High Exponent Research Articles

Lagrangian visualization of mixing enhancement induced by finite-time stretching in compressible vortex interaction

The investigation of the coupling relationship between mixing enhancement and convective stretching bears practical and scientific significance. In this paper, Lagrangian perspective is adopted for compressible vortex interaction visualization to emphasize the effect of inner flow dynamics other than the vorticity structure on mixing performance. Transport characteristics of the flow are studied through a blob of passive particles. The results demonstrate that Lagrangian coherent structures are faithfully tracked throughout the moving of particles, and the blob initially arranged upon the ridge of Lagrangian coherent structures exhibits efficient stretching. In this sense, the physical meaning of Lagrangian coherent structure as a material barrier and maximum stretching direction is highlighted. On this basis, mixing characteristics are investigated through the introduction of helium concentration blob considering viscous diffusion. Characterized by high growth rate of mixing and low mixing time, mixing enhancement is achieved with the aid of significant stretching directly reflected by high average finite-time Lyapunov exponent in the region of blob deposition, offering an optimal strategy for fluid mixing.

Planetary thermal evolution models with tectonic transitions

Thermal history calculations provide important insights into the interior evolution of planets, but incorporate simplified dynamics from the systems they represent. Planetary interiors typical incorporate complex rheologies, viscous layering, lateral heterogeneities, and time delays in processes, which have not been traditionally represented by parameterised approaches. Here we develop numerical models for planetary evolution, incorporating the physical complexity of Earth’s interior, and use them to generate statistically-based Nu-Ra scalings. These encapsulate the main effects of tectonic transitions, geometry, and depth-dependent rheology, and time-sensitivity. We find an exponent β of ~0.26 best describes the Nu-Ra relationship for evolving mobile lid systems, and β ~0.12 for stagnant-lid systems. Systems with time dependent subduction have β varying between ~0.26 during the Hadean, when external factors such as impacts facilitate tectonics, to ~0.12 during the Archaean, when the system is dominated by long periods of quiescence, and systems driven by external forcings (eg. due to impacts in the first 100Myr of Earth’s history) may exhibit much higher exponents. We also find a time-lag between Ra (which primarily depends on mantle temperature) and Nu (normalised surface heat flow) of around 200–300Myr, suggesting a significant delay between mantle thermal configuration, and its surface manifestation. These results provide an approach for the rapid characterisation of tectonic, volcanic, and atmospheric evolution of terrestrial exoplanets.

Frameworks for investigation of nonlinear dynamics: Experimental study of the turbulent jet

Analysis methods that have been developed in the field of nonlinear dynamics have provided valuable insights into the physics of turbulent flows, although their application to open flows is less well explored. The nonlinear dynamics of a turbulent jet with a low-to-moderate Reynolds number is investigated by using the single-trajectory framework and ensemble framework. We have used Lyapunov exponents to calculate the spectra of scaling indices of the attractor. First, we evaluated the frameworks on two theoretical models, one with a stationary attractor (Lorenz-63) and the other with time-varying characteristics (Lorenz-84). Theoretical studies showed that in dynamical systems with a stable attractor, both frameworks estimated the same largest Lyapunov exponent. The ensemble framework enables us to resolve the unsteady characteristics of a time-varying strange attractor. Second, we applied both frameworks to time-resolved planar velocity fields in a turbulent jet at local Reynolds numbers (Reδ) of 3000 and 5000. Time-resolved particle image velocimetry was utilized to measure streamwise and transverse velocity components. Results support the presence of a low-dimensional attractor in the reconstructed phase space with a chaotic characteristic. Despite considerable changes in the dynamics for the higher Reynolds number case, the system’s fractal dimension did not change significantly. We have used Lagrangian Coherent Structures (LCSs) to study the relationship between changes in the Lyapunov exponent with flow topological features. Results suggest that holes in the stable LCSs provide a path for the entrainment of the coflow, which is shown to be one of the main contributors to high Lyapunov exponents.

