Stuart I. Benton

Pulsed Jets Laminar Separation Control Using Instability Exploitation

The physics of control by pulsed blowing on a NACA 643-618 natural laminar flow airfoil is studied using hot-film anemometry. Measurements in the uncontrolled separated shear layer indicate that vortex shedding is taking place due to the Kelvin–Helmholtz-type inviscid instability. Steady and pulsed external acoustic excitation is used as well to decouple the frequency content of the perturbation from the vorticity introduced by the jet. Acoustic control introduces either the most unstable frequency or harmonics of carrier frequency in the most unstable range, which amplify exponentially in the separation region yielding a significant delay of the separation location to approximately 55% chord. Experimental data suggest that pulsed jets introduce higher-order harmonics of the low-frequency pulsing as well, amplifying the natural disturbances in the laminar separation. Phase-averaged wavelet analysis is used to study the control physics within a single pulsing period. It is shown that a jet-induced high-fre...

Large Low-Frequency Oscillations Initiated by Flow Control on a Poststall Airfoil

A large-amplitude low-frequency oscillation has been reported in the literature for some airfoils in a small range of angles of attack before stall. In the current study, the same low-frequency oscillations are shown to be triggered by two methods of active flow control at poststall angles. Active control is applied in the form of external acoustic excitation or a row of discrete jets that are operated in a pulsed or steady mode. Conditionally averaged particle image velocimetry and time-resolved surface pressure measurements are used to evaluate the spatial and temporal characteristics of the oscillation. By systematically increasing forcing amplitude, each flow control technique is shown to first excite low-frequency oscillation up to a maximum lift increase, after which subsequent increases in forcing amplitude attenuate and ultimately eliminate low-frequency oscillation altogether. At maximum oscillation amplitude, the behavior is highly periodic in the case of acoustic forcing and intermittent in the...

Parametric Optimization of Unsteady End Wall Blowing on a Highly Loaded Low-Pressure Turbine

Efforts to reduce blade count and avoid boundary layer separation have led to lowpressure turbine airfoils with significant increases in loading as well as front-loaded pressure distributions. These features have been independently shown to increase losses within the secondary flow field at the end wall. Compound angle blowing from discrete jets on the blade suction surface near the end wall has been shown to be effective in reducing these increased losses and enabling the efficient use of highly loaded blade designs. In this study, experiments are performed on the front loaded L2F low-pressure turbine airfoil in a linear cascade. The required mass flow is reduced by decreasing the hole count from previous configurations and from the introduction of unsteady blowing. The effects of pulsing frequency and duty cycle are investigated using phase-locked stereo particle image velocimetry to demonstrate the large scale movement and hysteresis behavior of the passage vortex interacting with the pulsed jets. Total pressure loss contours at the cascade outlet demonstrate that the efficiency benefit is maintained with the use of unsteady forcing. [DOI: 10.1115/1.4026127]

Three-dimensional Instabilities in Vortex/Wall Interactions: Linear Stability and Flow Control

The flow of interest is a streamwise vortex interacting with a wall. This flow is proposed as a simplified model of a generic secondary flow stemming from a blade side-wall junction such as those at the wing-body junction of an aircraft or the blade-hub junction of a compressor or turbine airfoil. The current computational campaign has shown that Crowtype and Widnall-type instabilities do exist between a vortex and a wall in a similar manner as a vortex pair. The results of the current modal analysis also provide the shape of the adjoint mode corresponding to the Crow and Widnall instabilities. These results are related to their usefulness in an ongoing research campaign to implement flow control that rapidly and efficiently dissipates these vortex structures.

Control of Three-dimensional Flow Instabilities at the Side-wall Junction of an Airfoil Cascade

Active flow control in the form of pulsed jets has been applied to the airfoil-sidewall junction of a cascade of low pressure turbine airfoils. The flow is characterized by a large streamwise vortex structure that interacts strongly with the airfoil suction surface boundary layer generating significant total pressure loss. Results from a previous study highlight a specific range of jet pulsing frequency that is highly effective at reducing generated losses through increased mixing. At this frequency, phase-locked stereo-PIV highlights differing dynamics as the vortex center moves due to the jet actuation. It is suggested that this behavior is due to an excited long-wavelength vortex instability that develops between the vortex and the strain applied by the neighboring walls. An inviscid vortex-line method for a vortex-corner interaction is developed that predicts the range of unstable frequencies and the expected dynamics of the instability. The unstable frequency range and expected dynamics are shown to agree very well. Finally, a simplified experiment is discussed with a generated vortex and a suspended wall. Stereo-PIV synchronized with a forcing subwoofer clearly extracts dynamics that suggest the development of a long-wavelength cooperative instability between the vortex and the wall. Future work on this facility will be used to inform the design of flow-control methods for secondary flows.

