Initial Transient Period Research Articles

State of the engineered barrier system and host rock of the HE-E experiment after 12 years of heating

This paper presents the status of the HE-E heater experiment, which is currently running at the Mont Terri Underground Research Laboratory (URL) in Switzerland. The experiment is located in a 10 m-long section of a micro-tunnel in the shaly facies of the Opalinus Clay and includes two 4 m-long heater sections isolated by plugs. Heater 1 is located in a sand/bentonite (65/35) buffer section and Heater 2 is located in a granular bentonite buffer section. Heating started on 28 June 2011, and this paper reports results up to 30 June 2023. During the 12 years of heating the system has otherwise been left undisturbed, with ongoing but very slow resaturation from the Opalinus Clay. The observed response to heating is considered in two phases: (1) the initial transient period of about 500 days when heater temperatures rose to the target 140°C and (2) the subsequent evolution. Recently a novel telescoped drilling procedure has been performed to recover and characterize material from the bentonite buffer without interruption of the heating. This paper describes the experimental long-term evolution and status prior to this disturbance, together with expectations regarding the current state of the buffer.

Turbulent natural convection in an air–water system with evaporation across the free surface

In this paper we report on direct numerical simulations of turbulent convection in a cuboidal pool that contains liquid water in the lower part and air in the upper one. The bottom wall is uniformly heated and natural convection is established in both phases, accompanied by water evaporation across the free surface of the water. The descent of the free surface due to evaporation is computed via a newly developed tracking algorithm based on the ghost-fluid method. We present results for three different cases which correspond to different pool heights. In all of them, the natural convection in the water lies in the soft-turbulence regime. Whereas in the gas, it lies in the laminar, transitional and soft-turbulence regimes, respectively. Our analysis focuses on the characteristics of the convective patterns in the two phases and the statistics of the various flow quantities of interest. According to our simulations, the flow in the water is organized in a single-roll Large-Scale Circulation (LSC). In the gas, it is organized in single or dual-roll LSCs, depending on the aspect ratio of the pool. Interestingly, the impingement points of the LSCs of the two phases at the free surface remain very close to one another, which is attributed to the continuity of the shear stresses across the free surface. Further, after the initial transient period, both the free-surface temperature and the evaporative mass flux are stabilized and remain almost constant, but they exhibit small-scale fluctuations in time due to turbulence. Also, the transport of water vapor in air has similar properties as the heat transport, and the ratio between the Sherwood and Nusselt numbers is very close to the Lewis number for air.

Dynamic MAIT Cell Recovery after Severe COVID-19 Is Transient with Signs of Heterogeneous Functional Anomalies.

Mucosal-associated invariant T (MAIT) cells are an abundant population of unconventional T cells in humans and play important roles in immune defense against microbial infections. Severe COVID-19 is associated with strong activation of MAIT cells and loss of these cells from circulation. In the present study, we investigated the capacity of MAIT cells to recover after severe COVID-19. In longitudinal paired analysis, MAIT cells initially rebounded numerically and phenotypically in most patients at 4 mo postrelease from the hospital. However, the rebounding MAIT cells displayed signs of persistent activation with elevated expression of CD69, CD38, and HLA-DR. Although MAIT cell function was restored in many patients, a subgroup displayed a predominantly PD-1high functionally impaired MAIT cell pool. This profile was associated with poor expression of IFN-γ and granzyme B in response to IL-12 + L-18 and low levels of polyfunctionality. Unexpectedly, although the overall T cell counts recovered, normalization of the MAIT cell pool failed at 9-mo follow-up, with a clear decline in MAIT cell numbers and a further increase in PD-1 levels. Together, these results indicate an initial transient period of inconsistent recovery of MAIT cells that is not sustained and eventually fails. Persisting MAIT cell impairment in previously hospitalized patients with COVID-19 may have consequences for antimicrobial immunity and inflammation and could potentially contribute to post-COVID-19 health problems.

