Transient growth mechanisms of low Reynolds number flow over a low-pressure turbine blade

TL;DR

This paper investigates how transient growth of perturbations can lead to flow transition over low-pressure turbine blades at low Reynolds numbers, using a linear analysis of flow stability.

Contribution

It extends transient growth analysis to higher Reynolds numbers and periodic flows, revealing potential transition routes in turbine blade flows.

Findings

Transient growth suggests plausible transition pathways.

Optimal modes are spatially localized and depend on spanwise wavelength.

Separation in shear layer excites wake modes at longer times.

Abstract

A direct transient growth analysis for three-dimensional perturbations to flow past a periodic array of T-106/300 low-pressure turbine fan blades is presented. The methodology is based on a singular value decomposition of the flow evolution operator, linearised about a steady or periodic base flow. This analysis yields the optimal growth modes. Previous work on global mode stability analysis of this flow geometry showed the flow is asymptotically stable, indicating a non-modal explanation of transition may be more appropriate. The present work extends previous investigations into the transient growth around a steady base flow, to higher Reynolds numbers and periodic base flows. It is found that the notable transient growth of the optimal modes suggests a plausible route to transition in comparison to modal growth for this configuration. The spatial extent and localisation of the optimal modes is examined and possible physical triggering mechanisms are discussed. It is found that for longer times and longer spanwise wavelengths, a separation in the shear layer excites the wake mode. For shorter times and spanwise wavelengths, smaller growth associated with excitation of the near wake are observed.