The Influence of Multimode Transition Initiated by Periodic Wakes on the Profile Loss of a Linear Compressor Cascade

Abstract

This paper presents results of boundary layer and loss measurements in a high speed cascade wind tunnel on a linear compressor cascade under the influence of unsteady, periodic wakes. The wakes of an upstream blade row were simulated by cylindrical bars moved by a belt mechanism upstream of the cascade. Extensive hot-film array, hot-wire and pressure measurements with variation of steady and unsteady inlet flow conditions have been performed for a better understanding of the transition and loss mechanisms on a blade row interacting with wakes. The incoming wakes are inducing early forced transition in the boundary layer followed in time by calmed regions. Due to its higher shear stress level and its fuller velocity profile, the calmed flow is able to suppress laminar separation bubbles and to delay transition in the region with undisturbed flow between wakes, playing a significant role in the loss generation process. At the investigated low Reynolds number, where the measurements for the steady flow case showed a well-developed laminar separation bubble, reductions of profile loss up to 20% were observed for the measured configuration. In the case of the high Reynolds number, where in undisturbed flow only a small separation bubble was detected, a profile loss rise up to 30% was measured. Beside a better understanding of unsteady flow physics the goal of these basic investigations of unsteady transition is to create a wide database for the improvement of transition modeling in unsteady CFD codes.