International Journal of Aeroacoustics

This work explores the use of bi-modal excitation to reduce the noise in a supersonic rectangular jet. The excitation is guided by previously published reduced-order modelling, suggesting that harmonic interactions can reduce the dominant noise sources in the jet. An unexcited jet is considered first as the baseline case, which showed Strouhal number 0.15 as the most dominant near field coherent structure, which also appeared as a prominent peak in the far field noise at low emission angles and as a secondary peak at the peak emissivity angle, thus it was taken as the fundamental frequency to target with excitation. The jet is then excited with the single-mode harmonic, St 0.30. Other cases are considered with bi-modal excitation at St’s 0.15 and 0.30 with various phase lags. Both single-mode and bi-modal excitation show a considerable reduction of the near field coherent structures at St 0.15 and amplification at 0.30. Reduction of the fundamental in the bi-modal cases shows a dependence on the initial phase lag whereby reduction is maximized when harmonic amplification is maximized. All excited cases are also shown to reduce the eddy convection velocity. Reduction in far field sound pressure levels at St 0.15 is obtained for all excitation cases up to 13 dB. For all metrics, it is shown that bi-modal excitation is more effective than single-mode excitation given an optimized phase lag. All results ultimately support the use of fundamental-harmonic interactions as a noise reduction mechanism.

The thermoacoustic effect provides a means to convert acoustic energy to heat and vice versa without the need for moving parts. This makes thermoacoustic devices of interest for multifunctional applications in harsh or remote service environments. Embedding thermoacoustic stacks within the framework of acoustic liners could provide opportunities to simultaneously optimize both acoustic absorption and thermal output as well as to integrate energy harvesting and sensing and monitoring functionalities. In this study, the influence of sound pressure level and frequency as well as stack length and position on the performance of a thermoacoustic liner is investigated using normal incidence impedance tube tests and DeltaEC simulations. Using Rott’s approximation, the stack’s pore width and wall thickness are optimized for additively manufactured thermoacoustic liner test articles. A peak steady-state temperature gradient of 9.5°C is obtained at 790 Hz for 140 dB tonal dwells. It is found that the temperature gradient correlates directly with the acoustic power available at the leading stack-face, which is highest when the stack is near the center of the resonator. In contrast, acoustic absorption diminishes when the stack is nearer to the closed-end of the resonator. At higher excitation pressure levels, the contribution of viscous heating to the deviation in the thermoacoustic gradient becomes significant. Further, the relationship between the acoustic absorption and the thermoacoustic temperature gradient was examined. In application scenarios requiring multifunctional optimization, a tradeoff may be required in terms of the absorption in order to maximize the temperature gradient. With current additive and hybrid fabrication processes and materials reaching commercial maturity, opportunities exist to realize thermoacoustic liners with enriched functionalities.

A computational fluid dynamics (CFD) solver based on Reynolds-averaged Navier–Stokes (RANS) equations and high-efficiency trim model is used to simulate the unsteady aerodynamics of coaxial rotor. Farassat 1 A equations are adopted for predicting rotor far-field aerodynamic noise. Forward flight cases of a two-bladed rigid coaxial rotor in different advance ratios and lift-offsets (LOS) are conducted. Sound pressure histories of different observation points and noise radiation maps are analyzed. Results indicate that, the intensity of rotor thickness noise moves towards the advancing side in the rotor disk plane, with the increase of advance ratio. Due to the superposition effect, the thickness noise of coaxial rotor is symmetrical, which is different with the single rotor. At low advance ratio, loading noise of the rigid coaxial rotor is enhanced near the blade-crossing azimuths caused by the unsteady interaction of the twin rotors. The enhancement turns weak with the increase of advance ratio. At high advance ratio, increasing LOS tends to enhance the rotor-self BVI noise, while weakening the inter-rotor interaction noise. At certain flight states, appropriate LOS can reduce the overall noise radiation intensity of rigid coaxial rotor.

