Showing papers in "Journal De Physique Iv in 2006"

Regional climate modeling: Status and perspectives

Abstract: This paper is presents a concise review of regional climate modeling, from its ensuing stages in the late 1980s to the most recent developments. A tremendous progress has been achieved in improving the performance of regional climate models, which are currently used by a growing research community for a wide range of applications, from process studies to paleoclimate and future climate simulations. Basic concepts underlying the nested modeling technique, along with the current debate on outstanding issues in regional climate modeling, are discussed. Finally, perspectives of future developments in this rapidly evolving research area are briefly outlined. An extensive reference list is provided to support the discussion.

OMEGA EP : High-energy petawatt capability for the OMEGA laser facility

Abstract: OMEGA EP (Extended Performance) is a petawatt-class addition to the existing 30-kJ, 60-beam OMEGA Laser Facility at the University of Rochester. When completed, it will consist of four beamlines, each capable of producing up to 6.5 kJ at 351 nm in a 1 to 10 ns pulse. Two of the beamlines will produce up to 2.6 kJ in a pulse-width range of 1 to 100 ps at 1053 nm using chirped-pulse amplification (CPA). This paper reviews both the OMEGA EP performance objectives and the enabling technologies required to meet these goals.

10-kJ PW laser for the FIREX-I program

Abstract: A 10-kJ PW laser (LFEX) is under construction for the FIREX-I program. This paper reports a design overview of LFEX, the technological development of a large-aperture arrayed amplifier with modified four-pass architecture, wavefront correction, a large-aperture Faraday rotator with a superconducting magnet, a new pulse compressor arrangement, and focus control.

Dynamics of the West African monsoon

Abstract: A review is given of the dynamical mechanisms responsible for the monsoon circulation over West Africa. Features of the circulation are first described, including the seasonal displacement of the rain bands, the structure of the heat low over the Sahara, the meridional circulation to the south and the associated zonal jets. Simple theories for the zonal-mean meridional circulation are then presented, using the principles of angular momentum conservation, thermal wind balance and moist convective equilibrium. The application of these theories to the West African monsoon reveals a sensitivity to the low-level meridional gradient of equivalent potential temperature, which helps explain observed variability in the monsoon onset. Processes leading to east-west asymmetries in the circulation are also described, and mechanisms linking West African rainfall anomalies with remote events in the tropics are discussed. These dynamical considerations are then placed in the broader context of the ongoing AMMA research program.

The Astra Gemini project - : A dual-beam petawatt Ti:Sapphire laser system

Abstract: The current Astra laser facility is capable of Terawatt operation at 10 Hz into one experimental area, and simultaneous 12 TW operation at 1 Hz into a second area. Both of these operational modes deliver a single beam with a pulse duration of 40 fs and a few times diffraction-limited quality. The Astra Gemini project will extend the capabilities of Astra to Petawatt level by providing two 0.5 PW beams in a completely new radiation-shielded interaction area. The new facility will be able to deliver two beams of 15 J in 30 fs at a rate of one shot per minute, and will support a wide range of experimental configurations, opening up many new possibilities for high-intensity laser-matter interaction studies.

Perforation of aluminium foam core sandwich panels under impact loading

Abstract: This paper reports an original inverse perforation tests on foam core sandwich panels under impact loading. The key point is the use of an instrumented Hopkinson pressure bar as a perforator and at the same time a measuring device. It aims at a high quality piercing force record during the whole perforation process, which is a lack of common free-flying projectile - target testing schemes. This new testing arrangement allows for the measurement of piercing force-displacement curves under quasi-static and impact loadings of sandwich samples, which is made of 40 mm AlSi7Mg0.5 Cymat foam cores and 0.8 mm thick 2024 T3 aluminium sheet as top and bottom skins. Compared with quasi-static top skin peak loads (the maximal load before the perforation of top skins) obtained under same geometric and clamping conditions and even in the case that the used foam core (Cymat) and aluminium skin sheet are known and have been confirmed rate insensitive, a significant enhancement under impact loading (25%) of the top skin peak load is found.

