Streamwise Vorticity, Laminar Separation and Transition in Flows over Turbomachinery Blades

Abstract

In low Reynol ds number fl ows over turbomachine bl ades streamwise vorticity is endemic. Recent observations have shown that, in addi tion to streamwise vorticity on the concave pressure surface, the convex suction surfaces are also influenced by streamwise vortices. These observati ons are based on surface flow visualization studies and computational work with highly resol ved Large Eddy Simulati on (LES ). Fine scale organized streamwise vorticity exists in the laminar regions of turbine and compressor blading and is predictable. For a turbine blade with a blunt leading edge the streamwise vorticity may persist on a time-average basis to influence the entire suction surface at suitably low Reynol ds numbers typical of aircraft cruise conditions. LES results emphasize the enormous computing resource required to resol ve the flow on a routine basis for design purposes. It is demonstrated computationally that streamwise vorticity interacts with spanwise vorticity in leading edge bubbles to promote early transition and bubble closure. Time resolution is required to capture the flow complexity that is fundamental for an understandi ng of the physical behavior of the lami nar boundary layer and its separati on and transition. Long duration ti me averaging is also required if the stable vortex organizati on seen in experi mental work is to be simulated. Computing over a narrow s panwise strip does not allow the streamwise vorticity to settle into the organized pattern. For streamwise vorticity to become org anized, an adequate spanwise domain and run duration for time averaging are both essential. Progress has also been made in the modeling and prediction of lami nar separation and boundary layer transition. This is allowed for in a pre diction method that takes account of the longitudinal merging of turbulent s pots. Such a procedure is essential as laminar separation usually occurs as a result of strong adverse pressure gradients and predictions are needed that incorporate the transition physics that occurs under these conditions. An accurate treatment of laminar boundary layers at low Reynol ds numbers needs to be performed three di mensionally and with a sufficiently fine spanwise spacing and run duration to resolve streamwise vortical structures.