The potentials of TiO2 nanocatalyst on HMX thermolysis

Even though HMX is one of the most powerful highly explosive materials; HMX-based propellants demonstrated complexity of burning rate control as well as high pressure exponent (n). In addition, HMX is insensitive to common catalyst. TiO2 can offer novel catalyzing ability for HMX. Highly-crystalline, mono-dispersed TiO2 NPs of 5.0 nm particle size with proper surface area (26.87 ± 0. 36 m2/g) were fabricated using hydrothermal processing. TiO2 NPs were re-dispersed in organic solvent and effectively-integrated into HMX via co-precipitation technique; the impact of TiO2 NPs on HMX thermal behavior was investigated using DSC and TGA. TiO2 NPs exposed superior catalytic performance; the endothermic phase change of HMX at 187 °C was decreased by 43.3%. The main exothermic decomposition peak was decreased by 10 °C with enhanced total heat release by 46.7%. The catalytic performance of TiO2 NPs could be ascribed to the release of active surface ȮH radicals that could induce HMX decomposition via hydrogen abstraction. Furthermore, TiO2 NPs could adsorb evolved NO2 on its surface with surge in total heat release in condensed phase.

A Novel Methodology to Calculate the Probability of Volatility Clusters in Financial Series: An Application to Cryptocurrency Markets

One of the main characteristics of cryptocurrencies is the high volatility of their exchange rates. In a previous work, the authors found that a process with volatility clusters displays a volatility series with a high Hurst exponent. In this paper, we provide a novel methodology to calculate the probability of volatility clusters with a special emphasis on cryptocurrencies. With this aim, we calculate the Hurst exponent of a volatility series by means of the FD4 approach. An explicit criterion to computationally determine whether there exist volatility clusters of a fixed size is described. We found that the probabilities of volatility clusters of an index (S&P500) and a stock (Apple) showed a similar profile, whereas the probability of volatility clusters of a forex pair (Euro/USD) became quite lower. On the other hand, a similar profile appeared for Bitcoin/USD, Ethereum/USD, and Ripple/USD cryptocurrencies, with the probabilities of volatility clusters of all such cryptocurrencies being much greater than the ones of the three traditional assets. Our results suggest that the volatility in cryptocurrencies changes faster than in traditional assets, and much faster than in forex pairs.

Random walks on dynamical random environments with nonuniform mixing

In this paper, we study random walks on dynamical random environments in $1+1$ dimensions. Assuming that the environment is invariant under space-time shifts and fulfills a mild mixing hypothesis, we establish a law of large numbers and a concentration inequality around the asymptotic speed. The mixing hypothesis imposes a polynomial decay rate of covariances on the environment with sufficiently high exponent but does not impose uniform mixing. Examples of environments for which our methods apply include the contact process and Markovian environments with a positive spectral gap, such as the East model. For the East model, we also obtain that the distinguished zero satisfies a law of large numbers with strictly positive speed.

Creep deformation behavior of nano oxide dispersion strengthened Fe–18Cr ferritic steel

The objective of present work is to investigate the creep behavior of nano oxide dispersion strengthened Fe–18Cr ferritic steel (Fe–18Cr–3W–0.3Ti–0.35Y2O3) over a range of temperatures 923–1023 K and range of stresses 175–400 MPa. The analysis of creep data according to the power law equation suggests high apparent stress exponents (17–26) and high apparent activation energy (~690 kJ/mol). Microstructural analysis indicated that no change in the grain and nanoprecipitate size occurred in the sample crept at 973 K and 225 MPa after 2600 h. Transmission electron microscopy studies on crept samples revealed substantial dislocation activity in the grains and the interaction between dislocations and nanoprecipitates. Based on the analysis of results, dislocation detachment mechanism is identified as a creep mechanism in the present steel. Accordingly, the creep results are rationalized based on the dislocation detachment model and mechanical threshold stress model.

Amphotericin B Loaded Polymeric Nanoparticles for Treatment of Leishmania Infections.

Fungal infections in immune-compromised patients are an important cause of mortality and morbidity. Amphotericin B (Amp B) is considered a powerful fungicidal drug but its clinical usage has certain limitations when administered intravenously due to its toxicity and poor solubility. In consideration of such challenges, in cutaneous leishmaniasis, the topical application of Amp B can be a safer option in many aspects. Thus, herein, biopolymer of polycaprolactone (PCL) nanoparticles (NPs) were developed with the loading of Amp B by nanoprecipitation for the treatment of topical leishmanial infections. Various parameters, such as concentration of PCL and surfactant Poloxamer 407, were varied in order to optimize the formation of nanoparticles for the loading of Amp B. The optimized formulation exhibited a mean hydrodynamic particle size of 183 nm with a spherical morphology and an encapsulation efficiency of 85%. The applications of various kinetic models reveal that drug release from nanoformulation follows Korsmeyer–Peppas kinetics and has a high diffusion exponent at a physiological pH of 7.4 as well a skin relevant pH = 5.5. The activity of the prepared nanoparticles was also demonstrated in Leishmania infected macrophages. The measured IC50 of the prepared nanoparticle formulation was observed to be significantly lower when compared to control free Amp B and AmBisome® for both L. tropica KWH23 and L. donovani amastigotes in order to demonstrate maximum parasite inhibition. The prepared topical nanoformulations are capable of providing novel options for the treatment of leishmaniasis, which can be possible after in vivo assays as well as the establishment of safety profiles.

