Solenn Moreau

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.

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.

Nonparametric spectral analysis of wideband spectrum with missing data via sample-and-hold interpolation and deconvolution

Spectral estimation of data sequences with randomly missing samples is considered in this paper. A nonparametric missing-data method is proposed based on interpolation followed by a deconvolution procedure. Sample-and-hold interpolation is considered here. The method is based on the analytic expression of the autocorrelation function of the interpolated data as a linear function of the autocorrelation function of the data to be estimated. Bias and standard deviation of both autocorrelation function and power spectral density are detailed for simulated data based on Monte Carlo analysis. The method is also compared with a fuzzy slotting technique with local normalization and weighting algorithm. Based on the results of these simulations, it is concluded that the performance of the proposed method is better than those of the slotting technique.

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.

Transition to turbulence and acoustic Rayleigh streaming in thermoacoustic devices

Thermoacoustic engines and refrigerators with practical levels of heating or pumping power must generally operate at high pressure amplitudes. When used to describe the behavior of such high‐amplitude thermoacoustic devices, the well‐established foundations of thermoacoustics, based on the acoustic approximation, reach their limits. It is necessary to gain a deeper understanding of the high‐amplitude phenomena in order to improve the performances of thermoacoustic devices, and efforts of several research groups have been directed towards this goal over the last decade. In this presentation, we will consider recent advances in the understanding of some of the gas‐dynamics phenomena leading to limitation of devices performances, namely transition to turbulence and acoustic Rayleigh streaming. The common point for these phenomena is that they owe their origin in the dynamic of oscillating flows in very near wall regions, so that their quantification implies measurements of acoustic particle velocity in adver...

Effect of an obstacle on Rayleigh acoustic streaming cells

Acoustic streaming has harmful consequences on thermoacoustic machines behaviour because of the associated heat transfers. A preliminary study was carried out in order to study the effect of an obstacle on the Rayleigh cells to help in understanding the role of such phenomena in thermoacoustic machines. An obstacle was introduced in a half‐wavelength cylindrical wave guide to study its effects on acoustic streaming. The obstacle was placed at various positions along the wave guide axis and experiments were carried out at various acoustic levels. The axial streaming velocity was measured using Laser Doppler Velocimetry (LDV). It was observed that adding an obstacle in the streaming pattern modifies the latter and that new streaming vortices appear in the vicinity of the obstacle. When the obstacle position approaches a maximum of the Rayleigh streaming velocity the number and the amplitude of acoustic streaming vortices at the ends of the obstacle increase. Similar tendencies were observed when the acousti...

Laser Doppler signal for high amplitude sound wave

Laser Doppler velocimetry (LDV) is now recognized as a very useful technique for estimating acoustic velocity with a good time resolution in many applications. Previous research showed that the most important parameter in LDV for acoustics is the particle displacement and particularly its magnitude compared with the size of the probe volume formed by the crossing of the laser beams. Specific techniques were developed to estimate the acoustic wave when the displacement is of the same order of magnitude as the probe diameter and when it is much smaller. In this study, we investigate situations where the displacement is much higher than the probe volume. The measuring process has been simulated numerically and it appears that the process leads to an under‐estimation of the velocity around zero. This under‐estimation is due to the processing of the laser Doppler signal in the case of high displacements. The associated signal is a non poissonnian randomly sampled signal to which classical processing methods ar...