Jean-Christophe Valière

Measurements of inner and outer streaming vortices in a standing waveguide using laser doppler velocimetry

Measurements of the axial streaming velocity are performed by means of laser doppler velocimetry in an experimental apparatus consisting of a waveguide having loudspeakers at each end for high intensity sound levels. Streaming is characterized by an appropriate Reynolds number Re(NL), the case Re(NL)<<1 corresponding to the so-called slow streaming and the case Re(NL)>/=1 being referred to as fast streaming. The variation of axial streaming velocity with respect to the transverse coordinate is compared to the available slow streaming theory. Streaming fluid flow is measured both in the core region and in the near wall region. Streaming velocity in the center of the guide agrees reasonably well with the slow streaming theory for small Re(NL) but deviates significantly from such predictions for Re(NL)>20 and its evolution for further increasing Re(NL) is discussed. Then streaming behavior in the near wall region is particularly studied. For Re(NL)<70, two vortices are present across the guide section as predicted by slow streaming theory. Then it appears that, when the Reynolds number is increased, two other vortices become visible in the near wall region. Different stages for the generation and evolution of these inner streaming vortices are presented.

Measurement of an aeroacoustic dipole using a linear microphone array

It is shown that the standard beamformer technique is inadequate for both the source location and the measurement of a simple dipole and that this is due to the assumption of monopole propagation in the calculation of the phase weights used to steer the focus of the array. A numerical simulation is used to illustrate the problem and to develop a correction to the signal processing algorithm to account for the dipole propagation characteristic. This is then applied to array measurements for an aeroacoustic dipole produced by a cylinder in a cross flow. The resulting source map and the beamformed spectrum are shown to give a true representation of the source energy and frequency content. A secondary effect of this correction is that the array becomes insensitive to other source types so that in addition to acting as a spatial filter, the array can perform as a source filter. This work also demonstrates how an array measurement can be misinterpreted if applied without consideration of the source mechanism.

Experimental investigation of acoustic streaming in a cylindrical wave guide up to high streaming Reynolds numbers.

Measurements of streaming velocity are performed by means of Laser Doppler Velocimetry and Particle Image Velociimetry in an experimental apparatus consisting of a cylindrical waveguide having one loudspeaker at each end for high intensity sound levels. The case of high nonlinear Reynolds number ReNL is particularly investigated. The variation of axial streaming velocity with respect to the axial and to the transverse coordinates are compared to available Rayleigh streaming theory. As expected, the measured streaming velocity agrees well with the Rayleigh streaming theory for small ReNL but deviates significantly from such predictions for high ReNL. When the nonlinear Reynolds number is increased, the outer centerline axial streaming velocity gets distorted towards the acoustic velocity nodes until counter-rotating additional vortices are generated near the acoustic velocity antinodes. This kind of behavior is followed by outer streaming cells only and measurements in the near wall region show that inner streaming vortices are less affected by this substantial evolution of fast streaming pattern. Measurements of the transient evolution of streaming velocity provide an additional insight into the evolution of fast streaming.

Fast acoustic streaming in standing waves : Generation of an additional outer streaming cell

Rayleigh streaming in a cylindrical acoustic standing waveguide is studied both experimentally and numerically for nonlinear Reynolds numbers from 1 to 30 [Re(NL)=(U0/c0)(2)(R/δν)(2), with U0 the acoustic velocity amplitude at the velocity antinode, c0 the speed of sound, R the tube radius, and δν the acoustic boundary layer thickness]. Streaming velocity is measured by means of laser Doppler velocimetry in a cylindrical resonator filled with air at atmospheric pressure at high intensity sound levels. The compressible Navier-Stokes equations are solved numerically with high resolution finite difference schemes. The resonator is excited by shaking it along the axis at imposed frequency. Results of measurements and of numerical calculation are compared with results given in the literature and with each other. As expected, the axial streaming velocity measured and calculated agrees reasonably well with the slow streaming theory for small ReNL but deviates significantly from such predictions for fast streaming (ReNL>1). Both experimental and numerical results show that when ReNL is increased, the center of the outer streaming cells are pushed toward the acoustic velocity nodes until counter-rotating additional vortices are generated near the acoustic velocity antinodes.

Development of Laser Techniques for Acoustic Boundary Layer Measurements. Part II: Comparison of LDV and PIV Measurements to Analytical Calculation

The adaptation of Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) for acoustic boundary layer measurements is considered. The specificities of acoustic boundary layer are presented and the theoretical expression of acoustic particle velocity is reminded. Appropriate parameters of the PIV system for sound measurements are determined. Results of LDV and PIV measurements of particle velocity profiles in acoustic boundary layers are compared with theoretical predictions based on the literature for different phases along the acoustic period. These results are very satisfactory and show that these two techniques are suitable for acoustic boundary layer measurements.

