Transient Evolution Research Articles

Numerical studies on applications of cavitation models in water hammer-induced cavitating flows in pipelines

Hydraulic transients caused by a fast-closing needle valve and accompanying water hammer-induced cavitation-vortex flow are numerically investigated using the shear stress transport (SST) k–ω model with a pressure–density equation considering the weak compressibility of water and the new cavitation models based on the Rayleigh–Plesset equation that takes into account the surface tension and kinematic viscosity effect on the bubble radius change rate, respectively. The predicted pressure evolutions exhibit reasonable agreement with the experimental data, and the interfacial surface tension-based model is the most accurate in predicting transient pressure evolutions, followed by the kinematic viscosity-based model and the Singhal cavitation model, based on three proposed error indices with minimum of 0.0515, 0.1337, and 0.0035. The wave speed in the upper part of the cavitation trailing edge is higher than that in the lower part, resulting in the evolution of inclination angle of the trailing edge. According to the convective transport mechanism of vorticity, the change rate of vorticity in non-cavitation regions is mainly dominated by the stretching–twisting term and the turbulent fluctuation–diffusion term. In cavitation regions, the dilatation–shrink term makes significant contributions to the change rate of vorticity, and the baroclinic-torque term mainly acts near the edge of the cavitation region.

Temperature dependence of resistivity of carbon micro/nanostructures: Microscale spatial distribution with mixed metallic and semiconductive behaviors

The temperature coefficient of resistivity (θT) of carbon-based materials is a critical property that directly determines their electrical response upon thermal impulses. It could have metal- (positive) or semiconductor-like (negative) behavior, depending on the combined temperature dependence of electron density and electron scattering. Its distribution in space is very difficult to measure and is rarely studied. Here, for the first time, we report that carbon-based micro/nanoscale structures have a strong non-uniform spatial distribution of θT. This distribution is probed by measuring the transient electro-thermal response of the material under extremely localized step laser heating and scanning, which magnifies the local θT effect in the measured transient voltage evolution. For carbon microfibers (CMFs), after electrical current annealing, θT varies from negative to positive from the sample end to the center with a magnitude change of >130% over <1 mm. This θT sign change is confirmed by directly testing smaller segments from different regions of an annealed CMF. For micro-thick carbon nanotube bundles, θT is found to have a relative change of >125% within a length of ∼2 mm, uncovering strong metallic to semiconductive behavior change in space. Our θT scanning technique can be readily extended to nm-thick samples with μm scanning resolution to explore the distribution of θT and provide a deep insight into the local electron conduction.

Operando neutron diffraction reveals mechanisms for controlled strain evolution in 3D printing

Residual stresses affect the performance and reliability of most manufactured goods and are prevalent in casting, welding, and additive manufacturing (AM, 3D printing). Residual stresses are associated with plastic strain gradients accrued due to transient thermal stress. Complex thermal conditions in AM produce similarly complex residual stress patterns. However, measuring real-time effects of processing on stress evolution is not possible with conventional techniques. Here we use operando neutron diffraction to characterize transient phase transformations and lattice strain evolution during AM of a low-temperature transformation steel. Combining diffraction, infrared and simulation data reveals that elastic and plastic strain distributions are controlled by motion of the face-centered cubic and body-centered cubic phase boundary. Our results provide a new pathway to design residual stress states and property distributions within additively manufactured components. These findings will enable control of residual stress distributions for advantages such as improved fatigue life or resistance to stress-corrosion cracking.

Effect of Ti3O5 addition on oxide evolution for HRB400

Abstract The transient evolution of oxide was studied after directly adding Ti3O5 powder into HRB400 steel. This experiment was carried out in magnesia crucible with vacuum induction furnace and the intermediate samples were taken at 1, 5, 10, and 15 min after Ti3O5 addition. At 20th min, the furnace was powered off to get furnace-cooled cast sample. For intermediate samples, it is found that with increasing treatment time, the titanium content increased though the acid–soluble aluminum content remained stable. Besides, the Ti-bearing oxides were observed by scanning electron microscope (SEM). Moreover, statistical analysis indicated that for Ti-bearing oxide, both number density and titanium content after further normalization increased with increasing processing time. For cast sample, the characteristic of Ti-bearing oxide at different heights are similar. These results confirmed the adding valid of Ti3O5, which may be due to its decomposition. Finally, after heat treatment, these introduced Ti-bearing oxides can induce the intragranular ferrite nucleation. This indicates the effectiveness of external adding method in oxide metallurgy.