Characteristics analysis of Isentropic Compression Experiments (ICE)

Abstract: Several new techniques for high-pressure (quasi-) isentropic compression of materials are becoming available: pulsed-power-driven magnetic pressure, graded-density impactors on gas-guns, and laser drive. These experiments are analysed to give sound speed as a function of particle velocity, which may then be integrated to pressure and volume, giving data for a complete isentrope with one shot. However, to do this the experimentally-measured free-surface - or window-interface - velocities must be corrected to give the in-situ material velocities in the absence of the surface or interface. This may be done by either "backward integration" (integrate the equations of motion in space rather than time using surface velocity vs time as the boundary conditions) or by a characteristics method. This presentation describes the characteristics method and its application to AWE experiments on the isentropic compression of lead. The experiment has produced results consistent with pre-existing experimental data within errors in sound speed and pressure of ∼4%. Iterative corrections to in-situ velocity, using the EOS calculated during the previous step, reduce - or do away with - any dependence of the results on an assumed EOS. Simultaneous characteristics calculations of the "input" pressure-time are used to estimate the errors.

Photonic doppler velocimetry in shock physics experiments

Abstract: Doppler velocimetry techniques are frequently used in Shock-Physics experiments to measure material velocities (as a function of time). With such diagnostics, there is no physical contact between the probe and the target, which presents the advantage of not intruding the observed phenomenon. They also provide very good precision on the velocities. Those techniques are either based on homodyne methods (such as Fabry-Perot and Visar) or on heterodyne methods (such as PDV-Photon Doppler Velocimetry). In particular, PDV technique has recently been developed at LLNL. We present in this paper a PDV system built at CEA DIF, along with our first experimental results which we obtained on a shock tube.

Fracture behaviour of High Performance Concrete (HPC) investigated with a Hopkinson-Bar

Abstract: This paper describes the measurement of the tensile strength and the fracture energy of High Performance Concrete (HPC) at high strain rates between 10/s and 100/s. Therefore a Hopkinson-Bar is modified in order to perform spall experiments. The spall strength is determined from the pull-back velocity at the free end of the specimen. The fracture energy is calculated from the dissipated energy while crack opening takes place. The paper provides Dynamic Increase Factors (DIF) for the tensile strength and the fracture energy of HPC.

The influence of temperature and strain rate on the tensile and compressive constitutive response of four fluoropolymers

Abstract: Fluoropolymers posses several advantageous properties for load-bearing structural components including higher strength at elevated temperatures and higher toughness at lowered temperatures. Stress-strain studies have been conducted on two homopolymers (Teflon (PTFE) and KEL-F 81 (PCTFE)) and two copolymers (KEL-F 800 and THV 500G) as a function of temperature and strain rate. A split-Hopkinson pressure bar (SHPB) was used to achieve a strain rate of ∼3200/s in compression, while screw-driven testing machines afforded a strain rate of 0.001/s. A tensile split-Hopkinson pressure bar (T-SHPB) was used for testing at strain rates of 800/s in tension. To address the large strains to failure (up to 700% extension) an optical extensometer was employed for a strain rate of0.005/s in tension. The effects of temperature and strain rate for this family of polymers is presented in detail, including an observed shift in glass transition (T g ) to higher temperatures with increased strain rate and rate insensitivity of thermally-induced phase transitions.

Study of parameters affecting the strength of yarns

Abstract: The purpose of this study is to gain an understanding of parameters that affect the strength of yarns. Quasistatic and dynamic strength of five different yarns were obtained using hydraulic and Hopkinson bar testing methods and the rate dependency of the failure strength of each yarn was quantified (average of 10 repeats). The scaling effect was also studied experimentally in order to relate the effect of specimen size to the failure stress of Kevlar 129. Single fibres were tested at five different gage lengths (5, 16, 25, 50, 100 mm) and multi fibre specimens were also tested at various gage lengths (24, 100, 170 mm). By studying the variation of statistical strength with increasing size of specimen, a decreasing trend for the tensile strength was observed. A three-parameter exponential growth model is proposed to relate the strength of the specimen to its volume in order to incorporate the increase in defect population.

