An experimental modal decomposition method to compute sound power of multi-dimensional acoustic waves from turbocharger compressors

The automotive turbocharger compressor in the present experimental study features a ported shroud casing treatment, which is known to elevate tonal noise at the blade-pass frequency (BPF) while reducing broadband whoosh noise and providing higher boost pressures at low mass flow rates. The high operating rotational speeds of such modern turbocharger compressors push the BPF to ranges where acoustic wave propagation is multi-dimensional within the compressor ducting. Simultaneously propagating acoustic modes at the BPF result in strong circumferential and axial variation of in-duct sound pressure levels. This poses a challenge for acoustic characterization and comparison of different hardware since typical measurement techniques do not consider the sensitivity of acoustic pressure to the measurement location. The current work utilizes a steady-flow turbocharger gas stand with a unique rotating compressor inlet duct fitted with multiple wall-mounted dynamic pressure transducers capable of performing a modal decomposition of the acoustic field. The decomposition is done using time-resolved acoustic pressure measurements from different orientations of the rotating inlet duct during steady compressor operation. The resulting modal amplitudes are then used to determine the sound power level, a quantity that is independent of the acoustic pressure measurement locations. Therefore, in addition to revealing the modal content of noise at the compressor inlet, the rotating inlet duct experimental setup better characterizes the acoustic field with sound power levels across the operating flow range at various compressor rotational speeds.