Experimental study of free-stream turbulence induced transition in an adverse pressure gradient

Results from Particle Image Velocimetry (PIV) investigations have been discussed in order to characterize the dynamics driving the transition process in a strong adverse pressure gradient condition exposed to high Free Stream Turbulence Intensity (FSTI) level. Measurements have been carried out in the suction side boundary layer of a low pressure turbine cascade. Two orthogonal planes are analyzed in order to study both lift-up mechanisms of low speed streaks (in the wall-normal plane) as well as instability mechanisms (in the wall-parallel plane) leading to the breakdown of streaks, thus transition. Proper Orthogonal Decomposition (POD) has been used to provide the reconstruction of the PIV data highlighting the instability motions as well as the occurrence of wall-normal oriented vortical filaments growing on the flanks of the streaks during their breakdown process. The dynamics characterizing the transition process in this strong adverse pressure gradient condition has been discussed in detail and analogies with recent models developed for flat plate are highlighted. The main streak dimensions (e.g. the spacing and wavelength during breakdown) are also compared with flat plate data reported in the literature. Once made dimensionless with the local boundary layer displacement thickness, the streak dimensions observed in the present case agree well with flat plate ones, making evident the self-similarity properties of the streaky structures irrespective of the strong adverse pressure gradient condition.