Philipp Tewes

On the Use of Sweeping Jets to Augment the Lift of a Lambda-Wing

Super-critical airfoils that are optimized for high speed subsonic flight require complex auxiliary high-lift systems for take-off and landing. A lambda wing model, based on such an airfoil, but containing simple flaps augmented by sweeping jet actuators, was constructed and tested. The purpose of these tests was to assess the efficacy of this method of separation control on a realistic wing configuration. Force and pressure measurements were carried out on this wing as well as surface flow visualization that used tufts and china clay. The strength of this actuation was altered and its effects were assessed. The orientation of the actuators was also altered for the outboard flap. The first flap had actuators aligned with the free stream while the second one had them parallel to the leading-edge that was swept back at 40°. The actuation from the second set of flaps turned out to be more effective because it affected only the decelerating flow component and no momentum was wasted on span-wise flow. These observations reaffirmed the ideas embedded in the boundary layer “independence principle” for large aspect ratio swept back cylinders.

Applying the Boundary-Layer Independence Principle to Turbulent Flows

Velocities measured in turbulent boundary layers over yawed flat plates confirmed that the mean velocity profiles normal to the leading edge are proportional to the velocity profiles parallel to it, with a proportionality constant depending on the yaw angle. This turned out to be the necessary and sufficient condition to make the wall stress components normal and parallel to the leading edge also proportional in the same manner, thus reaffirming the boundary-layer independence principle for turbulent and laminar flows alike. Reinterpretation of old experiments thus changed the mantra stating, “the independence principle does not apply to turbulent flow”, thus providing a new insight into three-dimensional boundary-layer flows on yawed, high-aspect-ratio wings. It explains the prevalence of attached spanwise flow near the trailing edges of such wings, and it provides a rationale for turbulence modeling on them. Furthermore, it indicates the direction along which active separation control should take place.

Feedback-controlled forcefully attached flow on a stalled airfoil

pressure coefficient to keep the flow attached. Sincea dimensionless pressure coefficient is required for this purpose, two similar sensors were installed in the pitot-static tube that monitored the freestream velocity. The impact of the time delay on the stability of the controller was briefly discussed and accounted for. The robustness of the controller was demonstrated under varying freestream velocities.

On the Effect of Sweep on Separation Control

The performance of a flapped wing based on a NACA 0012 airfoil section and equipped with a linear array of fluidic oscillators was investigated experimentally to assess the significance of wing sweep and aspect ratio on the efficiency of the actuation. The semi-span wing that was suspended from the wind tunnel ceiling through a six-component balance could be withdrawn partially from the test section and rotated in a plane parallel to the flow thus its sweep could vary from 0° to ±45° and its aspect ratio could change from 2.4 to 7.5. The wing incidence, its flap deflection, and the level and distribution of the actuation were the additional independent parameters investigated. The experiments were carried out at Reynolds numbers varying between 300,000 and 500,000. The boundary layer was tripped in order to fix the location at which transition to turbulence occurs. To overcome separation at high flap deflections in the absence of wing sweep, a minimum momentum coefficient of the order of 1% was required. However, on a swept-back wing a substantially lower input level could improve the lift generated by the wing by some 20% and alter the pitching moment provided the aggregate number of the actuators was small. Under these conditions, the actuators acted as fluidic boundary layer fences that can be switched ON or OFF on demand and change the aerodynamic characteristics of the wing for takeoff and landing purposes. An attempt was made to explain the mechanism that makes the fluidic oscillators so effective.

Control of Separation on a Swept Wing using Fluidic Oscillators

The performance of a flapped wing based on a NACA 0012 airfoil section and equipped with a linear array of fluidic oscillators was investigated experimentally to assess the significance of wing sweep and tip shape on the efficiency of this actuation technique. The semi-span wing was suspended from the wind tunnel ceiling through a six-component balance and could be rotated relative to the oncoming flow. Thus, its sweep could vary from 0° to ±45° while maintaining a constant aspect ratio of the wing. The experiments were carried out at a Reynolds number of approximately 500,000. The boundary layer was tripped in order to fix the location at which transition to turbulence occurs. The effects of incidence, flap deflection, and the level of the actuation were the independent parameters affecting the wing’s performance. In contrast to non-swept wings equipped with fluidic oscillators, the swept-back wing requires a substantially lower input level of actuation to improve the lift generated by the wing and alter its pitching moment provided the aggregate number of the actuators used was small. For this study, the number of active actuators did not exceed two. Under these conditions, the actuators acted as fluidic fences that reorientated the flow by reducing its spanwise component. Since this is an interim report in an ongoing investigation, it will focus on issues that were not reported at the AIAA meeting in Atlanta. For example, a two-actuator pattern (the spacing between them was kept constant) was moved along the wingspan and the impact of their location on performance was analyzed with respect to the sweep angle.

The Utility of Hysteresis for Closed-Loop Control Applications that Maintain Attached Flow under Natural Post Stall Conditions on Airfoils

** NASA Langley Research Center Much higher control input is required to attach separated flow than to keep an attached flow from separating under natural, post stall conditions. The experiments with slot suction applied near the leading-edge, revealed a hysteresis of lift and drag coefficients that depend on the level of suction. This offers an opportunity to keep the flow attached at minimal input levels, while guarantying that flow separation will be not be allowed to occur. A simple approach was adopted that uses a fast response pressure sensor located near the leading-edge of the airfoil for feedback control. Since a pressure coefficient is required for this purpose, two additional quick responding sensors were installed in the Pitot tube that measures the free stream velocity. The proposed controller used a prescribed pressure coefficient to keep the flow attached. The impact of the time delay on the stability of the controller is briefly discussed and accounted for. The robustness of the controller was demonstrated under varying free stream velocities.