Mechanism Of Acoustic Generation Research Articles

A general acoustic framework for the assessment of noise emitted by combustors: A first case study

This study proposes an alternative and general framework for the assessment of the sound generated within a combustion chamber. The novel approach is based on Doak’s Momentum Potential Theory and combines the advantage of using the single assumption of a time-stationary fluctuating flow with a clear separation of turbulent, thermal and acoustic dynamics. The latter can furthermore be quantified by defining a generalised acoustic variable, which can be identified in the fluctuations of the stagnation enthalpy. The proposed model provides a new and more comprehensive explanation for the generation of combustion noise and is used here for the first time to analyse the acoustic properties of the turbulent and non-isentropic flow in a simplified combustion chamber, represented by LES data of a non-reacting combustor simulator. The application highlights the ability of the model to identify and clearly separate the main dynamical effects characteristic for the flow. Finally, the acoustic production can be quantified using the concept of the generalised acoustic field and related to the turbulent dynamics, which seems to represent the major acoustic generation mechanism for the considered setup.

Acoustic Wave Generation in Nanofluids: Effect of the Kapitza Resistance on the Thermophone to Mechanophone Generation Mechanism Transition

High frequency, short wavelength acoustic wave generation in fluids, by means of immersed nanotransducers, is key to applications ranging from the biomedical to the ICT sector, all the way to investigating fundamental aspects of the mechanics of fluids. In this context, the commonly adopted generation mechanism is the thermophone, where the nanotransducer serves as a nanoheater for the surrounding fluid and the acoustic wave is launched by water thermal expansion. Its performance, however, is severely degraded when reaching up to hypersonic frequencies. We analytically investigate the thermoacoustic response in the frequency domain for the case of a gold nanofilm transducer in contact with water. We prove that another generation mechanism, the mechanophone, sets in this critical frequency range, widening the acoustic generation bandwidth. In the mechanophone, it is the nanotransducer thermal expansion that launches pressure waves in water. We find a threshold frequency discriminating between these two regimes, which can be tuned by acting on the Kapitza resistance at the solid–liquid interface. We then show how the mechanophone mechanism can be activated in the frame of photoacoustic generation with pulsed laser sources. The unveiled physics bears generality beyond the specific system and explains the acoustic generation mechanisms in nanofluids.

Aeroacoustic Characteristics of a Synchronized Fluidic Oscillator

A new opposite facing oscillator pair is presented where shared feedback channels enable synchronized sweeping of the exiting jets. The design has no moving parts and the oscillator pair is composed in a back-to-back configuration. The synchronized operation generates a near-field acoustic tonality and the objective is to determine the emitted directivity. The acoustic generation mechanisms were determined using compressible large eddy simulations, which was validated with hot-wire and microphone measurements. Different Reynolds number (Re) conditions up to 21,250 were analyzed for a synchronized oscillation with Strouhal number (St) of order 0.01. Dominant acoustic sources emerging during synchronized sweep oscillation were classified by the interpretation of directivity patterns for a selected flow rate. The near-field directivity pattern could be decomposed as a periodic signal consisting of one acoustic mode with a dipole-like pattern, showing two major lobes associated with the jet sweeping oscillation frequency: one acoustic monopole-like directivity for the first overtone; and dipole-like directivity with two lobes for the second harmonic. It is shown that the acoustic sources are generated by the synchronized pressure oscillation of the exiting jets. The vortical structures inside the fluidic oscillator interact with the non-slip walls. These manipulate the curvature of the central jet and causes an unsteady loading towards the discharge. The new fluidic oscillator design gives synchronized exiting sweeping jets and a large flow length scale near-field directivity pattern. These features give feedback type fluidic oscillators a wider application range.

Aeroacoustic resonance and self-excitation in screeching and impinging supersonic jets – A review

Supersonic jets, particularly shock-containing jets, often exhibit high-intensity, discrete-frequency acoustic tones. These tones are the signature of an aeroacoustic resonance loop established by the flow. This paper considers two of the classical forms of supersonic jet resonance: screech in shock-containing free jets and tones generated by the impingement of a jet against a surface. The first half of the paper provides a historical perspective on research into both forms of resonance, ranging from the seminal works of Alan Powell to recent contributions from high-fidelity numerical simulation, experiment, and stability theory. The second half of the paper provides a critical assessment of current understanding around the four processes that characterize jet resonance: a downstream propagation of energy, an acoustic generation mechanism, an upstream propagation of energy, and a receptivity mechanism.

