Aeroacoustic Sound Sources Research Articles

Reduced-order equivalent source modeling of aeroacoustic sources

The acoustic far-field prediction of high-fidelity aerodynamic simulation data is still not accurately possible when accounting for the surface reflections from an aircraft’s fuselage or wings. By presenting a general method for order-reduced equivalent source modeling of aeroacoustic sound sources, this work is bridging the gap between high-fidelity aerodynamic simulations and its integration to auralization frameworks. At the example of a numerically simulated jet, this study shows the computation of an equivalent source model with minimal complexity by means of spherical harmonic (SH) coefficients. The minimal source extension, in which all acoustic sources of a flow field are confined, is computed by using spherical Hankel extrapolation of sound pressure data from virtual concentric microphone arrays. The result of the SH transform shows that the dominant energy contribution can be associated to nine elementary sound sources. The resulting equivalent source model of jet noise provides a convenient date interface between large-scale computational fluid dynamics and acoustics simulations.

Experimental and numerical investigation of the blade tip-related aeroacoustic sound source mechanisms of a ducted low-speed axial flow fan

A combined aerodynamic and aeroacoustic study of a ducted low-speed axial flow fan at various operating points is presented herein, focusing on the blade tip-related noise sources. The investigation includes upstream and downstream phased array microphone measurements and computational fluid dynamic simulations. The downstream acoustic measurements are carried out by using an acoustically transparent duct. The resulting flow fields and noise source maps are evaluated simultaneously. The results show that at higher flow rates, the dominant noise source is the turbulent boundary layer – trailing edge noise. At lower flow rates, the dominating noise sources can be found at the blade tip’s leading edge. These noise sources are found to be tip leakage flow related. Also, downstream microphone array measurements indicate strong noise sources at the blade tip’s trailing edge. These are determined to be a combination of the turbulent boundary layer - trailing edge noise, tip leakage vortex impingement noise, and double-leakage noise.

Efficient shear layer corrections for acoustics in arbitrary jet flows

Localization of aeroacoustic sound sources in open jet wind tunnel experiments requires an accurate prediction of the acoustic propagation time. Most conventional predictions use either a ray-tracer, coupled with a modelled continuous velocity field, or use ray diffraction and a discretization of the velocity field by means of vortex sheets. In this work a novel method is proposed in which the continuous velocity field is discretized into blocks of constant velocity separated by velocity discontinuities, thus removing the requirement for the velocity to be parallel to the surface that separates the blocks. The acoustic ray is solved by minimization of the acoustic propagation time. The computational effort is low compared to ray-tracing methods while maintaining an improvement in accuracy compared to methodologies using vortex sheets. A specific continuous velocity field is derived that models a self-similar shear layer expanding asymmetrically from a rectangular nozzle. Subsequently, this velocity field is discretized to compute the acoustic rays. Experimental results with a loudspeaker source placed in the open jet of a large industrial wind tunnel showed a decrease in source localization uncertainty compared to techniques based on vortex sheets. This is attributed to the inclusion of the shear layer slanting.

Aerodynamic and acoustic characterisitics of glottal closure insufficiency

Glottal closure insufficiency (GCI) and asymmetric oscillations of the vocal folds are often responsible for low voice quality and increased phonation effort for concerned patients. In this study, four clinically observed types of GCI with increasing glottal closure insufficiency were reproduced in the aero-acoustic computer model simVoice. Each type was simulated with symmetric and asymmetric oscillations extracted from high-speed recordings obtained in ex-vivo porcine larynges. The results show a permanent glottal flow throughout the oscillation cycle and an increasing mean flow rate for increasing minimum glottal gap. Consequently, the aerodynamic work exerted onto the vocal folds decreases for an increasing gap corresponding to the reported patient’s effort increase. Moreover, the quality of the computed sound decreases with an increasing gap based on the cepstral-peak-prominence. Even worse, asymmetric oscillations further degrade both, the aerodynamic energy transfer and the acoustic quality. The temporal evolution of energy transfer shows that the main decay occurs during the opening of the glottis. This shows that the blockage of the flow during glottis closure and the resulting large temporal gradients in pressure and velocity in the larynx are the key features for an efficient phonation process which is also visible in the aero-acoustic sound sources.

