Spanwise Growth of the Turbulent Spot Pressure-Fluctuation Field in a Hypersonic Boundary Layer

The pressure-fluctuation field beneath wave packets and turbulent spots in a hypersonic boundary layer was studied on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. For a controlled study, the breakdown of disturbances created by spark perturbations was measured at various freestream conditions. A disturbance first grows into a linear instability wave packet and then quickly becomes nonlinear. At this point, the wave packet is still concentrated near the disturbance centerline, but weaker disturbances are seen spreading from the center. Breakdown to turbulence begins in the core of the wave packets where the wave amplitudes are largest. Second-mode waves are still evident in front of and behind the breakdown point and can be seen propagating in the spanwise direction at a spreading angle. The turbulent core grows downstream resulting in a turbulent spot with a typical arrowhead shape. However, the spot is not merely a localized patch of turbulence; instability waves are still an integral part of the disturbance.