33 research outputs found
Local evolution equations for non-Markovian processes
A Fokker-Planck equation approach for the treatment of non-Markovian
stochastic processes is proposed. The approach is based on the introduction of
fictitious trajectories sharing with the real ones their local structure and
initial conditions. Different statistical quantities are generated by different
construction rules for the trajectories, which coincide only in the Markovian
case. The merits and limitations of the approach are discussed and applications
to transport in ratchets and to anomalous diffusion are illustated.Comment: Latex, 11 pages, 1 eps figur
Lagrangian dispersion characteristics in the Western Mediterranean
Dispersion characteristics in the Western Mediterranean are analyzed using data from Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) surface drifters deployed in the period 1986–2017. Results are presented in terms of absolute dispersion A2 (mean-squared displacement of drifter individuals) and of relative dispersion (D2; mean square separation distance of drifter pairs). Moreover, the dispersion characteristics are estimated for different initial separation distances (D0) between particles: smaller, larger, or comparable with the internal Rossby radius of deformation. Results show the presence of a quasiballistic regime for absolute dispersion at small time scales and the nonlocal relative dispersion regime related to the submesoscale activities for scales smaller than the internal Rossby radius. At intermediate times, two anomalous absolute dispersion regimes (elliptic and hyperbolic regimes) related with the flow topology are observed, although the relative dispersion involves the Richardson and shear/ballistic regimes only for D0 smaller than the Rossby radius. During the subsequent 20–30 days, absolute dispersion shows quasirandom walk regime and relative dispersion follows the diffusive regime for scales larger than 100 km for which pair velocities are uncorrelated
Spreading of Lagrangian Particles in the Black Sea: A Comparison between Drifters and a High-Resolution Ocean Model
The Lagrangian dispersion statistics of the Black Sea are estimated using satellite-tracked drifters, satellite altimeter data and a high-resolution ocean model. Comparison between the in-situ measurements and the model reveals good agreement in terms of the surface dispersion. The mean sub-basin coherent structures and currents of the Black Sea are well reproduced by the model. Seasonal variability of the dispersion in the upper (15 m), intermediate (150 m) and deep (750 m) layers are discussed with a special focus of the role of sub-basin scale structures and currents on the turbulent dispersion regimes. In terms of the surface relative dispersion, the results show the presence of the three known turbulent exponential, Richardson and diffusive-like regimes. The non-local exponential regime is only detected by the model for scales <10 km, while the local Richardson regime occurs between 10 and 100 km in all cases due to the presence of an inverse energy cascade range, and the diffusive-like regime is well detected for the largest distance by drifters (100–300 km) in winter/spring. Regarding the surface absolute dispersion, it reflects the occurrence of both quasi-ballistic and random-walk regimes at small and large times, respectively, while the two anomalous hyperbolic (5/4) and elliptic (5/3) regimes, which are related to the topology of the Black Sea, are detected at intermediate times. At depth, the signatures of the relative and absolute dispersion regimes shown in the surface layer are still valid in most cases. The absolute dispersion is anisotropic; the zonal component grows faster than the meridional component in any scenario
Clustering dynamics of Lagrangian tracers in free-surface flows
We study the formation of clusters of passive Lagrangian tracers in a
non-smooth turbulent flow in a flat free-slip surface as a model for particle
dynamics on free surfaces. Single particle and pair dispersion show different
behavior for short and large times: on short times particles cluster
exponentially rapidly until patches of the size of the divergence correlation
length are depleted; on larger times the pair dispersion is dominated by almost
ballistic hopping between clusters. We also find that the distribution of
particle density is close to algebraic and can trace this back to the
exponential distribution of the divergence field of the surface flow.Comment: 5 pages, 5 Postscript figure
Dynamics of a small neutrally buoyant sphere in a fluid and targeting in Hamiltonian systems
We show that, even in the most favorable case, the motion of a small
spherical tracer suspended in a fluid of the same density may differ from the
corresponding motion of an ideal passive particle. We demonstrate furthermore
how its dynamics may be applied to target trajectories in Hamiltonian systems.Comment: See home page http://lec.ugr.es/~julya
The Mixing and Transport Properties of the Intra Cluster Medium: a numerical study using tracers particles
We present a study of the mixing properties of the simulated intra cluster
Medium, using tracers particles that are advected by the gas flow during the
evolution of cosmic structures. Using a sample of seven galaxy clusters (with
masses in the range of M=2-3 10^14Msol/h) simulated with a peak resolution of
25kpc/h up to the distance of two virial radii from their centers, we
investigate the application of tracers to some important problems concerning
the mixing of the ICM. The transport properties of the evolving ICM are studied
through the analysis of pair dispersion statistics and mixing distributions. As
an application, we focus on the transport of metals in the ICM. We adopt simple
scenarios for the injection of metal tracers in the ICM, and find remarkable
differences of metallicity profiles in relaxed and merger systems, also through
the analysis of simulated emission from Doppler-shifted Fe XXIII lines.Comment: 19 pages, 24 figures, Astronomy and Astrophysics accepted; Final
version after language editing and updating the bibliograph
Wafer-scale detachable monocrystalline Germanium nanomembranes for the growth of III-V materials and substrate reuse
Germanium (Ge) is increasingly used as a substrate for high-performance
optoelectronic, photovoltaic, and electronic devices. These devices are usually
grown on thick and rigid Ge substrates manufactured by classical wafering
techniques. Nanomembranes (NMs) provide an alternative to this approach while
offering wafer-scale lateral dimensions, weight reduction, limitation of waste,
and cost effectiveness. Herein, we introduce the Porous germanium Efficient
Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of
wafer-scale detachable monocrystalline Ge NMs on porous Ge (PGe) and substrate
reuse. We demonstrate monocrystalline Ge NMs with surface roughness below 1 nm
on top of nanoengineered void layer enabling layer detachment. Furthermore,
these Ge NMs exhibit compatibility with the growth of III-V materials.
