerged several interesting results related to the behavior of the laminar separation bubbles and their effect on the lift characteristics. A plateau hi the lift curve of symmetrical airfoils hi the vicinity of an angle of attack of 0 deg was found to be common in the Reynolds number range of 40,000 to 100,000. Through the use of zig-zag type boundary-layer trips, this nonlinearity can be reduced owing to a reduction in the size of the laminar separation bubbles. The influence of laminar separation bubbles was also found to dominate the performance of several high-lift airfoils in the Reynolds number range of 80,000 to 150,000. In particular, hysteresis loops in the lift curve were present, and these are related to the size of the laminar separation bubble as deduced from drag data. The data reveals that some airfoils exhibit both counterclockwise aoid clockwise hysteresis loops for a given Reynolds number. Moreover, depending on the airfoil, either type of loop can occur first. Introduction A wide variety of small unmanned aerial vehicles (UAVs) operate in the low Reynolds number regime in which the airfoil aerodynamics play a key role in aircraft performance and handling. In this regime, natural boundary-layer transition takes place through a laminar separation bubble that forms as the laminar boundary layer first separates, then becomes unstable, makes a transition to turbulent flow and reattaches to the airfoil to form a laminar separation bubble.' This bubble often results in a notable airfoil-performance degradation that is characterized by undesirable high drag, nonlinearCopyright © 1996 by Michael S. Selig, James J. Guglielmo, Andy P. Broeren and Philippe Giguere. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 'Assistant Professor. Member AIAA. fGraduate Research Assistant. Student Member AIAA. ^Graduate Research Assistant. ities in the lift-curve characteristics and sometimes static hysteresis in the section lift, drag and moment data.' In an effort to understand the flow phenomena at low Reynolds numbers, there have been numerous theoretical and experimental investigations that have resulted in three conferences" and a special AGARD publication. Because of the difficulty in modeling the laminar separation bubble, computational efforts, while vital for use in design, have not been entirely reliable hi predicting these complex flows. Nonetheless, considerable progress has been made in recent years." Despite the high level of interest in this area, few systematic experiments have been performed to document the performance of a wide selection of airfoils for use in conceptual and detailed design studies. This has been particularly problematic because comparisons of data between different wind tunnels facilities regularly show discrepencies owing to the documented difficulties in measuring low Reynolds number airfoil performance. Thus, the mixture of different data sets is not ideally suited for the purposes of examining performance trade-offs involving different airfoils. A key objective of the present research was to produce a large and consistent low Reynolds number airfoil performance database for use in the design of small UAVs. Specifically, the experiments involved measuring the lift and drag characteristics of many airfoils over the nominal Reynolds number range of 60,000 to 300,000. The collection of airfoils tested is depicted in Fig. 1. The more limited objective of this paper is to highlight and explain two interesting features observed in the lift characteristics of some of the airfoil tested. In particular, this paper presents and discusses (1) the nonlinear lift characteristics of two symmetrical airfoils (NACA 0009 and SD8020) as well as improvements made through the use of boundary-layer trips, and (2) the hysteresis in the lift characteristics of six high-lift airfoils (FX 63-137, S1210, modified FX 74CL5-140, CH 10-48-13, M06-13-128 and S1223). As will be described, both features, the nonlinearity and the hysteresis of the lift curves, can be linked to the behavior of the laminar separation bubbles. Anti-Turbulence Screens Diffuser Silencer N. / Frequency Controller / Fan \

Generalized multipoint inverse airfoil design

Abstract: In a rather general sense, inverse airfoil design can be taken to mean the problem of specifying a desired set of airfoil characteristics, such as the airfoil maximum thickness ratio, pitching moment, part of the velocity distribution, or boundary-layer development. From thie information, the corresponding airfoil shape is determined. We present a method that approaches the design problem from this perspective. In particular, the airfoil is divided into segments along which, together with the design conditions, either the velocity distribution or boundary-layer development may be prescribed