An interferometric study of unsteady passing shock flow in a turbine cascade

A single-plate interferometric system was developed for the evaluation of unsteady flow generated by a passing shock in a linear transonic turbine cascade. A 15 mW Helium-Neon laser was used as the light source. An intensified charge-coupled device (CCD) was used to capture images, allowing the required 100 ns shutter times to “freeze” the passing shock. High frequency Kulite pressure transducers and miniature hot wire probes were used to capture the required known density field for the quantitative evaluation of the flowfield. Variations in the density field data as the shock passes upstream of the studied blade passage are presented, showing the capabilities of this system and detailing the unsteady flow field. The automation of the interferogram interpretation is presented. Image analysis techniques used include adaptive binarization and Hilditch line thinning. Further areas for improvement are also discussed.

In addition to the interferometric study, a boundary layer transition study was also undertaken. Magnified boundary layer spark shadowgraphs were taken on both the pressure and suction surfaces of the turbine blade. Flow conditions including low (<1.0%) and high (5%) free stream turbulence values were investigated. The transition location was found to be dependent not only on the level of turbulence but also on the turbulence characteristics. Further, an investigation was made into the application of hydrodynamic stability theory to turbine blade flows. The results of this evaluation were used in the determination of appropriate length scales for tailoring free stream turbulence to more effectively induce suction surface transition. Little instability and no transition was calculated for the blade suction surface in agreement with the shadowgraph results. Free stream turbulence with frequencies near 40 kHz were found to most affect the instabilities in the boundary layer. Tailored free stream turbulence was found to produce transition on the blade suction surface.