In an ever-transforming sector such as that of private road transport, major changes in the propulsion systems entail a change in the perception of the noise sources and the annoyance they cause. As compared to the scenario encountered in vehicles equipped with an internal combustion engine (ICE), in electrically propelled vehicles the heating, ventilation, and air conditioning (HVAC) system represents a more prominent source of noise affecting a car's passenger cabin. By virtue of the quick turnaround, steady state Reynolds-averaged Navier Stokes (RANS)- based noise source models are a handy tool to predict the acoustic power generated by passenger car HVAC blowers. The study shows that the most eminent noise source type is the dipole source associated with fluctuating pressures on solid surfaces. A noise map is generated from the noise source models data, giving indications of how changes in operating conditions affect the acoustic output of the machine throughout its operating range. The capability to predict power spectra with steady state RANS is investigated, and the overall sound power level of several operating points is validated against experimental data, showing good match. The study aims at establishing steady state RANS noise source models as a valuable tool in preliminary acoustic analyses of HVAC blower designs, in particular in the early stage of new design studies, thus helping the industry to better target quieter operation and enhanced passenger comfort.

The rotational effect is a crucial factor that needs to be considered when dealing with acoustic wave scattering through high-speed rotors, e.g., in propulsion line simulations. Neglecting the rotational effect can lead to inaccurate results, especially in scenarios such as computing the transmission loss through compressors. Therefore, it is essential to incorporate the rotational effect in the governing linearized equations for fluid flow. This approach involves utilizing a rotating frame of reference in a stationary geometry, which has been demonstrated to be both straightforward and numerically inexpensive. Overall, the findings of this study highlight the importance of considering rotational effects in acoustic wave scattering simulations, particularly in high-performance turbo-machinery applications.

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.

This paper presents a significant advancement in micro-perforated panel absorber technologies. Two distinct types of sub-wavelength multi-chamber micro-perforated panel absorbers (MC-MPPA) for low-frequency broadband noise absorption under both normal and grazing acoustic incidence conditions are developed. Micro-perforated panel absorbers (MPPAs) exhibit versatility, eco-friendliness, and a straightforward, robust construction, making them ideal acoustic solutions. A graph-theory-based two-point impedance method (TpIM) and the Cremer impedance method are used for system modeling, enabling the design of the MC-MPPA to be optimized for maximum sound absorption in a chosen frequency range for both normal and grazing acoustic incidence. This graph theory approach represents a breakthrough, as an equivalent circuit model approach could not be applied to such complex models. Under normal incidence conditions, the experimental overall absorption coefficient is measured to be 0.8273 for a 22 mmthick MC-MPPA in the frequency range of [660 2000] Hz, and 0.8284 for a 52 mm thick MC-MPPA in the frequency range of [400 2000] Hz. Under grazing incidence conditions, an optimized MC-MPPA with a mere 30 mm sub-chamber depth achieves a transmission loss of 66 dB at 1180 Hz. Additionally, a 50 mm sub-chamber depth yields a transmission loss greater than 10 dB over an 880 Hz wide frequency range: [820 1700] Hz. The developed MC-MPPA technologies hold great promise for various duct noise attenuation applications including aeroengine acoustic liners.

In TRANSIT, experimental methods are developed to separate and characterise noise sources on moving trains. Improved microphone array techniques allow quantification of sound power and directivity. Source separation methods based on the Pass-By Analysis method, Advanced Transfer Path Analysis and the TWINS model are also developed. For trains at standstill, new test methods are developed to quantify noise transmission paths from sources to the standard microphone positions accounting for installation effects. Several measurement campaigns are used to demonstrate and verify these methods. In addition, innovative materials and methods are investigated for improved sound comfort in trains. Approaches considered include optimal sound absorption at the source, attenuation along ducts for air conditioning systems and innovative meta-structure designs for the car-body parts.

This study is concerned with reducing the tonal noise emitted by electric ducted fans which serve as the propulsion system for a variety of unmanned aerial vehicles (UAVs). The underlying concept is utilizing the shroud pressure fluctuation caused by the rotating blades to drive the over-the-rotor (OTR) acoustic treatment and generates a secondary sound field that cancels the primary ducted fan sound field. Experiment shows a considerable destructive effect can be made by meticulous tuning of the strength and phase of the secondary source. In view of the space constraints of a real electric ducted fan, the acous-tic treatment is reconfigured as a labyrinth-type acoustic metamaterial that reduces the structure size while maintaining the noise reduction performance at the same time. An advantage of the proposed tech-nique is that each tone can be dealt with by a specifically designed acoustic treatment occupying a small patch on the fan shroud. Hence, a compact multi-tonal noise control solution is possible by circumferen-tially deploying several OTR acoustic treatments.

This study is concerned with reducing the tonal noise emitted by electric ducted fans which is a popular choice for air mobility vehicles. Different from existing technologies, which are mainly based on lining sound absorption materials over the duct interior surface, a side-branch tube placed directly over the rotor is investigated. The underlying concept is that the aerodynamic pressure fluctuation induced by the rotating blades drives the tube through its opening and generates a secondary sound field. By carefully adjusting the depth, opening size and installation position of the tube, the secondary sound field makes a destructive effect on the primary sound field. Considering the space limit encountered by a small electric ducted fan in practical applications, an acoustic metamaterial with the same acoustic radiation characteristics as the side-branch tube is designed and tested. Experimental studies show that the proposed technology offers a space-saving solution to ducted fan tonal noise problems.

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.

One problem for railway noise predictions is to characterize noise from various auxiliary equipment, e.g., fans, compressors, transformers. The noise from such sources can be a dominating contribution under low-speed operation or stand still. To better handle this problem the EU-project TRANSIT investigates improved methods for acoustic source characterization. As a starting point it is assumed that an acoustic source is enclosed by a control surface. The surface is sub-divided into smaller areas and each area is assumed to act as an acoustic one-port coupled to all the other areas. The properties of each area can then be described by its volume flow and internal impedance. The resulting acoustic pressure at a receiving point, can finally be expressed as a product of the source volume flows and a matrix representing the acoustic installation effects (“source+radiation impedances”). To simplify the method one can assume uncorrelated sources and use an ISO procedure for sound power to determine the volume flows. The acoustic installation effects can be obtained using a monopole point source to measure or calculate the pressure at selected receiving positions.

In this work, the application of ultra-thin low frequency (UTLF) resonators to reduce low-frequency tonal noise components in HVAC systems is presented. A single UTLF absorber consists of a mass, a flexible membrane, a cavity, and a resistive layer. The resonator produces a near-perfect absorption in a narrow region around its resonance frequency. The proposed treatment consists of the addition, on one side of the HVAC duct, of an array of two kinds of UTLF resonators, tuned to two target frequencies. The acoustic performances of the system are mimicked in a three-dimensional numerical model, starting from a single resonator. The resonator impedance is tuned to the Cremer impedance of the duct by varying the design parameters to obtain optimal damping of the plane wave mode. The resonance frequency of the system is adjusted changing the mass and the depth of the cavity. Combining many resonators along the duct, one may achieve a high sound transmission loss in the target frequency due to the locally reactive behavior.