Direct observation and control of near-field radiative energy transfer in a natural hyperbolic material

Heat control is a key issue in nano-electronics, where new efficient energy transfer mechanisms are highly sought after. In this respect, there is indirect evidence that high-mobility hexagonal boron nitride (hBN)-encapsulated graphene exhibits hyperbolic out-of-plane radiative energy transfer when driven out-of-equilibrium. Here we directly observe radiative energy transfer due to the hyperbolic phonon polaritons modes of the hBN encapsulant in intrinsic graphene devices under large bias, using mid-infrared spectroscopy and pyrometry. By using different hBN crystals of varied crystalline quality, we engineer the energy transfer efficiency, a key asset for compact thermal management of electronic circuits.Comment: 21 pages including Supplementary Material (Main text: 10 pages, 4 figures

Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter

Data collected by the Pierre Auger Observatory through 31 August 2007 showed evidence for anisotropy in the arrival directions of cosmic rays above the Greisen-Zatsepin-Kuz'min energy threshold, \nobreak{$6\times 10^{19}$eV}. The anisotropy was measured by the fraction of arrival directions that are less than $3.1^\circ$ from the position of an active galactic nucleus within 75 Mpc (using the V\'eron-Cetty and V\'eron $12^{\rm th}$ catalog). An updated measurement of this fraction is reported here using the arrival directions of cosmic rays recorded above the same energy threshold through 31 December 2009. The number of arrival directions has increased from 27 to 69, allowing a more precise measurement. The correlating fraction is $(38^{+7}_{-6})$, compared with $21$ expected for isotropic cosmic rays. This is down from the early estimate of $(69^{+11}_{-13})$. The enlarged set of arrival directions is examined also in relation to other populations of nearby extragalactic objects: galaxies in the 2 Microns All Sky Survey and active galactic nuclei detected in hard X-rays by the Swift Burst Alert Telescope. A celestial region around the position of the radiogalaxy Cen A has the largest excess of arrival directions relative to isotropic expectations. The 2-point autocorrelation function is shown for the enlarged set of arrival directions and compared to the isotropic expectation.Comment: Accepted for publication in Astroparticle Physics on 31 August 201

Assessment of TELEMAC system performances, a hydrodynamic case study of Anglet, France

The wave propagation and flow modules of the TELEMAC system have been applied to the Adour River mouth and the adjacent beaches of Anglet (France). The wave propagation has been simulated using the phase-averaged model TOMAWAC. In addition, the phase-resolving REFDIF S model has been also used, as the diffraction of wind waves, which is not taken into account in TOMAWAC, can occur in the vicinity of the River mouth. Next, the hydrodynamics has been simulated using the two-dimensional depth-averaged flow TELEMAC 2D model.\ud \ud The numerical results are compared with field measurements collected in March 2003. Model performance statistics are calculated in order to assess the accuracy of the predictions at six locations. Regarding the computation of the wave propagation, a root mean square error (RMSE) is evaluated for the significant wave height, the mean period and the peak direction of propagation. For the flow computation, a mean absolute error (MAE) is used as it is applicable to vector quantities. A classification is also adopted that categorises the quality of the results.\ud \ud Regarding the significant wave height, the average relative RMSE is of 18.2% for the TOMAWAC computations whereas the error is of 30.8% for the REFDIF S simulations. Concerning the peak wave direction, the RMSE is about 10°. Both models predict the wave direction normal to the shore whereas the experimental parameters are more variable. However, the areas, where energy concentrates, are well reproduced.\ud \ud The classification of errors shows that TELEMAC 2D flow predictions are in a range from excellent to reasonable

Using ALS measurements for automatic coastline extraction

International audienc

Acoustic turbulence measurements of near-bed suspended sediment dynamics in highly turbid waters of a macrotidal estuary

