Spanwise-periodic roughness that was designed to excite selected wavelengths of stationary cross-flow modes was investigated in a 3-D boundary layer at Mach 6. The test model was a sharp-tipped 14-degree right-circular cone. The model, along with an integrated total pressure sensor traversing system, was placed in the United States Air Force Academy’s Mach 6 Ludwieg tube. The model was oriented at a 6-degree angle of attack to produce a mean cross-flow velocity component in the boundary layer over the cone. Passive surface roughness, consisting of indentations (dimples) was evenly spaced around the cone at an axial location that was just upstream of the linear stability neutral growth branch for stationary cross-flow modes. Two azimuthal mode numbers of the patterned roughness were examined. One had an azimuthal mode number that was in the band of initially amplified stationary cross-flow modes (‘critical’). The other azimuthal mode number was designed to suppress the growth of the initially amplified stationary cross-flow modes and thereby increase the transition Reynolds number (‘subcritical’). The results showed that the stationary cross-flow modes were receptive to the passive patterned roughness. Spatial amplitude distributions of the traveling fluctuations for the ‘critical’ and ‘subcritical’ roughness cone tips showed evidence of a nonlinear interaction between the linearly amplified stationary and traveling cross-flow modes.
Observation of this nonlinear phenomenon motivated the next part of the research effort to further investigate the stationary/traveling cross-flow mode interactions with the same experimental conditions, but with a dimpled plasma cone tip. This cone tip consisted of an active flow control mechanism; a plasma actuator that was located just downstream of the ‘critical’ 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. As a result, the nonlinear interaction between the stationary and traveling cross-flow modes was further enhanced by the excitation of the traveling cross-flow mode.