Time-resolved PIV investigation of the secondary instability of cross-flow vortices

Time-resolved PIV measurements of the secondary instability modes of cross-flow vortices are presented. Measurements are performed on a large scale 45o swept wing at chord Reynolds number of 2.17 million in a low turbulence wind-tunnel facility. Using acquisition frequencies of 20 kHz, the present study is the first experimental demonstration of spatio-temporally resolved measurements of these structures. Statistical and spectral analysis reveals a fluctuating velocity field, strongly conditioned in space by the primary stationary cross-flow vortex. The flow structures related to the type-I high-frequency instability and type-III are captured by Proper Orthogonal Decomposition of the instantaneous flow-fields. Their temporal evolution is analysed showing good agreement with previous studies thus confirming that POD is correctly representing the flow structures of the relevant instability modes. The low frequency meandering oscillation of the stationary vortices, first reported by Serpieri &Kotsonis (2016b), is observed and characterised.