Fast and Slow Responses of the Tropical Pacific to Radiative Forcing in Northern High Latitudes

Abstract This study investigates the transient evolution of tropical Pacific sea surface temperature (SST) responses to a constant northern high-latitude solar heating in fully coupled CESM 1.2. The study identifies two stages through multiple ensemble runs. 1) In the first 3 years, a hemispherically asymmetric pattern emerges, caused by air–sea interactions associated with the anomalous cross-equatorial Hadley cell. The northern tropics experience warming that is blocked north of the equator by the intertropical convergence zone. The southeast Pacific cooling reaches the equatorial region and is amplified by the equatorial Ekman divergence. 2) Within a decade, the equatorial cooling is replaced by warming in the eastern equatorial basin. The anomalous warming that appears faster than the time scales of the oceanic ventilation is attributed to anomalous meridional heat convergence and weakening of the northern subtropical cell. Our findings highlight the influence of ocean dynamics on the temporal and spatial evolution of tropical SST response to hemispherically asymmetric heating. The initial cooling caused by Ekman divergence delays the arrival of slow warming, while initial wind and temperature anomalies set the stage for the weakening of the subtropical cell. The results have important implications for understanding the evolution of tropical SST patterns in observational records and future climate change simulations, as they show strong interhemispheric temperature asymmetry in the extratropics.

Transient Simulation of Diffusion-Limited Electrodeposition Using Volume of Fluid (VOF) Method

A numerical model utilizing the volume of fluid (VOF) method is proposed to simulate the transient shape changes of the deposit front, considering the diffusion-limited electrodeposition process. Modeling equations are proposed to accurately handle transport phenomena in both electrolyte (fluid) and deposit (solid). Transient evolutions of field structures, including flow, concentration, electric current density, and electric potential, are computed considering electrodeposited copper bumps. Two cases, including single cavity and multiple cavities, are studied. Based on the modeling results, the maximum height of the hump and the thickness of the deposited layer in each consecutive cavity decreases going from upstream to downstream. Conversely, the location of the maximum height of the hump remains unchanged in all cavities. Results are validated against available experiments.

Analysis of the response of damped and parametrically driven, strongly anharmonic Klein-Gordon chain - Part 2: Beat-waves

This paper continues a series of publications devoted to the analysis of strongly nonlinear wave phenomena, emerging in genuinely anharmonic, Klein-Gordon cyclic chains, subjected to a traveling-wave parametric excitation. In this study, we analyze a special type of nonlinear waves, which will be further referred to as beat-waves. These are spatially extended, and strongly modulated waves manifested by the extreme amplitude modulation, which can be of stationary or nonstationary type. Stationary beat-waves of Klein-Gordon chains propagate with constant group velocity, that depends on the beat-wave amplitude. In the vicinity of 2:1 parametric resonance, we derive the parametrically driven, discrete p-Schrödinger (PD-DpS) model, which depicts the slow modulation of the response of a parametric oscillator chain. Special coordinates' transformation applied on the PD-DpS model allows the exact reduction of extremely modulated beat-waves to an invariant manifold of this slow-flow system. The system dynamics reduced to the beat-wave invariant manifold is depicted by three collective coordinates. In the undamped case, the analysis of the slow-flow system dynamics on the invariant manifold reveals a special family of nonlinear beat-waves characterized by an extreme amplitude modulation. Among these beat-wave solutions lying on the manifold, our analysis identifies some special solutions manifested by the solitary-pulse like, temporal evolution of the beat-wave amplitude which can either decay to zero or to a constant nonzero value. We derive the exact analytical solutions for these special types of waves as well as obtain an explicit analytical approximation for the beat-waves of other types. In the damped case, analysis of the slow-flow system dynamics on the invariant manifold fully recovers the mechanism of system synchronization on the special, stationary beat-wave solutions characterized by the harmonically modulated envelope. Importantly, the derived reduced order model accurately captures the transient evolution stage as well as the steady-state stage of the collective beat-wave response of PD-DpS model, initialized on the beat-wave invariant manifold. A detailed bifurcation analysis of the stationary undamped as well as the stationary damped beat-waves recovers their entire bifurcation structure and enables to fully characterize the zones of system parameters guaranteeing the existence of this special type of non-linear wave solutions. The results of analytical study of various types of beat wave solutions are in excellent agreement with the results of numerical simulation of both PD-DpS as well as the original, parametrically driven, genuinely an-harmonic, discrete Klein-Gordon model.

