Identification Of Different Regimes Research Articles

Decaying compressible turbulence with thermal non-equilibrium

The interaction of decaying turbulence with thermal non-equilibrium (TNE) is studied using direct numerical simulations. The focus is on energy exchanges and decay rates in decaying flows with initial vibrational excitation. A key finding is the identification of different regimes in the interaction and the nondimensional parameter (β) that controls it. The latter accounts for the degree of initial TNE as well as the ratio of timescales of turbulence and vibrational relaxation. For β < 1, TNE is essentially frozen and turbulence is largely unaffected by the decay of vibrational energy. For β > 1, TNE relaxation is relatively fast and produces an increase in translational–rotational energy, which, through changes in transport coefficients, leads to a temporary increase in dissipation leading to faster turbulence decay rates. Theoretical arguments are put forth to determine the asymptotic limits of this effect. TNE relaxation is also affected by turbulent fluctuations in unexpected ways. For example, although initial conditions are always vibrationally hot, the flow may undergo vibrationally cold transients, which are explained through simple models. The results presented here help explain disagreement between previous experimental and numerical data.

Photocarrier extraction in GaAsSb/GaAsN type-II QW superlattice solar cells

Photocarrier transport and extraction in GaAsSb/GaAsN type-II quantum well superlattices are investigated by means of inelastic quantum transport calculations based on the non-equilibrium Green's function formalism. Evaluation of the local density of states and the spectral current flow enables the identification of different regimes for carrier localization, transport, and extraction as a function of configurational parameters. These include the number of periods, the thicknesses of the individual layers in one period, the built-in electric field, and the temperature of operation. The results for the carrier extraction efficiency are related to experimental data for different symmetric GaAsSb/GaAsN type-II quantum well superlattice solar cell devices and provide a qualitative explanation for the experimentally observed dependence of photovoltaic device performance on the period thickness.

Mimicking bug-like surface structures and their fluid transport produced by ultrashort laser pulse irradiation of steel

Ultrashort laser pulses with durations in the fs-to-ps range were used for large area surface processing of steel aimed at mimicking the morphology and extraordinary wetting behaviour of bark bugs (Aradidae) found in nature. The processing was performed by scanning the laser beam over the surface of polished flat sample surfaces. A systematic variation of the laser processing parameters (peak fluence and effective number of pulses per spot diameter) allowed the identification of different regimes associated with characteristic surface morphologies (laser-induced periodic surface structures, i.e., LIPSS, grooves, spikes, etc.). Moreover, different laser processing strategies, varying laser wavelength, pulse duration, angle of incidence, irradiation atmosphere, and repetition rates, allowed to achieve a range of morphologies that resemble specific structures found on bark bugs. For identifying the ideal combination of parameters for mimicking bug-like structures, the surfaces were inspected by scanning electron microscopy. In particular, tilted micrometre-sized spikes are the best match for the structure found on bark bugs. Complementary to the morphology study, the wetting behaviour of the surface structures for water and oil was examined in terms of philic/phobic nature and fluid transport. These results point out a route towards reproducing complex surface structures inspired by nature and their functional response in technologically relevant materials.

Mimicking lizard-like surface structures upon ultrashort laser pulse irradiation of inorganic materials

Inorganic materials, such as steel, were functionalized by ultrashort laser pulse irradiation (fs- to ps-range) to modify the surface’s wetting behavior. The laser processing was performed by scanning the laser beam across the surface of initially polished flat sample material. A systematic experimental study of the laser processing parameters (peak fluence, scan velocity, line overlap) allowed the identification of different regimes associated with characteristic surface morphologies (laser-induced periodic surface structures, grooves, spikes, etc.). Analyses of the surface using optical as well as scanning electron microscopy revealed morphologies providing the optimum similarity to the natural skin of lizards. For mimicking skin structures of moisture-harvesting lizards towards an optimization of the surface wetting behavior, additionally a two-step laser processing strategy was established for realizing hierarchical microstructures. In this approach, micrometer-scaled capillaries (step 1) were superimposed by a laser-generated regular array of small dimples (step 2). Optical focus variation imaging measurements finally disclosed the three dimensional topography of the laser processed surfaces derived from lizard skin structures. The functionality of these surfaces was analyzed in view of wetting properties.

STM Imaging of Electronic Waves on the Surface ofBi2Te3: Topologically Protected Surface States and Hexagonal Warping Effects

Scanning tunneling spectroscopy studies on high-quality Bi2Te3 crystals exhibit perfect correspondence to angle-resolved photoemission spectroscopy data, hence enabling identification of different regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear, dispersing with a wave vector that may result from a hexagonal warping term.

A linear code parameter search algorithm with applications to immunology

The immune system is modeled by way of a system of ordinary differential equations involving a large number of parameters, such as growth rates and initial conditions. Key to successful implementation of the model is the estimation of such parameters from available data. A parameter search algorithm based on linear codes is developed having as aim the identification of different regimes of behaviour of the model, the estimation of parameters in a high dimensional space, and the model calibration to data.

