Numerical modeling of underwater parametric propagation to detect buried objects

In underwater acoustics, detection of buried objects in sediments (cables, mines,…) is a complex problem. One reason is that acoustic attenuation in these sediments increases with frequency. To ensure sufficient penetration depth in marine sediments, low frequencies have to be used, implying a low resolution. A solution proposed to solve this problem is the parametric emission based on the nonlinear properties of the propagation medium. This method can generate a low frequency wave from two directional high frequencies beams. The parametric propagation is simulated in seawater and marine sediments. The model developed is based on the fractional-step numerical method introduced by Christopher and Parker [1]. In this method, the normal particle velocity is calculated plane by plane from the surface of the transducer to a specified distance. The effects of nonlinearity, attenuation and diffraction are calculated independently for each spatial step. Moreover, to reduce the number of spatial steps, a second order operator splitting scheme is used. The diffraction computation is based on a method of angular spectrum in the frequency domain where the field across a source plane is described by a spatial frequency distribution. To improve code stability, the effects of nonlinearity and attenuation are calculated and associated in shorter propagation substeps. At the interface between water and marine sediments, the transmission conditions are applied. Several tests have been carried out in different configurations (changing the primary frequencies, the parametric frequency, the source geometry, the inclination of the source with the interface, the focal distance,…). The 3D velocity field is calculated in each case, thereby allowing to know the directivity of the source, the velocity amplitude in sediments and the performance

Anet – Sente des Duvaux

L’opération de fouille archéologique, d’une emprise de 3 400 m2, réalisée en 2017 sur le lieu-dit Les Terres Franche » à Anet (Eure-et-Loir) s’inscrit dans le même projet d’aménagement d’une déviation routière que le site protohistorique dit Les Durvys-Sente des Duvaux, fouillé à proximité par É. Fencke et fait suite à un diagnostic archéologique mené par cette dernière l’année précédente. Les parcelles s’étendent sur une succession de terrasses alluviales situées sur le versant est de la val..

The parametric propagation in underwater acoustics : experimental results

In underwater acoustics, detection of buried objects in sediments (cables, mines, . . . ) is a complex problem. Indeed, in order to ensure sufficient penetration depth in marine sediments, low frequencies have to be used, implying a low resolution. A solution proposed to solve this problem is the parametric emission based on the nonlinear properties of seawater. This method can generate a low frequency wave from two directional high frequencies beams. The aim of this work is to present experimental results of a parametric propagation. Experiments have been carried out in a water tank in various configurations. These experimental measurements are then compared with simulation results obtained with a numerical model based on a fractional-step method presented at the Underwater Acoustic Measurements conference in 2011

Numerical modeling of underwater parametric propagation to detect buried objects

In underwater acoustics, detection of buried objects in sediments (cables, mines,…) is a complex problem. One reason is that acoustic attenuation in these sediments increases with frequency. To ensure sufficient penetration depth in marine sediments, low frequencies have to be used, implying a low resolution. A solution proposed to solve this problem is the parametric emission based on the nonlinear properties of the propagation medium. This method can generate a low frequency wave from two directional high frequencies beams. The parametric propagation is simulated in seawater and marine sediments. The model developed is based on the fractional-step numerical method introduced by Christopher and Parker [1]. In this method, the normal particle velocity is calculated plane by plane from the surface of the transducer to a specified distance. The effects of nonlinearity, attenuation and diffraction are calculated independently for each spatial step. Moreover, to reduce the number of spatial steps, a second order operator splitting scheme is used. The diffraction computation is based on a method of angular spectrum in the frequency domain where the field across a source plane is described by a spatial frequency distribution. To improve code stability, the effects of nonlinearity and attenuation are calculated and associated in shorter propagation substeps. At the interface between water and marine sediments, the transmission conditions are applied. Several tests have been carried out in different configurations (changing the primary frequencies, the parametric frequency, the source geometry, the inclination of the source with the interface, the focal distance,…). The 3D velocity field is calculated in each case, thereby allowing to know the directivity of the source, the velocity amplitude in sediments and the performance

Experimental characterization of the 3D dynamics of a laminar shallow vortex dipole

Experimental results on the dynamics of a vortex dipole evolving in a shallow fluid layer are presented. In particular, the generation of a spanwise vortex at the front of the dipole is observed in agreement with previous experiments at larger Reynolds numbers. The results show that this secondary vortex is of comparable strength to the dipole. The present physical analysis suggests that the origin of this structure involves the stretching induced by the dipole of the boundary-layer vorticity generated by the dipole's advection over the no-slip bottom

In situ nanoindentation of Au crystals imaged by Bragg coherent X-ray diffraction

The mechanical properties of micro- and nanostructures were demonstrated to vary significantly from their bulk counterparts. Despite numerous studies, plasticity at the nanoscale is, however, not fully understood yet. In situ experiments are perfectly suited for the fundamental understanding of the onset of dislocation nucleation. Recently, we developed a scanning force microscope (SFINX) which is compatible with 3rd generation synchrotron beamlines allowing for in situ nano-mechanical tests in combination with nano-focused X-ray diffraction [1] such as coherent X-ray diffraction imaging (CDI). This novel lensless imaging method retrieves the sample scattering function from a coherent X-ray diffraction data set using computational inversion algorithms, thus determining the phase of the scattered amplitude, which is not directly measured by a detector. In Bragg condition, the retrieved phase is directly related to the displacement field and, hence to the strain within a crystal. Please click Additional Files below to see the full abstract

Simulation de la diffraction cohérente d'un film polycristallin

http://hdl.handle.net/2042/38756International audienceLa diffraction cohérente permet de caractériser les hétérogénéités de déformation dans un polycristal. La figure de diffraction dans l'espace réciproque dépend de la forme du grain illuminé et de son champ de déformation. Dans ce travail, La déformation d'un film mince polycristallin est calculée par élément finis et les figures de diffraction de certains grains en sont déduites en fonction de la déformation imposée. L'influence de la densité de maillage, de la taille et de la forme du grain illuminé est étudiée

Diffraction cohérente d'un film polycristallin

La diffraction cohérente permet de caractériser les hétérogénéités de déformation dans un polycristal. La figure de diffraction dans l'espace réciproque dépend de la forme du grain illuminé et de son champ de déformation. Dans ce travail, La déformation d'un film mince polycristallin est calculée par élément finis et les figures de diffraction de certains grains en sont déduites en fonction de la déformation imposée. L'influence de la densité de maillage, de la taille et de la forme du grain illuminé est étudiée

Synergistic influence of topomimetic and chondroitin sulfate-based treatments on osteogenic potential of Ti-6Al-4V

International audienceWe combined topographical and chemical surface modifications of Ti-6Al-4V (TA6V) to improve its osteogenic potential. By acid-etching, we first generated topomimetic surface features resembling, in size and roughness, bone cavities left by osteoclasts. Next, we coated these surfaces with biomimetic Layer-by-Layer films (LbL), composed of chon-droitin sulfate A and poly-L-lysine that were mechanically tuned after a post-treatment with genipin. The structural impact of each surface processing step was thoroughly inspected. The desired nano/microrough topographies of TA6V were maintained upon LbL deposition. Whereas no significant promotion of adhesion and proliferation of MC3T3-E1 preosteoblasts were detected after independent or combined modifications of the topography and the chemical composition of the substrates, osteogenic maturation was promoted when both surface treatments were combined, as was evi-denced by significant long-term matrix mineralization. The results open promising route toward improved osseointegra-tion of titanium-based implants. V C 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 00B:000-000, 2016