Controlled Cross-Flow Mode Interaction in Mach 6 Boundary Layer

Abstract

Experiments were performed to investigate a quadratic interaction between stationary and traveling cross-flow modes in the boundary layer on a sharp right-circular cone at an angle of attack at Mach 6.0. Passive discrete roughness was applied near the cone tip, just upstream of Branch I of the linear stability neutral curve to excite stationary cross-flow modes. A plasma actuator was located just downstream of the discrete roughness array. This was designed to produce an azimuthally uniform unsteady disturbance with a frequency that was at the center of the band of most amplified traveling cross-flow modes. Measurements consisted of off-wall azimuthal profiles of mean and fluctuating total pressure at different axial locations. Spectra of total pressure fluctuations verified the receptivity of the boundary layer to the unsteady excitation. This affected the azimuthal and streamwise development of the stationary cross-flow modes, with a general effect to move the transition location upstream by 9%. The quadratic interaction between the stationary and traveling cross-flow modes was verified through cross-bicoherence analysis. This presents a scenario where traveling cross-flow modes whose initial amplitudes are sensitive to free-stream disturbance levels, can interact with stationary cross-flow modes to produce rapid spectral broadening and accelerated turbulent transition.