High-resolution transmission electron microscopy (HRTEM) characterization shows
Ge NMs crystallinity and high-resolution X-ray diffraction (HRXRD) reciprocal
space mapping endorses high-quality GaAs layers. Finally, we demonstrate the
chemical reconditioning process of the Ge substrate, allowing its reuse, to
produce multiple free-standing NMs from a single parent wafer. The PEELER
process significantly reduces the consumption of Ge during the fabrication
process which paves the way for a new generation of low-cost flexible
optoelectronics devices.Comment: 17 pages and 6 figures along with 3 figures in supporting informatio
Lagrangian analysis of satellite-derived currents: Application to the North Western Mediterranean coastal dynamics
Photoconversion Optimization of Pulsed-Laser-Deposited p-CZTS/n-Si-Nanowires Heterojunction-Based Photovoltaic Devices
We report on the achievement of novel photovoltaic devices based on the pulsed laser deposition (PLD) of p-type Cu2ZnSnS4 (CZTS) layers onto n-type silicon nanowires (SiNWs). To optimize the photoconversion efficiency of these p-CZTS/n-SiNWs heterojunction devices, both the thickness of the CZTS films and the length of the SiNWs were independently varied in the (0.3–1.0 µm) and (1–6 µm) ranges, respectively. The kësterite CZTS films were directly deposited onto the SiNWs/Si substrates by means of a one-step PLD approach at a substrate temperature of 300 °C and without resorting to any post-sulfurization process. The systematic assessment of the PV performance of the ITO/p-CZTS/n-SiNWs/Al solar cells, as a function of both SiNWs’ length and CZTS film thickness, has led to the identification of the optimal device characteristics. Indeed, an unprecedented power conversion efficiency (PCE) as high as ~5.5%, a VOC of 400 mV, a JSC of 26.3 mA/cm2 and a FF of 51.8% were delivered by the devices formed by SiNWs having a length of 2.2 µm along with a CZTS film thickness of 540 nm. This PCE value is higher than the current record efficiency (of 5.2%) reported for pulsed-laser-deposited-CZTS (PLD-CZTS)-based solar cells with the classical SLG/Mo/CZTS/CdS/ZnO/ITO/Ag/MgF2 device architecture. The relative ease of depositing high-quality CZTS films by means of PLD (without resorting to any post deposition treatment) along with the gain from an extended CZTS/Si interface offered by the silicon nanowires make the approach developed here very promising for further integration of CZTS with the mature silicon nanostructuring technologies to develop novel optoelectronic devices
Turbulent dispersion properties from a model simulation of the western Mediterranean Sea
Using a high-resolution primitive equation model of the western Mediterranean Sea, we analyzed the dispersion properties of a set of homogeneously distributed, passive particle pairs. These particles were initially separated by different distances D-0 (D-0 = 5.55, 11.1 and 16.65 km), and were seeded in the model at initial depths of 44 and 500 m. This realistic ocean model, which reproduces the main features of the regional circulation, puts into evidence the three well-known regimes of relative dispersion. The first regime due to the chaotic advection at small scales lasts only a few days (3 days at 44m depth, a duration comparable with the integral timescale), and the relative dispersion is then exponential. In the second regime, extending from 3 to 20 days, the relative dispersion has a power law t(alpha) where alpha tends to 3 as D-0 becomes small. In the third regime, a linear growth of the relative dispersion is observed starting from the twentieth day. For the relative diffusivity, the D-2 growth is followed by the Richardson regime D-4/3. At large scales, where particle velocities are decorrelated, the relative diffusivity is constant. At 500m depth, the integral timescale increases (> 4 days) and the intermediate regime becomes narrower than that at 44m depth due to the weaker effect of vortices (this effect decreases with depth). The turbulent properties become less intermittent and more homogeneous and the Richardson law takes place