INSIGHTS INTO THE UNSTEADY SHOCK BOUNDARY LAYER INTERACTION

The flow through a transonic compressor cascade is characterized by high unsteadiness due to the shock boundary layer interaction. Investigations in recent years have shown that a detailed understanding of the causes of unsteady shock oscillation is necessary to develop successful approaches to influence it. Therefore, an experimental investigation of the unsteadiness of the shock boundary layer interaction in a transonic compressor cascade has been conducted within the transonic cascade wind tunnel at DLR in Cologne. At an inflow Mach number of 1.05, detailed measurements were carried out with a time-resolved PIV system in combination with a high-speed shadowgraphy setup. In this way it was possible to simultaneously measure both the shock movement and the flow field of the boundary layer under the shock. The analysis of the measured data showed a correlation between the oscillation behaviour of the passage shock and the unsteady flow behaviour within the boundary layer in front of the shock. In the shock oscillation spectra a dominat frequency at 1683 Hz and their first harmonic was found. This frequencies are also be found in the boundary layer flow below and in front of the shock with different amplitutes at three analysing points in the measured PIV Region. A detailed analysis of the measured data shows that the information of the unsteady shock oscillation propagates under the shock foot over the boundary layer upstream. It becomes clear that the propagation of the oscillating pressure information has an influence on the velocity component normal to the blade surface. This leads to a oscillating flow angle close to the blade. Through this effect, the inflow in itself interacts with the shock front and influences the shock position and structure. Based on this, a new thesis of self-exciting shock oscillation is developed. In addition, the used time-resolved PIV measurement enables an acquisition of the blade vibration behaviour. Within the results of the blade vibration four Eigenmodes are observable. In this context it has been shown that the Eigenmodes of the blades are not stimulated by the flow. On the other hand there is also no exciting interaction of the blades with the flow detectable. The measured data of transonic flow within a compressor cascade presented here are unique and provide new insight into shock movement and interaction with the boundary layer.