Insight into swept wing boundary-layer transition and turbulent flow physics from multi-point measurements

A review and fresh perspective are provided on a series of multi-point surface shear stress (hotfilm) and hotwire velocity measurements obtained within a three-dimensional, swept-wing boundary layer. The multi-point surface shear-stress measurements were performed in both the crossflow and streamwise directions. The measurements were extracted in regions on the wing surface upstream, through, and downstream of the region where the flow transitions from laminar to turbulent flow. Proper orthogonal decomposition (POD), spectra, and spatial correlations were used to identify the presence of flow structures and their spatial evolution. The POD streamwise spatial eigenmodes obtained through transition, track the start and finish of the transition process in the swept-wing boundary layer. The first POD crossflow spatial eigenmode obtained in the laminar region has a wavelength of 12 mm which is consistent with the crossflow vortex spanwise periodicity. The first POD crossflow spatial eigenmode obtained in the turbulent region is not nearly as distinct but a large spatial structure nearly 24mm in span is observed along with some smaller structures as well. This could indicate that through the transition process, two neighboring crossflow vortices are in the process of merging at this streamwise location. The POD was also applied in span to hotwire velocity measurements at 3 mm above the wing in both the laminar and turbulent regimes and through this analysis, the flow structure in the turbulent flow regime has been identified that is very similar to that obtained in the laminar transition zone. This suggests that the initial instability and transition physics may continue to play a key role even after the flow has transitioned to turbulence.