Dynamic stall is an incredibly rich fluid dynamics problem that manifests itself on an airfoil during rapid, transient motion in which the angle of incidence surpasses the static stall limit. It is an important element of many manmade and natural flyers, including helicopters and supermaneuverable aircraft, and low–Reynolds number flapping-wing birds and insects. The fluid dynamic attributes that accompany dynamic stall include an eruption of vorticity that organizes into a well-defined dynamic stall vortex and massive excursions in aerodynamic loads that can couple with the airfoil structural dynamics. The dynamic stall process is highly sensitive to surface roughness that can influence turbulent transition and to local compressibility effects that occur at free-stream Mach numbers that are otherwise incompressible. Under some conditions, dynamic stall can result in negative aerodynamic damping that leads to limit-cycle growth of structural vibrations and rapid mechanical failure. The mechanisms leading to negative damping have been a principal interest of recent experiments and analysis. Computational fluid dynamic simulations and low-order models have not been good predictors so far. Large-eddy simulation could be a viable approach although it remains computationally intensive. The topic is technologically important owing to the desire to develop next-generation rotorcraft that employ adaptive rotor dynamic stall control.

Dynamic Stall in Pitching Airfoils: Aerodynamic Damping and Compressibility Effects

Enhancing vertical axis wind turbine by dynamic stall control using synthetic jets

The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic sta...

Control of Thick Airfoil, Deep Dynamic Stall Using Steady Blowing

Effects of a moveable trailing-edge flap on the dynamic-load loops of an oscillating NACA 0015 airfoil were investigated at Re =1 .65 × × 10 5 . Both upward and downward flap deflections for different flap actuation start times, durations, and amplitudes were considered. Surface pressure measurements show that only upward flap deflections led to the reduction of nose-down pitching moment, and that the larger the flap deflection the more efficient the reduction mechanism was. The shorter the actuation duration the smaller the poststall lift loss. The reduction of the peak nose-down pitching moment was a consequence of the suction pressure appearing on the lower surface of the trailing-edge flap. The formation and detachment of the dynamic-stall vortex was not affected by the flap motion, whereas the low-pressure signature of the vortex was reduced by the upward flap deflection. The later the flap actuation during the upstroke, the greater the net torsional damping was. Optimum flap motion is also discussed.

Dynamic Stall Flow Control via a Trailing-Edge Flap

3to 200x10 3 and angles of attack from 0° to 18°. These data were used to characterize the separation bubble and determine lift coefficients. From these results, two distinct regions in the lift curves can be identified: a region of rapid and linear growth of the lift coefficients at low angles of attack and a region of more gradual and linear growth at higher pre-stall angles. Furthermore, the slope of the lift curve in each region is found to be linked to the rates of change in separation, transition, and reattachment locations with the angle of attack. These findings are substantiated by an analysis of the available experimental data for a NACA 0012 airfoil.

Aerodynamic Characterization of a NACA 0018 Airfoil at Low Reynolds Numbers

T HE dynamic overshoot in lift force and the accompanying large nose-down pitching moment observed on oscillating and pitching airfoils, as a result of the formation, convection, and shedding of an energetic dynamic-stall vortex (DSV), continue to make dynamic stall and its control an important topic in unsteady aerodynamics. Moreover, once the DSV passes the airfoil trailing edge and moves into the wake, the flow progresses to a state of poststall full separation over the upper surface and an abrupt loss of lift is incurred, causing a large degree of hysteresis in the lift coefficient. Various dynamic-stall flow control methods, such as trailing-edge flaps [1–5], pulsating and synthetic jets [6–8], leadingedge slots with and without blowing and suction [9,10], and dynamically deformable and variably drooped leading edges [11,12], etc., capable of minimizing, or eliminating, the hysteresis in the dynamic-Cl loop and the peak negative pitching-moment coefficientCm;peak, have been proposed. It should, however, be noted that for rotorcraft, the dynamic-stall flow control is aimed at the prevention of the DSV occurrence and the mitigation of nose-down pitching moment on rotor blades, whereas for highly maneuverable aircraft, the purpose is to delay the DSV spillage and to enhance the dynamic lift. The trailing-edge flap (TEF) dynamic-flow control concept has been considered widely for dynamic lift enhancement and nose-down pitching-moment suppression. Trailing-edge flaps have also been used extensively as a routine practice of controlling the lift by temporarily altering airfoil camber on an airplane in steadylow-speed operations, especially during takeoff and landing, without penalizing cruise performance. Rennie and Jumper [2] investigated the effectiveness of deflecting a 27% TEF control surface in negating the unsteady lift generated during a pitchmotion of a NACA 0009 airfoil atRe 2 10. They reported that the dynamic TEF effectiveness was larger than the steady-state value and had direct ramifications on unsteady lift control. The effectiveness of a deflecting control surface was also found to be higher while the control surface was in motion and was rapidly decreased at large deflections, as a result of the thickening or separation of the boundary layer on the trailing-edge flap. Furthermore, the motion of TEF appeared to delay the occurrence of dynamic stall on the airfoil. Gerontakos and Lee [5] examined the alleviation of the nose-downpitchingmoment by using a 25%c TEF, actuated dynamically in response to the airfoil oscillation, at Re 1:65 10. The prescheduled TEF motion consisted of a brief pulse, represented by a constant ramp-upmotion, remained steady briefly, and was followed by a constant ramp-down motion. They found that the reduction in j Cm;peakj was mainly a consequence of the suction pressure introduced on the lower surface of the upward deflected TEF and that a relatively early flap actuation (initiated between the static-stall angle ss and themaximum angle of attack max during pitch-up) with a rather long duration (about half of the oscillation cycle time) and a maximum deflection max 60% max should be more effective at reducing the nose-down pitching-moment excursion, and, in the meantime, provide a good compromise between the various aerodynamic requirements. Also, the magnitude of the maximum lift coefficientCl;max was found to be rather insensitive to the flap-actuation duration td, whereas the poststall lift was decreased with decreasing td. More important, the DSV formation and detachment were not affected by the upward TEF motion. In addition, dynamic leading-edge-flap lift control has also been employed by researchers elsewhere to allow high-performance aircraft to achieve a rapid and sustained high-lift coefficient. The purpose of leading-edge devices is to increase camber and thus suppress leading-edge separation during rapid arbitrary airfoil pitching maneuvers. Rennie and Jumper [13] reported an experimental determination of a 20% leading-edge flap schedule used to maintain attached flow during arbitrary dynamic pitching motions of a NACA 0009 airfoil at Re 2 10. The leading-edge flap (LEF) schedule kept the flow attached dynamically at angles of attack that were separated during static tests. The use of the LEF avoided catastrophic dynamic-stall flow-separation events. Also, for a static leading-edge flap schedule, the attached flow could be maintained by keeping the leading-edge flap aligned with the oncoming flow. In summary, it is known that both leadingand trailing-edge flaps can serve to increase the camber and manage the boundary layer effectively and thus increase the maximum lift of an airfoil. However, despite much predictive work, published experimental data on the unsteady aerodynamic loads induced by the dynamically deflecting leadingand trailing-edge flaps are still sparse. A preliminary experimental study was conducted to examine the effects of an 18%c leadingand a 25%c trailing-edge flap, actuated dynamically and independently, on the dynamic-load loops of a sinusoidally oscillatingNACA0015 airfoil in a subsonicwind tunnel at Re 2:86 10. The dynamic-load loops were obtained by integrating the unsteady surface pressure distributions. Both upward and downward flap deflections actuated at a fixed start time (ts 0 ) were investigated. This ts value was chosen to maximize the influence of the flap motion on the transient DSV-induced effects. Special emphasis was also placed on the simultaneousmeasurements of airfoil and flap deflection histories, synchronized with the surface pressure measurements.

