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 4-13 kHz range, where the lower frequency boundary is dictated by the cut-on frequency of the first multidimensional 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 near the shear layer between the bidirectional forward and reverse flow which results in the formation of rotating instability 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 levels decrease at elevated and reduced mass flow rates. To provide insight into the variation of broadband whoosh noise with flowrate, this work includes experimentally validated 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. Computational predictions capture the physical mechanism responsible for the dome-like character of whoosh noise as a function of flowrate at a given rotational speed.