Louis Boyer

Aix-Marseille Université | AMU · Faculté des Sciences

These results show a good qualitative agreement with the model of a threefold instability mechanism. The marginal stability is crossed when the strength of the hydrodynamic instability is increased following an increase in the normal flame speed. The critical velocity is lowered by decreasing the Lewis number due to the destabilizing influence of t...

Depuis sa "conquête", il y a environ un million d'années, le feu fait partie de l'expérience quotidienne de l'homme. Objet fascinant, insaisissable, diversement coloré, aux mouvements imprévisibles, la flamme, sa manifestation première, a suscité tout au long de l'histoire de l'humanité des questionnements, des mythes et des légendes. Si le feu est...

This paper reports experiments on initially quasi-conical premixed flames subjected to acoustic forcing of the cold gases with high amplitude and high frequency (≈ 1000 Hz). The resulting hemispherical-shaped flame, induced by a restabilization effect described previously by Durox el al., shows a parametric instability when stronger acoustic forcin...

This paper shows that the change in the number of moles of species during combustion can make a strong contribution to the acoustic power radiated by turbulent flames and cannot be systematically neglected. Starting from standard conservation equations, we derive an expression for the acoustic pressure radiated in the far field of a compact region...

A conical flame, in the presence of high-frequency ([approximate]1000 Hz) and high-amplitude acoustic modulation of the cold gases, deforms to a shape which is approximately hemispherical. It is shown that the acoustic level required to produce a hemispherical flame is such that the ratio of acoustic velocity to laminar combustion velocity...

The vortex street behind a ring of circular cross section and large aspect ratio is investigated experimentally. Different modes of annular and helical vortex shedding are identified by phase and frequency measurements. Their stability domains overlap in a large interval in Reynolds number where mode selection depends on initial conditions only. A...

The Ginzburg-Landau model for the wake of a straight circular cylinder is extended to the case of a torus, for which periodic boundary conditions apply. Experiments done on a real torus of dimensions 3-57 mm reveal the existence of different structures predicted by the model. There is an abridged English version. (English summary)

The dynamic and the shape of anchored premised flame are analyzed as the result of the sum of wrinkles convected along the flame and of the wrinkles locally induced by the flow. The linear approximation leads to an interference integral that describes the shape as a function of time. Three applications are given: An homogeneous oscillation of the f...

It is shown that experimentally observed three-dimensional effects associated with the wake of an elongated bluff body (i.e., slantwise shedding, bent vortices, and dislocations) can be described by the Ginzburg-Landau (G-L) equation. This model is obtained by local application of two-dimensional laws and a diffusive coupling. The numerical solutio...

The wake of a circular cylinder is investigated near the oscillation threshold by means of a laser probe. Above the threshold the transient regime is studied and described by a J. T. Stuart-L. Landau law (already found to be relevant in explaining free-oscillating regimes). Below the critical point, impulse and resonant regimes are examined, so the...

We calculate the spectrum of a signal derived from a continuous signal by ‘‘sample and hold’’ random sampling. It is seen that an undistorted spectrum is obtained only if the mean sampling frequency is more than an order of magnitude greater than the highest frequency in the original signal. The cases of signal‐independent and signal‐dependent samp...

A new insight into the classical problem of the Benard-Von Karman instability has been obtained with the help of a laser anemometer equipped with a rotating grating used as a well adapted frequency shifter. The experimental results refer to the spatial growth of the oscillation, the influence of both the aspect ratio and the blockage ratio, and to...

Four applications of laser tomography in combustion are presented. The principle of laser tomography which is utilized to obtain measurements of the position and velocity of a flame front is described. The stabilization of the position of an unattached flame by active control of the gas flow is examined. The interaction between combustion and turbu...

The most important probe to study the systems and their dynamics is the electromagnetic radiation. For instance until the last two decades any knowledge of the universe had been obtained through processing of its visible or non visible light. In the labs, where the experiments can be prepared, numerous investigations are based on interaction of lig...

The evolution of the velocity components of the wake behind a cylinder has been investigated in the range 40 < Re < 300 using a moving fringe Laser Doppler anemometer. The order parameter (amplitude of the perpendicular velocity fluctuations) obeys Landau's law. We confirm the existence of several modes and in particular, we observe and interpret a...

