Investigation of Shear Layer Induced Pressure Fluctuations in a Linear Compressor Cascade

Abstract In axial compressor stages so called rotating instabilities can be detected at aerodynamically highly loaded operating conditions. Multiple explanations have been proposed on the origin of these pressure fluctuations, among others, that rotating instabilities are caused by rotating source mechanisms, periodic vortex structures, periodic changes in the vortices or the bursting of the tip clearance vortex. There is also the idea that they are caused by shear layer instabilities. To investigate the hypotheses, a segment of the rotor of an axial compressor was transferred into a linear cascade in which the Reynolds number is the same and the Mach number also remains below 0.3. In order to achieve the effects of tip clearance and incidence variation, the blade tip section of the axial compressor was selected as a reference profile. In preliminary investigations, a configuration without tip clearance was also found to have spectral patterns similar to rotating instabilities, which supported the hypothesis of shear layer instabilities. To investigate the assumption, a Coandă nozzle was integrated into the endwall upstream of a compressor cascade to induce pressure fluctuations. The injected air creates a defined shear layer with the inlet main air flow. In the first step, a parameter study is carried out in which the position, injection velocity and inflow angle of the Coandă nozzle are varied. Subsequently, selected combinations are applied to different linear cascade configurations. Tip clearance and incidence of the cascade are varied. The main focus of the paper is the comparison between measurements with and without injection through the Coandă nozzle. It is analysed whether the injection can induce, suppress or intensify pressure fluctuations. By analyzing the time-resolved flow in the outlet plane, as well as the use of transient casing pressure measurements in the passage, the identification and occurrence of vortex systems and pressure fluctuations will be investigated.