A Summary: Dynamic and thermodynamic analysis of thermoacoustic and Stirling systems based on time-domain acoustic-electrical analogy

A unified fast transient simulation method for thermoacoustic and Stirling systems has been developed, named time-domain acoustic-electrical analogy (TDAEA) method. From a thermoacoustic theory point of view, the method captures the main acoustic impedance characteristics of the components and simplifies the calculations reasonably by using lumped models for components, thus remarkably reducing the computation. The TDAEA models of common components in thermoacoustic and Stirling systems are summarised with detail derivation. Two case studies are then carried out on free-piston Stirling generator (FPSG) and heat-driven thermoacoustic refrigerator (HDTR). Model verifications demonstrate the effectiveness and accuracy of the method: for the FPSG, the average deviation between the calculations and experiments in operating frequency, electric power, LA efficiency and thermal-to-electric efficiency are 5.0%, 16.3%, 2.4% and 26.9%, respectively; as for the HDTR, the deviation in cooling power lies in 29.9%. Using this method, transient evolutions can be obtained, which contains both start-up and steady stages. For start-up analysis, the transient operating frequency, pressure–volume diagram and limit cycle, etc, can be exhibited. Regarding the steady stage, the method is available to give the distribution of acoustic field and the losses in components. Moreover, the infulences of operating and geometric parameters on system performance can aslo be obtained. This study provides a new perspective and an effective method for fast transient simulation of thermoacoustic and Stirling systems, as well as gives a deeper understanding on the transient evolution of self-sustained oscillation and dynamic & thermodynamic cycles in these systems.

Enhancing the Thermal Dissipation in Batteries via Inclusion of Central Heat Sink

Abstract The generation of heat within the rechargeable batteries during the charge–discharge cycles is inevitable, making heat dissipation a very critical part of their design and operation procedure, as a safety and sustainability measure. In particular, when the heat gets the least possibility to escape from the electrode surface, the boundary of the packaging material remains the sole heat dissipator. In this regard, the heat gets accumulated in the central zone, making it the most critical, since it has the least possibility to escape to the surroundings. Anticipating such a heat trap, a central heat sink component is devised, where the role of its conductivity and the relative scale is analyzed based on the formation of transient and steady-state temperature profiles. Additionally, an analytical solution is attained for the location of the maximum temperature, where its value and correlation with the electrolyte conductivity, heat generation rate, and scale of the cell have been quantified. Due to the existence of the curved boundaries, it is shown that the time versus space resolution for capturing the transient evolution of the temperature is more strict than the flat surface and analytically acquired as ≈33% smaller value. Such enhanced design and subsequent analysis are critical for planning sustainable and cost-effective packaging to avoid the ignition and failure of the respective electrolyte.

Conceptualizing Aeolian Sediment Transport in a Cellular Automata Model to Simulate the Bio-Geomorphological Evolution of Beach–Dune Systems

Understanding the dynamics of beach–dune systems is crucial for effective coastal management. The cellular automata model DuBeVeg provides a powerful tool for simulating and understanding the bio-geomorphological evolution of these systems, capturing key interactions of aeolian, hydro-, and vegetation dynamics in a simplified manner. In this study, we present an alternative representation of the aeolian transport component in DuBeVeg, aiming to better capture the saltation transport mode that prevails on beaches. This new representation is compared with the original aeolian transport representation in DuBeVeg, which is inspired by ripple migration. For three beach width scenarios, we considered the effects of the different aeolian transport representations on the predicted foredune morphology after 50 years, as well as the spatio-temporal evolution of the beach–dune system leading to that morphologic state. The saltation transport representation resulted in a more realistic simulation of the seaward expansion of the foredune compared with the original representation, particularly in scenarios with wide and prograding beaches. The new representation also more accurately captured the amplitude of aeolian bedforms emerging across the beach. These findings highlight the importance of selecting the representative transport mode when simulating the transient bio-geomorphological evolution of beach–dune systems.

The Spatiotemporal Dynamics of Electron Plasma in Femtosecond Laser Double Pulses Induced Damage in Fused Silica

In this study, we employed the fs time-resolved shadowgraphy method to investigate the impact of the first pump pulse (DP1) on the transient temporal and spatial evolution of electron plasma induced by femtosecond (fs) laser double pulses (DPs) in fused silica. It was observed that the DP1-induced phase transition acted as a waveguide, confining the propagation of the second pump pulse (DP2) light inside the material and resulting in a decrease in the diameter of the DP2-induced electron plasma region. Moreover, the DP2-induced maximum peak electron density was higher than that induced by a single pulse (SP) at the same pulse energy, which may be explained by the DP1-induced highly absorbing semi-metallic state of warm dense glass in fused silica. Importantly, as the energy of DP1 increased, the mean diameter of the DP2-induced electron plasma region further decreased, and the maximum peak electron density increased. Compared with SPs, DPs more easily produced damage in fused silica. In addition, the mean diameter of the DP2-induced electron plasma region and the maximum peak electron density remained almost unchanged when the pulses’ time separation (ts) was changed from 1 to 50 ps, mainly due to the long relaxation time of the phase transition caused by DP1.

