The structure and instability of a laminar separation bubble in a low-pressure turbine cascade

A direct numerical simulation is performed on a T106A linear cascade at a low Reynolds number (Re) of 1.48×104 to investigate the connection between instantaneous structures and instability mechanisms of the trailing-edge laminar separation bubble. A sequential five-vortex pattern is identified, induced by the secondary vortex, comprising the main separation vortex, secondary vortex, braid-region vortex, shedding vortex, and trailing-edge vortex. Two-dimensional waves are investigated in physical and Fourier spaces using a double Fourier transform. The results reveal that the boundary-layer fluctuations are influenced by perturbations from the adjacent blade, and the separated boundary layer exhibits selective responses to waves of different frequencies. High-frequency fluctuations originating from the upstream attached flow are suppressed near the separation point, whereas the absolute and Kelvin–Helmholtz instabilities primarily stimulate the linear growth of two-dimensional fluctuations within the separated boundary layer. The near-wall absolute instability arises from the periodic motion of the secondary vortex, which is generated in both upstream and downstream portions of the primary separation bubble and subsequently convects downstream. The secondary vortex also contributes to the formation of the braid region and the shedding vortex, where three-dimensional fluctuations are generated via the hyperbolic and elliptical secondary instabilities, respectively. In the trailing-edge region, the shedding vortex is locked with the trailing-edge vortex at the fundamental frequency. The corresponding mode is extracted by the dynamic mode decomposition, which demonstrates that the interaction between the trailing-edge and shedding vortices further amplifies the three-dimensional fluctuations.