Sound generation by entropy waves accelerated by blades

Abstract

Entropy noise is the sound produced by the acceleration of convective temperature fluctuations (entropy waves) by non-uniform flows and is a major contributor to indirect combustion noise. Entropy waves are accelerated by nozzles in rocket engines or by the turbine blade rows in the combustors of gas turbines. While several analytical models have been developed for entropy noise generated by nozzles, not many efficient models exist for the case of blade rows. This is due to the more complex physical mechanisms involved: when entropy waves pass through cascades of blades, sound is produced due to the acceleration and turning of the entropy waves in the interblade region, but also unsteady forces are created in the blades and vorticity is shed from the trailing edge of the aerofoils. All these mechanisms are strongly coupled and affect the generated entropy noise. In this work, we explore numerically the importance of these mechanisms. To this end, we present time- and frequency-domain simulations of entropy waves interacting with several canonical aerofoils. Finally, a simple model is presented to estimate the entropy noise generated by each configuration.