Centrifugal compressors working in the surge side of the map generate a broadband noise in the range of 1 to 3 kHz, named as whoosh noise. This noise is perceived at strongly downsized engines operating at particular conditions (full load, tip-in and tip-out maneuvers). A 3-dimensional CFD model of a centrifugal compressor is built to analyze fluid phenomena related to whoosh noise. A detached eddy simulation is performed with the compressor operating at the peak pressure point of 160 krpm. A steady flow rig mounted on an anechoic chamber is used to obtain experimental measurements as a means of validation for the numerical model. In-duct pressure signals are obtained in addition to standard averaged global variables. The numerical simulation provides global variables showing excellent agreement with experimental measurements. Pressure spectra comparison is performed to assess noise prediction capability of numerical model. The influence of the type and position of the virtual pressure probes is evaluated. Pressure decomposition is required by the simulations to obtain meaningful spectra. Different techniques for obtaining pressure components are analyzed. At the simulated conditions, a broadband noise in 1-3 kHz frequency band is detected in the experimental measurements. This whoosh noise is also captured by the numerical model.

Turbocharger noise has become a major concern in downsized automotive engine development. In this paper, the analysis is focused on the whoosh noise produced by the compressor when it is working near surge. A centrifugal compressor has been acoustically characterized on a turbocharger test rig mounted on an anechoic chamber. Three in-duct pressure signals forming a linear array are registered in order to obtain pressure components. In this way, meaningful pressure spectra and sound intensity level (SIL) compressor maps are obtained, showing an increase of SIL in the frequency window corresponding to whoosh noise. Besides, detached eddy simulations (DES) of the centrifugal compressor flow in two operating conditions near surge are performed. Good agreement is found between the experimental measurements and the CFD solutions in terms of predicted pressure spectra. Flow analysis is used to identify patterns responsible for the different features of the pressure spectra. At the simulated conditions, rotating instabilities in the compressor diffuser and inducer cause pressure oscillations in the frequency range of whoosh noise.

This paper describes an experimental study carried out with the objective of characterizing flow instabilities in turbocharger compressors, specially the distribution of the high-temperature compressed backflow that appears upstream of the impeller at marginal surge conditions. The inlet of a test compressor was fitted with linear and circumferential thermocouple arrays in order to measure the temperature distribution caused by this backflow, whose independence of duct wall temperature was validated through thermographic imaging. Miniaturized pressure probes at the inducer and diffuser showed how pressure spectra varied during the different operating conditions. Induct acoustic intensity was measured in both the inlet and the outlet to investigate the correlation between a known super-synchronous broadband issue known as whoosh noise and the backflow behaviour as characterized by local pressure and temperature. Analysis of the results points to inlet whoosh noise being boosted by this reversed flow but not caused by it, the source probably being located at or downstream of the compressor impeller.

Centrifugal compressor aeroacoustics are analyzed by means of a 3-dimensional CFD model. Three operating points at nominal compressor speed are simulated ranging from best efficiency point to near-surge conditions. Experimental measurements are obtained using a steady flow rig mounted on an anechoic chamber. URANS and DES predictions of compressor global variables and pressure spectra are compared against experimental measurements. Flow-induced noise increases as the operating point moves towards surge line. Stall at the suction side of the blades exists even for high mass flow conditions, causing a high frequency boundary layer oscillation. Low momentum cells rotating at the diffuser are found at points closer to surge, causing the so-called whoosh noise. Inducer rotating stall is also present at these conditions. Point closest to surge shows a rotating tornado-type vortex at the inducer, determining a moving low pressure region that increases low frequency noise content.

In this article, the acoustic characterisation of a turbocharger compressor with ported shroud design is carried out through the numerical simulation of the system operating under design conditions of maximum isentropic efficiency. While ported shroud compressors have been proposed as a way to control the flow near unstable conditions in order to obtain a more stable operation and enhance deep surge margin, it is often assumed that the behaviour under stable design conditions is characterised by a smooth, non-detached flow that matches an equivalent standard compressor. Furthermore, research is scarce regarding the acoustic effects of the ported shroud addition, especially under the design conditions. To analyse the flow field evolution and its relation with the noise generation, spectral signatures using statistical and scale-resolving turbulence modelling methods are obtained after successfully validating the performance and acoustic predictions of the numerical model with experimental measurements. Propagation of the frequency content through the ducts has been estimated with the aid of pressure decomposition methods to enhance the content coming from the compressor. Expected acoustic phenomena such as ‘buzz-saw’ tones, blade passing peaks and broadband noise are correctly identified in the modelled spectrum. Analysis of the flow behaviour in the ported shroud shows rotating structures through the slot that may impact the acoustic and vibration response. Further inspection of the pressure field through modal decomposition confirms the influence of the ported shroud cavity in noise generation and propagation, especially at lower frequencies, suggesting that further research should be carried out on the impact these flow enhancement solutions have on the noise emission of the turbocharger.

Widely recognized since the beginning of air travel as a major issue, noise reduction remains nowadays a pressing concern for all stakeholders in the aviation industry. While aeroengine compressors, specially at the approach phase, have been historically identified as a leading source of noise, most of the research has been conducted on compressors of the axial type. However, radial compressors are found in a wide array of applications: smaller business jets, helicopters, unmanned aerial vehicles (UAVs), auxiliary power units (APUs), turbochargers for reciprocating engines, etc. Owing to their geometrical particularities, radial compressors feature flow patterns that differ from their axial counterparts, leading to different acoustic performance but also opening the door for different optimization approaches. Yet, classical modal decomposition techniques focused on duct propagation may fail to reveal the complex interactions between geometry and flow features that act as noise sources. In this paper we apply, in addition to the classical approach, a data-driven Dynamic Mode Decomposition (DMD) to pressure data coming from a Detached Eddy Simulation (DES), in which we have experimentally validated the correct reproduction of the modal behaviour of the compressor, thus obtaining in-depth details of the link between flow phenomena and noise generation and transmission across the inlet and outlet ducts.

In this paper, the unsteady behaviour of a compression-ignited (CI) engine combustion chamber is studied by analyzing the results of a Computational Fluid Dynamics (CFD) model through the application of different flow decomposition techniques, aiming to resolve the underlying modal structure of the process. Experimental validation for the combustion simulation is provided, and a methodology for extracting coherent pressure information is proposed in order to provide a suitable input for different analysis methods. These range from straightforward Fourier transform techniques to more sophisticated modal decomposition approaches. In particular Proper Orthogonal Decomposition (POD) is shown to provide valuable insight into the time-spatial structure of the combustion flow field, allowing the establishment of correlations between pressure modes and physical parameters of the combustion, such as the injection timing or the chamber geometry. Dynamic Mode Decomposition (DMD) on the other hand is proven to successfully highlight the link between the frequency of the unsteady energy components and their spatial distribution within the chamber. Advantage is then taken of the modal characterization of the unsteady behaviour in the chamber to showcase how physical parameters such as the spray angle can be modified to optimize the acoustic signature of the combustion process, helping CI internal combustion engines reduce their acoustic environmental impact.

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