Session: 41-07 Turbine Secondary Flows and Interactions II

Paper Number: 102213

102213 - Direct Numerical Simulation of a High-Pressure Turbine Stage: Unsteady Boundary Layer Transition and the Resulting Flow Structures 

Periodically passing wakes, together with incoming freestream turbulence (FST), interact with turbomachinery blades and may promote laminar-turbulent transition on the blade suction side. The unsteady transition in turbomachinery flows is of key importance to machine performance, as it governs the development of boundary layers, determining the onset of turbulent flows which induce much stronger wall friction and heat flux compared to laminar flows.

 

In the present study, we investigate the unsteady boundary layer transition based on the direct numerical simulation database of an HPT stage (Zhao and Sandberg, GT2021-58995), in which the HPT stage operates at engine-relevant conditions and turbulent fluctuations with different characteristics are introduced at the inlet. Previous work (Leggett et al., GT2022-82370) showed that the flow statistics are affected by the characteristics of FST and also the unsteady behavior of upstream wakes. Nevertheless, the underlying physical mechanisms of boundary layer transition are still unclear and need to be further investigated. Consequently, in the present study, we focus on the unsteady transition mechanism on the rotor blade, which is affected by the incoming periodic wakes and also the background FST. According to phase-lock averaged statistics, it is shown that the periodic wakes have a direct impact on the suction side boundary layer by greatly changing the friction and heat flux at the wall. In the separation region which periodically occurs on the blade suction side, the mean velocity profiles are inflectional due to an adverse pressure gradient in the tangential direction. The flow separation can be suppressed by the interaction between the incoming wake and the blade, which is mainly because the wake can induce an early transition in the suction side boundary layer. Based on the detailed analysis of the instantaneous flow structures, the incoming wake initially interacts with the leading edge, leading to formation of Klebanoff-type streaks in the blade boundary layer. Thereafter, the wake passes downstream and causes the earlier appearance of turbulent spots in the suction-side boundary layer. The breakdown mechanism responsible for the wake-induced turbulent spots seems to follow the path of the well-known varicose mode. Subsequently, the boundary layer flow turns turbulent, followed by a calm region as the wake travels away from the passage. It is also noted that the recovery to a calm region can be impacted by the FST. To be specific, the calm region in case with no FST is much longer compared to cases with stronger FST. The insights gained here are of great importance to both fundamental researchers and designers, and provide guidelines to accurately predict transition onset, to understand the flow physics of transition, and to even control transition.

Presenting Author: Richard D. Sandberg University of Melbourne

Presenting Author Biography: Richard is the Chair Professor of Computational Mechanics in the Department of Mechanical Engineering at the University of Melbourne. His main interest is in high-fidelity simulation of turbulent flows in turbomachinery and aerospace applications in order to gain physical understanding of flow and noise mechanisms. He also uses the data to help assess and improve low-order models that can be employed in an industrial context, in particular by pursuing novel machine-learning approaches.

He received his PhD in 2004 in Aerospace Engineering at the University of Arizona and prior to joining the University of Melbourne, he was a Professor of Fluid Dynamics and Aeroacoustics in the Aerodynamics and Flight Mechanics research group at the University of Southampton and headed the UK Turbulence Consortium (www.turbulence.ac.uk), coordinating the work packages for compressible flows and flow visualisations and databases. He was awarded a veski innovation fellowship entitled: "Impacting Industry by enabling a step-change in simulation fidelity for flow and noise problems" and has been granted an Australian Research Council Future Fellowship for 2020-2023. He is the Program Lead for Power Generation and Transport at the Melbourne Energy Institute.

Authors:

Taiyang Wang Peking University
Yaomin Zhao Peking University
John Leggett University of Melbourne
Richard D. Sandberg University of Melbourne