Breakdown of tip clearance vortices in periodically unsteady flow

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This project shall contribute to a better understanding of the complex interaction of various flow phenomena in highly loaded axial compressors. While the sequence of rotors and stators itself already leads to an influence of the upstream wakes onto the flow around a subsequent blade, additional inherent secondary flows contribute significantly to the performance and flow stability of the compressor stages. A major impact stems from the flow across the gap between the profiles and the hub respectively casing of the flow channel, which is becoming more and more important due to the high loading of todays profiles. The results are strong systems of vortices, interacting with the main flow. In preceding research projects in an annular wind tunnel at the chair of aero engines, it was found, that especially the breakdown of these vortices in itself leads to severe changes of the flow within the cascade und thus its performance in terms of flow stability, blockage and pressure increase. The objective of the proposed project is the determination of the impact of periodical wakes coming from an upstream rotor onto the formation of the existing vortex systems and the resulting flow pattern. The wind tunnel is extremely modular and allows for the integration of a rotor featuring bars with different diameters and running at variable rotational speeds. Thus a direct comparison between the steady and the unsteady flow within the cascade becomes possible. Further on, it can be assessed, if and how the incoming wakes lead to fortification respectively damping of the breakdown of the vortices caused by the leakage flow across the profiles gaps. The variability of the rotor configuration can be used to study the effect of different parameters on the performance of the cascade. The investigations will be performed by means of state-of-the-art measurement technology, including stereo-PIV-systems, hot wire anemometry and piezoresistive sensors. Due to their strong time-resolving capabilities, these systems are highly suitable for this type of flows and are well established at the institute.