George T. K. Woo

Transient Separation Control Using Pulse-Combustion Actuation

The transitory response of the flow over a stalled, two-dimensional (NACA 4415) airfoil to pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale is investigated experimentally (Re = 570, 000). Actuation is effected by momentary [O(1 ms)] pulsed jets that are generated by a spanwise array of combustion-based actuators integrated into the center section of the airfoil. The flowfield in the cross-stream plane above the airfoil and in its near wake is computed from multiple high-resolution particle image velocity images that are obtained phase locked to the actuation waveform and allow for tracking of vorticity concentrations. The brief actuation pulse leads to a remarkably strong transitory change in the circulation about the entire airfoil that is manifested by a severing of the separated vorticity layer and the subsequent shedding of a large-scale clockwise vortex that forms the separated flow domain. The clockwise severed vorticity layer that follows behind this detached vortex has a distinct sharp streamwise edge that grows and rolls up as the layer is advected along the surface. It is shown that the shedding of the severed vortex and the accumulation of surface vorticity are accompanied by a transitory increase in the magnitude of the circulation about the airfoil that lasts 8—10 convective time scales. The attached vorticity layer ultimately lifts off the surface in a manner that is reminiscent of dynamic stall, and the flow separates again.

Transitory Separation Control over a Stalled Airfoil

Transitory attachment of the flow over a stalled, 2-D airfoil is investigated in wind tunnel experiments using pulsed actuation. The impulse of the momentary control jets is produced by combustion based actuators on characteristic time scales O[1 ms] that are an order of magnitude shorter than the convective time scale of the flow. A single actuation pulse results in transitory flow attachment that is manifested by rapid increase in the global circulation and aerodynamic forces and persists for about ten convective time scales before the flow becomes fully stalled again. Large-scale changes in vorticity accumulation that are associated with repetitive, burst-modulated actuation pulses are exploited for significant extension of the streamwise domain and duration of the attached flow with a corresponding increase in the peak circulation. High-resolution PIV measurements of the interaction between the pulsed jets and the cross flow reveal details of the severing and collapse of the separated flow domain and the dynamics of vorticity accumulation within the attaching boundary layer.

Transient Control of Separating Flow over a Dynamically-Pitching Airfoil

Transitory separation control of the flow over a 2� D airfoil that is dynamically pitching beyond the stall limit is investigated in wind tunnel experiments using pulsed actuation. Actuation is effected by a spanwise array of momentary, combustionbased actuator jets having a characteris tic time scale O(1 ms) that is an order of magnitude shorter than the convective time scale of the flow. The present experiments consider two stall configurations. In the first configuration, the actuation is nominally twodimensional and the actuator spans a stalled flow domain that is bounded by spanwise fences. In the second configuration the fences are removed and the actuation covers about 20% of the entire span of a fullystalled airfoil. In the nominally 2�D config uration, singlepulse actuation can significantly increases the lift over most of the oscillation cycle not only at poststall but also at angles of attack that are be low stall. In the 3�D configuration, a single pulse can increase the lift during a significant fractions of the oscillation cycle despite the massive separation on the outboard segments of the unactuated airfoil. Several actuation pulses distributed during the oscillation cycle can lead to a remarkable increase in lift over most of the cycle including at angles of attack that are below stall ostensibly by controlled trapping vorticity over the entire oscillation cycle.

Combustion-Powered Actuation for Dynamic-Stall Suppression: High-Mach Simulations and Low-Mach Experiments

An investigation on dynamic-stall suppression capabilities of combustion-powered actuation (COMPACT) applied to a tabbed VR-12 airfoil is presented. In the first section, results from computational fluid dynamics (CFD) simulations carried out at Mach numbers from 0.3 to 0.5 are presented. Several geometric parameters are varied including the slot chordwise location and angle. Actuation pulse amplitude, frequency, and timing are also varied. The simulations suggest that cycle-averaged lift increases of approximately 4% and 8% with respect to the baseline airfoil are possible at Mach numbers of 0.4 and 0.3 for deep and near-deep dynamic-stall conditions. In the second section, static-stall results from low-speed wind-tunnel experiments are presented. Low-speed experiments and high-speed CFD suggest that slots oriented tangential to the airfoil surface produce stronger benefits than slots oriented normal to the chordline. Low-speed experiments confirm that chordwise slot locations suitable for Mach 0.3-0.4 stall suppression (based on CFD) will also be effective at lower Mach numbers.

Transitory Control of Dynamic Stall on a Pitching Airfoil

Transitory attachment of the flow over a stalled, 2-D airfoil is investigated in wind tunnel experiments using pulsed actuation. Actuation is provided by a spanwise array of momentary, combustion-based actuator jets having a characteristic time scale O[1 ms] that is an order of magnitude shorter than the convective time scale of the flow. It is shown that a single actuation pulse results in transitory flow attachment that is manifested by rapid increase in the global circulation and aerodynamic forces and persists for about ten convective time scales before the flow becomes fully stalled again. Large-scale changes in vorticity accumulation that are associated with repetitive, burst-modulated actuation pulses are exploited for significant extension of the streamwise domain and duration of the attached flow with a corresponding increase in circulation. The effects of the transitory actuation are further amplified when the airfoil is mounted on a dynamic 2DOF (pitch and plunge) traversing mechanism and the actuation is tested with pitch oscillations beyond the stall limit. In this configuration, the actuation is nominally two-dimensional within a spanwise domain measuring 0.21S that is bounded by end fences. It is shown that pulse actuation significantly increases the lift not only at post-stall but also at angles of attack that are below stall (ostensibly by trapping vorticity over the entire oscillation cycle).

