Fluid dynamic and thermal characterization of a turbulent shear layer

Planar particle image velocimetry (PIV) and hot-wire anemometry are used to study the turbulent shear layer that forms naturally from an initial laminar state due to velocity differences between a jet and its surrounding quiescent region. Spectral Proper Orthogonal Decomposition (SPOD) applied to PIV data reveals the presence of dominant large-scale coherent structures in the post-transition turbulent shear layer. These structures span the full thickness of the shear layer and account for approximately 30% of the total turbulent kinetic energy. They are associated with a Strouhal number of approximately 0.2, similar to that of Kelvin–Helmholtz instabilities typically observed in pre-transition regimes. This observation raises questions about the underlying growth mechanism: while the elongated spatial patterns are consistent with continuous structure growth, the Strouhal number suggests a behaviour more typical of pairing-dominated dynamics. The estimated structure spacing supports the hypothesis of a nearly constant spacing-to-thickness ratio. The thermal shear layer, characterized with CCA and CVA cold wires, was shown to exhibit self-similarity. The growth rate of the thermal shear layer differs from that of the velocity shear layer. New analytical formulations for universal non-dimensional profiles of mean temperature and standard deviation are presented.