Transient evolution of non-metallic inclusions in molten high aluminum and high manganese steel contacting with slag and crucible: experimental investigation and FactSage macros modeling

In this work, high temperature experiments of high Al molten steel contacting with crucible and slag were performed to investigate the effect of crucible–slag–steel reaction on transient evolution of inclusions in the high Al molten steel. Processing samples were taken for the chemical compositions of steel and also the inclusions have been characterized by using SEM and EDS AZtec. FactSage macros have been applied to simulate the evolution of molten steel composition and inclusions in molten steel during steel-slag reaction and comparing to the experimental results. Due to the steel–crucible–slag reactions especially under the condition of the high Al molten steel, the Ca and Mg transferred from slag and crucible to the molten steel and then subsequently led to the evolution of Mg and Ca-containing inclusions. The Mg inclusions transferred from MgAl2O4 to MgO inclusions while Ca transferred to steel and modified MgAl2O4 at crucible interface to form CaMgAl2O5 phase. The FactSage macros modeling did agree reasonably well with the experimental results.

High-fidelity simulations of the aeroacoustic environment of the VEGA launch vehicle at lift-off

The lift-off of space launch vehicles generates strong acoustic waves that interact in a complex and potentially dangerous way with the launch facility and the launcher itself. Engineering tools developed in the past to predict the strong acoustic radiation and the peak acoustic loads during the first seconds of the launch have a limited validity and are not able to provide reliable predictions. For this reason, in order to better identify the noise generation sources and to assess the effects of acoustic mitigation measures, it is fundamental to develop and validate more advanced computational models able to capture the transient flow induced by the ignition of the motors. In this work, we present high-fidelity 3D Large Eddy Simulations of the acoustic field produced by the lift-off of a realistic space launcher. A state-of-the-art, high-order, GPU accelerated, compressible solver is used to simulate the highly unsteady interaction of the exhaust plume from the launcher’s nozzle with a realistic launch pad, whose geometry has been modelled by Immersed Boundary Method. The results obtained demonstrate the capability of our solver to provide accurate predictions compared to flight measurements of real configurations, despite the challenging scenario in terms of operating conditions and geometry. Moreover, wavelet analysis proves to be an appropriate tool to pinpoint and characterise the overpressure mechanisms that take place in the transient evolution of the flow.

The Transient Evolution of Flow Due to the Excitation Pulse in Oscillating Drop Experiments in Microgravity Electromagnetic Levitation

In oscillating drop experiments, surface oscillations in a molten sample are captured and analyzed to determine the surface tension and viscosity of a melt without the need to contact the liquid sample. In electromagnetic levitation, surface oscillations are initiated using an excitation pulse in the electromagnetic field. The variation in the electromagnetic force field drives rapid acceleration in the melt while also changing the flow pattern. During the quasi-static flow conditions prior to the excitation pulse, the flow displays a “positioner-dominated” flow pattern with 4 recirculation loops in the sample hemisphere. However, the accelerating flow of the excitation pulse transitions into a “heater-dominated flow” pattern in which there are only 2 recirculation loops in the sample hemisphere. Following the excitation pulse, the flow rapidly slows and quickly returns to the conditions present before the excitation pulse. For many combinations of parameters, the transition in the flow pattern results in a very complicated variation in velocity with time; that variation is the topic of this paper.

Development of a Multi-oxide Kinetic Model to Study Inclusion Transient Evolution in Molten Steel: Application to Si–Mn Killed Steel

Development of a Multi-oxide Kinetic Model to Study Inclusion Transient Evolution in Molten Steel: Application to Si–Mn Killed Steel

Limited Impact of Thwaites Ice Shelf on Future Ice Loss From Antarctica

AbstractThwaites Ice Shelf (TWIS), the floating extension of Thwaites Glacier, West Antarctica, is changing rapidly and may completely disintegrate in the near future. Any buttressing that the ice shelf provides to the upstream grounded Thwaites glacier will then be lost. Previously, it has been argued that this could lead to onset of dynamical instability and the rapid demise of the entire glacier. Here we provide the first systematic quantitative assessment of how strongly the upstream ice is buttressed by TWIS and how its collapse affects future projections. By modeling the stresses acting along the current grounding line, we show that they deviate insignificantly from the stresses after ice shelf collapse. Using three ice‐flow models, we furthermore model the transient evolution of Thwaites Glacier and find that a complete disintegration of the ice shelf will not substantially impact future mass loss over the next 50 years.