Aeroacoustic topology optimization of noise barrier based on Lighthill's acoustic analogy

Topology optimization for the shape of noise barrier taking into account the aeroacoustic sound source is presented based on Lighthill's acoustic analogy approach. In the previous acoustic topology optimization based on the Helmholtz equation, only simple acoustic sound sources were taken into account. To overcome this limitation, the present aeroacoustic topology optimization is performed using Lighthill's equation that represents the sound propagation by an aeroacoustic sound source. In this study, aeroacoustic sound sources are randomly generated through the stochastic noise generation and radiation method. As an interpolation method of material properties by continuous design variables, the linear filter and the sigmoid penalty function are used. The top shape of noise barrier is designed by the optimization examples for various conditions, such as random aeroacoustic sound sources, interpolation methods, and target frequencies.

Acoustic modeling of fricative /s/ for an oral tract with rectangular cross-sections

Fricative /s/ is known to be pronounced by jet generation and subsequent impact on walls of the oral cavity. The prediction of acoustic characteristics of /s/ is an ongoing research topic due to the aeroacoustic nature of the sound source. In this study, acoustic characteristics are modeled using the multimodal theory with monopole and dipole sources positioned in the oral tract waveguide. The oral tract geometry of fricative /s/ was simplified by concatenating rectangular channels whose cross-sectional areas and vertical height are derived from medical imaging. To validate the model accuracy, transverse and sagittal directivity patterns (49 cm every 15°) were measured for flow supplied to a realistic oral tract replica. Comparison between measured and modeled spectra showed that the modeling with the dipole source predicted the pressure amplitude within a discrepancy of ±5 dB up to 14 kHz. Modeled acoustic directivity patterns using a dipole source reflected main tendencies observed on measured directivity patterns in both the transverse and sagittal planes. The proposed modeling approach enables a systematic analysis of the high frequency (>5 kHz) acoustic characteristics as a function of geometrical details for speech production due to an aeroacoustic sound source.

Aeroacoustic source localization and source level estimation during wind tunnel testing of a flat plate with and without a gap

Acoustic measurements in wind tunnels are notoriously difficult because of machinery noise from the tunnel, and aeroacoustic noise generated within the tunnel and by the test model; and because the potentially-weak signals of interest may share a common frequency range with these noise sources. Thus, when model changes are made, localizing any new aeroacoustic source(s) and determining their level(s) are challenging tasks. This presentation provides such experimental results for the aeroacoustic source associated with addition of a 6 mm gap in a 0.5-m-by-1.0-m flat plate aligned with the flow direction. When present, the gap was located 0.40-m from the plate’s leading edge. The measurements were collected in the Anechoic Flow Facility of the Naval Surface Warfare Center – Carderock Division at nominal air speeds of 19 and 30 m/s using a ∼0.6-m-diameter spiral microphone array with 24 elements placed 1.24 m from the plate and separated from the air flow by a thin barrier. Measurements in the 4-to-12 kHz frequency range were processed using conventional and high-resolution beamforming methods with and without noise-reference subtraction. Source location and level estimation for the gap-induced aeroacoustic sound source was successful at nominal signal-to-noise ratios of –20 dB. [Work supported by NAVSEA through the NEEC.]

Extraction and imaging of aerodynamically generated sound field of rotor blades in the wind tunnel test

Abstract The acoustic beamforming has been widely applied in the imaging of flow-induced aeroacoustic sound sources. However, the measured signals of the rotor blades often accompanied with the unwanted interference from other components of the experimental setup in the wind tunnel test (for example, the rotor shaft and the stand), which results in the undistinguished sources in the beamforming result. In this paper, the signals of rotor blades are defined first as the cyclostationary process based on the Ffowcs Williams-Hawkings (FW-H) equation in the form of Wold-Cramer decomposition, which connects the statistical definition of rotor blades signals with the wave propagation model of moving sources. Then the developed cyclostationary signal processing tools of a second order, specifically the reduced-rank cyclic Wiener filter, can be applied in the wind tunnel test of rotor blades. The rotor blades signals can be extracted from the noisy measurements with other interferences, which aide to purify the image results of beamforming in the final experiment of wind tunnel test.

