C. Lempereur - Academia.edu (original) (raw)

Papers by C. Lempereur

Pour essayer de mieux apprehender les mecanismes mis en jeu dans la perte de connaissance sous ac... more Pour essayer de mieux apprehender les mecanismes mis en jeu dans la perte de connaissance sous acceleration +Gz, une etude biomecanique s'interesse au comportement mecanique de l'encephale sous hypergravite et aux perturbations de l'ecoulement sanguin au niveau de la circulation cerebrale. L'evaluation des contraintes intracerebrales sous facteur de charge est obtenue grâce au couplage d'une experimentation realisee sur des cerveaux animaux en centrifugeuse avec une modelisation aux elements finis. Il est alors mis en evidence une augmentation des contraintes de compression a la base des hemispheres et du cervelet et une augmentation des contraintes de traction et de cisaillement au voisinage de l'incisure de la tente du cervelet. Ces contraintes peuvent etre assimilees a des forces surfaciques qui tendent a collaber la paroi des vaisseaux. Elles sont donc importantes a prendre en compte dans la modelisation de la circulation cerebrale.

Experiments in Fluids, 2021

In order to study spray combustion, an experimental test rig was developed at ONERA to partially ... more In order to study spray combustion, an experimental test rig was developed at ONERA to partially characterize the flow conditions inside the combustion chamber of a gas turbine. Experimental campaigns using laser-based diagnostics were performed to provide an experimental database under reacting and non-reacting conditions. The paper first describes the Mie scattering image-processing to detect the droplets in the spray, and to calculate 2D maps of droplet number density and mean inter-droplet distance. The method is subsequently used to investigate the spray behavior under both reacting and non-reacting conditions according to global-averaging and phase-averaging methods. Experimental findings on the spatial droplet distribution in the spray are compared to the simple regular grid distribution and the Hertz-Chandrasekhar distribution. Results show that, under both conditions, there is an affine relationship between the inverse square root of the mean droplet number density and the nearest-neighbor inter-droplet distance. Moreover, observations suggest that the droplet spatial distribution fits more closely to a Hertz-Chandrasekhar distribution than a simple regular grid distribution, which may bring new insight for spray modeling.

Applied Physics B, 2019

Fluorescence-based techniques are commonly implemented to characterize droplet sprays in flows at... more Fluorescence-based techniques are commonly implemented to characterize droplet sprays in flows at temperature above 293 K. Nevertheless, in the case of gas turbine altitude relight, aeroengines are exposed to critical operating conditions where the liquid fuel temperature can reach values as low as-40°C. As a result, it is necessary to address the influence of this parameter on fluorescence intensity for temperatures below room values. In this paper, an innovative SLIF (Saturated Laser-Induced Fluorescence) excitation scheme is proposed in order to overcome the strong temperature dependence of fluorescence in liquid kerosene seeded with a dye. The findings of this work are based on gated sequential images recorded with an intensified camera during and after the laser pulse duration so as to sample the temporal evolution of the fluorescence emission. The results show that, for low laser excitation intensity, the fluorescence intensity remains proportional to the laser power. However, a non-linear response is observed in the case of high excitation intensity. This difference can be used to limit the fluorescence dependence on temperature by carefully selecting the temporal window where fluorescence is detected. Finally, although the method presented in this paper is applied to cold liquid kerosene, its range of application can be extended to other liquids and other temperature ranges.

Doppler Global Velocimetry is a laser measurement technique able to provide planar velocity field... more Doppler Global Velocimetry is a laser measurement technique able to provide planar velocity fields in a flow seeded with micronic particles. Its main advantage as regards wind tunnel experiments lies in the reduction of tests duration, as several thousands velocity vectors can be obtained in a single shot. The principle lies on a direct measurement of the frequency shift of the light scattered by moving particles (Doppler effect). A narrow absorption band of iodine serves as a frequency-to-intensity converter in the form of a cell placed in front of a CCD camera. The laser has to be tuned and thoroughly stabilized on one of these absorption bands. As frequency drifts can never be really avoided, a patented device, described in this paper, has been designed to give realtime relationship between the intensity measurement and the Doppler shift. The last experiments carried out in ONERA facilities with a two-component DGV system are reported here. First, an OAT15A 2D airfoil has been te...