Rotorcraft Fuselage Drag Reduction using Combustion Powered Actuators

Separation control of the 3-D flow over the aft body of a scale model of the NASA ROBIN mod7 rotorcraft fuselage is investigated in wind tunnel experiments. Pulsed actuation is effected by arrays of momentary, combustion-based actuator jets having a characteristic time scale O[1 ms] that is an order of magnitude shorter than the convective time scale of the flow. The actuators are placed upstream of the transition region between the fuselage and the tail boom and their interactions with the massively separated cross flow in this domain and effects on the global aerodynamic forces and moments are measured using an onboard six-axis load cell and high resolution PIV that is acquired phase-locked to the actuation waveform. The present investigations have demonstrated that the actuation can significantly mitigate separation, and lead to a reduction in drag (although flow attachment is accompanied by some lift penalty). It also is shown that transitory aft flow attachment using burst-modulated actuation can be exploited for effecting significant steering aerodynamic side forces for improved flight maneuverability.

Combustion Powered Actuators for Separation Control (Invited)

An overview is presented of research involving the development and application of combustion powered actuation (COMPACT) for active control of separated flows. This actuation approach produces high-speed, pulsed actuation jets through the ignition of a mixture of gaseous fuel and air within a cubic-centimeter-scale combustion chamber. The basic characterization of the actuator including several geometric and chemical factors that affect the actuator performance is described. Dynamic pressure measurements and PIV show how these factors alter the dynamic pressure pulse within the combustor and the ensuing actuation jet. Environmental testing performed on the actuator is discussed for several harsh environmental conditions with sample results presented for rain exposure. Aerodynamic applications of COMPACT to mitigate both 2-D and 3-D separated flows are also described for airfoils and fuselage integration into a generic rotorcraft body. Flow field measurements show that reattachment of large-scale flow separation is accomplished for each of these applications with the reattachment taking place over a much larger time scale than that of the actuation pulse or the convective time over the surface, resulting in enhanced transitory aerodynamic performance.

Three-Dimensional Transitory Control of Flow Separation over a 2-D Airfoil

The dynamics of controlled transitory 3-D flow attachment within a separated flow domain over a stationary and pitching 2-D airfoil is investigated in wind tunnel experiments using a spanwise-compact pulsed actuation having a characteristic time scale that is an order of magnitude shorter than the convective time scale. It is shown that spanwise spreading of the induced transitory 3-D flow attachment beyond the spanwise edges of the actuator can amplify the aerodynamic performance of the airfoil compared to 2-D, spanwise-bounded actuation. The dynamics of the 3D attachment domain and some features of its spanwise structure above the airfoil and in its near wake are investigated using PIV. The temporal and spanwise variations of the 3-D attached domain result in corresponding variations of the vorticity over the both surfaces of the airfoil and consequently in significant changes in the time-dependent sectional circulation. Coupling of the pulsed actuation to the airfoil’s motion by sequenced actuation during the pitch cycle enhances the control authority of 3-D unsteady separation and can significantly mitigate the effects of dynamic stall and improve the unsteady aerodynamic lift and pitching moment.

Transitory Control of Unsteady Separation using Pulsed Actuation

The dynamic mechanisms of transitory flow attachment effected by pulsed actuation of the separated flow over a stalled airfoil are investigated experimentally. Actuation is effected by momentary pulsed jets generated by a spanwise array of combustion-based actuators such that the characteristic time of jet duration is nominally an order of magnitude shorter than the flows convective time scale. The transitory flow field in the cross stream plane above the airfoil and in its near wake is investigated using multiple high-resolution PIV images that are obtained phase-locked to the actuation for continuous tracking of vorticity concentrations. The brief actuation pulse leads to severing of the separated vorticity layer and the subsequent shedding of large-scale vortical structures owing to the collapse of the separated flow domain which is accompanied by strong changes in the circulation about the entire airfoil. By exploiting the disparity between the characteristic times of flow response to actuation and relaxation, it is shown that successive actuation pulses can extend the flow attachment and enhance the global aerodynamic performance. It is also shown that coupling of the actuation to the airfoils motion during cyclical pitch enhances the effect of transitory flow control and leads to a significant suppression of dynamic stall.

Pulsed Actuation Control of Flow Separation on a ROBIN Rotorcraft Fuselage

Separation control of the three-dimensional flow over the aft segment of a NASA Rotor-Body-Interaction (ROBIN) rotorcraft fuselage is investigated in wind-tunnel experiments using arrays of miniature combustion-based actuators. Successive pulsed actuation is effected by momentary jets each having a characteristic time scale O[1  ms] that is an order of magnitude shorter than the convective time scale of the flow. The actuators are placed upstream of the transition ramp between the ROBIN’s fuselage and the tail boom. The jet-induced interactions with the massively separated crossflow over the ramp and their effects on the global aerodynamic forces and moments are investigated using high-resolution particle image velocimetry that is acquired phase-locked to the actuation waveform in conjunction with simultaneous force and moment measurements using an onboard load cell. The present investigations show that symmetric actuation about the model’s centerline can significantly mitigate separation, lead to a reduc...