Julio Apolinario Cordioli

We investigate the influence of realistic sheared grazing flow on acoustic propagation in three-dimensional rectangular ducts. We show that the conclusions reached in the literature about the effects of sheared grazing flow on acoustic propagation in lined ducts are dependent on the flow profiles used in those studies and that significantly different conclusions are reached once a realistic flow profile is used. We particularly focus on small ducts typical of most experimental impedance eduction facilities, for which velocity gradients are relevant in a significant fraction of the duct cross section. We assess the effect of simplifying the velocity distribution in the cross section to either a one-dimensional (two-dimensional, spanwise-infinite duct) or a uniform flow profile. Three flow profiles are considered, namely, i) the tensorized hyperbolic tangent, ii) the law of the wall, and iii) one obtained from a RANS simulation. These flow profiles are used as input in numerical simulations, based on the solution of the three-dimensional Pridmore–Brown equation, to perform in silico impedance eduction experiments. Results show that realistic flow profiles can be well approximated for acoustic wave propagation in ducts by uniform or one-dimensional flow profiles, provided that the bulk Mach number is correctly accounted for, which contrasts with previous findings considering more simplistic flow profiles. The key conclusion of this work is that if viscous effects are negligible and acoustic impedance is a good representation of a lined wall with grazing flow, then the simplification to a uniform flow is a reasonable approximation, and traditional eduction methods are sufficiently accurate.

This study examines the hydrodynamic–acoustic feedback mechanisms responsible for whistling in double-neck Helmholtz resonators under bias flow, with emphasis on long-cavity configurations representative of human whistling. Combined flow and acoustic experiments were performed using particle image velocimetry and pressure measurements to characterize vortex dynamics, sound generation, and oscillation regimes across a range of Reynolds numbers and geometric parameters. Within the self-sustained oscillation regime, sound pressure levels scale consistently with the Strouhal number, exhibiting a peak near St = 0.32. Experimental data collapse is achieved using a modified Reynolds number that incorporates cavity-length effects. Moreover, two-point coherence and cross-spectral analyses show that acoustic generation occurs predominantly inside the resonator chamber, with maximum sound production located approximately two orifice diameters downstream of the upstream neck. Flow visualization reveals that vortices dissipate before reaching the downstream neck, indicating a feedback mechanism distinct from classical hole-tone and Rossiter-type oscillations. To interpret these findings, a reduced-order analytical model was developed, consisting of a two-degree-of-freedom lumped acoustic system coupled to a nonlinear oscillator representing vortex shedding. The model reproduces the onset, lock-in, saturation, and abrupt extinction of self-sustained oscillations, capturing the essential coupling that transfers energy from the mean flow to the acoustic field. These results provide new insight into long-cavity whistles and offer a predictive framework for Helmholtz-type resonators under bias flow.

This work adopts a mid-fidelity numerical framework, coupling Reynolds-Averaged Navier-Stokes simulations with frequency-domain Linearised Navier-Stokes equations, to assess the impact of conventional flow and acoustic propagation assumptions on educed impedance. First, the approach is validated by simulating a three-dimensional acoustic liner geometry and comparing the results with experimental and high-fidelity scale-resolved simulation data. It is demonstrated that the framework reproduces the frequency-dependent acoustic impedance behaviour, capturing the impedance's directional dependence on acoustic wave propagation, but showing some differences in terms of absolute values. Then, the liner geometry is substituted by an impedance wall boundary condition with a frequency-dependent impedance value equal to the one obtained for the explicit liner configuration. The impact on the acoustic field of assuming uniform flow with the Ingard-Myers boundary condition or a sheared flow with an impedance boundary condition without the thin boundary layer assumption is evaluated. It is found that, in both cases, the predicted transmitted and reflected fields remain direction dependent, but the transmission–reflection partition depends on the selected combination of flow profile and acoustic formulation. Based on this outcome, a FEM-based impedance-eduction method employing a dual-stage optimisation (global followed by local refinement) is introduced and applied with a shear-resolved, viscous forward model. Compared with conventional eduction approaches, this procedure shows a less relevant propagation-direction dependence of the educed impedance.

