The Influence of Unsteady Acceleration and Turbulence Intensity on Transition in Low-Pressure Turbines

Abstract

Wakes generated by upstream airfoil rows cause an unsteady flow field at the downstream rows in a Low Pressure Turbine (LPT). The pressure gradients and the local turbulence intensity levels experienced by the airfoil surface boundary layers change as the wakes pass through the passage. Laminar-toturbulent transition onset, transition path characteristics and separation and reattachment locations (if the flow separates) also change during the wake passing cycle. In this paper, the effects on transition onset and transition path of unsteady acceleration (composed of spatial acceleration and temporal acceleration), as well as turbulence intensity, are described. Results come from an experiment in which the wakes and passage flow of an LPT are simulated. Spatial acceleration and temporal acceleration are influenced by both the geometry and the instantaneous flow field. Thus, both are dynamic, driven by the change in streamwise momentum associated with the passing wakes. The total acceleration is the sum of the spatial acceleration and the temporal acceleration. Since the boundary layer responds to the instantaneous total acceleration field, this is the term to monitor. However, it is instructive to study its two components separately. Another feature of the wakes which is important to transition is the turbulence field. The local turbulence intensity increases then drops as the wake passes overhead. Deceleration and high turbulence levels, as experienced by the boundary layer as the wake approaches, promote transition whereas acceleration and a low level of turbulence, as experienced as the wake departs, can delay transition. Also, it is well documented that immediately behind a strip of transitional flow (a turbulent strip or turbulent spot), the boundary layer is especially calm (the calmed region), a feature which adds to the dynamics of transition in the LPT. Nomenclature