Experimental investigation of transition as applied to low pressure turbine suction surface flows

Results are presented of an experimental study of separation and transition within the flow over the suction surface of a low-pressure turbine airfoil. Detailed velocity profiles, measured in the near-wall region with the hot-wire technique, and surface static pressure distributions are presented. Flow transition is documented using measured intermittency distributions in the attached boundary layer and within the separated shear layer. Cases for Reynolds numbers based on exit velocity and suction surface length of 50,000, 100,000, 200,000, and 300,000 under low Free Stream Turbulence Intensity (FSTI = 0.5%), moderate-FSTI (2.5%), and high-FSTI (10%) are reported. Cases of FSTI = 2.5%, which, due to wakes, are most representative of low-pressure turbine flows, are discussed in detail. Comparisons are made for cases of differing Reynolds numbers and FSTI values. Flow separation, with transition of the shear layer over the separation bubble, is observed for the lower-Re cases. Enhanced transport after flow transition reduces the separation bubble size and eventually accelerates the near-wall flow to attached boundary layer status. Elevated FSTI and increased Re promote earlier transition, smaller separation bubbles, and an increased possibility that the boundary layer will remain attached and transition as such. Models for intermittency distribution, transition onset location, and transition length are assessed.