Stationary and Traveling Vortical Structures on Swept Cylinders and Turbine Blades

Abstract

Suction surface flow visualization on turbine blades at subsonic and transonic speeds showed robust streamwise streaks on a lengthy time-average basis. The normal flow past a circular cylinder is a more canonical case and testing was undertaken at high speeds on a 38 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. Although their work was related to unswept circular cylinders it also provides an excellent benchmark for sweep effects on cylinders and turbomachinery blading. The observations of streaks on turbine blades and unswept cylinders provided a firm basis for referencing the influence of sweep. Experiments on a circular cylinder were performed over a range of sweep angles from zero to 61 o giving results for lateral spacing and angular orientation of the streaks. At high-sweep angles the results are consistent with those of Poll. The introduction of sweep brings consideration of a wide range of instabilities; streamwise and crossflow structures are present on the suction surface of turbine blades. Although the available information on fine structures comes from surface flow visualization, work is now progressing on hot wire measurements away from the surface. The aim is to demonstrate the relationship between the structures and the surface traces. Analysis of the data is complemented by ongoing theoretical work. It is also hoped to provide information on the changing behavior of the vertical structures as the sweep angle is increased. The streamwise disturbance in the unswept case was found to be stationary in nature and to be resilient, often persisting from leading edge to trailing edge. Crossflow instability becomes more significant as sweep is increased. It grows aggressively and rapidly, being predominantly of a traveling nature, and has a major role to play in the transition process. The observed streaks could be of particular concern for the thermal design of turbine blades. It is hoped to give designers confidence about the flow regimes they might anticipate for a given sweep angle and particularly of when and how the aggressive crossflow instability mode is likely to be encountered. Nomenclature