Aeroacoustic analysis of the human phonation process based on a hybrid acoustic PIV approach

The detailed analysis of sound generation in human phonation is severely limited as the accessibility to the laryngeal flow region is highly restricted. Consequently, the physical basis of the underlying fluid–structure–acoustic interaction that describes the primary mechanism of sound production is not yet fully understood. Therefore, we propose the implementation of a hybrid acoustic PIV procedure to evaluate aeroacoustic sound generation during voice production within a synthetic larynx model. Focusing on the flow field downstream of synthetic, aerodynamically driven vocal folds, we calculated acoustic source terms based on the velocity fields obtained by time-resolved high-speed PIV applied to the mid-coronal plane. The radiation of these sources into the acoustic far field was numerically simulated and the resulting acoustic pressure was finally compared with experimental microphone measurements. We identified the tonal sound to be generated downstream in a small region close to the vocal folds. The simulation of the sound propagation underestimated the tonal components, whereas the broadband sound was well reproduced. Our results demonstrate the feasibility to locate aeroacoustic sound sources inside a synthetic larynx using a hybrid acoustic PIV approach. Although the technique employs a 2D-limited flow field, it accurately reproduces the basic characteristics of the aeroacoustic field in our larynx model. In future studies, not only the aeroacoustic mechanisms of normal phonation will be assessable, but also the sound generation of voice disorders can be investigated more profoundly.

Efficient and Noise Reduced Design of a Side Channel Blower Considering Psychoacoustic Evaluation Criteria

Commonly used turbo machines like axial fans or side channels blowers have a large impact on their environment: Large demand for electrical power and production of strong aeroacoustic sound sources, generating a significant noise emission, lead to a environmental footprint. The current work addresses both tasks: improvement of efficiency and reduction of noise emission. In this work a side channel blower is in the focus of the investigations. Since the noise emission of side channel blowers mainly occurs in the region of the stripper, pressure measurements on the blade surface are performed. The pressure trend during the blade passage along the stripper provides information on the exact location of and the contribution to the acoustic noise generation. Modifications of the stripper geometry with the objective of pressure drop linearization along the stripper are evaluated with respect to efficiency, pressure rise and noise reduction. All acoustic measurements are also evaluated with psychoacoustic quantities, like loudness and sharpness, to account for the human perception.

A hybrid approach to the computational aeroacoustics of human voice production.

The aeroacoustic mechanisms in human voice production are complex coupled processes that are still not fully understood. In this article, a hybrid numerical approach to analyzing sound generation in human voice production is presented. First, the fluid flow problem is solved using a parallel finite-volume computational fluid dynamics (CFD) solver on a fine computational mesh covering the larynx. The CFD simulations are run for four geometrical configurations: both with and without false vocal folds, and with fixed convergent or convergent-divergent motion of the medial vocal fold surface. Then the aeroacoustic sources and propagation of sound waves are calculated using Lighthill's analogy or acoustic perturbation equations on a coarse mesh covering the larynx, vocal tract, and radiation region near the mouth. Aeroacoustic sound sources are investigated in the time and frequency domains to determine their precise origin and correlation with the flow field. The problem of acoustic wave propagation from the larynx and vocal tract into the free field is solved using the finite-element method. Two different vocal-tract shapes are considered and modeled according to MRI vocal-tract data of the vowels /i/ and /u/. The spectra of the radiated sound evaluated from acoustic simulations show good agreement with formant frequencies known from human subjects.

