Insights Into the Unsteady Shock Boundary Layer Interaction

Abstract

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.

Effect of Leading-Edge Erosion on the Performance of Transonic Compressor Blades

“…The gradient difference can be attributed to a slight change in the structure of the lambda shock above the blade surface. Even small deviations in the numerical and experimental boundary layer can lead to a changed shock boundary layer interaction and thus to a changed shock structure, as described by Hergt et al [16]. Furthermore, this position of the gradient can also be used to determine the average position of the shock foot on the blade suction surface.…”

“…The experiments were performed in the Transonic Cascade Wind Tunnel (Hergt et al [16]) at DLR in Cologne. This tunnel is a closed-loop, continuously running facility with a variable nozzle, an upper transonic wall in order to reduce shock reflection, and a variable test section height.…”

The Current Gap between Design Optimization and Experiments for Transonic Compressor Blades

“…The main difficulty throughout this experimental campaign was related to the fact that the measurement techniques employed for this study are not time-resolved and measured only the steady properties of the flow over the cascade. This was complicated because a high level of unsteadiness was expected during the tests from previous experiences at the TGK [7]. The type of unsteadiness observed in these configurations emerges from the strong shock-boundary layer interaction (SBLI) that occurs between the passage shock and the boundary layer on the suction surface of the blade.…”

“…In the past, research institutions and organizations like ONERA [4] and General Motors [5] have designed and operated linear cascade wind tunnels for high-speed flow applications. Today, on the other hand, research projects that advance the fundamental understanding of these types of flows reside in the facilities of just a few research institutions, such as the IMP PAN's Transonic Wind Tunnel [6] and the DLR's own Transonic Cascade Wind Tunnel (TGK) in Cologne [7].…”