Numerical Prediction of the Sources and the Modal Content of the Acoustic Field in a Radial Compressor Outflow

Abstract

In large modern turbochargers, transonic compressors often constitute the main source of noise, with a frequency spectrum typically dominated by tonal noise at the blade passing frequency (BPF) and its harmonics. Inflow BPF noise is mainly generated by rotor locked shock fronts. Outflow noise, while also dominated by BPF tones, is linked to more complex source mechanisms. Its modal structure and the relationships between sources and modal sound pressure levels (SPL) are less well understood, and its numerical analysis is, in general, significantly more complex than for compressor inflows. To shed some light on the outflow acoustic characteristics of radial machines, transient simulations of a 360 deg model of a radial compressor stage, including its vaned diffuser and volute, were carried out. Four increasingly finer grids were used for this purpose. On all grids, numerical damping had detrimental effects on prediction quality. A simple and mathematically sound method is proposed to account for this damping. With it, the global outflow acoustic power level (PWLg) is predicted to within an accuracy of 2 dB of the experimental result on the finest grid. This shows that satisfactory accuracy can be obtained with state-of-the-art computational fluid dynamics (CFD) codes if care is taken with the simulation setup. The simulations are further validated with experimental data from 17 transient wall pressure sensors.

Dynamic mode decomposition of the acoustic field in radial compressors

“…Later, authors such as Raitor & Neise [6] implemented these techniques in the experimental characterization of centrifugal fans and turbocompressors. Most recently, Limacher, Banica et al investigated the sound field of centrifugal turbochargers through the least-square fitting of select blade passing frequencies to Tyler-Sofrin modes from both numerical [21] and experimental [22] points of view.…”

“…It can be seen in the aforementioned studies [6,21,22] that when noise phenomena other than BPF harmonic tones are decomposed in this way, a large number of mode orders become excited, this is, a large sum along the m and n orders is required in order to explain the observed pressure pattern at the measured cross-section.…”

“…As a first step towards understanding the structure of sound transmission through the ducts, the classical Tyler-Sofrin modal decomposition was performed, following specifically the method proposed by Banica & Limacher [21,22], in which modal distributions ϕ m (r j ) are obtained at discretized radial positions r j by means of circumferential Fourier transforms, allowing the least-squares estimation of A mn by:…”

On the iInfluence of Inlet Geometry on Turbocharger Compressor Noise.

“…Later, authors such as Raitor & Neise [26] implemented these techniques in the experimental characterization of centrifugal fans and turbocompressors. Most recently, Limacher, Banica et al investigated the sound field of centrifugal turbochargers through the least-square fitting of select blade passing frequencies to Tyler-Sofrin modes from both numerical [96] and experimental [97] points of view.…”

“…It can be seen in the aforementioned studies [26,96,97] that when noise phenomena other than BPF harmonic tones are decomposed in this way, a large number of mode orders become excited, this is, a large sum along the m and n orders is required in order to explain the observed pressure pattern at the measured cross-section.…”