TRANSIENT CONTROL OF THE SEPARATING FLOW OVER AN AIRFOIL

Abstract

The effects of pulsed actuation on stalled NACA 4415 airfoil is investigated in wind tunnel experiments. The actuation results in transitory flow attachment that is manifested by rapid changes in the global circulation and aerodynamic forces. Actuation is applied by a momentary [O(1 msec)] jet produced by a combustion-based actuator such that the characteristic duration of the impulse is an order of magnitude shorter than the characteristic convective time over the airfoil. The present work has shown that large-scale changes in vorticity accumulation and flux can be effected by successive repetitions of a single actuation pulse and are accompanied by significant shedding of CCW vorticity concentrations on the pressure side coincidently with the trapping of CW vorticity concentrations, hence extending the streamwise domain of the attached vorticity layer towards the trailing edge. BACKGROUND Traditional approaches to control of separation on stalled airfoil have focused on quasi-steady actuation within two distinct frequency regimes. “Low-frequency” actuation has relied on receptivity of the separated, wake-dominated flow to external actuation within a narrow-band of Strouhal numbers that effectively correspond to unstable frequencies of the near wake, Stact ~ O(1) (e.g., Neuburger and Wygnanski, 1987, and Seifert et al., 1996). “High-frequency” actuation is decoupled from global flow (wake) instabilities and emphasizes fluidic modification of the “apparent” aerodynamic shape of the surface upstream of separation at actuation frequencies that are at least an order of magnitude higher than the characteristic wake frequency [i.e., Stact ~ O(10)] (e.g., Honohan et al., 2000, and Glezer et al., 2005). Actuation is effected by forming a controlled interaction domain of trapped vorticity between a surface-mounted fluidic actuator and the cross flow above the surface that displaces the local streamlines of the cross flow and thereby induces a ‘virtual’ change in the shape of the surface. The separated flow is extremely susceptible to transitory actuation such that substantial control authority can be achieved when the actuation input is applied on time scales that are significantly shorter than the characteristic advection time over the separated flow domain. Brzozowski and Glezer (2006) exploited the receptivity of separated flow over a stalled airfoil and showed that a single actuation pulse [O(0.05Tconv)] could lead to brief, partial collapse of the separated flow domain and a momentary increase in circulation on time scale of 10Tconv. The recent work of Woo et al. (2008) demonstrated significant pressure and lift recovery of a stalled airfoil with successively pulsed actuation that are applied Tconv apart, and with burstmodulated actuation. The present work is motivated by the previous investigations of pulsed actuation. The major focus of the present work is on the transient aerodynamic effects of repetitive pulsed actuation on the separated flow over an airfoil. Current work also explores the dynamics of the actuation on the large coherent structures in the separating shear layer, and their role in the momentary attachment. EXPERIMENTAL SETUP AND PROCEDURES The experimental setup is described in detail in the earlier work of Woo et al. (2008). The 2-D airfoil (Figure 1a) has a fixed cross section based on a NACA 4415 configuration (c = 457 mm, 1 m span). The airfoil model is comprised of three spanwise segments where the center segment is instrumented with a spanwise array of seven combustion-based jet actuators. The center section is also instrumented with 75 static pressure ports located circumferentially at mid-span.