Interplay between streaks and vortices in shock-boundary layer interactions with conditional bubble events over a turbine airfoil

TL;DR

This study investigates the complex interactions between streaks, vortices, and bubble events in shock-boundary layer interactions over a turbine airfoil using LES data, revealing how these dynamics influence separation bubbles and turbulent stresses.

Contribution

It provides new insights into the conditional behavior of extreme bubble events and the role of streaks and vortices in mass flux and turbulence during shock-boundary layer interactions.

Findings

High-speed streaks penetrate bubbles during contraction, increasing turbulence.

Streamwise vortices induce fluid mixing and affect mass flux during bubble events.

Bubble expansion and contraction are linked to vortex dynamics and fluid entrainment.

Abstract

The shock-boundary layer interaction over the convex wall of a supersonic turbine vane is studied with a focus on extreme separation bubble events and the interplay between the bubble, streaks, and streamwise vortices. The present analysis is performed on a dataset computed by a LES of a supersonic turbine at Ma = 2.0 and Re = 395000. Building on findings related to near-wall streaks and vortices that drive the bubble breathing motion, here, we employ conditional analysis to study extreme bubble events. In events where the recirculation region is small, near-wall high-speed streaks penetrate the bubble and lead to higher tangential Reynolds stresses upstream of the incident shock compared to those with large recirculation. The streaks are accompanied by streamwise vortices that meander and induce intense fluid mixing, leading to higher wall-normal and spanwise Reynolds stresses, and consequently, higher turbulent kinetic energy. This turbulent activity also causes intense fluctuations in wall pressure and skin-friction coefficient along the separation region. In contrast, during events with bubble expansion, high-speed streaks are advected over the separation region, with streamwise vortices appearing only downstream of the shock, resulting in minimal fluid mixing inside the bubble. The analysis of mass flux along the bubble surface reveals that during its contraction phase, mass flux out of the bubble occurs predominantly upstream of the incident shock because of high-speed streaks dragged towards the wall by the streamwise vortices. In the expansion phase, pronounced mass flux into the bubble is observed downstream of the shock, near reattachment. This increased fluid injection is associated with the presence of streamwise vortices and low-momentum flow structures near reattachment, suggesting that fluid entrainment by vortices plays a key role in the mass flux into the bubble.