Base Pressures and Energy Separation in Transonic Turbine Blading

This paper concerns unsteady near-wake flows on, and close to, the thick trailing edges of turbine blades, circular cylinders and similar bodies. Subsonic surface base pressures, and Eckert-Weise energy separation in the wake, are principal manifestations of the same phenomenon. Both are a direct result of von Kármán vortex shedding. The subsonic flow past a turbine blade having a thick trailing edge is still not well-predicted and this results from a lack of understanding of the flow past the trailing edge and into the wake. It is here argued that von Kármán vortex shedding is the principal cause of the subsonic base pressure deficit and the related energy separation in the wake. Parallels can be found in the behaviour of elastically-mounted circular cylinders and the caudal fin oscillation propelling fish. These should also affect supersonic flows although the physical causes are different. At supersonic speeds the trailing edge base pressure, and the energy separation in the downstream wake, exhibit different characteristics from the subsonic behavior and need to be treated differently. For supersonic flows, shock waves from a blade trailing edge may impinge on the adjacent suction surface adversely affecting the downstream boundary layer. Supersonic flows most often involve shock and expansion waves. Exotic vortex shedding also has an important role to play. In addition to experimental observation, the guidance of an analytical framework is needed. The eventual goal is accurate computational prediction for validation of computer models and prediction of flow behaviour.