A Promising Fuel for Variable Thrust Motor: HTPB Propellant with High Pressure Exponent

AbstractIn this paper, we developed a HTPB propellant containing energetic compound hydrated‐tetra‐(‐amino‐1,2,4‐triazole) copper perchlorate (ACP) as the fuel for variable thrust motor with the purpose of controlling gas flow. Formulation of the propellant was designed and then various performances of this propellant were investigated. Results show that HTPB propellant containing 2 % ACP has a pressure exponent of 0.57 (static) or 0.56 (dynamic), suggesting an excellent capacity to control the burning rate. Thermal decomposition of ACP and the decomposition of ammonium perchlorate (AP) in high temperature are demonstrated to be sensitive to pressure by PDSC analysis, moreover, decomposition of AP in high temperature (around 400 °C) could be accelerated by ACP, φ 50 mm motors charged with this propellant work stably in the pressure range from 1.18 MPa to 21.72 MPa and less aluminum (Al) deposits in chambers and nozzles. Mechanical performance of the propellant in −40 °C and 20 °C turns out to be excellent with σm≥0.96 MPa and ϵb≥20.92 %, and is better than that in 50 °C. Low impact sensitivity (H50, 72 cm) and friction sensitivity (P, 31 %) of HTPB propellant containing 2 % ACP are found in the security tests, the propellant is also insensitive to shock wave (G50, 23.8 mm).

Fabrication and investigation of 3D-printed gun propellants

Gun propellant is the energy source of barrel weapons. A 3D printed gun propellant based on “Stereolithography (SLA)” was developed to overcome the limitation of traditional extrusion technologies. A demonstration formulation composed of 50 wt% 1,3,5-Trinitro-1,3,5-triazinane (RDX), 25 wt% epoxy acrylate, 12.5 wt% N-n-butyl-N-(2-nitroxyethyl) nitramine (Bu-NENA), 12.5 wt% reactive diluent and additives were fabricated. The 3D-printed propellants were characterized in terms of inner structure, mechanical strength and combustion behavior. Finally, a ballistic assessment of multi-perforated disk (MPD) propellant was carried out using a 30 mm test gun. The results showed that no signs of porosity or defects existed in the propellant printed by the SLA technique. The compression strength and tensile strength were 21.6 MPa and 7.3 MPa, respectively. The 3D-printed propellant exhibited a relatively low linear burn rate and a high pressure exponent value of 1.46. In the gun test, a muzzle velocity of 420 m/s was achieved without crushing the MPD.

On the Effectiveness of Using the Real Law of Solid Propellant Burning Rate As a Function of Solid Rocket Motor Pressure

This paper presents a comparative analysis of the maximum possible solid rocket motor pressures obtained using the exponential and real laws of the propellant burning rate as a function of pressure. It is shown that at high exponents in the burning rate law, there is a large (tens of percent) difference between the maximum motor pressure obtained by the above two methods. In the case where the exponent is in the range 0.3–0.4, the exponential burning rate law overestimates the motor pressure by only 2–3%.

Chaotic Image Encryption Using Hopfield and Hindmarsh-Rose Neurons Implemented on FPGA.

Chaotic systems implemented by artificial neural networks are good candidates for data encryption. In this manner, this paper introduces the cryptographic application of the Hopfield and the Hindmarsh–Rose neurons. The contribution is focused on finding suitable coefficient values of the neurons to generate robust random binary sequences that can be used in image encryption. This task is performed by evaluating the bifurcation diagrams from which one chooses appropriate coefficient values of the mathematical models that produce high positive Lyapunov exponent and Kaplan–Yorke dimension values, which are computed using TISEAN. The randomness of both the Hopfield and the Hindmarsh–Rose neurons is evaluated from chaotic time series data by performing National Institute of Standard and Technology (NIST) tests. The implementation of both neurons is done using field-programmable gate arrays whose architectures are used to develop an encryption system for RGB images. The success of the encryption system is confirmed by performing correlation, histogram, variance, entropy, and Number of Pixel Change Rate (NPCR) tests.