Acoustic signature of flow instabilities in radial compressors

Rotating stall and surge are flow instabilities contributing to the acoustic noise generated in centrifugal compressors at low mass flow rates. Their acoustic generation mechanisms are exposed employing compressible Large Eddy Simulations (LES). The LES data are used for calculating the dominant acoustic sources emerging at low mass flow rates. They give the inhomogeneous character of the Ffowcs Williams and Hawkings (FW-H) wave equation. The blade loading term associated with the unsteady pressure loads developed on solid surfaces (dipole in character) is found to be the major contributor to the aerodynamically generated noise at low mass flow rates. The acoustic source due to the velocity variations and compressibility effects (quadrupole in character) as well as the acoustic source caused by the displacement of the fluid due to the accelerations of the solid surfaces (monopole in character) were found to be not as dominant. We show that the acoustic source associated with surge is generated by the pressure oscillation, which is governed by the tip leakage flow. The vortical structures of rotating stall are interacting with the impeller. These manipulate the flow incidence angles and cause thereby unsteady blade loading towards the discharge. A low-pressure sink between 4 and 6 o'clock causes a halving of the perturbation frequencies at low mass flow rates operating conditions. From two point space-time cross correlation analysis based on circumferential velocity in the diffuser it was found that the rotating stall cell propagation speed increases locally in the low pressure zone under the volute tongue. It was also found that rotating stall can coexist with surge operating condition, but the feature is then seen to operate over a broader frequency interval.

Investigation and Analytical Description of Acoustic Production by Magneto-Acoustic Mixing Technology

Magneto-Acoustic Mixing Technology is a novel manufacturing method that combines two magnetic fields to produce high-intensity sonication for liquid-state materials processing. This method may be adapted to the manufacture of various materials that benefit from a combination of high temperature, magnetic fields, and acoustic energy. In this work, acoustic generation mechanisms are described in detail and found to be dependent on the skin depth of the induction currents. Analytical models of acoustic pressure are derived, based on two mutually exclusive vibration mechanisms, crucible and melt vibration. Additionally, grain size evidence of acoustic pressure distribution is presented as preliminary model validation.

Numerical study on acoustic generation of a supersonic jet impinging to deflectors

Acoustic wave generated from a M = 1.8 ideally expanded jet impinging on flame deflectors is investigated numerically in order to obtain the knowledge of flame deflector design to mitigate the acoustic loading on launch systems and payloads. Mechanism of acoustic wave generated from a 45-degree-inclined flat plate placed 5D downstream from the nozzle exit is analyzed first, and it is revealed that source of the acoustic wave is located where shock waves is formed due to the jet impingement. In this study, effect of the deflector shape will be discussed for better understanding the acoustic generation mechanism.

S0505-3-4 CFDを用いたロケット打上げ時の音響環境予測に関する研究(車両・機体・流体機械に関連した騒音問題へのアプローチと対策(3))

Since acoustic wave radiated from the rocket plume causes severe acoustic loading on the payload at lift-off, prediction and reduction of acoustic level is an important issue in the design phase. Acoustic environment is empirically predicted as of now. However, the acoustic generation mechanism is not clearly understood, which results in unsatisfactory prediction accuracy. In order to overcome the limitations of the empirical method, CFD study is carried out in JAXA. Through the comparison with the measured result in the static-firing tests, it is found that prediction accuracy of the CFD superiors to the empirical method. In addition, the acoustic generation becomes clear, which gives us the idea of developing the launch-pad which reduces acoustic level of the vehicle.

Acoustic monitoring of microplasma formation and filamentation of tightly focused femtosecond laser pulses in silica glass

Contact acoustic technique has been employed to perform spatially resolved in situ detection of microplasma formation and filamentation of tightly focused femtosecond laser pulses with supercritical pulse powers in bulk dielectrics, via corresponding acoustic emission. Investigation of acoustic generation mechanisms related to the plasma formation and filamentation effects reveals the critical character of the opaque microplasma and provides estimates of its gigapascal-level pressures and energy densities of a few kJ∕cm3. The acoustic measurement enables real-time in situ monitoring and revealing of basic mechanisms of ionization and filamentation in bulk dielectrics.

