Acoustic Pressure Fluctuations Research Articles

Nonlinear fluid-structure interaction of an elastic panel in an acoustically excited two-dimensional inviscid compressible fluid

The focus of this paper is on the asymptotic investigation of the nonlinear fluid-structure interaction of an acoustically excited clamped panel immersed in an inviscid compressible fluid. A multiple-scales analysis of the corresponding two-dimensional unsteady potential flow initial-boundary-value-problem is employed to investigate both primary resonance and a 3:1 internal resonance between the panel fifth and ninth modes. Validation of the asymptotic structural response and the fluid pressure shows good agreement with numerical solution of a weakly nonlinear panel in a quadratic Euler field. The results shed light on the intricate acoustic interaction bifurcation structure which exhibits coexisting bi-stable periodic solutions, and quasiperiodic response reflecting spatially periodic modal energy transfer for both panel and fluid. This behavior is found to occur for panel excitation by finite level acoustic pressure waves that can be a crucial factor for design of high integrity structural systems required for aviation or space where light structures are exposed to intensive acoustic pressure fluctuations.

Experimental localization of an acoustic sound source in a wind-tunnel flow by using a numerical time-reversal technique

The possibility of using the time-reversal technique to localize acoustic sources in a wind-tunnel flow is investigated. While the technique is widespread, it has scarcely been used in aeroacoustics up to now. The proposed method consists of two steps: in a first experimental step, the acoustic pressure fluctuations are recorded over a linear array of microphones; in a second numerical step, the experimental data are time-reversed and used as input data for a numerical code solving the linearized Euler equations. The simulation achieves the back-propagation of the waves from the array to the source and takes into account the effect of the mean flow on sound propagation. The ability of the method to localize a sound source in a typical wind-tunnel flow is first demonstrated using simulated data. A generic experiment is then set up in an anechoic wind tunnel to validate the proposed method with a flow at Mach number 0.11. Monopolar sources are first considered that are either monochromatic or have a narrow or wide-band frequency content. The source position estimation is well-achieved with an error inferior to the wavelength. An application to a dipolar sound source shows that this type of source is also very satisfactorily characterized.

Hybrid Analytical/Numerical Prediction of Propeller Broadband Noise in the Time Domain

This work deals with the numerical prediction of the trailing edge noise from rotating blades. A semi-empirical model of the wall pressure spectrum in proximity of the trailing edge is used in connection with an analytical flat-plate aerodynamic response function to compute the acoustic pressure fluctuations on the surface of the blade. Pressure signals with a broadband spectral content are generated by superimposing Fourier components of the blade pressure waves with a stochastic phase distribution. Finally, the far-field noise is computed through a numerical integration of the Ffowcs-Williams and Hawkings equation in the time domain. The hybrid analytical/numerical broadband noise prediction method is verified by computing the trailing-edge noise from a flat plate and comparing its spectrum to the one obtained through a direct analytical extrapolation of the wall pressure spectrum in the frequency domain. The same flat-plate results are then validated against measurements of trailing edge noise spectrum from a NACA-0012 airfoil. The method is finally used to compute the broadband noise from an aircraft propeller and results are compared with analytical results obtained through a blade strip approach. The method and the results described in this paper have been presented at the 14th CEAS-ASC workshop held on 7–8 October 2010 at the Institute of Aviation in Warsaw, Poland.

Unsteady response of hydrogen and methane flames to pressure waves

Recent measurements of the direct response of premixed hydrocarbon flames to acoustic pressure fluctuations have shed doubt on the validity of analytical models that use irreversible one-step chemistry (Wangher et al., 2008) [1], and suggest that more realistic chemical kinetic models are needed to fully describe the unsteady dynamics of premixed flames. In this paper we present experimental results and numerical simulations for planar hydrogen flames which have simpler chemical kinetics than hydrocarbon flames. The simulations employ detailed chemical kinetics, including OH∗ chemiluminescence chemistry, so that the validity of using the emission from the excited OH∗ radical as a marker of the reaction rate can be assessed. By comparing our numerical results with measurements on hydrogen, and with previous measurements on methane, we show that OH∗ chemiluminescence does not always provide a reliable measure of heat release rate in the presence of a pressure driven interaction. Finally, our results are compared to the predictions of an analytical model for two-step chemistry with a chain-branching and a chain-breaking reaction (Clavin and Searby, 2008) [2]. We conclude that multi-step chemistry must be taken into account when evaluating the unsteady response of flames to pressure waves.

