Tomographic PIV investigation of crossflow instability of swept wing boundary layers

The boundary layer evolving on the pressure side of a 45° swept wing at Re = 2.17 · 106 is experimentally investigated. This flow is characterized by laminar-turbulent transition, dominated by crossflow instability. This mechanism manifests, in low freestream turbulence flows, as stationary waves aligned approximately with the flow direction. These waves grow along the chord and deeply modify the boundary layer causing the destabilization of secondary high frequency vortices. The boundary layer at the location where primary modes saturation occurs, has been investigated here with hotwire anemometry and tomographic PIV. The optical technique allows the simultaneous measurement of all the velocity components within a volume of fluid. The comparison with the hotwire scans shows a very good match. The possibility of applying reduced order analyses based on the flow spatial coherence, such as proper orthogonal decomposition, led to the first experimental description of the spatial arrangement of the secondary instability modes under natural flow conditions.