Velocity measurements of wake-induced unsteady flow in a linear turbine cascade

Extensive velocity measurements have been taken in a linear turbine cascade with unsteady oncoming wakes. The unsteady wakes were generated by moving cylinders on a squirrel cage device. The Reynolds number was 1.1 × 10⁵, and the Strouhal number varied from o to 7.36. The blade-to-blade flow and the boundary layers on the suction side were measured with a hot-wire anemometer. The results were obtained in ensemble-averaged form so that periodic unsteady processes can be studied. Of particular interest was the transition of the boundary layer. The boundary layer remained laminar in the case without wakes. The passing wakes caused transition, and the beginning of transition moves forward as the wake-passing frequency increases. Unlike in the flat plate study of Liu and Rodi (1991a) the boundary layer state hardly changed with time, although the turbulence level in the boundary layer showed clear periodic response to the passing wakes. The work reported here was sponsored by the German Federal Ministry of Research and Technology through program TURBOTHERM under contract no. 0326501D. The authors should like to thank Mr. D. Bierwirth for his excellent technician work on this project, Dr. N. H. Cho for his help with the preparation of the plots and Mrs. R. Zschernitz for her expert typing of the text.