Underwater acoustic field and pressure fluctuation on ship hull due to unsteady propeller sheet cavitation

The main objective of this paper is to develop an efficient numerical method which can predict the underwater acoustic field and pressure fluctuation on a ship hull due to unsteady propeller sheet cavitation by linear acoustic theory. In addition, the noise scattered from the ship hull and reflected from the free surface are included. Concerning the computation of the acoustic field induced by unsteady sheet cavitation and forces of a marine propeller, a method is derived without making any approximation about the distance function between the noise source and field point. Thus, this method can be used to predict acoustic pressure at both far and near fields, and this is very important for the scattering problem because the ship hull is located very close to the propeller. For the computation of the scattering problem, a more efficient and robust method is derived in time domain, which can treat multi-frequency waves scattered from underwater obstacles. The acoustic fields of a container ship radiated by the propeller and scattered from the ship hull with free surface is investigated in this paper. The pressure fluctuations of low blade rate on the ship hull induced by the propeller are also computed by the present method and are found to be similar to the results obtained by a panel method satisfying the Laplace equation for the points near the propeller due to the small retarding time. However, for the points on the ship hull away from the propeller, the differences of the results between two methods will increase.

Correlation of Flowfield and Acoustic field Measurements in High-Speed Jets

The present study demonstrates the capabilities of an optical deflectometry (OD) technique to provide flow field turbulence measurements that can be successfully correlated with far-field acoustic field measurements. Results from supersonic fully expanded jets are presented and show variations in the frequency content of the coherence between the flow field and the acoustic field with OD positioning. The method is applied to jets with supersonic convection speed, producing Mach wave radiation, and show the strong directionality of the emitted noise. Similar measurements performed in shock-containing jets with microphones in the forward arc reveal enhanced coherence at the same frequency as the Broad Band Shock Associated Noise. Measurements are also made with the optical sensors scanning radially outside of the jet in an attempt to obtain a measurement of the spectra in the acoustic near field. These results show an exponential decay of the signal characteristic of the hydrodynamic region, where an evanescent pressure field dominates the total pressure fluctuation. The high level of skewness in these signals also support the concept that the crackle observed in the far-field originates in the near field. Further radially, acoustic pressure fluctuations are measured and successfully correlated to the acoustic far-field, thus demonstrating another application of this non-intrusive measurement technique in jet acoustics diagnostics.

Low Mach number analysis of idealized thermoacoustic engines with numerical solution

A model of an idealized thermoacoustic engine is formulated, coupling nonlinear flow and heat exchange in the heat exchangers and stack with a simple linear acoustic model of the resonator and load. Correct coupling results in an asymptotically consistent global model, in the small Mach number approximation. A well-resolved numerical solution is obtained for two-dimensional heat exchangers and stack. The model assumes that the heat exchangers and stack are shorter than the overall length by a factor of the order of a representative Mach number. The model is well-suited for simulation of the entire startup process, whereby as a result of some excitation, an initially specified temperature profile in the stack evolves toward a near-steady profile, eventually reaching stationary operation. A validation analysis is presented, together with results showing the early amplitude growth and approach of a stationary regime. Two types of initial excitation are used: Random noise and a small periodic wave. The set of assumptions made leads to a heat-exchanger section that acts as a source of volume but is transparent to pressure and to a local heat-exchanger model characterized by a dynamically incompressible flow to which a locally spatially uniform acoustic pressure fluctuation is superimposed.

Fan noise visualization and noise source identification by using nearfield acoustical holography.

A nearfield acoustical holography (NAH) method combined with a non-stationary source compensation procedure is applied to identify the noise sources of two open fans. The measured noise data of a low-speed, small fan indicate the highest peak levels at the blade passing frequencies (BPFs) of 102 and 204 Hz. It is observed that there is no noise component induced by vortex shedding at the blade tips, or leading or trailing edges of the small fan. It is also observed that the structural resonance frequencies of the small fan blades are not coincident with the BPFs: thus, the structural vibration responses of the blades contribute insignificantly to the fan noise. The NAH results of the small fan show acoustic pressure fluctuations on the blade surfaces at the BPFs. The measured noise data of a high-speed, large fan show the frequency components associated with BPFs as well as vortices shed at blade edges. Similar to the small fan case, the resonance frequencies of the fan blades cannot be observed in the measured acoustic data. The NAH results of the high-speed, large fan successfully exhibit acoustic pressure fluctuations at BPFs as well as noise patterns radiated from the vortex shedding.

