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.

Fluid machines are an integral part in energy conversion with applications from pumps, fans, propellers, compressors and turbines. In the automotive industry, turbochargers are commonly employed to counteract the effect of engine downsizing. However, designing efficient compressors with wide operating ranges and reduced noise emissions consitute a challenge.

This thesis investigates flow instabilities and sound generation in turbocharger compressors, utilizing compressible Large Eddy Simulations (LES). The numerical approach is validated through sensitivity studies and comparison with measurement data. Three different compressor designs used in both light-duty and heavy-duty applications are examined with the aim of enhancing the understanding of rotating stall mechanism in real-world configurations and their impact on aerodynamically generated noise.

The analysis employs compressible Navier-Stokes equations with a scale-resolving model, evaluating its robustness in comparison to other computational methods under various operating conditions. The system's response to time-varying boundary conditions is assessed, and the effect of pulse amplitude is quantified.

Subsequently, the mechanism for aerodynamically generated noise, focusing on the broadband components are explored through analysis of the recirculation region. Resolving the Taylor micro-scale in the recirculation region enhances the understanding of the dynamics in this zone. It is demonstrated that an inlet recirculation zone develops near surge conditions, which is highly sensitive to the choice of boundary conditions and turbulence formulation. Passive flow control, such as the ported-shroud, are considered to illustrate their influence on performance, stability and noise.

Finally, the system is studied using a two-port method, accounting for rotational effects. This provide insights into the transmission poperties at low frequencies (< 3 kHz) and the mechanism of sound generation. It is demonstrated that the use of Computational Fluid Dynamics can improve the understanding of flow-acoustic interaction in complex geometries. Additionally, the developed numerical simulation and post-processing methods have potential application in a range of turbochargr systems, from hybrids to fuel cell application.

The use of turbochargers with downsized internal combustion engines improves road vehicles’ energy efficiency but introduces additional sound sources of strong acoustic annoyance on the turbocharger’s compressor side. In the present study, direct noise computations (DNC) are carried out on a passenger vehicle turbocharger compressor. The work focuses on assessing the influence of grid parameters on the acoustic predictions, to further advance the maturity of the acoustic modelling of such machines with complex three-dimensional features. The effect of the boundary layer mesh structure, and of the spatial resolution of the mesh, on the simulated acoustic signatures is investigated on detached eddy simulations (DES). Refinements in the core mesh are applied in areas of major acoustic production, to generate cells with sizes proportional to the local Taylor microscale values. Such an educated guess allows for quality enhancement with a smaller increase in computational costs as compared to more general overall refinements. The reflection-free simulation results are validated against experiments. The experimental data were post-processed with methods from the two-port theory to represent pure acoustic source power density for the acoustic modes, cleaned from test-domain-specific reflections. A detailed comparison between experiments and numerical simulations is carried out. As a result of this study, the most critical parameters for the numerical prediction of turbocharger noise are presented. The results can, furthermore, be used to improve the understanding of grid construction when predicting noise signature for compressor flows.

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.

Existing models of extended-tube liners are mainly applicable to the normal-incidence case, and the influence of the grazing-flow effect is often ignored. In the current paper, an impedance model is developed based on the transfer matrix method, and the grazing-flow effect on the surface impedance is taken into account in terms of the end corrections. The validity of the model is examined on a flow-duct facility, and the liner impedances are obtained from an impedance eduction method. The proposed model shows a reasonable agreement with the educed data, and better accuracy is found in terms of the transmission loss. Geometric parameters including the length of the extended tube and the grazing-flow speed are then investigated. It is found that the frequency for maximum attenuation is shifted to lower frequencies for a longer extended tube, but it is less sensitive to the grazing-flow speed. The effects of the sound pressure level and nonlinearities are also investigated.

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.

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.

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.