Shock-wave initiation of nuclear transmutation of chemical elements

Abstract: Experimental data are given about the synthesis of chemical elements Mn, S, Na, Cu, Al, Ne, Rn etc. during superdeep penetration of lead particles into targets of Armco iron (the SDP phenomenon). It is supposed that flows of dense high-temperature plasma occur at the interface between the penetrating microparticle and the target. Consolidation of the internal energy of the plasma flows as a result of such collisions can cause nuclear transmutation of chemical elements.

Numerical modelling of orthogonal cutting: Influence of cutting conditions and separation criterion

Abstract: Chip formation is a high strain rate process studied with analytical and numerical models. Analytical models have the advantage of a small calculation time, however, they are often based on some assumptions which are difficult to verify. Finite element modelling (FEM) of chip formation process provides more details on the chip process formation, such as plastic strain, strain rate or stress fields. FEM can be used to improve the analytical models' assumptions. There is still a wide dispersion of formulations and numerical parameters adopted in order to obtain accurate results in numerical models. In the Lagrangian approach, it is of crucial importance to establish realistic criteria for element deletion, allowing chip separation from original workpiece. In the arbitrary Lagrangian Eulerian (ALE) formulation no element deletion is needed. This work is focused in modelization of orthogonal cutting. A comparison between both numerical approaches, Lagrangian and ALE is shown. The effects of geometrical parameters, erosion criterion and cutting speed are evaluated. Comparisons between numerical and theoretical results are performed, and the results obtained from the numerical approach are used as an input of analytical model, improving its accuracy.

Mechanical properties of an iron oxide formed by corrosion in reinforced concrete structures

Abstract: The paper deals with elastic properties of iron oxides formed in reinforced concrete structures. Due to the difficulty to perform mechanical tests on the real oxides presented in the form of (multiple) laminated stratums, the elastic modulus of iron oxides remains unknown. Young's moduli of porous compact "synthetic oxides" in powder form, obtained in laboratory conditions, were measured from both acoustic measurements and oedometric tests. The elastic modulus of the compact polycrystalline iron oxide is deduced with respect to two models, a micromechanical one and a Hertz' theory. The full method is validated on a well-known material, the alumina.

On the use of combined finite/discrete element method for impacted concrete structures

Abstract: A combined finite/discrete element method is proposed for the prediction of reinforced concrete structure response under severe dynamic lo ads as impacts due to natural or anthropic hazards. A bridging subdomain is used, the Hamiltonian is chos en as a linear combination of discrete and continuu m Hamiltonians. Discrete Element displacements and rotations are linking to Finite Element displacements by means of Lagrange multipliers. The discontinuiti es of discretization introduce spurious reflection ; numerical methods are proposed to attenuate it. The modeling of the whole structure under such impacts needs on one hand a model able to describe the occurrence of severe damage (fragmentation, spa lling, tunneling) in the zone of impact, and on another hand a model able to capture the global res ponse of the structure. The study aims at showing how the proposed combined finite/discrete element method satisfies both conditions. Locally, the discrete element model deals with nonlinear phenomena, far from impacted zone the finite element method allows the reduction of both times of modeli ng and computation. Using the combined method allows the prediction of projectile penetration, st ructural damage and global displacements. The efficiency of DEM to study rocks, plain concret e and reinforced concrete has already been shown in 2D, Camborde (1), Potyondy (2) and in 3D, Hentz (3). The applications of DEM to large scale structures were limited due to computing costs; the number of elements increase reduces drastically computational efficiency. To improve that point, th e region without any assumed damage is modeled by means of the FEM. The same media is model with two different methods; the discretization size at the interface is discontinuous. In the vicinity of impact the DE discretization is fine, whereas FE s ize is adapted to the structural scale and is larger th an DE. Discretization discontinuity induces spuriou s reflections.

