Bertrand Mercier

Bio: Bertrand Mercier is an academic researcher from Delft University of Technology. The author has contributed to research in topics: Schlieren & Shock (fluid dynamics). The author has an hindex of 4, co-authored 13 publications receiving 77 citations. Previous affiliations of Bertrand Mercier include University of Lyon & École centrale de Lyon.

Experimental characterisation of the screech feedback loop in underexpanded round jets

Abstract: Near-field acoustic measurements and time-resolved schlieren visualisations are performed on 10 round jets with the aim of analysing the different parts of the feedback loop related to the screech phenomenon in a systematic fashion. The ideally expanded Mach number of the studied jets ranges from to . The single source of screech acoustic waves is found at the fourth shock tip for A1 and A2 modes, and at either the third or the fourth shock tip for the B mode, depending on the Mach number. The phase of the screech cycle is measured throughout schlieren visualisations in the shear layer from the nozzle to the source. Estimates of the convective velocities are deduced for each case, and a trend for the convective velocity to grow with the axial distance is pointed out. These results are used together with source localisation deduced from a two-microphone survey to determine the number of screech periods contained in a screech loop. For the A1 and B modes, four periods are contained in a loop for cases in which the radiating shock is the fourth, and three periods when the radiating shock tip is the third, whereas the loop of the A2 mode contains five periods.

Density Fluctuations Measurement by Rayleigh Scattering Using a Single Photomultiplier

Abstract: Rayleigh scattering provides a direct nonintrusive method for density measurement, which is of particular interest in aeroacoustics. However, two major concerns arise linked to the use of photomult...

Cross-spectral analysis of PSP images for estimation of surface pressure spectra corrupted by the shot noise

Abstract: We proposed applying the cross-spectrum analysis to the unsteady pressure-sensitive paint (PSP) images to eliminate the electronic shot noise component from the pressure fluctuation spectrum. The data of surface pressure fluctuation behind a square cylinder which measured by means of the polymer/ceramic PSP with a frequency response of approximately 5 kHz which is sufficiently high for studying the Karman vortex shedding was used for the validation of this analysis method. The cross spectrum is compared with the auto-spectrum of PSP images and that of the Kulite pressure transducer that corresponds to the reference signal. The cross spectrum can drastically reduce the electronic shot noise component in the pressure fluctuation spectrum. The second peak of vortex shedding which is not observed in the result of auto-spectrum can be observed thanks to the noise reduction. The convergence of calculation is faster as the size of the spatial area for calculating cross spectrum increases.

A schlieren and nearfield acoustic based experimental investigation of screech noise sources

Abstract: A series of experiments coupling high speed schlieren visualizations and near field acoustic investigations of three underexpanded jets are used to determine the screech acoustic source location. The jets exhibit screech tones of mode A1, A2, and B when the exhaust conditions correspond to the perfectly expanded jet Mach numbers Mj equal to 1.13, 1.15, and 1.35. The source was sought at a shock tip, the fourth shock is most likely found to radiate screech associated acoustic feedback for all modes. This is deduced from nearfield phase measurements. Complementary, high-speed schlieren movies help in estimating the convection velocity of screech associated hydrodynamic structures. From this analysis, the duration of the whole screech loop is found to consist of four periods of screech for modes A1 and B, and five periods for mode A2.

Experimental investigation of the turbulent density – Far-field sound correlations in compressible jets

Abstract: Jet mixing noise is experimentally investigated by means of cross-correlations between density fluctuations inside the turbulent jet flow and the far-field acoustic pressure. The time-resolved density fluctuations are measured by an experimental device based on Rayleigh scattering, which is mounted in the large anechoic wind tunnel of Ecole Centrale de Lyon. An original signal processing developed in a previous study is implemented for the photon counting, combined with the use of a single photomultiplier to remove shot noise. A high-speed subsonic jet and a perfectly expanded supersonic jet with a subsonic convective velocity are considered to characterize mixing noise sources. In order to go beyond the classical Fourier analyses, conditional cross-correlations are determined, and the signature of turbulent events linked to the noise emission in the downstream direction is extracted.

