The role of streamwise vorticity in flows over turbomachine blade suction surfaces

Streamwise streaks and vortices are frequently encountered in low Reynolds number flows over blading. Observations have shown that, in addition to flows over concave pressure surfaces, convex suction surfaces are also influenced by streamwise vortices. These observations are based on surface flow visualization studies and computational work with highly resolved Large Eddy Simulation. Fine scale organized streaks exist in the laminar regions of turbine and compressor blading and are predictable. For a turbine blade with a blunt leading edge, at Reynolds numbers typical of aircraft cruise conditions, the streamwise vorticity may persist, on a time-average basis, to influence the entire suction surface. Time resolution is required to capture the flow complexity that is fundamental for an understanding of the physical behavior of the laminar boundary layer and its separation and transition. Progress has been made in modeling and predicting transition and laminar separation and the new findings of interesting vortical behavior need to be incorporated. In the leading edge region spanwise vorticity may promote early transition and bubble closure; further downstream streamwise vorticity may become established. The physics of this streamwise vorticity imposes severe requirements on the temporal and spatial resolution of both experimental and computational methods. A narrow spanwise computational strip does not allow the streamwise vorticity to settle into an organized pattern; if it is to become organized, an adequate spanwise domain is required.