Streamwise and crossflow vortical structures on turbine blades and swept cylinders

Flow visualization on a lengthy time-average basis on the suction surface of turbine blades showed robust and consistent streamwise streaks at subsonic and transonic speeds. The normal flow past a circular cylinder is a more canonical case and testing was undertaken at high speeds on a 37.23 mm diameter cylinder and at low speeds on a 152 mm diameter cylinder. The lateral spacing between streaks on cylinders had been predicted by Kestin and Wood and the present tests gave excellent agreement with their theory. Their work on unswept circular cylinders provides a good baseline model for understanding and predicting sweep effects on cylinders and turbomachinery blading. Experiments on a circular cylinder were performed over a range of sweep angles from zero to 61°, giving results for lateral spacing and angular orientation of the streaks. At high sweep angles, the results are consistent with those of Poll. Hot wire measurements away from the surface indicate a variable flow structure in the spanwise direction with a wavelength matching that of the surface traces. The streamwise disturbance was predominantly stationary in nature and resilient, often persisting from leading edge to trailing edge. Crossflow instability becomes more significant at high sweep angles. It grows aggressively and rapidly, being predominantly of a traveling nature. The observed streaks could be of particular concern for the thermal design of turbine blades.