「レイリー散乱」(Rayleigh Scattering)

Abstract: • Rayleigh scattering is an approximation used to predict scattering from particles much smaller than an electromagnetic wavelength • The approximation is based on quasi-static ideas: in regions of space much smaller than a wavelength, fields act approximately like static fields • Basic procedure: solve statics problem to determine field inside scatterer when a uniform field (plane wave) impinges from outside • This determines field inside particle so we can then compute scattering cross sections, etc, using our previous equations • This is a nice method because we have lots of analytical techniques for solving the Laplace equation, also usually will predict constant fields inside scatterer

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

Abstract: 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 ...

Upstream-travelling acoustic jet modes as a closure mechanism for screech

Abstract: Experimental evidence is provided to demonstrate that the upstream-travelling waves in two jets screeching in the A1 and A2 modes are not free-stream acoustic waves, but rather waves with support within the jet. Proper orthogonal decomposition is used to educe the coherent fluctuations associated with jet screech from a set of randomly sampled velocity fields. A streamwise Fourier transform is then used to isolate components with positive and negative phase speeds. The component with negative phase speed is shown, by comparison with a vortex-sheet model, to resemble the upstream-travelling jet wave first studied by Tam & Hu (J. Fluid Mech., vol. 201, 1989, pp. 447–483). It is further demonstrated that screech tones are only observed over the frequency range where this upstream-travelling wave is propagative.

Topic Review: Application of Raman Spectroscopy Characterization in Micro/Nano-Machining

Abstract: The defects and subsurface damages induced by crystal growth and micro/nano-machining have a significant impact on the functional performance of machined products. Raman spectroscopy is an efficient, powerful, and non-destructive testing method to characterize these defects and subsurface damages. This paper aims to review the fundamentals and applications of Raman spectroscopy on the characterization of defects and subsurface damages in micro/nano-machining. Firstly, the principle and several critical parameters (such as penetration depth, laser spot size, and so on) involved in the Raman characterization are introduced. Then, the mechanism of Raman spectroscopy for detection of defects and subsurface damages is discussed. The Raman spectroscopy characterization of semiconductor materials’ stacking faults, phase transformation, and residual stress in micro/nano-machining is discussed in detail. Identification and characterization of phase transformation and stacking faults for Si and SiC is feasible using the information of new Raman bands. Based on the Raman band position shift and Raman intensity ratio, Raman spectroscopy can be used to quantitatively calculate the residual stress and the thickness of the subsurface damage layer of semiconductor materials. The Tip-Enhanced Raman Spectroscopy (TERS) technique is helpful to dramatically enhance the Raman scattering signal at weak damages and it is considered as a promising research field.

Screech-tone prediction using upstream-travelling jet modes

Abstract: The purpose of this paper is to characterise and model the A1 and A2 screech modes in supersonic jets operating at off-design conditions. The usual screech-modelling scenario involves a feedback loop between a downstream-travelling Kelvin–Helmholtz instability wave and an upstream-travelling acoustic wave. We review state-of-the-art screech-frequency prediction models and associated limitations. Following the work of Edgington-Mitchell et al. (J Fluid Mech 855, 2018), a new prediction approach is proposed where the feedback loop is closed by the upstream-travelling jet modes first discussed in Tam and Hu (J Fluid Mech 201:447–483, 1989) in lieu of the free-stream sound waves. The Kelvin–Helmholtz and upstream-travelling jet modes are obtained using a cylindrical vortex-sheet model. The predictions provide a better agreement with experimental observations than does the classical screech-prediction approach. Screech dynamics associated with the staging process is explored through a wavelet analysis, highlighting that staging involves mutually exclusive switching that is underpinned by non-linear interactions.