Unsteady Airfoil with Dynamic Leading- and Trailing-Edge Flaps

The effects of upward ramp rate, actuation start time, and duration of a moveable trailing-edge flap on the critical aerodynamic values of an oscillating wing were investigated. The largest improvement in the peak negative pitching moment was obtained with a fast ramp rate and a start time near the mean angle. The largest value of net work coefficient, however, was generated with a delayed start time and a slower ramp rate. The peak lift coefficient decreased with increasing ramp rate and decreasing start time. The inclusion of a short steady-state portion in the flap motion was beneficial. An upward flap deflection initiated slightly after the mean angle during pitch-up, moved to its maximum deflection at a moderate speed, remained steady for a rather short period, and then returned to its initial position, which spanned half the oscillation cycle, was found to provide a best compromise between the various aerodynamic requirements.

Trailing-Edge Flap Control of Dynamic Pitching Moment

Introduction T HE possibility of the suppression or elimination of vortex shedding is of considerable practical interest from the standpoint of wake modification and reduction of drag, as well as of flowinduced vibration. Various passive and active control schemes have been attempted to affect the vortex wake formation process. Recently, it has been shown that a considerable amount of control of the cylinder vortex wake can be achieved through rotational oscillation and that when the flow takes place in a fluid relative to a rotationally oscillating cylinder, a new series of flow phenomena could arise.1−4 Tenada1 studied the effects of rotational oscillation for Re(= du∞/ν, where ν is the fluid viscosity) ranging between 30 and 300, and indicated that, at very high oscillation frequency f0 and amplitude, the dead-fluid region behind a cylinder and the vortex-shedding process could be nearly eliminated. The amplitude is given by 1 = π θ f0d/2u∞ > 7–27, where θ is the peak-to-peak amplitude in degrees, d is the cylinder diameter, and u∞ is the freestream velocity. Recently, Filler et al.2 studied experimentally the frequency response of the shear layers separating from a circular cylinder subjected to small-amplitude oscillations, that is, 1 ≤ 0.03, for Re = 250–1200, and observed a coupling of the cylinder oscillations with both modes of vortex shedding, that is, shear-layer vortices and primary Karman vortex shedding. Filler et al. also reported that by rotationally oscillating the cylinder at or near the natural Karman frequency, the shear-layer instability was promoted and the Karman mode of vortex shedding was also affected. On the other hand, Tokumaru and Dimotakis3 have shown that large-amplitude ( 1 ≤ 16) and highfrequency ( f0/ fk = 1–20, where fk is the natural Karman shedding frequency) rotational oscillations, together with the spinning of the cylinder, can suppress the vortex shedding and produce significant reduction in the profile drag acting on the cylinder at Re = 1.5 × 104. The objective of this study was to investigate the variation of the lateral and longitudinal spacings of the shed vortices and the vortex formation with the cylinder oscillation at Re = 1.75 × 103 by the use of hydrogen-bubble and particle streakline flow visualization methods. Special attention was given to the study of the type of wake synchronization that might occur throughout an intermediate oscillation range ( f0/ fk < 4 and 1 ≤ 1.0), between those of Filler et al. and Tokumaru and Dimotakis.

P. Gerontakos

Papers

Dynamic Stall Flow Control via a Trailing-Edge Flap

Unsteady Airfoil with Dynamic Leading- and Trailing-Edge Flaps

Trailing-Edge Flap Control of Dynamic Pitching Moment

Vortex shedding and spacing of a rotationally oscillating cylinder

Effect of Flap Motion on Unsteady Aerodynamic Loads