Experiments in Fluids, 2005

A specially designed Doppler global velocimetry system (DGV, planar Doppler velocimetry) was deve... more A specially designed Doppler global velocimetry system (DGV, planar Doppler velocimetry) was developed and installed in a high-speed cryogenic wind tunnel facility for use at free stream Mach numbers between 0.2 and 0.88 and pressures between 1.2 and 3.3 bar. The necessary particle seeding was achieved by injecting a mixture of gaseous nitrogen and water vapor into the dry and cold tunnel flow which then immediately formed a large amount of small ice crystals. As operational and access conditions are very restrictive with respect to other facilities, DGV is currently considered the best choice for the non-intrusive measurement of flow fields.

Image Processing: Machine Vision Applications V, 2012

1998 IEEE Southwest Symposium on Image Analysis and Interpretation (Cat. No.98EX165), 1998

ICIASF 99. 18th International Congress on Instrumentation in Aerospace Simulation Facilities. Record (Cat. No.99CH37025), 1999

A one-component Doppler Global Velocimeter has been designed and tested in an empty wind tunnel g... more A one-component Doppler Global Velocimeter has been designed and tested in an empty wind tunnel generating a perfectly calibrated flow. Most of the technical choices involving light sheet generation, optical components, cameras, iodine cell, are described and argued, as well as geometrical calibration and processing steps. A specific device has been developed, but not yet described, to relax constraints on the stabilization of laser frequency and iodine cell temperature. Velocity maps obtained by this D.G.V. system are compared to the known Mach number. These promising results show the interest of following such a procedure to qualify and optimize a prototype

ICIASF 2001 Record, 19th International Congress on Instrumentation in Aerospace Simulation Facilities (Cat. No.01CH37215), 2001

A Doppler Global Velocimetry System (DGV) has been developed and qualified at ONERA. It can perfo... more A Doppler Global Velocimetry System (DGV) has been developed and qualified at ONERA. It can perform planar three component mean velocity measurements in air flows. Our system was designed with special regards to wind tunnel applications. The technical performance of this system is reported as well as the successful tests in wind tunnel flows. DGV measurements were performed in the

ICIASF 2005 RecordInternational Congress onInstrumentation in AerospaceSimulation Facilities, 2005

ABSTRACT The need to qualify the now field around a model is a constant objective for research ac... more ABSTRACT The need to qualify the now field around a model is a constant objective for research activities and industrial aeronautical development programs. The progress in global optical techniques such as Doppler Global Velocimetry (DGV) and Particle Image Velocimetry (PIV) has permitted to make comparative tests in F1 of these two non-intrusive methods and to compare these results to a reference case investigated by a five-hole probe. The ONERA F1 wind tunnel is a low-speed pressurized wind tunnel used by industry as well as research activities. The three components of the velocity field have been measured, half a chord behind the wing of a generic civil aircraft in a landing configuration, where the different vortices emitted from the wing tip and from the flaps are visible. These global methods are of great interest for large wind tunnels, by reducing the measurement time to some seconds for a mean flow field map, compared to hours with the probing systems.

ICIASF'97 Record. International Congress on Instrumentation in Aerospace Simulation Facilities, 1997

Measurement Science and Technology, 2008

Thermal paints are used by most aeronautical manufacturers in dedicated tests to record temperatu... more Thermal paints are used by most aeronautical manufacturers in dedicated tests to record temperature profiles over the surface of engine components. The analysis of these coatings, which irreversibly change colour with temperature, is at present performed by a skilled operator who draws isothermal lines directly on the components surface. The technique suffers from a lack of accuracy and automation, as