Linear Stability of the flow past a low pressure turbine blade

The present work is concerned with linear BiGlobal instabilities of flow in a cascade of Low-pressure turbine (LPT) blades. In particular the three-dimensional instability of twodimensional time-periodic basic states past a T106/3 blade is investigated using Floquet theory and a high-order spectral/hp-element scheme, used for both the basic flow and the instability calculations. The two-dimensional bifurcation of steady to time-periodic flow has been identified in the framework of earlier work on the present project. The present Floquet analysis shows that the time-periodic flow becomes three-dimensionally unstable soon after the two dimensional transition. Two modes of significance have been identified, associated to the separation bubble and the wake of the basic state. Three-dimensional DNS initialized on the identified Floquet eigenmodes at low amplitudes confirms the results of linear stability analysis. Besides the completion of the eigenvalue-based analyses, first-ever transient-growth results of an essentially nonparallel flow have been obtained. In particular, the pseudospectrum of the flow under consideration has been identified and new mechanisms have been revealed, demonstrating the potential for three-dimensional transition as consequence of transient growth.

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