Role of Tip Clearance Flow in Rotating Instabilities and Nonsynchronous Vibrations

Single- and multiple-blade-passage simulations of an isolated subsonic axial compressor rotor show that flow oscillations in the tip region, known as rotating instabilities and a driver for nonsynchronous vibrations, occur when only one of the two criteria for short-length-scale rotating stall inception is satisfied. This criterion is tip clearance backflow below the trailing-edge blade tip. The flow oscillations associated with rotating instabilities most likely result from impingement of this tip clearance backflow on the rear pressure side of the blade. This phenomenon could plausibly be modeled with an impinging jet subject to a lateral pressure gradient and lateral shear flow. The findings have important practical implications on the prediction and suppression of nonsynchronous vibrations.