Overview of Alfven eigenmode experiments in JET

Results from the first experiments to drive Alfven eigenmodes (AEs) with antennas external to a tokamak plasma are presented. In ohmically heated plasma discharges, direct experimental measurements of the damping of toroidicity induced AEs (TAEs) have allowed an identification of different regimes corresponding to different dominant TAE absorption mechanisms with a wide range of damping rates, 10-3 ⩽ γ/w ⩽ 10-1. In plasmas heated by ion cyclotron resonance heating, neutral beam injection heating, lower hybrid heating and high plasma current ohmic heating, a new class of weakly damped Alfven eigenmodes, the kinetic Alfven eigenmodes, predicted in theoretical models that include finite Larmor radius and finite parallel electric field effects, has been identified experimentally

Statistical control chart for regime identification in volcanic time series

In an important paper, Mulargia et al. (1987) address the importance of quantitative and objective identification of different regimes of a volcano. They develop a procedure based on the two-sample Kolmogorov-Smirnov (K-S) statistic. The K-S test is a general-purpose test that discriminates between two data sets as belonging to two different regimes based on their empirical distribution functions. The empirical distribution function is designed to describe the aggregate behavior of the volcanic activity, and it is constructed from the orders of the length of the collected repose times in each data set. In this article, we use the idea of statistical process control to distinguish between the variation inherent in the observed repose times and the extraordinary variation that signals a real change in the regimes. We construct a table of control limits, and we demonstrate the procedure of regime identification based on a simple control chart. It shows a point outside the control limits almost as soon as the process enters a new regime. The basis of the statistical process control mechanism is a simple Poisson process, which is state of the art. The proposed control charting procedure is an eruption by eruption procedure, which follows the original chronological order of the eruptions. This procedure is applied to the eruptive history of the Mount Etna volcano. The application shows schematically that the procedure presents a visual interpretation of the identified regimes and can be practically translated for tabular or manual use.

Identifying different regimes in eruptive activity: An application to Etna volcano

The objective identification of different regimes in the eruptive time-history of a volcano is crucial to the understanding of its physics. While a problem well-known in statistical literature under the name of change-point or scan-point problem, no method of general applicability exists for the identification of different regimes in a time-series. In particular, the available techniques seem unsuitable to the volcanological case. We developed an original procedure based on two-sample Kolmogorov-Smirnov statistics which offers satisfactory accuracy in a broad range of conditions with a minimum of assumptions and is expressly tailored to the study of geophysical phenomena. Our procedure requires neither the a priori knowledge of the number of regimes nor of the statistical distributions governing the whole process, which can be of different type. The parent distribution of each regime is inferred through a goodness-of-fit test, and this in turn allows the confidence intervals for each of the change-points identified to be estimated by numerical simulation. This procedure is applied to the eruptive history of Mount Etna volcano. Available data allow the analysis of flank eruptions in the period 1600–1980, while the total output (summit and flank activity) can be studied only in the period 1971–1981. Information on eruptive history can be therefore obtained at two different timescales. Since no univocally accepted catalog exists except for the last few decades, we use two different sets of data, which practically exhaust all the available information. The results are interpreted by a stability analysis, and only stable results are retained. Our analysis yields that: • - The inter-event times of flank eruptions in the period 1600–1980 follow two regimes before and after year 1865, while the eruptive activity in the period 1971–1981 follows four different regimes. In each regime eruptions occur according to a Poisson process and Etna behaves as a random nonstationary volcano both at long (few centuries) and short (few years) time-scales. The eruptions appear therefore to be triggered by several combining effects with a balance showing fluctuations at both time-scales. • - The volume output of flank eruptions in the period 1600–1980 follows three different regimes, with change-points at 1670 and 1750. The volume output in the period 1971–1981 follows a single regime. Least squares regression and the related confidence bounds, calculated for each regime, suggest that, even accounting for summit activity, the output rate in the period 1971–1981 is not anomalously high. Provided that no further change in regime takes place, the confidence bounds can be used to cast estimates on the expected future activity. • - The series of the mean effusion rates (erupted volume/duration of the eruption) applied to flank eruptions in the period 1600–1980 follows two different regimes, with a change-point around year 1950. There is a net decrease in the mean effusion rate from over 24 m 3 s −1 to about 7 m 3 s −1, which suggests a change in the eruptive style to lower mean effusion rates. The total output (flank + summit) data of the period 1971–1981 confirms this issue, with averaged mean effusion rates lower than 12 m 3 s −1. • - The points of change in regime in the time-series of the inter-eruption times and volume outputs do not coincide, thus implying that eruptions are governed by other factors than the input of magma. At the same time, the analysis of seismic activity does not suggest the stress field as a most important factor. The eruptive activity does not appear to be controlled by a single factor, but by the balance of several contributing factors, further corroborating the conclusion that Etna behaves as a random nonstationary volcano. • - The relatively small fluctuations in each of the periods identified in the flank output series suggest a constant input within each output regime, although the unavailability of both a reliable model and accurate estimates on historical summit eruptions recommends prudence in accepting this conclusion. • - There appears to be no significant correlation between the volume of a “large” flank eruption and the subsequent inter-event time. This, with the above proviso, suggests the absence of any high-level magma reservoir.