Flow-Excited Acoustic Resonance Excitation Mechanism, Design Guidelines, and Counter Measures

The excitation mechanism of acoustic resonances has long been recognized, but the industry continues to be plagued by its undesirable consequences, manifested in severe vibration and noise problems in a wide range of industrial applications. This paper focuses on the nature of the excitation mechanism of acoustic resonances in piping systems containing impinging shear flows, such as flow over shallow and deep cavities. Since this feedback mechanism is caused by the coupling between acoustic resonators and shear flow instabilities, attention is focused first on the nature of various types of acoustic resonance modes and then on the aeroacoustic sound sources, which result from the interaction of the inherently unstable shear flow with the sound field generated by the resonant acoustic modes. Various flow-sound interaction patterns are discussed, in which the resonant sound field can be predominantly parallel or normal to the mean flow direction and the acoustic wavelength can be an order of magnitude longer than the length scale of the separated shear flow or as short as the cavity length scale. Since the state of knowledge in this field has been recently reviewed by Tonon et al. (2011, “Aeroacoustics of Pipe Systems With Closed Branches”, Int. J. Aeroacoust., 10(2), pp. 201–276), this article focuses on the more practical aspects of the phenomenon, including various flow-sound interaction patterns and the resulting aeroacoustic sources, which are relevant to industrial applications. A general design guide proposal and practical means to alleviate the excitation mechanism are also presented. These are demonstrated by two examples of recent industrial case histories dealing with acoustic fatigue failure of the steam dryer in a boiling water reactor (BWR) due to acoustic resonance in the main steam piping and acoustic resonances in the roll posts of the Short Take-Off and Vertical Lift Joint Strike Fighter (JSF).

Localization of aeroacoustic sound sources in viscous flows by a time reversal method

A time reversal procedure used previously for aeroacoustic source localization is extended to the viscous flows case. Successive flow configurations including steady flows and unsteady ones have been performed to test and validate the proposed procedure. It is demonstrated that it provides a useful methodology for the detection and the localization of aeroacoustic sources, not only in presence of a dissipative medium but also in unsteady flow configurations. When dealing with dissipative media, the application of time-reversal procedure allows for the recovery of both the shape and the location of the source of sound even if the source amplitude is damped due to the viscous energy loss. For unsteady flows (in presence or not of a dissipative medium), the aeroacoustic source detection method remains always effective even if some theoretical assumptions are not fully satisfied.

Experimental investigation of unsteady flow in axial skewed fans according to flow rates

Unsteady flow in axial fans with circumferential skewed blades were measured at off-design conditions using a hot-wire test, to investigate the effect of sweep on aerodynamic limit and aeroacoustic sound source. Two circumferential skewed fans, with the blade skew angles at 8.3° forward and backward, respectively, were investigated in this study. The instantaneous flow field in the upstream and downstream of these fans was measured at off-design operations, using a two-dimensional (2D) probe based on a Constant Temperature Anemometry (CTA) system, and a NI PXI data collecting system. From the measured results, the characteristics of time domain and frequency domain were presented. The three-dimensional (3D) structure and turbulent characteristics of inlet and outlet flow in circumferential skewed fans were analyzed according to flow rate. Unsteady flow instability and its relation with circumferential skewed blades, to expand stall-free operation range and suppress noise of axial fans, were discussed. The sweep blade is found to be effective in controlling unsteady flow to improve aerodynamic limit and reduce noise source in these rotors at off-design operations.

Microphone Array Wind Tunnel Measurements of Reynolds Number Effects in High-Speed Train Aeroacoustics