Response of pre-loaded laminate composite plates subject to high velocity impact

Abstract: An experimental and analytical study of two cases of static preload (uniaxial and biaxial) was made to determine the response of structural plate elements subjected to high velocity impacts under inplane tensile preloading conditions. The results were compared with those obtained in non-preloading specimens. Rectangular specimens were used for the uniaxial and non-preload tests, while cross-shaped specimens were used for the biaxial tests. The impacts were made by spherical projectiles travelling at velocities from 140 to 525 m/s. As a general result, the existence of a static preloading on the specimen was found to affect the ballistic limit and the damaged area. The biaxial preload specimens showed the higher ballistic limit and the damaged area was slightly bigger in the non-preloaded specimens. From the analytical model, the ballistic limit calculated for the non-loaded specimens showed a difference of 16% from the experimental values, and in the preloaded specimens the model did not show showed the same tendency as that observed experimentally.

Modifications of a high pressure device for speed of sound measurements in liquids

Abstract: Modifications of a measuring set for measurements of the speed of sound in liquids under elevated pressures are described. The reliability of the proposed experimental device was checked under pressures up to 100 MPa in the temperature range from 293 K to 318 K.

A new experimental technique for the analysis of concrete under high triaxial loading

Abstract: Concrete is a building material used for sensitive infrastructures like dams or nuclear power reactors; however its behaviour remains badly known under extreme dynamical loading like rock falls, explosions or ballistic impacts. It is due both to the difficulty of reproducing experimentally such a loading and to the intrinsic complexity of concrete behaviour. Predicting its response under dynamic loading needs the experimental characterization of its static behaviour in compression under very high confinement. This paper first presents a new large capacity triaxial press and the manufacturing and testing procedures developed to perform the tests. Plain concrete specimens (centimetric aggregate dimension) were submitted to different loading paths up to an ultimate state associated to failure.

Analysis of the concrete behaviour in tension at high strain-rate by a modified Hopkinson bar in support of impact resistant structural design

Abstract: The strain-rate sensitivity of concrete in tension has been investigated employing Hopkinson bar techniques. The devices used have allowed the successful testing of large specimens with real size aggregates. Full stress-strain curves, including the softening branch, have been obtained for several strain rates. The use of bar bundles for holding the specimens has further provided insight on the fracturing mechanism inside the specimen, revealing amply the crack multi-activation process under dynamic loading. Such data are considered essential in the impact and blast resistant design of concrete structures..

Vulcan petawatt-operation and development

Abstract: Petawatt capability on the Vulcan laser facility has been available to the international plasma physics community for over two years. This has enabled novel experiments to be carried out and new regimes of physics to be explored. During that time, there have been 10 successful user experiments with 89% of shots delivered within the requested energy limits. In the autumn of 2004, pulses with powers of more than a petawatt (10 15 Watts) were delivered to target with energies greater than 400 J and pulse widths shorter than 500 femtoseconds (10 -15 ) on target. In parallel to the development of ultra-high intensity pulses is a programme to enhance Vulcan's long pulse capabilities. This paper will present an overview of the current capabilities of the Vulcan Petawatt facility and discuss some of the recent technological advances that have enabled the generation of Petawatt pulses.

Mechanical behaviour of polymers at high rates of strain

Abstract: The stress-strain behaviours of polycarbonate (PC) and polyvinylidene difluoride (PVDF) have been measured over a range of strain rates at room temperature and a range of temperatures at high strain rate. Both materials show an approximately bilinear dependence of yield stress on strain rate over the rates examined. The experiments at different temperatures allow the high strain rate glass and β transitions to be identified in PC, and the melting point and glass transition to be identified in PVDF. These can be confirmed by comparison to Dynamic Mechanical Analysis (DMA) measurements on the materials. Applying a time-temperature superposition to the data shows that these transitions are the cause of the bilinearity in the strain rate dependence of the materials. The behaviour of nominal (or engineering) stress in PC is also examined.

