Transition in low-pressure turbines: Effects of unsteady acceleration and turbulence intensity

Wakes generated by upstream airfoil rows create an unsteady flowfield to downstream rows of a low-pressure turbine. Pressure gradients and local turbulence intensity levels imposed on the airfoil surface boundary layers change as the wakes pass through the passage. Separation and laminar-to-turbulent transition onset locations and transition path characteristics also change during the wake passing cycle. The effects of unsteady acceleration (composed of spatial acceleration and temporal acceleration) and time variations in turbulence intensity on transition onset and transition path are described. Results come from an experiment in which the wakes and passage flow of a low-pressure turbine are simulated. The total acceleration field is separated into spatial and temporal components. Also important is the changing turbulence field as affected by passing wakes. Deceleration and high turbulence levels, as experienced by the boundary layer as the wake approaches, promote transition, whereas acceleration and low levels of turbulence, experienced as the wake departs, can delay transition.