Swept-wing boundary-layer transition and turbulent flow physics from multipoint measurements

Multipoint surface shear stress and hot-wire velocity measurements were obtained within a three-dimensional, swept-wing boundary layer. Proper orthogonal decomposition, spectra, and spatial correlations were used to identify the presence of flow structures and their spatial evolution. The first proper orthogonal decomposition crossflow spatial eigenmode from the shear stress obtained in the laminar region has a wavelength of 12 mm, which is consistent with the crossflow vortex spanwise periodicity. The first proper orthogonal decomposition crossflow spatial eigenmode obtained in the turbulent region is not nearly as distinct, but a large spatial structure approximately 24mm in span is observed, suggesting that, through the transition process, two neighboring crossflow vortices are interacting with a stronger one, canceling a close weaker one. The proper orthogonal decomposition was also applied in span to hot-wire velocity measurements at 3 mm above the wing surface in both the laminar and turbulent regimes. 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 continue to play a key role even after the flow has transitioned to turbulence.