Impedance eduction methods are the current standard approach to measure the impedance of acoustic liner under sheared grazing flow. The dedicated facilities for these methods consists on a waveguide with rectangular cross-section, which implies a sheared grazing flow. A current debate in the literature is the effect of this sheared flow in the impedance eduction methods. We assess the impact of the flow profile shape on acoustic propagation in a two-dimensional duct within the typical operating range of impedance eduction facilities. Firstly, a numerical experiment is proposed in which the Pridmore–Brown equation is assumed to represent the true physical behaviour, and is used with both simplified flow profiles commonly used in the literature and a realistic representation of a turbulent boundary layer using a van Driest universal law of the wall model. The data from these numerical experiments are then used with a traditional impedance eduction process, and the resulting variation in obtained impedances are investigated. Secondly, we apply a less-traditional impedance eduction method that incorporates the sheared velocity profile to data obtained from real-world experiments. The results suggest that the Ingard–Myers boundary condition remains a good approximation to a realistic boundary layer profile, such as the universal law of the wall, at least in the two-dimensional case. However, it is also shown that the simplified flow profiles often used in the literature can lead to significant deviations from the results obtained using a realistic velocity distribution.

The so-called Ingard–Myers boundary condition is commonly used to represent an acoustic impedance in the presence of grazing flow. However, recent studies have shown that it may be inadequate under certain conditions. The main problem has been identified as the assumption of a boundary layer of infinitesimal thickness, and several alternative boundary conditions that address this shortcoming have been proposed. In this work, we present a methodology to implement an alternative boundary condition in the context of the finite element method. The acoustic impedance boundary condition for finite boundary layers in straight circular ducts known as the Brambley boundary condition is selected for this purpose. The procedure to obtain an adequate finite element formulation of the chosen boundary condition is described. The new formulation is implemented in the commercial finite-element solver COMSOL Multiphysics and initially validated by comparison with a simple circular duct analytical model. The alternative boundary conditions is then used to predict liner attenuation in a realistic turbofan engine geometry and operating conditions and in a scaled-down fan rig. Results suggest that the effect of the boundary layer is nonnegligible, specially when higher-order modes are involved, with the Brambley boundary condition outperforming the Ingard–Myers boundary condition.

The experimental characterization of acoustic liners has garnered significant attention within the research community. Typically, such characterization involves measuring the acoustic impedance of liners using various techniques under conditions similar to those found in turbofan engines. This study aims to offer a comparative analysis of educed acoustic impedance for two liner samples, manufactured by the same provider and tested in the UFSC Impedance Test Rig and the NASA GFIT. Leveraging the distinct geometries of these test rigs, the investigation considers the impact of different grazing flow profiles in terms of the boundary-layer displacement thickness on the educed impedance. Notably, impedance measurements exhibit congruent outcomes in the absence of grazing flow but diverge in terms of resistance values when grazing flow is present, with the UFSC Impedance Test Rig consistently indicating higher values. The postprocessing algorithms of both institutions are uniform. In addition, a comparison involving a semiempirical predictive model has been conducted. The findings suggest that the observed disparities relate to the grazing flow boundary-layer profile. In summary, it is important to report both the mean flow Mach number and the displacement thickness to enable proper comparison between impedances educed from data acquired in different test rigs.

O avanço tecnológico permitiu o monitoramento simultâneo de milhares de máquinas, possibilitando o treinamento de algoritmos de aprendizado de máquina para realizar a detecção e o diagnóstico de falhas de forma automatizada. Contudo, um dos principais desafios tem sido a falta de dados rotulados, além do fato de que um mesmo tipo de máquina pode ser exposto a diferentes condições de operação, alterando a distribuição dos atributos dos sinais adquiridos. Uma possível solução é utilizar técnicas de adaptação de domínio, que podem melhorar o desempenho dos modelos em novos tipos de máquina, mesmo sem dados rotulados. No entanto, muitos dos artigos apresentados na literatura enfrentam problemas de vazamento de dados, o que impos sibilita a comparação entre técnicas, pois o desempenho dos modelos é irrealista. Diante disso, este artigo propõe uma metodologia para comparar técnicas de adaptação de domínio, evitando esse problema. Foram implementadas oito técnicas de adaptação de domínio e avaliadas em dois conjuntos de dados, resultando em ganhos significativos em um dos cenários de avaliação.