GPU-optimized approaches to molecular docking-based virtual screening in drug discovery: A comparative analysis

Finding a novel drug is a very long and complex procedure. Using computer simulations, it is possible to accelerate the preliminary phases by performing a virtual screening that filters a large set of drug candidates to a manageable number. This paper presents the implementations and comparative analysis of two GPU-optimized implementations of a virtual screening algorithm targeting novel GPU architectures. This work focuses on the analysis of parallel computation patterns and their mapping onto the target architecture. The first method adopts a traditional approach that spreads the computation for a single molecule across the entire GPU. The second uses a novel batched approach that exploits the parallel architecture of the GPU to evaluate more molecules in parallel. Experimental results showed a different behavior depending on the size of the database to be screened, either reaching a performance plateau sooner or having a more extended initial transient period to achieve a higher throughput (up to 5x), which is more suitable for extreme-scale virtual screening campaigns.

Dynamics of an Evaporating Drop Migrating in a Poiseuille Flow

Abstract The evaporation of a liquid drop of initial diameter (Ddrop) migrating in a tube of diameter (D0) is investigated using the coupled level set and volume of fluid method focusing on determining the heat and mass transfer coefficients for a deforming drop. A robust phase change model is developed using an embedded boundary method under a finite difference framework to handle vaporizing flows. The model is extensively validated through simulations of benchmark problems such as arbitrary evaporation of a static drop and reproduction of psychrometric data. The results show that the Sherwood number and the Nusselt number reach a steady value after an initial transient period for the drop subjected to Hagen-Poiseuille flow. A parametric study is conducted to investigate the effect of drop deformation on the rate of evaporation. It is observed that Stefan flow due to evaporation has a negligible impact on the drop deformation dynamics. We also observed that, for different values of Ddrop/D0, the Sherwood number follows a linear correlation with Re1/2Sc1/3.

Measurement of the thermal diffusivities of insulating materials using boiling water

We present a method and corresponding experimental setup for estimating thermal diffusivities of insulating materials. The setup consists of a boiling water container placed on a heating plate; the test plate is placed atop the boiling water container. Under this condition, both the steady and transient input-response temperature characteristics are measured. We find that the water in its boiling state causes a nearly stable temperature at one side of the test plate. However, this stable condition is reached only after an initial transient period. Rather than using either steady or transient behavior of the response temperature characteristics exclusively in the analysis, we show the potential of using both by combining the experimental observations with the subsequent theoretical and optimization analyses. The proposed method and experimental setup do not require any prior estimation of other parameters. All the necessary information, including the thermal diffusivity, can be estimated from the input-response temperature characteristics only, as we show here.

Effects of boron addition on the high temperature oxidation of MoSi2 alloys

Boron containing MoSi2 is a promising material for applications at high temperature, but the oxidation mechanism is still unclear. In this work, the high temperature (1100 °C) oxidation of B doped MoSi2 in synthetic air has been investigated. A (boro)silicate layer is formed on the surface of the alloy, which features a mixture of amorphous SiO2 and cristobalite. After an initial transient period, the oxidation kinetics follows a parabolic growth rate law. The growth rate constant of the oxide layer is enhanced by the boron in the alloy by 90 % per at.% B. The increase in growth rate is associated with boron mitigating the formation of cristobalite thereby promoting the formation of amorphous SiO2.

On the analysis of plasticity induced crack closure in welded specimens: A mechanism controlled by the stress intensity factor resulting from residual stresses

The aim of this paper is to clarify how and to what extent varied welding residual stresses (WRSes) affect the plane stress plasticity induced crack closure (PICC) simulation. A well-characterized, representative WRS field with tensile and compressive reversals was imported and the predicted PICC results were evaluated against the relevant WRS-free ones. It turned out that instead of WRS typically reported in the literature, there is actually a Kres (stress intensity factor resulting from residual stresses) controlled crack closure mechanism when investigating the PICC behavior in the presence of a WRS field, i.e., WRS influences PICC in the form of Kres. Four major categories where the crack tip was located in different Kres zones were defined using MT (middle tension) and SENT (single edge notch tension) specimens to investigate the mechanism in detail. In summary, the Kres value determined the general steady-state PICC level in the WRS field, i.e., a diminished steady-state opening stress level was predicted in the WRS field with a positive Kres regardless of whether the crack is located in a tensile or a compressive WRS zone, and an elevated value was estimated when the Kres was negative. Besides, the initial transient period was scrutinized that occurred in some WRS scenarios and the concept of primary and secondary plastic wake evolution was proposed to explain its formation. It was found that the transient behavior could be eradicated by tailoring the secondary plastic wake employing appropriate constitutive models and mesh refinement levels. The results attained in the present work provide some guidelines for the WRS-PICC modeling and simulation in the fracture mechanics community.

