Rotating instabilities in an axial compressor originating from the fluctuating blade tip vortex

Rotating instabilities (RIs) have been observed in axial flow fans and cent rifugal compressors as well as in low-speed and high-speed axial compressor s. They are responsible for the excitation of high amplitude rotor blade vi brations and noise generation. This flow. phenomenon moves relative to the rotor blades and causes periodic vortex separations at the blade tips and a n axial reversed flow through the tip clearance of the rotor blades. The pa per describes experimental investigations of RIs in the Dresden Low-Speed R esearch Compressor (LSRC). The objective is to show, that the fluctuation o f the blade tip vortex is responsible for the origination of this flow phen omenon. RIs ha ve been found at operating points near the stability limit o f the compressor with relatively large tip clearance of the rotor blades. T he application of time-resolving sensors in both fixed and rotating frame o f reference enables a detailed description of the circumferential structure and the spatial development of this unsteady flow phenomenon, which is lim ited to the blade tip region. Laser-Doppler-anemometry, (LDA) within the ro tor blade passages and within the tip clearance as well as unsteady pressur e measurements on the rotor blades show the structure of the blade tip vort ex. It will be shown that the periodical interaction of the blade tip vorte x of one blade with the flow at the adjacent blade is responsible for the g eneration of a rotating structure with high mode orders, termed a rotating instability.