Long-Time Memory in Drought via Detrended Fluctuation Analysis

The persistence of drought events largely determines the severity of socioeconomic and ecological impacts, unfortunately the performance of current weather forecasting models (WFM) to simulate such events is subject to great uncertainties. This study is investigating time-domain characteristics of drought persistence over Turkey by applying the detrended fluctuation analysis (DFA) method to the Palmer drought severity index (PDSI). The existence of long-range power-law correlation in PDSI fluctuations is demonstrated for time scales ranging from monthly to decadal. Understanding of such statistical patterns in PDSI values can definitely be a step forward in drought predictability. From a climatological point of view, it is found that the areas with high level DFA scaling exponent (generalized Hurst) indicate the areas of higher sensitivity to droughts and associated risks. Furthermore, the characteristics of the persistence of the PDSI in climate zones have also been examined by applying the Holdridge Life Zones (HLZ) classification. HLZ classification over Turkey leads to two climate-zones: cool-temperate and warm-temperate. In addition, when topography is taken in account, montane (cool-temperate) and lower-montane (warm-temperate) climate zones can be treated as two different zones. It has been observed that the predictable index (PI) of the PDSI derived from the DFA Hurst exponent is relatively high in the cool-temperate and montane climate zones compared to others. In fact, very different PI values were also obtained in a few HLZ climate classes within the same climate zone and with same vegetation index (i.e. steppe, dry-forest, warm-forest etc.).

Development of enhanced multiwalled carbon nanotube (MWCNT) conductive polymeric nanocomposites by using acidified derivative of MWCNT as dispersant

The agglomeration of pristine multiwalled carbon nanotubes (MWCNTs) within a polymer matrix has largely limited the efficient conductive reinforcement of MWCNTs-based polymeric nanocomposites. In this paper, the pristine MWCNTs realized well-dispersed with the assistance of nitric acid acidified MWCNTs. The as-prepared hybrid MWCNTs were used to prepare hybrid MWCNTs buckypaper and hybrid MWCNTs/polyvinyl butyral (PVB) nanocomposites. The hybrid MWCNTs dispersion maintained good dispersing stability over 3 months. The fabrication of hybrid MWCNTs buckypaper shows that the intrinsic conductivity of the hybrid MWCNTs is 25.1 ± 0.2 S/cm, higher than the pristine ones of 23.3 ± 0.2 S/cm. The SEM images of hybrid MWCNTs/PVB nanocomposites show that hybrid MWCNTs are distributed homogenously in the PVB matrix. The conductive performance of nanocomposites is significantly enhanced with a low percolation of 0.44 ± 0.05 wt. % and a high critical exponent of 3.57.

Hydrogen Production from the LOHC Perhydro-Dibenzyl-Toluene and Purification Using a 5 µm PdAg-Membrane in a Coupled Microstructured System.

Hydrogen bound in organic liquid hydrogen carriers (LOHC) such as dibenzyl-toluene enables simple and safe handling as well as long-term storage. This idea is particularly interesting in the context of the energy transition, where hydrogen is considered a key energy carrier. The LOHC technology serves as a storage between volatile energy and locally and timely independent consumption. Depending on the type of application, decisive specifications are placed on the hydrogen purity. In the product gas from dehydrogenation, however, concentrations of 100 to a few 1000 ppm can be found from low boiling substances, which partly originate from the production of the LOHC material, but also from the decomposition and evaporation of the LOHC molecules in the course of the enormous volume expansion due to hydrogen release. For the removal of undesired traces in the LOHC material, a pre-treatment and storage under protective gas is necessary. For purification, the use of Pd-based membranes might be useful, which makes these steps less important or even redundant. Heat supply and phase contacting of the liquid LOHC and catalyst is also crucial for the process. Within the contribution, the first results from a coupled microstructured system—consisting of a radial flow reactor unit and membrane separation unit—are shown. In a first step, the 5 µm thick PdAg-membrane was characterized and a high Sieverts exponent of 0.9 was determined, indicating adsorption/desorption driven permeation. It can be demonstrated that hydrogen is first released with high catalyst-related productivity in the reactor system and afterwards separated and purified. Within the framework of limited analytics, we found that by using a Pd-based membrane, a quality of 5.0 (99.999% purity) or higher can be achieved. Furthermore, it was found that after only 8 hours, the membrane can lose up to 30% of its performance when exposed to the slightly contaminated product gas from the dehydrogenation process. However, the separation efficiency can almost completely be restored by the treatment with pure hydrogen.

