Silencer for high-frequency turbocharger compressor noise via an acoustic straightener

Decades of successful research and development on automotive silencers for engine breathing systems have brought about significant reductions in emitted engine noise. A majority of this research has pursued airborne noise at relatively low frequencies, which typically involves planar wave propagation. However, with the increasing demand for downsized turbocharged engines in passenger cars, high-frequency compressor noise has become a challenge in engine induction systems. Elevated frequencies promote multi-dimensional wave propagation rendering at times conventional silencer treatments ineffective due to the underlying assumption of one-dimensional wave propagation in their design. The present work focuses on developing a high-frequency silencer that targets tonal noise at the blade-pass frequency within the compressor inlet duct for a wide range of rotational speeds. The approach features a novel "acoustic straightener" that creates exclusive planar wave propagation near the silencing elements. An analytical treatment is combined with three-dimensional (3D) acoustic finite element method to guide the early design process. The effects of mean flow and nonlinearities on acoustics are captured by 3D computational fluid dynamics (CFD) simulations which are then utilized to introduce geometry modifications that reduce flow losses and suppress noise generation due to flow-acoustic coupling. The effectiveness of the silencer in the presence of mean flow is demonstrated by computing the transmission loss across the configuration from 3D CFD predictions.