Aerodynamic Stability Investigations of an Annular Compressor Cascade Based on Unsteady Pressure Measurements

The use of larger more flexible blades in turbomachines requires detailed knowledge about flutter phenomena especially for compressor cascades in transonic flow where vibrating shocks may drastically change the aerodynamic behaviour. In order to investigate the aerodynamic behaviour, basic experiments were performed to measure unsteady pressure distributions for subsonic and transonic flow conditions. Experimental investigations on a two-dimensional section of a compressor cascade were carried out at EPF Lausanne in the annular wind tunnel for non-rotating cascades. In this wind tunnel real flow conditions are simulated by using a spiral flow. The twenty blades of the cascade were mounted on elastic spring supensions which allowed for torsional motion about the midchord. For the measurement of the unsteady pressure distributions, the blades were excited to harmonic torsional vibrations in all possible twenty travelling wave modes of the cascade. In order to ensure precise aerodynamic stability investigations, it is essential to know the complete unsteady pressure distribution around the profile. In this cascade piezoelectric pressure transducers were mounted close together around the blades in order to resolve pressure variations caused by a vibrating shock. These measured pressure distributions enable the evaluation of local stability/instability contributions using the local work coefficient and furthermore allow an integration into reliable global stability parameters such as the out-of-phase component of the unsteady moment coefficients. The influence that front shock on the suction side as well as the blade channel shock have on the stability behaviour is shown for the transonic case