Analysis of maximum flood records in the arid environment of Saudi Arabia

A collection of rainfall data and flood records for ephemeral channels in the Kingdom of Saudi Arabia (KSA) provided a unique opportunity to analyze the extreme records in such arid environment. Data of 3121 records were collected. A regional flood envelope relationships and other envelope curves in similar arid areas were compared with the current study. We found that the envelope of the maximum-recorded peak discharge versus basin area follows The exponent falls in the range of the exponents in other arid basins. The enveloping curve for the volume versus basin area is The parameters of the equation are significantly different from that of the arid Negev desert, Israel. Derived peak flow–volume relationship has the form We found such relation is similar to arid/semi-arid southwestern United States but with slightly higher exponent and constant. When the peak discharge and maximum volume are plotted against their accompanied rainfall depth, p, linear envelope in log- log plot are developed for KSA region: and The equations predict the upper limit of the expected peak discharges and volumes that can be used for hydro-engineering applications in ungauged basins in the study region and regions a like. The developed envelope curves can also be used as an alternative or guide to performing rainfall-runoff modeling in KSA. The volume-area envelope equations have high degree of credibility since, 92% of the published data used for validation falls within the upper and lower envelope limits. These envelope curves have to be updated when more records are available.

Creep analysis in thick composite cylinder considering large strain

Cylindrical vessels are widely used in petroleum or aerospace industry. In these applications, the cylinders have to work under harsh mechanical and thermal loadings where creep plays an important role. The present paper analyzes creeps behavior of an Al–SiCp composite cylinder under internal pressure. The strains are assumed to be large which necessitate the use of finite strain theory. Threshold creep law is used for analysis because the creep rate in aluminum-based composites using Norton’s law was not approved due to apparently high stress exponent and high apparent activation energy. Such analysis will help the designers in the prediction of correct creep rate and stresses in cases where large creep deformation of a cylinder is permissible. With the use of finite strain theory, it is found that design of the cylinder based on small strain theory leads to unsafe cylinder design.

Deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions

We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1GPa, 400°C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500°C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust.

Stable second phase: The key to high-temperature creep performance of particle reinforced aluminum matrix composite

High-temperature creep behavior of 45vol%-SiCp/2024Al was studied at 250–350 °C under 40–140 MPa. True stress exponent and activation energy were measured 8 and 227.7 kJ/mol, respectively. The feature substructure of SiCp/2024Al was the second phase Al2Cu dispersed at the interface. The evolution of microstructure and the reason for its high creep activation energy were both analyzed and discussed. θ′ phase and S′ phase underwent different evolution processes during creep. θ′ phase gradually transformed to θ phase and remain stable from the steady-state creep until creep rupture. High content of the interface facilitated the dispersion of Al2Cu. The relationship between true stress exponent and interface content as well as the key to high true stress exponent of high volume fraction particle reinforced aluminum matrix composite were also discussed. This research provided new perspectives and guidance for designing and fabricating composites with superior high-temperature creep properties.

Micro‐Seismic Monitoring of a Shear Fault Within a Floating Ice Plate

AbstractThe deformation of a circular fault in a thin floating ice plate imposed by a slow rotational displacement is investigated. Temporal changes in shear strength, as a proxy for the resistance of the fault as a whole, are monitored by the torque required to impose a constant displacement rate. Micro‐seismic monitoring is used to study the relationship between fault average resistance (torque) and micro‐ruptures. The size distribution of ruptures follows a power‐law scaling characterized by an unusually high exponent (b≃3), characteristic of a deformation driven by small ruptures. In strong contrast to the typical brittle dynamics of crustal faults, an 'apparently aseismic' deformation regime is observed in which small undetected seismic ruptures, below the detection level of the monitoring system, control the slip budget. Most (≃71%) of the detected ruptures are organized in bursts with highly similar waveforms, suggesting that these ruptures are only a passive by‐product of apparently aseismic slip events. The seismic signature of this deformation regime has strong similarities with crustal faulting in settings characterized by high temperature and with non‐volcanic tremors.