Time Resolved Stereo Particle Image Velocimetry Measurements of the Instabilities Downstream of a Backward-Facing Step in a Swept-Wing Boundary Layer

Time-resolved particle image velocimetry (TRPIV) measurements are performed downstream of a swept backward-facing step, with a height of 49% of the boundary-layer thickness. The results agree well qualitatively with previously reported hotwire measurements, though the amplitudes of the fluctuating components measured using TRPIV are higher. Nonetheless, the low-amplitude instabilities in the flow are fairly well resolved using TRPIV. Proper orthogonal decomposition is used to study the development of the traveling crossflow and Tollmien-Schlichting (TS) instabilities downstream of the step and to study how they interact to form the large velocity spikes that ultimately lead to transition. A secondary mode within the traveling crossflow frequency band develops with a wavelength close to that of the stationary crossflow instability, so that at a certain point in the phase, it causes an increase in the spanwise modulation initially caused by the stationary crossflow mode. This increased modulation leads to an increase in the amplitude of the TS mode, which, itself, is highly modulated through interactions with the stationary crossflow. When the traveling crossflow and TS modes align in time and space, the large velocity spikes occur. Thus, these three instabilities, which are individually of low amplitude when the spikes start to occur (urms/Ue <0.03), interact and combine to cause a large flow disturbance that eventually leads to transition.