Measured wavenumber: Frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.

대와류모사법을 이용한 원주 주위의 공력소음 특성에 관한 기초연구

As a basic study of the aero-acoustic noise, large eddy simulations were carried out for a fixed circular cylinder at Reynolds number(Re=9.0×10⁴) using commercial CFD code, FLUENT. The subgrid-scale turbulent viscosity was modeled by Smagorinsky- Lilly model adapted to structured meshes. The results of LES analysis showed that time-averaged value, C<SUB>D</SUB> was approximately 1.47 which was considerably adjacent with the experimentally measured value of 1.32 in comparison with the results of twoequation turbulent models performed by previous researchers. It was observed that there were the very small acoustic pressure fluctuations with the same frequency of the Karman vortex street. In conclusion, the analysis of LES provided the improved results in the prediction of drag and lift coefficient in addition to acoustic pressure distribution than two other turbulent models.

Interaction Between the Acoustic Pressure Fluctuations and the Unsteady Flow Field Through Circular Holes

Gas turbine combustion systems are prone to thermo-acoustic instabilities, and this is particularly the case for new low emission lean burn type systems. The presence of such instabilities is basically a function of the unsteady heat release within the system (i.e., both magnitude and phase) and the amount of damping. This paper is concerned with this latter process and the potential damping provided by perforated liners and other circular apertures found within gas turbine combustion systems. In particular, the paper outlines experimental measurements that characterize the flow field within the near field region of circular apertures when being subjected to incident acoustic pressure fluctuations. In this way the fundamental process by which acoustic energy is converted into kinetic energy of the velocity field can be investigated. Experimental results are presented for a single orifice located in an isothermal duct at ambient test conditions. Attached to the duct are two loudspeakers that provide pressure fluctuations incident onto the orifice. Unsteady pressure measurements enable the acoustic power absorbed by the orifice to be determined. This was undertaken for a range of excitation amplitudes and mean flows through the orifice. In this way regimes where both linear and nonlinear absorption occur along with the transition between these regimes can be investigated. The key to designing efficient passive dampers is to understand the interaction between the unsteady velocity field, generated at the orifice and the acoustic pressure fluctuations. Hence experimental techniques are also presented that enable such detailed measurements of the flow field to be made using particle image velocimetry. These measurements were obtained for conditions at which linear and nonlinear absorption was observed. Furthermore, proper orthogonal decomposition was used as a novel analysis technique for investigating the unsteady coherent structures responsible for the absorption of energy from the acoustic field.

Invariant features of deep ocean ambient noise.

Very low‐frequency, deep ocean ambient noise is generated by non‐linear interactions (collisions) between surface waves with nearly equal frequency, propagating in nearly opposite direction. It is shown that the usual “standing wave” approximation used to compute this noise predicts that the ratios of entries in the power spectral density matrix, obtained from auto and cross‐correlations of acoustic velocity and pressure fluctuations, are universal constants. This prediction is too strong and not observed. A careful consideration of bottom effects invalidates the standing wave approximation. Nevertheless a weak version of the standing wave approximation still holds and has interesting implications. While the ratios of entries in the power spectral density matrix due to surface wave generated noise are not universal constants, they are insensitive to the details of the surface wave spectrum. This theoretical prediction is borne out by data from the Hawaii‐2 Observatory. [Work supported by ONR.]

Ablation of Chronic Total Occlusions Using Kilohertz-Frequency Mechanical Vibrations in Minimally Invasive Angioplasty Procedures

Certain minimally invasive cardiology procedures, such as balloon angioplasty and stent implantation, critically require that the site of an arterial blockage be crossed by an intraluminal guidewire. Plaques resulting in near or totally occluded arteries are known as chronic total occlusions, and crossing them with conventional guidewires is a significant challenge. Among the most promising proposed solutions is the delivery of high-power, low-frequency ultrasonic vibrations to the occlusion site via an intraluminal wire waveguide. The vibrating distal tip of the ultrasound wire waveguide is used to transmit energy to the surrounding plaques, tissues, and fluids to ablate or weaken atherosclerotic plaque. Potential mechanisms of interaction with the plaque and adjacent fluids identified in the literature include: (i) direct contact with the waveguide distal tip, (ii) subcavitational acoustic fluid pressure fluctuations, (iii) cavitation, and (iv) acoustic streaming. We summarize developments in this area over more than two decades, describing experimental methods for device performance characterization, preclinical tests, early clinical investigations, and, later, full clinical trials. The article also reviews theoretical foundations and numerical models suitable for device design and analysis. Finally, important issues for future research and for the development of this technology will be considered.