The present study focuses on the Reynolds number dependence of high-speed train aeroacoustic sound sources. To cover a wide range of Reynolds numbers the experimental investigations are carried out on a 1: 25 scale-model of the high-speed train Inter City Express 3 by conducting microphone array measurements in two wind tunnels. The latter are the Aeroacoustic Wind tunnel (AWB) of the German Aerospace Center (DLR) in Brunswick, providing nearly perfect acoustical conditions, and the Cryogenic wind tunnel (DNW-KKK) of the DNW (German -Dutch wind tunnels) in Cologne, allowing measurements at higher Reynolds numbers. Two types of sound sources with different characteristics at Reynolds numbers of up to Re = 0.46 × 106 have been identified by measurements in the AWB. It was found, that the aeroacoustic noise from the bogie section is dominant for frequencies f < 5 kHz and can be characterised by cavity mode excitation. Further, the pantograph is the dominant sound source above f = 5 kHz with an Aeolian tone characteristic. Additional aeroacoustic measurements have been conducted in the cryogenic wind tunnel DNW-KKK in order to analyse the Reynolds number dependence of the noise generated at the first bogie, for higher Reynolds numbers of up to Re = 3.70 × 106. The DNW-KKK admits varying the Mach and Reynolds numbers independently. These measurements reveal only a weak Reynolds number dependence of the noise source generated at the first bogie.

Vortex sound in unconfined flows: Application to the coupling of a jet-slot oscillator with a resonator

The aeroacoustical coupling of a jet-slot oscillator to the acoustic resonances of the flow-supply duct is studied in this paper. This configuration, producing self-sustained tones, is original in particular because the source domain is unconfined and the coupling occurs to high-order acoustic resonances. The development of a model, based on the Howe's corollary of the vortex-sound theory permits the flow-acoustic interactions induced in the production of such tones to be modelled theoretically. This model, together with a careful experimental exploration, provided a description of this aeroacoustic sound source. The findings show that the combination of two mechanisms determines the operating frequency that maximizes the acoustic fluctuations responsible for the production of the tones: (i) the energy exchange between the hydrodynamic and the acoustic fluctuations of the fluid, and (ii) the enhancement of the acoustic oscillation by the resonances. Some differences from the usual behaviour of acoustically coupled self-sustained oscillations—in particular, related to the oscillation amplitude and the synchronization between the vortical and acoustical fields—are highlighted.

Wind Tunnel Tests on Reduction of Aeroacoustic Noise from Car Gaps and Bogie Sections

In this study, we propose techniques for reducing aeroacoustic noise from gaps between cars and bogie sections. A microphone array placed adjacent to the main flow of the wind tunnel is used as a measurement device to improve the spatial resolution and the accuracy of identifying the noise sources. The results show that aeroacoustic sound sources at gaps between the cars are sufficiently weakened by rounding corners of cars, because the impact of impinging vortices at the corners is relieved and efficiency of acoustical radiation is reduced. Noise from the bogie sections is also reduced by installing side covers, slopes and round corners.

Active control of the blade passage frequency noise level of an axial fan with aeroacoustic sound sources

This paper presents results of a research project on active control of the blade passage frequency tone of an axial fan. The secondary sound field is generated by aeroacoustic sources, which are produced by actively controlling the flow around the impeller blade tips. Both amplitude and phase can be controlled in such a way that a destructive superposition with the primary sound field is possible. The flow distortions can be achieved by using different actuators; results using steady and unsteady jets of compressed air and piezo-electric actuators are presented.

127 数値解析による車両台車部空力騒音の可視化(騒音解析技術(1),騒音・振動評価・改善技術)

127 数値解析による車両台車部空力騒音の可視化(騒音解析技術(1),騒音・振動評価・改善技術)

An aeroacoustically driven thermoacoustic heat pump

The mean flow of gas in a pipe past a cavity can excite the resonant acoustic modes of the cavity--much like blowing across the top of a bottle. The periodic shedding of vortices from the leading edge of the mouth of the cavity feeds energy into the acoustic modes which, in turn, affect the shedding of the next vortex. This so-called aeroacoustic whistle can excite very high amplitude acoustic standing waves within a cavity defined by coaxial side branches closed at their ends. The amplitude of these standing waves can easily be 20% of the ambient pressure at optimal gas flow rates and ambient pressures within the main pipe. A standing wave thermoacoustic heat pump is a device which utilizes the in-phase pressure and displacement oscillations to pump heat across a porous medium thereby establishing, or maintaining, a temperature gradient. Experimental results of a combined system of aeroacoustic sound source and a simple thermoacoustic stack will be presented.