Direct numerical simulations of separation-induced transition in a low-pressure turbine under varying compressibility

The present study investigates the influence of compressibility on separation-induced transition in a low-pressure turbine (LPT) cascade using high-fidelity direct numerical simulations of the T106A blade. Simulations are performed for inlet Mach numbers, Ms ranging from 0.15 to 0.35 at a fixed Reynolds number and high incidence, representative of off-design LPT operation. A dispersion-relation-preserving numerical framework is employed to accurately capture instability waves, separation bubbles, and separation-induced transition to turbulence. A comprehensive analysis is carried out using surface pressure and skin-friction distributions, boundary-layer integral parameters, spectral analyses, and budgets of compressible enstrophy. Increasing Ms systematically reduces the streamwise extent of both leading-edge and trailing-edge separation bubbles and promotes earlier transition and reattachment, consistent with trends observed under increased free-stream disturbances. Despite shorter separation regions, suction-side momentum thickness at trailing edge increases by nearly 350% from Ms=0.15 to 0.35, indicating higher profile losses at elevated Ms. Spectral analyses demonstrate a redistribution of turbulent spatial and temporal scales, with energy injection occurring at progressively larger scales as Ms increases. Flow-field visualizations reveal a transition pathway that shifts from two-dimensional spanwise rolls and intermittent turbulent spots at low Ms to streak-dominated, bypass-like transition at higher Ms. Compressible enstrophy budgets reveal that viscous–compressible coupling and baroclinic mechanisms dominate vorticity dynamics. Thus, compressibility fundamentally alters transition mechanisms and loss generation in LPT flows, underscoring the need for vorticity- and enstrophy-based analyses in addition to conventional boundary-layer characterization.