The present study focuses on the acoustics of a turbocharger centrifugal compressor from a spark-ignition (SI) internal combustion (IC) engine for passenger car applications. Whoosh noise is typically the primary concern for this type of compressor, which is loosely characterized by broadband sound elevation in the 4 to 13 kHz range. To identify the generation mechanism of broadband whoosh noise, the present study combines three approaches: three-dimensional (3D) computational fluid dynamics (CFD) predictions, experiments, and modal decomposition of 3D CFD results. CFD predictions include four operating points at the (relatively low) 80 krpm rotational speed, spanning from near choke to the peak pressure ratio, along with an additional point at the peak whoosh noise (from experiments) at 140 krpm. Predicted compressor performance, along with noise in the compressor inlet duct agree reasonably well with the corresponding experimental results.
After establishing the accuracy of predictions, flow structures and time-resolved pressures are closely examined in the vicinity of the main blade leading edge. This reveals the presence of rotating instabilities that may interact with the rotor blades to generate noise. An azimuthal modal decomposition is performed on the predicted pressure field to determine the number of cells and the frequency content of these rotating instabilities.
The strength of the rotating instabilities and the frequency range in which noise is generated as a consequence of the rotor-rotating instability interaction, is found to correspond well with the qualitative trend of the whoosh noise that is measured several duct diameters upstream of the rotor blades. The variation of whoosh frequency range between low (4 to 6 kHz at 80 krpm) and high (4 to 13 kHz at 140 krpm) rotational speeds is interpreted through this analysis. It is also found that the whoosh noise primarily propagates along the duct as acoustic azimuthal modes. Hence, the inlet duct diameter, which governs the cut-off frequency for multi-dimensional acoustic modes, determines the lower frequency bound of the broadband noise.