Boundary layer transition induced by surface roughness distributed over a low-pressure turbine blade

TL;DR

This study uses direct numerical simulations to systematically analyze how distributed surface roughness affects boundary layer transition on a low-pressure turbine blade, revealing complex interactions between roughness size, distribution, and flow phenomena.

Contribution

It provides a comprehensive analysis of boundary layer transition mechanisms induced by distributed roughness on turbine blades, highlighting the roles of roughness height and streamwise wavenumber.

Findings

Large roughness heights cause violent disturbances and maintain turbulence across the blade surface.

Small roughness heights can suppress transition until near the trailing edge due to pressure gradients.

Larger streamwise slope of roughness promotes earlier transition through shear layer instability.

Abstract

Direct numerical simulations of a low-pressure turbine with roughness elements distributed over the blade surface have been performed. A series of fifteen cases with varying roughness heights and streamwise wavenumbers are introduced to present a systematic study of the effect of roughness on the various transition phenomena in the suction-side boundary layer. For cases with large roughness heights, the boundary layer is violently disturbed by the wake of rough elements in the leading edge (LE) region, and maintains the turbulent state over the whole blade suction-side. For cases with small roughness heights, however, the disturbances induced by the LE roughness are suppressed by the favourable pressure gradient in the downstream boundary layer, and the relaminarized flow does not undergo transition until the separation near the blade trailing edge (TE). Furthermore, the streamwise wavenumber of the distributed roughness plays an important role in cases with intermediate roughness height. Specifically, cases with larger streamwise slope show earlier transition induced by strong shear layer instability, which manages to suppress the mean flow separation near the TE region. Overall, the combined effect of several factors, including the geometric effect at the blade LE and TE, the complex pressure gradient distribution across the turbine vane, and the various roughness configurations, is responsible for the intriguing boundary layer behaviours in the present study.