Two-step simulation of slat noise

The flow field and the acoustic field of a high-lift configuration consisting of a slat and a main wing are numerically investigated by a hybrid method. In a first step, the unsteady flow field is computed via a large-eddy simulation (LES) and in a second step, the acoustic field is determined by solving the acoustic perturbation equations (APE). The mean flow field is compared to experimental findings followed by an investigation of the turbulent structures which are visualized by λ 2 contours. The analysis of the acoustic field shows that at the main wing trailing edge acoustic pressure fluctuations of approximately 5 kHz are generated. Correlations between the noise sources and the acoustic pressure identify the slat-gap region to be responsible for the mixture of broadband and tonal noise between 1 and 3 kHz. The decay of the pressure spectrum is found to be approximately f −2 which is in agreement with the literature.

Quantification of the transient mass flow rate in a simplex swirl injector

When a heat release and acoustic pressure fluctuations are generated in a combustor by irregular and local combustions, these fluctuations affect the mass flow rate of the propellants injected through the injectors. In addition, variations of the mass flow rate caused by these fluctuations bring about irregular combustion, which is associated with combustion instability, so it is very important to identify a mass variation through the pressure fluctuation on the injector and to investigate its transfer function. Therefore, quantification of the variation of the mass flow rate generated in a simplex swirl injector via the injection pressure fluctuation was the subject of an initial study. To acquire the transient mass flow rate in the orifice with time, the axial velocity of flows and the liquid film thickness in the orifice were measured. The axial velocity was acquired through a theoretical approach after measuring the pressure in the orifice. In an effort to understand the flow area in the orifice, the liquid film thickness was measured by an electric conductance method. In the results, the mass flow rate calculated from the axial velocity and the liquid film thickness measured by the electric conductance method in the orifice was in good agreement with the mass flow rate acquired by the direct measuring method in a small error range within 1% in the steady state and within 4% for the average mass flow rate in a pulsated state. Also, the amplitude (gain) of the mass flow rate acquired by the proposed direct measuring method was confirmed using the PLLIF technique in the low pressure fluctuation frequency ranges with an error under 6%. This study shows that our proposed method can be used to measure the mass flow rate not only in the steady state but also in the unsteady state (or the pulsated state). Moreover, this method shows very high accuracy based on the experimental results.

Revisiting a Model for Combustion Instability Involving Vortex Shedding

We revisit a simple reduced order model (Matveev, K., and Culick, F. E. C, 2003, A Model for Combustion Instability Involving Vortex Shedding, Combust Sci. and Tech., 175, 1059) and re-examine its implications in light of the non-normal and nonlinear nature of combustion acoustic interactions. To this end, one-dimensional linear acoustic equations are used to model the acoustic field in an open-open duct. The Galerkin technique is then implemented to expand the acoustic pressure and velocity fluctuations in terms of the natural acoustic modes. The coupled thermoacoustic system is shown to be non-normal and nonlinear. This leads to complicated but interesting physics that were not examined in the earlier study. Examples showing transient growth leading to instability and bootstrapping in an initially decaying system are then presented. It is also shown that a vortex-based combustor reaches different limit cycle amplitudes for the same system parameters when subjected to different initial conditions. Further, the effect of damping on the non-normal behavior of the system is studied. A test case is presented that shows that for a lowly damped system, a slight increase in damping leads to high amplitude unstable oscillations. Finally, pseudospectral analysis is presented to study the non-normal behavior of such systems.

