Transition on the T106 LP Turbine Blade in the Presence of Moving Upstream Wakes and Downstream Potential Fields

Boundary layer measurements were performed on a cascade of the T106 high lift low-pressure (LP) turbine blades that was subjected to upstream wakes and a moving downstream potential field. Tests were carried out at a low level of inlet freestream turbulence (0.5%) and at a higher (4.0%). It is found that perturbations in the freestream due to both disturbances are superposed on each other. This affects the magnitude of the velocity perturbations at the edge of the boundary layer under the wakes as well as the fluctuations in the edge velocity between the wakes. Furthermore, the fluctuations in the adverse pressure gradient on the suction surface depend on the relative phase of the upstream and downstream disturbances, providing an additional stimulus for clocking studies. Time-mean momentum thickness values calculated from laser Doppler anemometry (LDA) traverses performed near the suction surface trailing edge are used to identify the optimum relative phase angle of the combined interaction. Unsteady suction surface pressures, quasiwall shear stress and LDA data illustrate the resulting multimode process of transition, which is responsible for the observed clocking effects. The optimum relative phase angle of the upstream wake and the downstream potential field can produce 0.25% of efficiency improvement through the reduction of the suction surface boundary layer loss. This reduction is mainly related to the calmed region and the laminar flow benefits that can be more effectively utilized than when only the upstream wakes are present. During the remaining parts of the cycle, the features that are usually associated with the wake and the potential field effects are still present.