BLEED CONTROL OF PITCHING AIRFOIL AERODYNAMICS BY VORTICITY FLUX MODIFICATION

Abstract

Distributed active bleed driven by pressure differences across a pitching airfoil is used to regulate the vorticity flux over the airfoil’s surface and thereby to control aerodynamic loads in wind tunnel experiments. The range of pitch angles is varied beyond the static stall margin (14° <  < 22°) of the 2-D VR-7 airfoil at reduced pitching rates up to k = 0.42. Bleed is regulated dynamically using piezoelectric louvers between the model’s pressure side near the trailing edge and the suction surface near the leading edge. The timedependent interactions of the bleed with the cross flow and its effects on the production, accumulation, and advection of vorticity concentrations during the pitch cycle are measured using phase-locked PIV. These interactions mitigate the impact of abrupt transitions between attachment and separation by reducing the peak lift and moment loads that can lead to pitch instabilities. As a result, the stability of the pitch cycle can be improved (negative damping reduced) by as much as E = 1.21 (at k = 0.25). I. OVERVIEW Aerodynamic bleed effected by pressure differences over a lifting surface interacts with the local surface vorticity layer to produce significant changes in vorticity flux and therefore in global aerodynamic forces and moments. Though passive bleed through porous surfaces for flow control was investigated as early as the 1920s (e.g., Lachmann, 1924) and since then by a number of researchers (e.g., Hunter, Viken, Wood, and Bauer, 2001, Han and Leishman, 2004, Lopera, Ng, and Patel, 2004), active distributed bleed by time-dependent regulation of surface porosity for mitigation of adverse aerodynamic effects on static and pitching airfoils has only been demonstrated recently by Kearney and Glezer (2012, 2013, 2014). These investigations revealed that such bleed can modify the formation and advection of surface vorticity concentrations and thereby alter the timing and strength of the dynamic stall vortex and aerodynamic loads during the pitch cycle. Excursions in pitch through the static stall angle can result in high, transitory aerodynamic loads due to the rapid buildup and shedding of vorticity concentrations. When the pitch motion through stall is oscillatory (i.e., during dynamic stall), especially at rapid pitch rates (“reduced” frequencies of k = c/2U∞ ≳ 0.1), the alternating attachment and flow separation produce periodic forcing that can lead to structural instabilities that are manifested by severe torsion or flutter (Carta, 1967, Johnson and Ham, 1972, McCroskey, Carr, and McAlister, 1976, Ericsson and Reding, 1988). Therefore, the occurrence of dynamic stall on the retreating blade imposes limitations on rotorcraft forward flight speeds (Raghav and Komerath, 2013). The earlier work on dynamic stall has indicated that these adverse effects can be mitigated by modifying the evolution of the unsteady vorticity concentrations that arise due to the blade’s motion. The present investigations build on the earlier findings of Kearney and Glezer and focus on the timedependent mechanisms by which bleed affects vorticity concentrations over the airfoil and in the near wake during oscillatory pitching. The alteration of the vorticity flux near the surface can have significant effects on the evolution and timing of the dynamic stall vortex that are manifested by changes in the lift hysteresis and pitch stability during the cycle. II. EXPERIMENTAL SETUP AND PROCEDURES The present investigation is conducted in a low-speed wind tunnel (U∞ = 15 m/s, Rec = 190,000) having a rectangular test section measuring 25 x 47 x 132 cm with optical access from all sides. The VR-7 airfoil model (c = 20 cm, tmax = 0.12c) spans nearly the entire width of the test section (s = 24 cm, with endplates). The model is mechanically isolated from the test section and is mounted on a shaft through x/c = 0.25. Dynamic pitch is driven by two synchronized, computer controlled servo motors, one on each side of the tunnel, that are each connected to the model through a load cell that measures lift and drag (within 0.011 N) and pitching moment (within 0.001 N·m), with frequency response of up to 500 Hz. The system can pitch the model sinusoidally at frequencies in excess of 25 Hz (corresponding to a reduced frequency of k  1). High-resolution PIV measurements of the velocity field above the airfoil and in the near wake are obtained using a 1600 x 1200 pixel, 14bit CCD camera and an Nd:YAG laser with cross stream views ranging in width from 0.1c to 0.6c (magnifications of 69 and 8 px/mm, respectively). The airfoil model is fabricated using stereolithography (SLA) and includes several spanwise rows of lowresistance bleed ports through the pressure and suction surfaces that are open to the inner volume of the model (Figure 1). Each spanwise row is comprised of 16 10 mm-wide ports whose streamwise length increases with distance from the leading edge from 2 mm (at x/c = 0) to 2.5 mm (at x/c = 0.95). The bleed configuration in the present work leverages the pressure difference upstream of the trailing edge (on the pressure June 30 July 3, 2015 Melbourne, Australia 9 5B-5