Lutz Taubert

Improving Rudder Effectiveness with Sweeping Jet Actuators

The application of active flow control on a vertical tail of a typical twin engine aircraft was investigated. Sweeping jets installed into the rudder surface were used and their effect was assessed by force measurements, flow visualization and local pressure distributions. The airfoil forming the tail is a NACA 0012 with a rudder using 35% of its chord. The tests were carried out at the Lucas Wind Tunnel at the California Institute of Technology at representative Reynolds numbers of up to Re=1.5 million. Multiple flap deflections and spanwise actuator configurations were tested resulting in an increase of up to 50-70% in side force depending on the free stream velocity and momentum input.

Single dielectric barrier discharge plasma actuators for improved airfoil performance

The applicability of single dialectic barrier discharge plasma actuators for use as active flow control devices, capable of enhancing the performance of airfoils, was assessed in this investigation. Measurements were carried out on two thick airfoils with simple flaps: a NACA0021 and an airfoil that is similar to those commonly used on tiltrotor aircraft. The chord length of the airfoils was approximately 0.3 and 0.25 m, respectively, and the span was approximately 0.6 m. They were both tested in the same wind tunnel with a test section of 0.6 x 1.1 m. Freestream velocities varying from 5 to 15 m/s were tested, corresponding to chord Reynolds numbers ranging between 0.8 × 10 5 and 3 × 10 5 . The lift, moment, and form drag were obtained from the pressure distributions over the airfoils surface, and the total drag was calculated from a wake survey. The range of incidence angles α varied from ―4deg <α < +20 deg and flap deflections δ f of 0 and 15 deg were tested. The location of the actuation was also altered. Two data sets are presented: one in which the actuator was placed at approximately 5 % of the chord and the other in which it was located just upstream of the flap shoulder at a chord location corresponding to about 75 %. The momentum input of the single dialectic barrier discharge plasma actuators was measured with a hot wire and was in good agreement with previously published results. The input momentum is very weak and is not sufficient to prevent separation at Reynolds numbers greater than 100,000. The single dialectic barrier discharge plasma actuators used in this study may only provide sufficient momentum to be effective at very low Reynolds numbers, such as those appropriate to micro air vehicles. Under special circumstances, their passive presence on the surface may trip the boundary layer, making it more resistant to separation, but in those cases, a proper roughness strip or vortex generators may delay separation more effectively.

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.

Discrete Sweeping Jets as Tools for Separation Control

Experiments aimed at delaying flow separation through discrete jets pointing in the direction of streaming and sweeping side-to-side along the span were conducted on two airfoils, a NACA0015 and a V-22 airfoil with and without deflected trailing edge flaps. The results indicated substantial drag reduction and lift increase at moderately low inputs of mass and momentum. Additional experiments were carried out on a semi-span V-22 wingnacelle combination and they too provided an increase in L/D of approximately 60%, even with the presence of nacelle drag and induced drag. The effectiveness of the sweeping jets on reducing the download force acting on a V-22 powered model in hover was also examined. A 29% reduction in download was realized using the embedded sweeping jets.

Manipulating the Flow over Spherical Protuberances in a Turbulent Boundary Layer

Means of controlling the flow over a large spherical protuberance were examined. The role of suction around the base of the protuberance in reducing or even eliminating the necklace vortex created by the protuberance was considered. In the absence of suction, this vortex was lifted by the low base pressure existing behind the protuberance into the wake, thus affecting the turbulence level along its path. Large vortex generators placed upstream of the protuberance were able to delay local separation of the flow over the protuberance, thus affecting the symmetry of the wake and the level of turbulence on one side or the other. Observations made using flow visualization were supplemented by hot-wire measurements. The experiments were carried out at low speed at Reynolds numbers that did not exceed 3 x 105.

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.

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.

GENERIC BLUFF BODIES WITH UNDETERMINED SEPARATION LOCATION

The parameters governing forced attachment of flow to a flat, inclined surface, were determined by Nishri 1 . The addition of convex curvature is investigated presently using the circular cylinder as a model. In both flows the forcing consisted of two-dimensional, periodic oscillations emanating from a narrow slot. Naturally the flow separates from the surface of a smooth circular cylinder around 70° from the leading stagnation point when the Reynolds number is approximately 40,000. Periodic excitation from a slot located some 40° further downstream from the natural separation location altered very significantly the pressure distribution on that surface. On the opposite side of the cylinder neither the location of separation on the opposite side nor the pressure distribution was substantially affected, but a major change in the base pressure was observed. The cylinder started to lift and the typical vortex shedding from the cylinder was altered if not entirely eliminated. Experiments were carried out using both pressure measurements and particle image velocimetry (PIV). Numerical simulations were also carried out using an unstructured mesh finite element method with dynamic and constant coefficient Smagorinsky large eddy simulation (LES) turbulence models.

Control of Vortex Shedding from a Cylinder at different Sweep back Angles

The control of vortex shedding from a yawed cylinder at different sweep back angles 0°, 45° and 60°, is currently investigated at the University of Arizona. Active flow control is applied on the cylinder in the form of oscillatory, zero mass flux excitation through two slots located symmetrically on the circumference. Surface and wake pressure measurements are carried out, as well as application of flow visualization and Particle Image Velocimetry.