Controlled leading-edge suction for management of unsteady separation over pitching airfoils

Abstract

The effect on the unsteady surface pressures of controlled suction from a span wise slot, located at 2% chord in the suction surface of a two-dimensional NACA 0012 airfoil model, was examined in detail for a wide range of pitch rates with a constant velocity ramp motion. The experiments were conducted in the Andrew Fejer Wind 1\mnel at the Illinois Institute of Technology's Fluid Dynamics Research Center. The optimum suction required to meet three different control objectives, suppression of the dynamic-stall vortex, delaying detachment of the vortex from the airfoil surface, and maximizing the unsteady lift, was determined for different pitch rates and angles of attack. The pressure data were used to develop specifications for the flow state over the airfoil surface that would meet these objectives. Such specifications are necessary for the development of on-line flow management systems. A procedure was also developed to account for variations in suction and motion history.= dimensionless suction flow rate, QsKU^c 1 ) <2 opt = optimum suction rate for a given objective Q s = volumetric suction flow rate Re c = chord Reynolds number, U^c/v S = surface vorticity flux, (l/p)(dp/Qs) S + = dimensionless surface vorticity flux, 2cS/ Uŝ = coordinate along airfoil surface t p = time period for pitch-up motion through an angle Aa UQQ = freestream velocity a = angle of attack a + = dimensionless pitch rate, (Aac)/(^(7 00 ) Aa = change in angle of attack during pitch up p = fluid density v = fluid kinematic viscosity Introduction and Background

Unsteady RANS Simulations of Strong and Weak 3D Stall Cells on a 2D Pitching Aerofoil

“…A low reduced frequency may cause the disappearance of LEV, resulting in the lack of a sudden increase of C L . An even lower reduced frequency could eliminate the dynamic stall features completely and make the flow quasi-steady, which was also verified by the experimental study of Alrefai and Acharya [6]. However, Lee and Gerontakos [7] found in their experiments that the reduced frequency had little impact on the location and size of LEV.…”

Role of Synthetic Jet Frequency & Orientation in Dynamic Stall Vorticity Creation

“…Control of dynamic stall and its distinctive vortex has consequently been investigated using passive methods such as vortex generators [11] and slats [12] as well as active blowing [13], suction [7,14] and buzzing [15] methods with the aim of maintaining beneficial lift increases while decreasing drag and pitching moment. While these studies have shown some success, flow control using synthetic jets [16][17][18][19] has shown great potential for managing dynamic stall aerodynamic coefficient excursions due to their actuation versatility [20][21][22].…”

Compressible Dynamic Stall Control: Comparison of Two Approaches

“…It appears that shedding the unsteady vorticity from some downstream location in smaller chunks of uid can provide an acceptable solution to prevent the vortex from forming. Then the drastic consequences of the vortex-induced pitching moment variations can be avoided, even if the ow separates partially.As the equationindicates,using either surfaceacceleration 3 or surface mass transfer such as blowing and/or suction 6 enables some degree of vorticity ux manipulation. The latter has been a popular approach to control of steady ows, but has only been partly satisfactory in dynamic stall control for incompressible conditions.…”