Transient wheel–rail rolling contact theories

This paper provides an overview of different theories to analyse unsteady rolling contact phenomena between wheel and rail: the exact formulation by Kalker, the simplified model based on the Winkler approximation, and the recent two-regime model. The classic solution to the transient problem derived by Kalker using the complete theory of elasticity is first recalled. The more involved situation of combined creepages and spin is analysed using Kalker’s simplified model. Analytical solutions are reported in integral form concerning the time-varying and constant creepages. Qualitative results are additionally provided for the case of a time-varying contact patch. Finally, a novel theory, which describes the transient evolution of the force-creepage characteristics using a system of ordinary differential equations (ODEs), is introduced.

Single-Molecule Ultrafast Fluorescence-Detected Pump-Probe Microscopy.

We introduce fluorescence-detected pump-probe microscopy by combining a wavelength-tunable ultrafast laser with a confocal scanning fluorescence microscope, enabling access to the femtosecond time scale on the micrometer spatial scale. In addition, we obtain spectral information from Fourier transformation over excitation pulse-pair time delays. We demonstrate this new approach on a model system of a terrylene bisimide (TBI) dye embedded in a PMMA matrix and acquire the linear excitation spectrum as well as time-dependent pump-probe spectra simultaneously. We then push the technique toward single TBI molecules and analyze the statistical distribution of their excitation spectra. Furthermore, we demonstrate the ultrafast transient evolution of several individual molecules, highlighting their different behavior in contrast to the ensemble due to their individual local environment. By correlating the linear and nonlinear spectra, we assess the effect of the molecular environment on the excited-state energy.

Thermoporoelastic response of a semi‐permeable wellbore subjected to convective cooling and non‐hydrostatic in situ stresses

AbstractExisting models of wellbore stability idealize the borehole wall as either a perfectly permeable or an entirely impermeable surface. The widespread observation that a shale borehole allows solvent molecules to pass through but impedes solutes suggests that the borehole wall should be regarded as a non‐ideal semi‐permeable medium. To address the magnitude and rate of fluid penetration into the formation when a semi‐permeable wellbore is exposed to a non‐isothermal drilling fluid quantitatively, this work develops analytical solutions for a semi‐permeable borehole undergoing convective cooling and far‐field non‐hydrostatic in situ stresses in the framework of fully coupled thermoporoelasticity. Integral transform and load decomposition techniques are employed to facilitate the derivation of analytical solutions. The results show that, in contrast to the permeable or impermeable borehole models, the semi‐permeable model predicts significantly different stress and pore pressure fields. The transient evolution of temperature, pore pressure, and stresses is predominantly governed by two non‐dimensional numbers: the Biot number Bi and a newly identified number πf that characterizes the capability of shale to transport fluid across the solid‐fluid interface.

Melting performance enhancement in a thermal energy storage unit using active vortex generation by electric field

Latent heat thermal energy storage (LHTES) devices aid in efficient utilization of alternate energy systems and improve their ability to handle supply–demand fluctuations. A numerical analysis of melting performance in a shell-and-tube LHTES unit in the presence of a direct current (DC) electric field has been performed. The governing equations of fluid flow, heat transfer, electric potential and charge conservation are solved using a customized finite-volume solver built in the open-source framework of OpenFOAM. Enthalpy-porosity method based fixed grid approach is used to track the melt interface. Primary objective of the study is to highlight the interface and flow morphology evolution in the presence of electric field induced flow and to evaluate the melting performance of the LHTES unit. The transient evolution of the melting process in the presence of electric field has been mapped in terms of total liquid fraction, kinetic energy density and mean Nusselt number. The charge injection from the tube surface generates multiple electrohydrodynamic (EHD) flow vortices in the liquid region. Thus, the inherent uni-cellular flow structure of the natural convection driven melting is disrupted. The multi-cellular flow structure with stronger velocity distribution enhances mixing and heat transfer. Melting performance at various levels of applied voltages (0≤V≤10kV) in both vertical and horizontal orientations of the LHTES unit has been quantified in terms of charging time and total power storage. The charging time gets shorter and total power storage gets higher with increasing applied voltages. In the vertical orientation, a maximum 82.52% reduction in charging time and 80.85% increase in net power storage is achieved. In the horizontal orientation, weaker buoyancy force leads to stronger influence of the electric field. A maximum of 89.61% reduction in charging time and 88.35% increase in power storage is achieved in the horizontal orientation. The results of this study aid in understanding the mechanism of EHD flow assisted melting and provide a reference for design of a shell-and-tube LHTES unit with improved performance.