Organized Streamwise Vorticity on Convex Surfaces With Particular Reference to Turbine Blades

Experiments were conducted on the flow through a transonic turbine cascade and at subsonic speeds past a circular cylinder in cross-flow. These followed extensive work on vortex shedding behind these bodies, displaying the phenomena of energy separation at subsonic speeds; the turbine blades also exhibited exotic vortex-shedding modes in transonic flow. Surface flow visualization was undertaken on the suction surface of the turbine blade and on the circular cylinder. This was effective in providing a time-average mapping of the vortical structures within the blade passage and around the cylinder. The usual phenomena of horseshoe vortices, secondary flows, passage vortices and wall and corner vortices were observed. In addition, and more surprisingly, organized systems of fine-scale streamwise vortices were observed for both cases. Under the influence of the strong favorable pressure gradients on the turbine blade suction surface, the vortices persisted to the trailing edge. For the circular cylinder work, undertaken at an inlet Mach number of 0.5, the streamwise vortices occupied the forward portion of the cylinder, almost to the 83 degree azimuth, and re-appeared after laminar separation. This streamwise vorticity had been predicted and observed previously for low speed flows, with attendant theories for wavelength. The present results have been compared with the predictions giving reasonable agreement.