The present study focuses on the acoustics of a turbocharger centrifugal compressor without casing treatment and with a vaneless diffuser. This is the design adopted for the vast majority of spark-ignition (SI) internal combustion (IC) engines 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 12 kHz range. However, at low rotational speeds, the upper frequency boundary is often observed to reduce to as low as 6 kHz. Earlier experimental results from the compressor of the present study revealed that reversed flow first appeared upstream of the inducer within the mid flow range (at a blade tip incidence angle of 15°), and the peak total sound pressure level in the 4 to 12 kHz range occurred at flow rates just below (incidence angles just above) the onset of inlet reversal. This secondary, recirculating flow structure occurs in an annular region near the periphery.

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. The 3D CFD model is constructed to be identical to the corresponding turbocharger gas stand experimental setup. The mesh of this CFD model, along with the selected solvers and settings are specified to directly predict the unsteady flow field and acoustic propagation throughout the computational domain. Therefore, this direct noise calculation approach considers all aeroacoustic aspects, including noise generation and radiated sound waves. 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 the compressor inlet duct total sound pressure levels in the 0 to 12 kHz and 4 to 12 kHz ranges agree reasonably well with the corresponding experimental results.   

After establishing the accuracy of the predictions, flow structures and time-resolved pressure contours are closely examined at the vicinity of the shear layer between the forward moving core flow and annular reversed region on a plane located 0.5 mm upstream 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 stall 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 12 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.


Generation Mechanism of Broadband Whoosh Noise in an Automotive Turbocharger Centrifugal Compressor

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.