Acoustic Wave Generation in a Compressible Wake

The compressible Navier-Stokes equations are numerically solved to study the acoustic generation mechanism associated with the evolution of the structure in a compressible plane wake undergoing transition to turbulence. High-order compact finite difference schemes are used for spatial derivatives and a 4th-order Runge-Kutta scheme is employed for time advancement. Navier-Stokes characteristic boundary conditions are used in the vertical direction and periodic boundary conditions in the streamwise and spanwise directions. Three-dimensional structures of the wake are studied by means of temporally evolving plane wakes forced with a combination of unstable modes obtained from linear stability theory using a mapped Fourier method for the viscous compressible equations. Forcing with a pair of oblique subharmonic unstable modes yields streamwise/vertical counter- rotating vortices in the saddle region. As the streamwise/vertical vortices evolve outside, their self-induction causes inclined braidlike structures to form in the wake, which are similar to observations in the experimental supersonic flat wake transition. The oblique subharmonic unstable modes also cause the spanwise variations in the core that lead to roller distortion. Acoustic waves of plane wakes are generated when two-dimensional rollup structures appear and rotate in such wakes. Near-field sound wave pressure decreases downstream due to the three-dimensional evolution of the wake.

Generation of frequency-tunable nanoacoustic waves by optical coherent control

We have developed a system to generate arbitrary wave-form nanoacoustic waves (NAWs) with a piezoelectric InGaN∕GaN single-quantum well. Based on an optical coherent control technique, acoustic frequency tunability in the subterahertz range is realized within only one fixed sample. The acoustic generation mechanisms, especially the in-well piezoelectric field Coulomb screening which tends to be saturated at high carrier concentrations, are discussed with optical power dependency. With the generated NAWs propagating in the c axis of a GaN thin film, the lifetime of the 500 GHz longitudinal-acoustic phonon pulses in GaN is measured to be longer than 420 ps, corresponding to a GaN depth more than 3.3μm.

Photoacoustic study of KrF laser heating of Si: Implications for laser particle removal

A photoacoustic study of KrF laser heating of Si has revealed that the dominant mechanism of acoustic generation is thermoacoustic with a considerable contribution from the concentration–deformation mechanism at laser fluences below the Si melting threshold of 0.5 J/cm2. Upon Si melting the contraction of the molten material contributes significantly to acoustic generation. At fluences above 1.4 J/cm2 laser ablation of the molten layer enhances the amplitude of the compression pulse and diminishes that of the rarefaction pulse. The results of photoacoustic measurements allow optimization of experimental conditions for dry laser particle removal.

Arc–acoustic interaction in rare gas flashlamps

High frequency oscillations have been observed in rare gas flashlamp voltage and light output pulses. Experiments have shown that the frequency of the oscillations increases with the square root of the input electrical energy density. At fixed energy density input, the period of the oscillations increases linearly with the cylindrical lamp radius and with the square root of the atomic mass of the rare gas. These measured dependences suggest an acoustic generation mechanism with the gas temperature proportional to the input energy density. This interpretation allows a determination of the instantaneous gas temperature from the measured oscillation frequency.

Experimental Studies of the Acoustic Signature of Proton Beams Traversing Fluid Media

The work reported here establishes that a detectable sonic signal is produced by protons while traversing through or stopping in a fluid medium. Experiments exploring the global characteristics of both the acoustic generation mechanism and the radiation pattern have been performed at three different accelerators. The results are consistent with a simple thermal model for the transformation of the energy of moving charged particles into acoustic energy. This novel phenomenon could be exploited in several forseeable applications: (1) as a charged particle monitor in accelerator beams, (2) as a heavy ion detector sensitive to nuclear charge (signal infinity Z/sup 2/), e.g. in measuring the cosmic ray isotope abundances, (3) as an inexpensive shower detector in massive (approx. 10/sup 4/ Ton) neutrino detectors at the next generation of high energy accelerators, e.g. the Fermilab energy doubler and (4) as the shower calorimeter (and perhaps the muon detector) in massive (greater than or equal to 10/sup 9/ Ton) deep underwater detectors of cosmic neutrino and muon interactions.