The ICF Status and Plans in the United States

Abstract: The United States continues to maintain its leadership in ICF as it moves toward the goal of ignition. The flagship of the program is the National Ignition Facility (NIF) presently under construction at LLNL. Experiments had begun on the first four beams of the National Ignition Facility just at the time of the last IFSA Conference. Several new successful campaigns have been conducted since then in planar hydrodynamics and hohlraums as well as activating the VISAR diagnostic for equation of state experiments. Highlights of these results will be reviewed. Presently, the four beam experimental capability has been suspended while the first eight beams are being installed as the first step in building out the project. Meanwhile, much progress has been made in developing ignition designs for using NIF. An array of designs having several ablator materials have been shown computationally to ignite with energies ranging from the design energy to as low as 1 MJ of laser energy. Alternative direct drive designs in the NIF indirect drive configuration have been developed by LLE. This wide array of design choices has increased the chance of achieving ignition sooner on the facility. Plans are now being developed to begin an ignition experimental campaign on NIF in 2010, a little over a year after completion of the facility. Other US facilities are also implementing improved capabilities. Petawatt lasers are now under construction at the University of Rochester and Sandia National Laboratory. The Z pulsed power machine at Sandia National Laboratory is being refurbished to improve its performance. The ongoing research program at the OMEGA laser at the University of Rochester and the Z machine at Sandia National Laboratory as well as at the Nike, Trident and Janus lasers remain strong, performing experiments supporting the NIF ignition plan and direct drive ignition. There also is an active program in the broader field of high energy density science on these facilities. These activities will be reviewed here as well as presented in talks throughout the conference.

A modified Johnson-Cook model for strain rates ranging from 10$^{\mbox{-}3}$ to 10$^{5}$ s$^{\mbox{-}1}$

Abstract: A modified Johnson-Cook model is proposed to take into account the thermally activated and viscous regimes that can be easily integrated in numerical codes. The model was calibrated using experimental data generated at temperatures ranging from 23 to 1000°C and strain rates ranging from 0.002 to 3 x 10 4 s -1 and validated through numerical simulations of symmetric Taylor tests for a pure nickel and a high strength nickel alloy.

Polar-direct-drive experiments on OMEGA

Abstract: Polar direct drive (PDD), a promising ignition path for the NIF while the beams are in the indirect-drive configuration [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)], is currently being investigated on the OMEGA laser system by using 40 beams in six rings repointed to more uniformly illuminate the target [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005).]. The OMEGA experiments are being performed with standard, "warm" targets (865-μm-diam, 20-μm-thick, CH shells filled with 15-atm D 2 ) with and without the use of an equatorial "Saturn-like" toroidally shaped CH ring [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 095002 (2005)] (nominal dimensions: 2.2-mm diam measured to ring center, 0.3-mm thick). For the Saturn case, the plasma formed around the ring refracts light toward the target equator as the ring plasma expands. The nominal laser drive is a 1-ns flat pulse, ∼400 J per beam, employing 1-THz, 2-D SSD with polarization smoothing. Target implosion symmetry is diagnosed with framed x-ray backlighting using additional OMEGA beams and by time-integrated x-ray imaging of the stagnating core. The best results have been obtained with Saturn targets by varying the beam pointing and ring diameter, achieving ∼75% of the fusion yield from symmetrically illuminated targets with the same total energy (60 beams, 15.3kJ).

A new approach of high speed cutting modelling: SPH method

Abstract: The purpose of this study is to introduce a new approach of high speed cutting numerical modelling. A lagrangian Smoothed Particle Hydrodynamics (SPH) based model is carried out using the Ls-Dyna software. SPH is a meshless method, thus large material distortions that occur in the cutting problem are easily managed and SPH contact control permits a “natural” workpiece/chip separation. Estimated chip morphology and cutting forces are compared to machining dedicated code results and experimental data. The developed SPH model proved its ability to account for continuous and shear localized chip formation and also correctly estimates the cutting forces, as illustrated in some orthogonal cutting examples.