Development of Laser Techniques for Acoustic Boundary Layer Measurements. Part I: LDV Signal Processing for High Acoustic Displacements

A statistical model of the Laser Doppler signal in the case of pure acoustics is proposed. It appears that two different cases should be considered depending on the ratio of acoustic displacement amplitude to probe volume diameter. The processing of Laser Doppler Velocimetry signal in the case of high particle displacements (with oscillations across the measuring volume) is then considered. A specific signal post-processing strategy is proposed to determine the acoustic frequency, the acoustic velocity amplitude and its phase. First, the acoustic frequency is estimated by means of synchronous analysis weighted by arrival times. Then, the signal is uniformly re-sampled and the phase of the acoustic velocity is calculated. Lastly, a least-square method weighted by local probability density function is used to determine the acoustic velocity amplitude. This method permits an accurate estimation of the three acoustic parameters (frequency, velocity amplitude and phase) even in the adverse conditions induced by the proximity of a wall and is applied to oscillating viscous boundary layer measurements.

Analysis of the Acoustic Flow at an Abrupt Change in Section of an Acoustic Waveguide Using Particle Image Velocimetry and Proper Orthogonal Decomposition

In the vicinity of an abrupt change in cross section, an acoustic wave generates a nonlinear flow. This is investigated experimentally using Particle Image Velocimetry (PIV). The effect of both the acoustic level and the radius of curvature of the abrupt change in section on the flow is studied. At sufficiently high acoustic levels, and past a value of about 0.5 for the Strouhal number, the flow separates and a vortex is formed. Its evolution with the different parameters is studied. Proper Orthogonal Decomposition (POD) is applied to the ensemble of phase-averaged flow fields in the vicinity of the abrupt change. It is used as a means to separate the global acoustic movement from the localized non linear movements induced by it. The quantity of energy flowing from the first (acoustic) mode toward the higher (nonlinear) modes is calculated and is shown to be largely governed by the Strouhal number.

Evidence of Hydrodynamic Instability over a Liner in a Duct with Flow

Lined ducts are widely used to reduce noise radiation from ducts, from air conditioning systems to turbofan engines. Liners may sustain some instabilities although there is no definite knowledge on that subject. Some analytical studies based on simple flow models have predicted the existence of unstable hydrodynamic modes, but their prediction capability for realistic conditions is difficult to assess. Experimentalevidence of the existence of instable modes in realistic flow conditions exist but is rare. While large increase in acoustic transmission across a lined part of ducts have been measured in the past, the flow itself over the liner has never been investigated. In the present paper an experimental study of the flow over a liner is made in such a configuration where the acoustic transmission is large, and evidence of an underlying hydrodynamic instability is provided, based on optical flow measurements.

Effect of a stack on Rayleigh streaming cells investigated by laser Doppler velocimetry for application to thermoacoustic devices (L)

A preliminary study was conducted to observe the influence of a stack on the Rayleigh streaming pattern for application to thermoacoustic devices. The velocity field was estimated from laser Doppler velocimetry measurements in a resonator first without a stack; then a stack was placed at various positions along the resonator axis for various acoustic levels. It was observed that adding a stack locally modifies the streaming pattern and that new streaming vortices appear. When the stack position approaches the location of the streaming velocity maximum or when the acoustic velocity amplitude is increased, the amplitude of additional acoustic streaming vortices at the ends of the stack increases.

Acoustic Pots in Ancient and Medieval Buildings: Literary Analysis of Ancient Texte and Comparison with Recent Observations in French Churches

During the last decade, acoustic pots inserted into the walls and roofs of medieval and modern churches havebeen the focus of renewed scientific curiosity after a long time of relative silence. Traditionally, authors fromthe Middle Ages to the present time considered that Vitruvius established the relation between the “vasa aerea”(bronze vessels) in ancient Greek theatres and the “fictilibus doliis” (earthen vessels). Vitruvius’s text and phi-losophy is analysed with regards to acoustics, and his recommendations are compared with medieval, modernand contemporary texts about acoustic pots. Most of them hints at an acoustical purpose for the pots. Therefore,the literature survey is supplemented with an acoustical survey of 25 French churches where pots still remain.Recent measurements and observations in some those churches, completed with recent data from foreign studies,are consistent with an acoustical purpose of the pots to decrease the reverberation time at frequencies stronglyexcited by the spoken voice.