Prediction of low-speed aerodynamic load and aeroacoustic noise around simplified panhead section model

This paper deals with a low-speed aerodynamic and aeroacoustic analysis of the two-dimensional panhead section model that is a simplification of the pantograph used in high-speed railway (HSR). As a preliminary study of HSR whose actual operating speed is over 300 km/h, the present study considers the low-speed aerodynamics and aeroacoustics in which the speed range is between 120 and 180 km/h. The computational fluid dynamics-based analysis is performed using FLUENT and the low-speed wind tunnel test is conducted to verify the flow simulation as well. The aerodynamic noise is predicted using Ffowcs Williams—Hawkings equation without the quadrupole term. The present study suggests the use of a thin plate (i.e. lift generator) to generate the positive up-lift force that can provide the stable aerodynamic contact between the panhead strips and the catenary system. For a number of panhead section models, acoustic pressure fluctuations and sound pressure levels are analysed to identify the optimum plate length of the lift generator in terms of aerodynamic stability and aeroacoustic noise reduction.

Fluctuations under turbulent flows: Enhanced methods for separation of propagative wavenumbers from wall pressure dataset

This work is a part of a more general study on automobile interior noise due to acoustic and aerodynamic wall-pressure fluctuations. Using experimental data of wall-pressure fluctuations measured with a microphone array beneath several kinds of flows, a wave-number analysis based on recently developed signal processing methods -- the spatial Empirical Mode Decomposition (sEMD) and the Ensemble Empirical Mode Decomposition (E-EMD) -- is carried out, in order to separate acoustic and aerodynamic pressure fluctuations. In opposition with an existing classical method previously used, based on the spatial correlogram, these methods do not require a stationary uniform flow. A turbulent boundary layer on a flat plate and a detached/reattached flow downstream three different forward-facing steps are tested, with flow velocities from 0 to 40 m.s-1, with or without a well-controlled artificial acoustic source. The sEMD method is first used as a wavenumber filter, and is shown to improve the detection of acoustic fluctuations of about 5 dB on classical wavenumber (k,f) representations. The E-EMD method is developed in order to decompose the (x,t) representation to make possible the separation of the acoustic and aerodynamic components, regardless of the stationarity or the uniformity of the flow.

The acoustic wave propagation equation in a turbulent combusting flow

Sound generation by turbulent flames originates from the fluctuating heat release in the flame. The description of this fluctuating heat release and its effect on acoustics in turbulent flames is complicated due to the interaction of chemical reactions with turbulence, mixing and pressure fluctuations. In a turbulent flame the instantaneous density, velocity, pressure, temperature and species concentrations are determined by the transport equations for mass, momentum, enthalpy and species and by the equation of state. In the proposed paper an equation is formulated that describes the propagation of acoustic pressure fluctuations, and that determines the source terms. These source terms are compared with various terms derived in the literature. Significant differences are found between several approaches. An effort is made to come to a reliable and generally acceptable formulation. Subsequently events are ordered on basis of their typical time scale. That way source terms can be evaluated for the situation where the combustion is described with the use of time averaged chemical reaction progress variables and a mixture fraction variable. Subsequently the consequences of Reynolds and Favre averaging on these source terms and conservation of acoustical variables in a domain with turbulent flow are discussed.

Transcranial Doppler and acoustic pressure fluctuations for the assessment of cavitation and thromboembolism in patients with mechanical heart valves

The formation and collapse of vapor-filled bubbles near a mechanical heart valve is called cavitation. Such microbubbles are suspected to have strong pro-coagulant effects. Therefore, cavitation may be a contributing factor to the pro-thrombotic effects of mechanical valves. Herein, we systematically review the available evidence linking cavitation and thrombosis. We also critically appraise the potential usefulness of transcranial Doppler and other new non-invasive diagnostic methods to study cavitation and cerebral embolism in mechanical valve patients. Experimental studies indicate that cavitation microbubbles cause platelet aggregation, complement-activation, fibrinolysis, release of tissue-factor, and endothelial damage. Administration of 100% oxygen to mechanical valve patients during transcranial Doppler examination can transiently decrease the counts of Doppler-detected cerebral microemboli compared with room air. This is associated with removal of most circulating gaseous emboli from cavitation. This method may therefore be applied to the study of cavitation and thromboembolism. Additionally, the analysis of high-frequency acoustic-pressure fluctuations detected from the implosion of cavitation bubbles is a promising method for assessment of cavitation in vivo; however, this requires further development. A better understanding of cavitation is important in order to adequately investigate its role in the overall pro-thrombotic effects in mechanical valve patients. Such studies may allow establishing guidelines for new valve designs.