Turbocharger compressor simulations can improve the development of compressors with broad operating ranges and improved noise signatures. At low mass flow rates aerodynamic instabilities, known as rotating stall and surge, limit the operating range of the compressor. However, the triggering factors for these instabilities are currently unknown. High quality numerical methods are therefore needed to capture instabilities and to simulate reliable performance maps. The aim of this study is to improve the understanding of the chosen numerical approach going from design to off-design operating conditions. It is shown that flow structures are under-predicted using URANS k-ω SST and that a LES WALE method better captures the flow structures in the blade region. Further, the level of fluctuations in different parts of the compressor is presented. By increasing the predictive quality of the numerical simulations, the need for empirical calibration can be reduced and turbocharger matching can be improved.

This numerical study focuses on using Large Eddy Simulations to investigate the influence of pulsating backpressure on turbocharger compressor performance, stability and noise generation. This to simulate the effects of the valve train. The study investigates how pulsating backpressure effects compressor performance, especially in the design and near surge conditions. Previous research has indicated that compressor performance deviated from continuous operation under pulsating conditions, resulting in a hysteresis loop. Additionally, the surge margin is affected due to time-history effects. The study employs a pulsating frequency of 100 Hz and examines the sensitivity to pulse amplitude. These results contribute to an improved understainding of the impact of pulsating backpressure on compressor performance and stability, quantifying the effects and providing valuable insights.

A numerical study is presented where Large Eddy Simulation (LES) calculations are used in an effort to characterise flow-acoustic interaction in a turbocharger compressor operating at near surge conditions. Under such off-design operation, an inlet recirculation zone develops upstream of the compressor following the compressor inlet duct. Previous studies have shown that the development of reversed flow can impact the sound power level of the blade passing frequency (BPF) tones and broadban whoosh noise. In this study local mesh independence studies are performed to capture the vortical structures in the recirculation region, which are often not properly resolved. Computational grid refinements in the fluid domain are applied based on extimated of the local Taylor microscale in the flow. Moreover, the grid resolution is enhanced in the region where domainant acoustic sources are to be expected according to Proadman noise source model. The temperature field and the velocity components with correponding standard deviations are quantified and validated against avaliable experimental data. The analysis of the LES data allows quantifying the dynamics of the developed recirculation zone in space and time and enables a detailed assessment on the impact it has on the incoming flow, explaining its contribution to the flow-induced noise mechanisms in a centrifugal compressor.

Centrifugal compressors experience flow instabilities like rotating stall and surge at low mass flow rates. In this study, we use Large Eddy Simulations (LES) to investigate the impact on ported-shroud passive flow control on these instabilities. At low mass flow rates, flow reversal over the blade tips create a shear layer that generates vortical stuructures. These structures persist downstream of the radial diffuser. Additionally, the tip leakage flow has angular momentum imparted by the impeller, causing the incidence angles to the blade tip to deteriorate due to an imposed swirling component in the incoming flow. The impeller cannot maintain a constant efficiency at near suthe conditions due to the extreme alteration of the incidence angle. This leads to unsteadu flow momentum transfer downstream, resulting in compression waves at the compressor outlet, travelling towards the impeller. The pressure oscillations govern the tip leakage flow and, consenquently, the incidence angles at the impeller. However, standing waves are observed in the ported shroud increasing the noise levels in the duct. This study highlights the role of the ported-shroud in the generation and maintenance of flow instabilities in a centrifugal comprssor under design and off-design conditons.

Numerical solutions of acoustic wave scattering are often used to describe sound propagation through complex geometries. For cases with flow, various forms of the convected equation have been used. A better alternative that includes vortex-sound interaction is instead to use the linearized and harmonic forms of the unsteady fluid flow governing equations. In this paper, a formulation of the linearized equations that include rotational effects, in an acoustic computation using a rotating frame of reference in a stationary geometry, is presented. We demonstrate that rotational effects can be important, e.g., when computing the transmission loss through high-speed compressors. The implementation of the proposed addition to the existing schemes is both simple and numerically inexpensive. The results are expected to have an impact on the research and development related to noise control of high-performance turbo-machinery, e.g., used in automotive or aviation applications at operating conditions that can be represented by steady background flows.