Controlled transients of flow reattachment over stalled airfoils

Abstract

The flow transients associated with controlled reattachment and separation of the flow over a stalled airfoil are investigated in wind tunnel experiments. Control is effected using surface-mounted synthetic jet actuators that are typically operated at frequencies, which are at least an order of magnitude higher than the characteristic shedding frequency of the airfoil. While at these actuation frequencies the circulation (and hence the lift) of the attached flow is nominally time invariant, actuation at lower frequencies that are commensurate with the shedding frequency results in a Coanda-like attachment of the separated shear layer, organized vortex shedding and substantial oscillation of the circulation. The transients associated with flow reattachment and separation are investigated using amplitude modulation of the actuation waveform. Phase-locked measurements of the velocity field in the near wake of the airfoil and corresponding flow visualizations show that the transients that are associated with the onset of reattachment and separation are accompanied by the shedding of large-scale vortical structures and oscillations of the circulation. Pulsed modulated actuation of the actuation waveform is used to capture these transient effects and augment the increase in lift that is obtained by conventional time-harmonic actuation.

Introduction

Active manipulation of separated flows over lifting surfaces at moderate and high angles of attack to achieve complete or partial flow reattachment with the objective of improving the aerodynamic performance and extending the flight envelope has been the focus of a number of investigations since the early 1980s. Coanda-like reattachment is normally effected by exploiting the receptivity of the separating shear layer to external excitation which affects the evolution of the ensuing vortical structures and their interactions with the flow boundary. Active flow control schemes that rely on the instability of the separating shear layer was demonstrated in a number of earlier investigations of separated flow (e.g., Ahuja and Burrin, 1984; Huang et al., 1987; Hsiao et al., 1990; Williams et al., 1991; Seifert et al., 1996).

Separation control by means of internal acoustic excitation (e.g., Huang et al., 1987) has typically employs an acoustically driven cavity within the airfoil in which (normally time harmonic) acoustic excitation is applied through a spanwise slot upstream of separation (typically near the leading edge of the airfoil). The work of Chang et al. (1992) confirmed earlier results of Hsiao et al. (1990), namely, that at low excitation levels, excitation at or near the unstable frequency of the separating shear layer (St=f_actc/U_∞=2, where f_act is the actuation frequency, c is the chord and U_∞ is the free stream velocity) can lead to a 50% increase in post-stall lift. Similar approach for the coupling of internal forcing to the predominant instabilities of the separating shear layer was also employed in the experiments of Wygnanski and Seifert (1994) and Seifert et al. (1996). These authors used unsteady jet blowing over several airfoil models to achieve various degrees of separation control by employing dimensionless actuation frequencies St∼O(1) (instead of St, these authors chose to denote the dimensionless actuation frequency as F⁺; in their work the characteristic length scale x_s is the streamwise extent of the separated flow domain). However, an important contribution of the work of Chang et al. (1992) was the demonstration that the application of acoustic excitation at levels that are somewhat higher than those of their baseline experiments resulted in effective control of separation over a broad range of excitation frequencies (up to St=20) that far exceed the unstable frequency of the separating shear layer.

Smith et al. (1998) and Amitay et al. (2001) demonstrated the utility of synthetic (zero mass flux) jet actuators for the suppression of separation over an unconventional airfoil at moderate Reynolds numbers (up to 10⁶) resulting in a dramatic increase in lift and decrease in pressure drag. The jets are typically operated at dimensionless frequencies that are an order of magnitude higher than the shedding frequency of the airfoil (i.e., St∼O(10)) and because they are zero net mass flux in nature, their interaction with the crossflow leads to local modification of the apparent shape of the flow surface. Full or partial reattachment including the controlled formation of a closed separation bubble, can be controlled by the streamwise location and the strength of the jets. The excitation is effective over a broad streamwise domain that extends well upstream of where the flow separates in the absence of actuation and even downstream of the front stagnation point on the pressure side of the airfoil. Furthermore, the work of Amitay and Glezer (1999) showed that while the circulation of the attached flow when actuation is applied at St∼O(10) is nominally time invariant, the shedding of organized vortical structures when actuation is applied at St∼O(1) leads to a time-periodic variations in the circulation (and therefore in the lift).

The sensitivity of the attached flow (and the restored lift) to the excitation frequency is also demonstrated in the numerical simulation of Donovan et al. (1998) who investigated flow reattachment over a NACA 0012 airfoil using time-harmonic zero mass flux blowing at St=1. These simulations showed a 20% post-stall increase in lift at α=22°. However, the reattachment was similar to a Coanda-like effect where the forced shear layer deflected towards the airfoil surface, and the time-periodic vortex shedding from the top surface of the airfoil, led to 20% oscillations in the lift coefficient. Similarly, the more recent numerical simulations of Wu et al. (1998) reaffirmed that a separated flow can be effectively manipulated by low-level periodic blowing/suction near the leading edge. The forcing modulates the evolution of vortical structures within the separated shear layer and promotes the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices and thereby alter the global stalled flow. In a certain range of post-stall angles of attack and actuation frequencies, the flow becomes periodic and is accompanied by a significant lift enhancement.

The present work builds on earlier results of Amitay and Glezer, 1999, Amitay and Glezer, 2001, Amitay and Glezer, 2002 and focuses on the transients associated with flow reattachment and separation when actuation is applied at frequencies for which St∼O(1) and also St∼O(10) (Section 3). As shown in Section 4, the transients associated with flow reattachment and separation can exploited to achieve an improvement in the efficiency of the jet actuators by pulsed modulation of the excitation input.