Sediment-turbulence interactions near the bed are still poorly understood in highly turbid estuaries, especially in the presence of fluid mud layers. This results primarily from the difficulty in measuring co-located velocity and suspended sediment concentration (SSC) at sufficiently high rate to resolve small turbulent flow scales. In this paper, we show how a set of commercial acoustic and optical bacicscattering systems known as ADCPs, ADVs and OBSs, can be deployed and used in a complementary way to perform large-scale profilings of tidal current and SSC combined with high-resolution velocity and SSC measurements in the highly turbid near-bed zone. The experiment was done in the Gironde estuary (France) which is well known for its turbidity maximum zone characterized by high SSC values, above 1 g l(-1) near the surface. A first simple inversion method is proposed to convert the backscattered acoustic intensity measured with ADV into SSC data in the highly turbid near-bed zone. Near-bed SSC data from the OBS are used to compensate for the important acoustic sediment attenuation effect at an acoustic frequency of 6 MHz. No a priori knowledge of acoustic backscattering properties of mud suspensions is required with this calibration procedure. We obtain an attenuation coefficient for mud suspensions of 0.28 m(2)/kg at 6 MHz leading to a good agreement between the SSC timeseries from the three ADV receivers and the OBS over the entire tidal cycle. The obtained SSC data are then analyzed with respect to the near-bed velocity, Reynolds shear stress and turbulent kinetic energy (TKE) timeseries in order to identify the relevant sediment transport processes during the tidal cycle. Significant differences in bed shear stress and TICE levels are found between ebb and flood stages with effects on near-bed sediment dynamics. During the ebb, maximum levels of tidal current, bed shear stress and TKE are associated with a reduction of near-bed sediment concentration (from 400 kg m(-3) down to 100 kg m(-3)). Bed liquefaction process is assumed to occur at this moment with the presence of highly concentrated mud layer and a possible lutocline at a distance of less than 20 cm above the bed. During the first 1.5 h of flood, turbulent activity remains moderate. The near-bed flood current is then inhibited very abruptly while a sudden increase in SSC occurred above the bed. Assuming that the ADV is able to estimate relevant turbulent erosion fluxes, the co-located velocity and SSC are multiplied and compared with settling flux measurements made onboard under quiescent water conditions. The mean sediment settling fluxes (averaged over 3 min) increase with SSC and are in relative good coherence with fluxes in quiescent water below the hindered regime, for SSC below 15 g l(-1). Reducing averaging time from 3 min to 30 s allows to increase the range of turbulent fluxes and SSC values, up to 99 g l(-1). At this scale, fluxes keep increasing quasi-linearly at higher SSC, suggesting the inhibition (delay or reduction) of the hindered settling regime as previously shown by Gratiot et al. (2005) from laboratory experiments. However, the 3-min averaged concentration field remained too low to conclude definitively on the effectiveness of such a process. Further analysis conducted at higher SSC regimes and under fully verified equilibrium are necessary

Sediment budget and morphological evolution in the Bay of Mont-Saint-Michel (Normandy, France): aerial (LIDAR) and terrestrial laser monitoring

We propose a study on the use of laser techniques to monitor altimetric variations in the tidal flat of the Bay of Mont-Saint-Michel. The Bay of Mont-Saint-Michel has been strongly anthropised. Because of impoldering, wandering rivers were not able to sap salt-meadow and modern tidal flooding of the Mont-Saint-Michel has lowered. Through modern studies and projects aimed at restoring the marine nature of the bay it appears that flushes are useful to discard sediment tending to silt the bay. The major aim of our work consists in the better understanding of the effect of the dam built recently (May 2009) in that purpose. Laser scanning is commonly used for topographic surveys and the generation of Digital Elevation Model (DEM). Repeating surveys, allow to quantify topographic changes and therefore sediment budgets. Our study is based on aerial topographic surveys of the intertidal zone acquired before the operational start up of the dam (in 1997, 2002, 2007 and February 2009). Sediment budgets computations confirm that the bay tends to accrete but at annual rates quite variable in time. The value computed between 2002 and 2007 is 2.3 times and 3.5 times smaller than the deposition rates computed for the 1997/2002 and 2007/2009 periods. Terrestrial laser surveys acquired on a more restricted area after the beginning of the flushes are also used in a more qualitative way to investigate their impacts. The preliminary results show that flushes allow to reduce sediment deposition but also that their efficiency is geographically contrasted. Another remarkable result consists in the identification of coastal dynamics: location of erosion versus deposition zones, of evolutions of vegetation cover and drainage channels activity

Using ALS measurements for automatic coastline extraction

International audienc

MODELING OF ALS INTENSITY BEHAVIOR AS A FUNCTION OF INCIDENCE ANGLE FOR COASTAL ZONE SURFACE STUDY

Backscatter intensity of the LiDAR, recorded simultaneously with the discrete topography measurements, was recently investigated, providing complementary information to study coastal environments. The aim of this study was (i) to analyze the intensity-incidence angle behavior for coastal environments (sea and moist sand surfaces) and other surfaces (fields and urban area) with different reflectance models and (ii) to choose the best modeling for all these surfaces. We compared three reflection models. Firstly, diffuse reflection is introduced by Lambert's cosine law. The illuminated surfaces are composed of numerous small mirror-like facets inducing a dominant specular diffusion. This scattering of micro-facets is defined by the Beckmann distribution. The last model is a combination of diffuse and specular reflection. In this paper, we found that Beckmann distribution well simulates the intensity-incidence behavior for specular and diffuse mechanisms with the advantage to have only one parameter to characterize the studied surfaces