Acoustic Separation Control for a Laminar Flow Airfoil

Acoustic flow control is investigated on the NACA 643 − 618 laminar airfoil. This airfoil has received considerable interest of late for its unique stall characteristics and the opportunities it presents for the design of a robust active flow control mechanism. At low Reynolds numbers (6.4x10), the airfoil exhibits laminar separation, turbulent separation, and/or the presence of a re-attaching laminar separation bubble, depending on α. This presents a rich environment for the study of active flow control. The focus of this work is to study the effects of pure frequency excitation through the use of acoustics. Angles of attack of 10◦ and 22◦ are studied. In both cases, significant improvements in lift and drag are obtained. Conclusions are drawn about the opportunity to exploit instability frequencies through the use of more practical flow control methods.

Parametric Optimization of Control for a Post-Stall Airfoil Using Pulsed Jets

The performance of active flow control on a NACA 643-618 laminar airfoil at post-stall angles of attack is evaluated using discrete, wall-normal pulsed jets. Actuation is implemented near the leading edge of the airfoil. The effect of actuation duty cycle and blowing ratio on lift coefficient are studied for an actuation period equal to the convective period of the flow. Lift coefficient shows a large dependence on both blowing ratio and duty cycle for α ≥ αstall, and improvements increase as the duty cycle is reduced for a given blowing ratio. Phase-locked particle image velocimetry shows the development of two vortices associated with the onset and termination of actuation. For low duty cycle forcing, the early interaction of the two vortices cause a roll up of the separated shear layer, and increased mixing and momentum entrainment in the boundary layer. Data taken at a higher Reynolds number (128,000 vs. 64,000) suggest that the same mechanisms for control exist.

Large Low-Frequency Oscillations Initiated by Sub-Optimal Flow Control on a Post-Stall Airfoil

A large amplitude low frequency oscillation in the lift force is detected for an airfoil experiencing leading edge stall. This behavior has been documented experimentally and numerically in the literature, but with an incomplete understanding as to its origin. In the current study this behavior is shown to interact with standard methods of active flow control. Active control is applied in the form of external acoustic excitation or a row of discrete jets that are operated in a pulsed or steady mode. Time-resolved PIV and surface pressure measurements are used to evaluate the spatial and temporal characteristics of the oscillation. The use of forcing allows for further investigation into the physics of this oscillation, specifically in the dynamic behavior and primary time scale. The paper is concluded with a discussion of implications for active flow control. Each flow control technique is shown to excite the oscillation at moderate forcing amplitudes and remove the oscillation with higher forcing amplitude. At maximum oscillation amplitude, the behavior is highly periodic in the case of acoustic forcing and intermittent in the case of pulsed or steady jets. Unsteady behavior of this amplitude could have adverse effects for structural mechanics and should be considered in the design of a robust active flow control system.

Effect of Spanwise Jet Spacing on Separation Control for Swept and Unswept Airfoils

The performance of active flow control on a NACA 643-618 laminar wing at an effective streamwise Reynolds number of 64,000 with and without sweep (Λ = 30° and 0°) is evaluated at post-stall angles of attack. Actuation is implemented near the leading edge using discrete, wall-normal, steady vortex generating jets (VGJs). The effect of increasing spanwise distance between jets along the leading edge of the wing is studied. For the swept wing configuration, lift coefficient shows a strong dependence on the number of vortex generating jets distributed near the leading edge. While holding blowing ratio constant and increasing spanwise jet spacing, small performance gains are noted while significantly reducing the required mass flow across a wide range of angles of attack (22o ≤ α ≤ 35o). While holding the total mass flow rate constant and increasing the spanwise distance between jets, significant performance gains are seen. The controlled straight wing configuration demonstrated a similar trend with increased spanwise jet spacing, but only over a narrow range of angle of attack (centered at α = 18o). Above this threshold, the flow displays a tendency toward multiple stable states in a seemingly unpredictable manner.

The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade

An experimental investigation of the effect of acoustic excitation on the boundary layer development of a highly loaded low-pressure turbine blade at low-Reynolds number is investigated. The aim of this work is to study the effect of excitation at select frequencies on separation which could give indications about active flow control exploitation. The front-loaded L2F blade is tested in a low-speed linear cascade. The uncontrolled flow presents a separation bubble on the suction surface at Reynolds numbers below 40,000. For these conditions, the instability of the shear layer is documented using hot-wire anemometry. A loudspeaker upstream of the cascade is directed towards the passage inlet section. A parametric study on the effect of amplitude and frequency is carried out. The effect of the excitation frequency is observed to delay separation for a range of frequencies. However, the control authority of sound is found to be most effective at the fundamental frequency of the shear layer. The amplitude of perturbation is significant in the outcome of control until a threshold value is reached. PIV measurements allow a deeper understanding of the mechanisms leading to the reduction of separation. Data has been acquired with a low inlet turbulence level (<1%) in order to provide a cleaner environment which magnifies the effects of the excitation frequency, and with an increased turbulence intensity level of 3% which is representative of more typical engine values. Integrated wake loss values are also presented to evaluate the effect on blade performance.