Michael Desalvo

High-Lift Enhancement using Fluidic Actuation

The performance of an airfoil equipped with a high-lift system based on a single-element flap configuration is enhanced by the manipulation of vorticity concentrations near the airfoil surface using fluidic actuation. The actuation, which is effected downstream of the juncture between the main airfoil body and the flap, leads to a significant extension of the attached flow domain along the flap. Consequently, a substantial increase in lift at large flap deflections is produced (at δ = 40o and α = 4o, the lift is increased by 49% relative to baseline). Actuation is effected using spanwise arrays of fluidic oscillators consisting of jets oscillating in the spanwise direction. It is shown that the momentum coefficient Cµ required for a given lift increment can be significantly reduced through appropriate design of the interface geometry between the jet and the boundary layer. I. INTRODUCTION The performance of high-lift systems has a significant impact on the design and performance of transport aircraft. Maximum takeoff weight, required runway length and stall speeds are some of the parameters influenced by high-lift system performance. Historically, high-lift systems have consisted of complex, multi-element designs with intricate positioning mechanisms to derive maximum performance and efficiency. While high-lift systems have been simplified significantly on modern transport aircraft, there remains room for improvement in terms of weight, parts, fabrication costs and cruise efficiency. Active flow control provides an avenue for such improvements and creates the potential for high-lift performance beyond what is possible for conventional systems. A study conducted in 1999 by Boeing and NASA (McLean et al. 1 ) estimates that, among other things, a simplified high-lift system using unsteady flow control can reduce the empty weight of a 737-class aircraft by 3.3%. This weight reduction, along with the aerodynamic advantages of a simplified high-lift design, such as the removal of fairings required for the external mechanisms of conventional high-lift systems, equate to a bestcase cruise drag reduction of 3.2%. The authors also conclude that a further study is required to define the aerodynamic performance of high-lift systems using unsteady flow control. Gomes et al. 2 have concluded that it is possible to design a practical AFC highlift system based on synthetic jets. However, the performance benefits of such a system still remained to be established and would determine whether or not the technology could be integrated on an aircraft. They have also suggested that the application of AFC to the trailing-edge flap would yield the greatest benefit to the airplane as far as high-lift system application was concerned. Strategies for active flow control on lifting surfaces have primarily focused on mitigation of partial or complete flow separation over stalled flaps or wing sections, where the separating shear layer is dominated by a strong coupling to the instability of the wake that

Airfoil Aerodynamic Performance Modification using Hybrid Surface Actuators

Global modifications of the aerodynamic characteristics of a commercial transport swept airfoil at cruise (low) angles of attack (when the baseline flow is fully attached) are achieved without moving control surfaces using hybrid actuators that are surface mounted on the pressure side of the airfoil 0.21c downstream of its leading edge. Control is effected by the manipulation of trapped vorticity concentrations that are induced by leveraging the presence of a miniature, O(0.01c) obstruction integrated with a synthetic jet actuator. At Rec = 1.3·10 6 , the operation of the hybrid actuators with maximum Cµ = 0.5 10 -3 results in continuous variation of the pressure drag from 15% greater to 50% less than the pressure drag of the baseline airfoil with minimal lift penalty and consequently an increase of the lift to pressure drag ratio in excess of a factor of two. High-resolution particle image velocimetry measurements at Rec = 6.7·10 5 are used to characterize the evolution of the boundary layer on the surface of the airfoil and estimate the total drag coefficient. It is shown that the overall drag is reduced by 29% and yields an increase in L/D of 27%.

High-lift Enhancement Using Active Flow Control

The high-lift performance of an airfoil with a single-element flap is enhanced using fluidic actuation based on synthetic jet technology. Acutation is implemented using a spanwise array of individually controlled discrete synthetic jets with variable spanwise spacing that issue in a direction nominally-tangential to the flap immediately upstream of separation. The jets are used to engender and manipulate concentrations of vorticity in a manner that leads to improved flow attachment. The resulting increase in suction upstream and downstream of the jet array leads to a substantial increase in lift (at  = 25o, Rec = 3.3∙10 5 and α = 4o, CL of up to 0.82 relative to the unactuated flow can be realized). The effect of the spanwise actuation wavelength  is investigated with the objective of optimizing the actuation momentum coefficient C. It is shown that for a given CL, C has a minimum for some spanwise actuation wavelength. Measurements of the three-dimensional flow field in the vicinity of an actuator jet show that flow attachment is accompanied by the formation of a counterrotating streamwise vortex pair, and favorable streamwise pressure gradient downstream of the actuator.

Enhancement of a High-Lift Airfoil using Low-Power Fluidic Actuators

The performance of an airfoil equipped with a high-lift system based on a single-element flap configuration is enhanced by the manipulation of vorticity concentrations near the airfoil surface using spanwise arrays of fluidic jet actuators. Spanwise periodic flow injection downstream of the juncture between the main airfoil body and the flap leads to a significant increase in the extent of flow attachment along the flap. Consequently, a substantial increase in lift at large flap deflections is produced (at δ = 40o and α = 4o, the lift is increased by up to 66% relative to baseline). It is shown that the momentum coefficient Cμ required for a given lift increment can be reduced significantly through the use of multiple actuators and by appropriate integration of the actuator jets.

Aerodynamic Control at Low Angles of Attack using Trapped Vorticity Concentrations

The aerodynamic characteristics of a swept wing are significantly altered at low angles of attack (when the baseline flow is fully attached) by vorticity concentrations that are engendered using integrated hybrid actuators each comprised of a miniature [O(0.01c)] small obstruction and a synthetic jet actuator. The present work demonstrates that the presence of controllable trapped vorticity concentrations near the leading and trailing edges on the pressure side of the airfoil can lead to a significant simultaneous increase in lift and reduction in drag compared to the baseline airfoil (L/Dp increases by a factor of 2.6 at α = 6°). It is also shown that the aerodynamic performance can be maintained at substantially reduced actuation power when the actuation waveform of the trailing edge actuator is pulse-modulated at a frequency that is commensurate with the unstable frequency of the wake.