Separation induced transition in a low pressure turbine under varying compressibility

TL;DR

This study examines how increasing Mach number affects flow separation and transition to turbulence in a low pressure turbine blade, revealing earlier transition, altered turbulence scales, and increased profile losses with higher compressibility.

Contribution

It provides high-fidelity numerical insights into the effects of compressibility on separation-induced transition in turbine flows, highlighting changes in transition mechanisms and turbulence characteristics.

Findings

Higher Mach numbers shorten separation bubbles and promote earlier transition.

Increased Ms leads to larger turbulent energy scales and altered transition pathways.

Elevated Ms results in higher profile losses despite shorter separation regions.

Abstract

The present study investigates influence of compressibility on separation induced transition in a low pressure turbine 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 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 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.