Centrifugal compressors for automotive turbochargers must operate over wide speed and flow ranges to provide the required air pressure and mass flow rate to the intake manifold of the internal combustion engines. At a fixed rotational speed, the flow field near the inducer of the impeller becomes increasingly unstable with decreasing flow rate, as the incidence angle grows between the air flow approaching the impeller, relative to the tangent of the main impeller blades at the leading edge. Flow field measurements conducted earlier have revealed that once the incidence angle exceeds a critical value (nearly independent of rotational speed) of approximately 15°, reversed flow near the periphery (blade tips) starts penetrating upstream of the impeller, with a high tangential velocity in the direction of impeller rotation. As the incidence angle is increased towards this critical value, whoosh noise elevates, where it remains high for a significant portion of the mid-flow operating range, before decreasing at further elevated incidence angles. To understand this phenomenon further, a detailed, three-dimensional (3D) computational fluid dynamics (CFD) model of the experimental setup was constructed, and simulations were completed at four flow rates along a constant rotational speed. Predictions from this 3D CFD model agree reasonably well with experiments, including the steady-state performance, time-averaged flow field, and noise as captured from the pressure transducer installed in the compressor inlet duct. Near the peak whoosh noise of the studied speed, the impeller flow field was closely examined. Predictions reveal the highest total sound pressure level in the whoosh frequency range occurs near the inducer plane, within the shear layer between the concentric, bi-directional flow structure, with forward flow closer to the axis and reversed flow around the periphery.