The dynamical properties of the hydrodynamic discontinuity associated with a slow-burning premixed gaseous flame front are studied with use of laser diagnostics. The fluctuating motion of the front around its equilibrium position and its effects on the flow are studied in the case of an intrinsically stable front propagating downwards in a weekly t...

Using the laser tomographic method, the local and instantaneous velocity of the front of a turbulent premixed flame is compared to the longitudinal fluctuation velocity of the turbulent stream in the fresh mixture just ahead the flame front, this latter quantity being measured by L.D.A. The experiments are carried out on a lean flame of a propane a...

We report the first light-scattering study of a critical mixture (aniline-cyclohexane) under shear flow $S$ which affects critical fluctuations with relaxation time $$\tau${}$ only in the region $S$\tau${}>1$. All anomalies can be expressed as a function of $S$\tau${}$ only and can be interpreted by a lowering of the critical temperature $$\Delta${...

The influence ofinhomogeneity and of anisotropy of the flux scattered in the aperture of the spectrometer on the position and the lineshape of the recorded Brillouin spectrum is analysed for the plane and spherical Fabry-Pérot interferometers. A method of calculating the spectrum is presented as well as its results for numerous standard cases. The...

A strong anisotropy of the intensity with crystal orientation is observed in the depolarized Rayleigh line. This effect is attributed to the existence of preferred orientations of molecules in the lattice and allows the determination of crystal orientation. The orientational order parameter and two relaxation times have been measured. L'étude expér...

The model of the standard linear solid is extended to the viscoelastic anisotropic medium by defining a relaxation-time tensor. The velocity and attenuation of sound waves are calculated as a function of frequency from the components of this tensor and low- and high-frequency elastic constants. The anisotropic behavior of relaxation curves can be d...

After a short outline of the principle and advantages of real time analysis, an original device designed for very low frequency analysis of random signals is described. Two examples of hydrodynamic velocity measurements are presented. Firstly, as a test of the device, a spectrum obtained from a random distribution of particles with uniform known ve...

Both the polarized and depolarized spectra of Rayleigh scattering from pivalic acid have been measured with high precision. The relaxation time for molecular reorientation and the energy of activation are deduced directly from these measurements for both the liquid and plastic phases of the crystal. Les spectres de diffusion Rayleigh polarisés et d...

Starting from the measurement of the Brillouin-line intensities, the photoelastic constants of lithium acetate have been determined. Because of the strong optical anisotropy of the material, the effect of the rotational part of the gradient of the displacement vector on the photoelastic properties is important. The sign of the photoelastic constant...

After an outline of the Brillouin effect and of elastic waves in crystals, a method for the determination of elastic and photoelastic constants is analyzed. The authors propose a set of conditions with a view to obtaining accurately the numerical values of elastic and photoelastic constants and to ascertain their sign. The Brillouin-line intensitie...

The numerical values of the elastic constants at room temperature are given. The significance of the results and the various causes of uncertainty are discussed. Indices of refraction and density are also measured.

It is shown that the dispersion of the velocities observed in a plastic crystal may be interpreted by assuming a reaction mechanism. Starting from the equations of the thermodynamics of irreversible processes, frequency dependent elastic constants of the crystal are defined. Measurements performed in the 6 MHz (ultrasonic waves) — 14 GHz (Brillouin...

The Rayleigh line scattered by succinonitrile is studied by means of a Fabry-Pérot interferometer having a resolving power of 5 × 105. The line is found to be Lorentzian with a 5.4-GHz width and a depolarization ratio 0.77. The experimental data are in good agreement with the orientational relaxation theory in liquids.

On décrit la construction d'un dispositif destiné à refroidir un photomultiplicateur à des températures comprises entre - 20° et — 100 °C. Le principe de fonctionnement est celui d'un thermosiphon : le liquide (éther) circule par convection entre l'enceinte à refroidir et une enceinte refroidie par l'azote liquide. Un dispositif de régulation simpl...

Analysis of the different means of filtering Rayleigh line has brought out the various advantages of the Fabry-Pérot interferometer used by reflection. A theoretical study of the device enables us to calculate the filtering coefficient which proves in agreement with the first experimental results. Une analyse des différents procédés de filtrage de...

After a short outline of the Brillouin effect and optical heterodyning, the problem of heterodyne spectrometry under scattering angles of about 90° is described. According to the classical device, the beat between the scattered wave and the coherent wave supplied by the laser would be around 30 GHz. But at this frequency the signal is too weak to b...