Analysis of the Reynolds stress component production in a laminar separation bubble

Proper Orthogonal Decomposition has been applied to Time-Resolved Particle Image Velocimetry data describing the dynamics of laminar separation bubbles. The mutual orthonormality of the POD modes of the velocity components has been accounted for to separate the contributions to the Reynolds stress tensor due to the different modes, thus to the stress production and the mean flow energy dissipation. The low frequency motion of the separated shear layer, the shedding phenomenon and the formation of finer scales in the rear part of the bubble have been clearly isolated, and their role in the turbulence production identified by means of reduced order models. The low frequency activity observed in the fore part of the separated flow region drives the turbulence production through the normal strain mechanism. Only in the rear part of the bubble the high shear between adjacent vortices establishes the more common shear strain production mechanism, that definitively dominates the transition process. A limited number of modes captures almost the whole process responsible for stress production, even though both Reynolds number and free-stream turbulence intensity levels affect the number of modes involved in the stress generation for different dynamics.