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.

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.

Mode merging and the creation of exceptional points can be used to create optimum damping in a lined duct, as pointed out by Cremer [Acustica 3, 249-263 (1953)]. The effect of a mean flow has traditionally been analyzed by assuming the Ingard-Myer boundary condition at the wall. For low frequencies, however, the classical boundary condition is a better alternative. This paper shows that this choice removes two problems with the low-frequency solution: the negative real part of the optimum wall impedance and the non-valid solution for the upstream case. Theoretical derivations are complemented by numerical results to support these conclusions.

The "Flying Laptop"small satellite was developed as a technology demonstration platform and is operated by the University of Stuttgart's Institute of Space Systems (Germany) and was launched on July 14th 2017 into a 600 km Sun synchronous Low Earth Orbit (LEO). For data exchange with ground stations, the satellite is equipped with the S-band transceivers of type SSTRX-1100. The receiver operates at a centre frequency of 2083.5 MHz and additionally provides various information about the received signals. A power indicator value is used to determine the incoming radio power on this frequency. Whereas this information normally is disregarded if the satellite is not communicating with a ground station, for this analysis, the power indication value has been used to derive the background noise temperature within 100 kHz bandwidth. For the period between February 2018 and January 2021, the incoming signal power was recorded every 20 s and later extended by the corresponding satellite position, based on on-board GPS measurements. Although the exact direction of the incoming signal cannot be determined due to the quasiomnidirectional antenna design, a statistical analysis has been performed for the large amount of measurement points for a 1° Longitude/Latitude grid. Finally, the background noise temperature is derived. Several maps show interferences and background noise, and variations over time. The presented results provide an important insight on the radio environment for a commonly used frequency for Earth-observation LEO satellites.

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.

The last twenty years have seen a great development in inverse techniques for the determination of liner impedance under grazing flow conditions, so called impedance eduction techniques. This paper contributes to a continuing effort to gain confidence in the results obtained, specifically in the dependence of the results on fabrication, data acquisition and analysis. It is part of the IFAR Acoustic Liner Challenge were data from multiple test rigs with similar liner configurations fabricated using 3D printing are gathered and compared. Experimental results are reported for two liner configurations obtained in KTH’s advanced impedance eduction flow rig.