Transient Control of Separating Flow over a Dynamically-Pitching Airfoil

Woo, George T. K.; Glezer, Ari

doi:10.2514/6.2010-861

Cited by 22 publications

(5 citation statements)

References 2 publications

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“…14 For example, the manifestation of dynamic stall during retreating blade stall imposes the primary limitation on helicopter forward flight speeds. 15 A number of numerical and experimental investigations have sought to reduce or eliminate the variations in lift and negative damping associated with dynamic stall by using active flow control, including zero net mass flux or synthetic jet actuation, 16,17 steady and pulsed blowing, 18,19 variable-geometry leading 20 and trailing edges, 21 combustion pulsed actuation, 22 and plasma actuation. 23 Several of these investigations indicated that time-periodic excitation of the separating shear layer can increase the steady and unsteady stall angles and the post-stall lift and mitigate unfavorable temporal variation in the induced pitching moment.…”

Section: Overviewmentioning

confidence: 99%

Aerodynamic Control of a Pitching Airfoil by Active Bleed

Kearney

Glezer

2014

32nd AIAA Applied Aerodynamics Conference

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The aerodynamic loads on an airfoil that is pitching beyond its static stall margin are controlled in wind tunnel experiments by regulation of surface vorticity flux using distributed bleed actuation. The airfoil model is based on a two-dimensional VR-7 configuration that is pitching time harmonically over a broad range of reduced frequencies and angles of attack (up to  = 22°).Bleed is driven by pressure differences between surface ports upstream of the pressure side trailing edge and downstream of the suction side leading edge and is regulated by integrated low-power piezoelectric louver actuators. The time-dependent evolution of the outer flow over the airfoil during the pitch cycle is investigated in the absence and presence of bleed using high-speed PIV to resolve transitory formation and shedding of vorticity concentrations during the onset and termination of dynamic stall. The timing of the dynamic stall vorticity flux into the near wake and its effect on the flow field are analyzed in the presence and absence of bleed using proper orthogonal decomposition (POD). It is shown that bleed actuation alters the production, accumulation, and advection of vorticity concentrations near the surface with significant effects on the evolution, and, in particular, the timing of the dynamic stall vortex. These changes are manifested by alteration of the lift hysteresis and pitch stability during the cycle. The timeperiodic changes in lift during the up-and downstroke segments of the pitch cycle are accompanied by mitigation of sharp excursions of the pitching moment in the base flow, and in complete reversal of the "negative damping" while keeping the cycle-averaged lift to within 5% of the base flow level. Such control of the pitch stability can lead to significant improvement of the stability of flexible wings and rotor blades. I. OverviewThe interactions between the outer flow over the aerodynamic surfaces and controlled distributed aerodynamic surface bleed that is engendered by pressure differences in flight can result in significant alteration of the global flow and therefore of the global aerodynamic loads. 1-3 Exploiting this approach for aerodynamic control is attractive because the primary actuation power is derived from the motion of the vehicle, and control is applied by regulation of the bleed flow through surface ports and therefore requires relatively little power. Aerodynamic flow control using bleed has been demonstrated in a number of earlier investigations using continuous, time-invariant bleed through passive porous surfaces by leveraging high-and low-pressure domains over lifting surfaces. The bleed flow results in modifications of the local pressure distributions and thereby alters the global aerodynamic forces. Early investigations of passive (nearly time-invariant) bleed for flow control realized lift enhancement and drag reduction using multiple slots in wings, 4,5 while more recent efforts have demonstrated control of boundary-layer instabilities, 6 rotor blade tip vortices, 7 and leading edge...

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Section: Overviewmentioning

confidence: 99%

Aerodynamic Control of a Pitching Airfoil by Active Bleed

Kearney

Glezer

2014

32nd AIAA Applied Aerodynamics Conference

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“…19 Hassan et al numerically analyzed the effectiveness of SJ on dynamic stall suppression for VR-7 airfoil, 20 and the benefits of SJ for improving the aerodynamic characteristics of airfoil with onset of the dynamic stall were well validated. Woo and Glezer further carried out wind tunnel tests on SJ control of NACA 4415 airfoil dynamic stall, 21 which showed that SJ could significantly reduce the nose-down moment and increase lift coefficient of airfoil at the same time.…”

Section: Introductionmentioning

confidence: 99%

New combinational active control strategy for improving aerodynamic characteristics of airfoil and rotor

Zhao

2019

Proceedings of the Institution of Mechanical Engineers, Part G:

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In order to improve the aerodynamic characteristics of rotor, a new active flow control strategy by combining a synthetic jet actuator and a variable droop leading-edge or a trailing-edge flap has been proposed. Their control effects are numerically investigated by computational fluid dynamics (CFD) method. The validated results indicate that variable droop leading-edge and synthetic jet can suppress the formation of dynamic stall vortex and delay flow separation over rotor airfoil. Compared with the baseline state, Cdmax and Cmmax are significantly reduced. Furthermore, parametric analyses on dynamic stall control of airfoil by the combinational method are conducted, and it indicates that the aerodynamic characteristics of the oscillating rotor airfoil can be significantly improved when the non-dimensional frequency ( k*) of variable droop leading-edge is about 1.0. At last, simulations are conducted for the flow control of rotor by the combinational method. The numerical results indicate that large droop angle of variable droop leading-edge can better reduce the torque coefficient of rotor and the trailing-edge flap has the capability of increasing the thrust of rotor. Also, the synthetic jet could further improve the aerodynamic characteristics of rotor.

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“…The present investigation builds on the findings of Woo et al (2010 and2011) who used pulsed actuation to control flow separation over an airfoil undergoing time-periodic pitch oscillations beyond the static stall margin, and demonstrated significant improvements in its aerodynamic performance. The paper focuses on the changes induced in the unsteady baseline flow of the pitching airfoil (in the absence of actuation) by controlled flow transients with emphasis on the connection between the time dependent flow field and the instantaneous global aerodynamic forces, and the mitigation of dynamic stall and reduction in negative pitch damping.…”

Section: Overviewmentioning

confidence: 99%

“…Five instances during the cycle (i through v) are marked for reference in connection with the discussion of Figure 10. In the absence of actuation, the lift and pitching moment of the model (measured by load cells and torque sensors) exhibit hysteretic effects that are associated with dissimilar shedding of vorticity concentrations during the up-and down-strokes of the oscillation cycle (e.g., Woo et al, 2010). Figure 9 also shows the effects of pulsed actuation that is applied over the center 0.21S section of the airfoil using a burst of eight actuation pulses that is triggered as the airfoil pitches up through (t = t start ) = 14 o such that successive pulses are T pulse = 1.4lT conv apart.…”

Section: Control Of Dynamic Stall Using Stage Pulsed-actuationmentioning

confidence: 99%

“…Finally, the lift begins to recover slowly as  drops below 12 o , and the accumulation of vorticity commences, and eventually, the flow returns to the fully attached state as the airfoil pitches through  = 10 o and the cycle repeats. Woo et al (2010 and2011) showed that pulsed actuation can be used to mitigate some of the undesirable aerodynamic effects that are associated with dynamic stall on an oscillating airfoil. Specifically, the actuation reduces the magnitude of lift reduction during the downstroke, and the effects of "negative damping" in the pitching moment.…”

Section: Control Of Dynamic Stall Using Stage Pulsed-actuationmentioning

confidence: 99%

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