Physical Insight Into Whoosh Noise in Turbocharger Compressors Using Computational Fluid Dynamics

Whoosh is typically the primary noise concern for turbocharger centrifugal compressors without ported shroud recirculating casing treatments, utilized for spark-ignition automotive applications. Whoosh is characterized by broadband elevation of noise in approximately the 4 to 13 kHz range, where the lower frequency boundary is dictated by the cut-on frequency of the first multi-dimensional acoustic mode. At mid to low compressor flow rates, swirling, reversed flow emanates from the leading-edge tip region of the main impeller blades, comprising an annular zone at the inducer plane. High velocity gradients are observed within the shear layer between the bi-directional forward and reverse flow which results in the formation of rotating instability (RI) cells. Whoosh noise is generated due to the interaction of these rotating instabilities with the leading edge of main impeller blades. Along a line of constant rotational speed, whoosh noise exhibits a dome-like character, where its maximum value occurs in the mid to low flow region and the whoosh noise levels decrease at elevated and reduced mass flow rates. The present work includes experimentally validated three-dimensional computational fluid dynamics predictions for five mass flow rates at a fixed rotational speed. These predictions span the constant speed flow range from just below the peak efficiency to near the surge boundary. A modal decomposition is performed on the predicted pressures from a circular array of points near the inducer plane to characterize the modal content of the RI cells at each of the studied flow rates. At the highest flow rate studied (near peak efficiency), flow reversal is present only intermittently near the inducer plane, resulting in weak RI cells and therefore, low levels of whoosh noise. As the flow rate is reduced, sustained flow reversal is present within the annular region near the inducer blade tips, which strengthens the RI cells. For the three lowest flow rates considered, the strength of the RI cells is somewhat similar, but they are characterized by lower mode numbers and frequencies as the flow rate is reduced. This shift is physically interpreted as a reduction in the number of RI cells when moving from peak whoosh noise in the mid to low flow range to lower flow rates approaching the surge line. Due to the shift in mode number and frequency of the RI cells, the modal content of the resulting impeller and RI interaction noise shifts to higher mode numbers and frequencies as the flow rate is reduced. At the peak whoosh noise, the interaction modes occur at frequencies above their cut-off, and thereby propagate upstream through the inlet duct. At the two lowest flow rates, on the other hand, the modal content of the interaction noise increasingly shifts to higher mode numbers and frequencies where they are cut-off. Thus, the computational predictions capture the physical mechanism responsible for the dome-like character of whoosh noise as a function of flow rate at a given rotational speed.