Carbonation and historical buildings made of concrete

Abstract: The most ancient buildings made of reinforced concrete encountered in the field of historical monuments date back to the beginning of the 20 th century. Carbonation, inducing a corrosion of the rebars, is the main weathering mechanism of those edifices. Even if largely studied, this decay phenomenon remains quite complex, field studies being sometimes in contradiction with theories on carbonation progression for example, as high depths of carbonation can be reached within 50 years. Several case studies of buildings of different ages will illustrate the complexity of the carbonation development, strongly linked to environmental conditions, but also to concrete performance.

Polarization Smoothing on the National Ignition Facility

Abstract: We have recently implemented polarization smoothing (PS) on one quad of the NIF laser. Specially cut KDP and DKDP crystals at 420 x 420 mm sizes were used to scramble the incident 351 nm laser polarization over the beam aperture. The intensity contrast deduced from the measured focal spot images for one of the NIF beams is in very good agreement with the expected contrast. KDP and DKDP crystals are known to produce considerable amount of transverse stimulated Raman scattering (SRS) when irradiated with large beams at {approx}1-2 GW/cm2. In order to measure the transverse SRS, we attached optical fibers on the side of one of the PS crystals. The KDP PS crystal showed > 1 J/cm2 side scattered SRS at irradiances of 1.2 GW/cm2. The DKDP (70% deuteration level) PS crystal showed significantly less SRS. Detailed analysis of the SRS scattering in the PS crystal is in progress.

Mechanical behaviour and temperature measurement during dynamic deformation on split Hopkinson bar of 304L stainless steel and 5754 aluminium alloy

Abstract: The forming process of massive products at ambient temperature and at high speed of loading has not led to many scientific investigations up to now. Its understanding involves mechanical and thermal aspects that are strongly linked together (thermo-mechanical coupling). The adiabatic process generated at high strain rates, due to the short duration of the test that does not allow for thermal equilibrium, can induce thermal softening in the billet and modifications of the metallurgical microstructures. The tests are done with 304L stainless steel and 5754 aluminium alloy. A split Hopkinson bar is used for strain rates up to 2000 s-1 . During the test, the temperature at specimen surface was measured with an infrared multi-detector (with a resolution area 43 μm×43μm and an frequency acquisition equal to I MHz). The measurement system allows for a temperature measurement along a line of the specimen surface. The focusing system is designed to eliminate the geometric and chromatic aberrations induced by the lenses and it allows for measurements at high strain rates with short specimens. With this system, it is shown that the temperature field is homogeneous along the sample during the complete duration of loading. Consequently, the Taylor-Quinney coefficient can be deduced from temperature measurements.

The physical parameters estimation of physiologically worked heart prosthesis

Abstract: One of possible cardiac failure therapy is mechanical heart supporting. The following types of ventricular assist devices (VAD) are clinically used: diaphragm displacement, centrifugal and axial pumps. Each of supporting devices produces different hemodynamical effect and affects the circulatory system in various ways. It causes impossibility of therapeutic effect comparison obtained by different pumps' treatment. A lack of defined physical parameters describing phenomena inside the pump and its influence on circulatory system are an obstacle during new supporting devices designing. The goal of investigations is to create a set of physical parameters which characterized pump's operating and its cooperation with circulatory system.

A general equation of state for polymeric materials

Abstract: Prediction of the high strain rate response of polymer and polymer-composite materials is necessary in a variety of important defence and industrial applications. The response of such materials is dominated by the interchain van de Waal's bonding and this implies that a general form for the pressure-volume relation should exist. Well-validated molecular modelling has allowed the form of such relations to be explored. A relation that links pressure, volume and energy for polymers via a generalised potential function is presented and demonstrated for materials of interest. The parameters used in the relation can all be determined from the chemical structure of the polymer, and additivity rules are used to apply the model to unreacted PBX material from its components. Good agreement with experimental data is found.