The present study provides comprehensive experimental investigations into the use of wavy leading edge (LE) and trailing edge (TE) serrations as a passive means for augmenting the reductions of fan broadband noise. The findings clearly indicate that the wavy LE – TE serrated fan could yield greater noise reduction performance than the un-serrated and wavy LE serrated fan, over a wide range of frequencies. In general, the wavy LE-TE serrated fans offer a maximum noise reduction of about 10 - 6 dB and an average reduction of about 4 - 5 dB, over a broad range of frequencies. For the range of frequencies from about 3 to 8 kHz, the wavy LE – TE serrated fan delivers a notable additional noise reductions of about 1-2 dB as compared to LE serrated ones, which is observed for all rpm values. For all rpm values, the un-serrated and serrated fans exhibit maximum / minimum directivity at an emission angle of about 127.5^o / 77.5^o. The lower far-field acoustic emissions (i.e., interaction noise + self-noise) offered by the wavy LE-TE serrated fans arises due to intense far-field destructive interference as a result of the faster spanwise phase variation of the velocity/pressure as well as the higher spanwise de-coherence. Further, the wavy LE-TE serrations mitigate direct scattering at both the LE and TE by dispersing sound energy over a wider area, which results in less intense noise signature in the far field. Thus, it clearly demonstrates that the second generation wavy LE-TE serrations could provide the substantial reduction of the far-field noise as compared to first-generation wavy serrations over a broad range of frequencies.

This study employs a numerical approach that combines compressible detached eddy simulation (DES) with the Ffowcs Williams-Hawkings (FW-H) equation to investigate the potential of employing a slot jet at the leading edge of the bogie cavity for reducing bogie aerodynamic noise. The simulation is conducted on a 1:8 scale shortened train model, and two jet fluxes are considered. The permeable surface formula of the FW-H equation is adopted for far-field noise calculation to account for the scattering and shielding effects of the vehicle body and track structures, as well as the quadrupole source contribution. The results reveal that the slot jet induces a notable lifting effect on the shear layer formed at the leading edge of the bogie cavity, consequently reducing the impact of the shear vortex structures on the lower part of the bogie and the rear wall of the bogie cavity. However, as the shear vortex structures are guided towards the track, their interactions with the track structures are intensified. Due to the slow attenuation of the turbulent wake of the bogie region, it is difficult to avoid the spurious sound sources (hydrodynamic pressure fluctuations) induced by the wake passing through the integral surface by merely adjusting the position of downstream integral surface, while the end cap averaging technology can effectively filter out these spurious sources. The introduction of the jet primarily reduces the far-field noise below 1600 Hz of the bogie region, with higher jet velocity resulting in improved noise reduction effects.

This paper introduces a passive protrusions to reduce undesirable structural loading and high-amplitude noise radiation in axisymmetric cavities, are common in industries, including pipelines and aerospace. A comprehensive experimental investigation of the pipe-cavity noise control system is conducted, exploring various protrusion locations and lengths at different Nozzle Pressure Ratios (NPRs). The study examines protrusions placed at the leading, trailing, and both leading and trailing edges of the axisymmetric cavity. Additionally, detailed experiments are performed to analyze the impact of protrusion lengths on unsteady cavity pressure and far-field noise radiation. Cavity pressure fluctuations and far-field noise levels are measured for both cases: with and without protrusions in the pipe-cavity setup. The Proper Orthogonal Decomposition (POD) technique is applied to the time-resolved Schlieren images to show the effect of passive protrusions on the exit jet flow structure. Results demonstrate that a protrusion located at the trailing edge yields superior noise reduction compared to other configurations. The efficacy of this protrusion in mitigating cavity noise is attributed to its ability to efficiently alleviate the feedback mechanism responsible for cavity noise generation and break down the recirculation region within the cavity system. Significant noise reduction, approximately 4 dB, is achieved at lower NPRs, and substantial noise reductions are also observed for the underexpanded jet condition.