Active Boundary-Layer Separation Control on a Multi-Element Airfoil

A fundamental and perennial motivation underlying research in fluid mechanics, especially in the field of aerodynamics, is the investigation of techniques that can be used to improve the high-lift performance of aircraft. Much research has been done in the area of modifying the design lines of various aircraft components to augment the total lift, most notably, the wings and other lifting surfaces. Advancements in this area have been aided mainly through the efforts of both experimentalists using wind tunnels and researchers applying computational fluid dynamics. Additionally, research has also proceeded along the path of employing variousflow control techniques for the purpose of improving the high-lift characteristics of aircraft. It is important now to capitalize on this knowledge and to add to the existing database by exploring and devising new and improved techniques of controlling the flow field affecting the performance of lifting surfaces. This should be done in an attempt to produce an improved lift capability of aircraft in an efficientmanner. The experimental investigation presently described is an attempt to do just that by employing a flow control technique to influence the flow field surrounding a standard, multielement airfoil geometry at a subsonic speed. The primary motivation behind the development of high-lift systems is to ensure that high aerodynamic lift at low aircraft speeds is maintained during the liftoff and landing phases of the flight. Of course, high-lift could simply be achieved by adding area to thewing, but this has the obvious disadvantage of increasing the weight and possible complexity of the lifting system. One option would be to employ conventional lift-augmenting devices such as leading-edge slats and trailing-edgeflaps.Over the years, various configurations of slats and flaps have been devised to achieve high-lift and yet the challenge has always been to minimize their adverse effects on the other aircraft systems due to aerodynamic interference. Active flow control can be applied to achieve a variety of objectives. This investigation was primarily concerned with flow control using piezoelectric devices as a means of achieving sizable increments in lift for aircraft, specifically at subsonic speeds by transferring turbulent kinetic energy and momentum into the boundary layer. Researchers have discovered that physicalmechanisms associated with the dynamics of vortices produce favorable effects with regards to the generation of high-lift [1]. Experiments have been conducted which have studied how vortex interactions in the shear layer of a separated boundary layer can contribute to the enhanced spreading rate of the shear layer [2]. Previous research results have shown that this phenomenon promotes boundary-layer reattachment. Bhattacharjee [3] observed that the process of “vortex pairing” increases the shear-layer-spreading rate. This phenomenon associated with vortices can be described as the amalgamation of neighboring vortex lumps into larger ones. The underlying premise is that vortex amalgamation reenergizes a separated boundary layer, which produces the favorable effect of an increment in aerodynamic lift. Experimental data from a previous study [4] have indicated that vortex amalgamation occurs when the forcing frequency is a subharmonic of the vortex passage frequency associated with the test article for a specified set of test conditions and model geometry.