On the Control of Leading Edge Vortex Structures on Swept Bodies

Abstract

Tailless λ-wing planforms have gained significant interest since the early 2000s due to their potential to combine stealth with aerodynamic performance, as exemplified by the X-47B, Blended Wing Body model (BWB), and Swept WIng Flow Test model(SWIFT). The current research explores flow control on configurations featuring a high sweep (Λ) inner wing Leading Edge (LE) (Λ = 60◦) transitioning into a lower sweep outer wing (Λ = 30◦) via a crank. The SWIFT model was extensively investigated at higher Reynolds number (Re) at the National Transonic Facility (NTF), and a scaled model retrofitted with Active Flow Control (AFC) was tested at California Institute of Technology (Caltech) and University of Arizona (UA). A simplified sharp leading edge flat surface -λ model was also tested to fix the primary flow separation at LE. Despite significant sectional geometry differences between the models, with the spanwise thickness variation and wing twist on SWIFT affecting its Leading Edge Vortex (LEV) development, the two wings exhibited similarities in the lift-slope (∂CL/∂α ), drag slope (∂CD/∂α ) and the departure of trimmed pitch (∂CLM/∂CL= 0). A single jet influenced the dynamics of the LEV structures, generating an on-demand nose-up or nose-down pitch moment at low incidence angles. The LEV primarily consists of a separating shear layer that forms the primary vortex and a smaller and weaker counter-rotating secondary vortex. Surface flow visualization distinctly identified two LEVs on the flat-λ model, one for each inner and outer wing; and was complimented with two-dimensional Particle Image Velocimetry (PIV) normal to the inner wing LE at 38% span, and stereoPIV in a plane parallel to the wing’s suction surface, at 8 y-normal distances. A Steady Supersonic Nozzle (SSN) mounted on a rotating platform, located near the Secondary Separation line(SS) at 39% span on the inner wing, affected the evolution of the inner wing LEV and its interaction with the outer wing LEV, thus affecting the lift distribution over the outer wing and thus the pitch moment. Two nozzle orientations were selected for detailed investigations, β = 330◦ and 270◦ that generated nose-up and nose-down pitch moment respectively. Actuation at β = 330◦ normal to outer wing LE, displaced the secondary vorticity of the inner wing LEV thus weakening the LEV and increasing its interaction with the outer wing LEV. Whereas actuation at β = 270◦ energized the axial momentum of the inner wing LEV core thus delaying its lift-off. The blunt LE on SWIFT caused the Primary Separation line (PS) line to be on the suction surface, resulting in a smaller LEV. The LEV lifted-off at α = 13◦ from near the crank region, highlighted by the curved streaklines in the surface flow visualizations. LEV-liftoff triggered strong flow oscillations over the outer wing, with a dominant frequency of ≈ 16hz−17hz. Preliminary attempts are made to relate it to the Kelvin-Helmholtz (K-H) instability. These oscillations were limited by the use of a single SSN located at 61% span (outboard of the crank), blowing at β = 105◦ (towards the inner wing), and also generated a nose-down pitch moment. The flow oscillations and the effect of AFC were characterized by stereo-PIV parallel and normal to the surface, hotwire and force balance measurements, along with smoke flow visualization.