Understanding and modelling wear rates and mechanisms in fretting via the concept of rate-determining processes - Contact oxygenation, debris formation and debris ejection

A new framework which describes the role of three key processes in fretting wear of metals is proposed, with these three processes being: (i) oxygen transport into the contact; (ii) formation of oxide-based wear debris in the contact and (iii) ejection of the wear debris from the contact. Based upon a physical understanding, rate equations for the three key processes are proposed, which provide a basis for the influence of test parameters (such as contact geometry, applied load, slip amplitude, fretting frequency etc) on the three rates to be understood. To maintain system equilibrium in steady-state fretting, the three processes must operate at the same rate as each other (debris cannot be ejected from the contact faster than it is formed, and debris cannot be formed faster than it is ejected). Accordingly, the observed wear rate is the rate of the process with the lowest rate of the three processes, with this process being termed the rate-determining process. The effect of test parameters on the three key processes differs, and thus the effect of changes in any test parameter on the observed rate of wear will itself be dependent upon which of the three processes is rate-determining.A number of assumptions have been made in deriving the equations which describe the key processes and it is recognised that these equations themselves may be refined in light of future research; however, any such revised equations can simply replace those proposed as part of the rate-determining process framework.The framework can be applied to both conforming and non-conforming contact geometries. In tests with non-conforming contact geometries, the contact size increases with wear; since it is proposed that the rates of two of the three processes (oxygen transport and debris ejection) are dependent upon the size of the contact, the rate determining process can change during such a test and the rate of wear will continually fall when either of these processes are rate-determining. This complexity means that the evolution of the wear volume can only be evaluated numerically through the use of a time-marching analysis in such cases. In contrast, in tests with conforming contact geometries, the contact size does not change as wear occurs, and so there will be no changes in the rate-determining process as wear proceeds.It is recognised that this framework addresses wear under steady-state conditions and does not consider the initial period of exposure where steady-state conditions are being developed. In the case of conforming contacts where the contact size is large, the duration of the initial transient period may be substantial and may form a significant proportion of the test duration or component lifetime. This needs to be recognised both in the design of tests and in the application of this framework.

Flexible Kinetic Energy Release Controllers for a Wind Farm in an Islanding System

To improve frequency nadir following a disturbance and avoid under-frequency load shedding, two types of flexible kinetic energy release controllers for the doubly fed induction generator (DFIG) are proposed. The basic idea is to release only a small amount of kinetic energy stored at the DFIG in the initial transient period (1–3 s after the disturbance). When the frequency dip exceeds a preset threshold, the amount of kinetic energy released is increased to improve the frequency nadir. To achieve the goal of flexible kinetic energy release, a deactivation function based integral controller is first presented. To further improve the dynamic frequency response under parameter uncertainties and external disturbances, a second flexible kinetic energy release controller is designed using a proportional-integral controller, with the gains being adapted in real-time with the particle swarm optimization algorithm. Based on the MATLAB/SIMULINK simulation results for a local power system, it is concluded that the frequency nadir can be maintained around the under-frequency load shedding threshold of 59.6 Hz using the proposed controllers.

The investigation of evaporation behavior of lubricating oil droplet at high ambient temperature

The formation and vaporization of combustible mixture was crucial for the auto-ignition of lubricating oil droplets. In this study, a quartz glass tube was used as the evaporation chamber, and a micro-syringe was used to extract a specific volume of oil from the cross-fiber. The evaporation process of lubricating oil droplets at atmospheric pressure and different ambient temperatures was investigated by using the cross-fiber technique. The experimental results showed that the evaporation lifetime of lubricating oil droplets could be divided into three phases: initial transient heating period, equilibrium evaporation period, and stable evaporation period. The bubble generation and puffing behavior were observed during the evaporation phase. The equilibrium evaporation rate (Kave) was proportional to the ambient temperature. Compared to Kave, the stable evaporation rate (Ksta) was slightly influenced by the ambient temperature. The absolute rate of equilibrium evaporation (kave) and the absolute rate of stable evaporation (ksta) increased with an increase in initial diameter of the lubricating oil droplets. In order to describe the evaporation characteristics of lubricating oil droplets, a new correlation between evaporation absolute rate, initial diameter, and ambient temperature was obtained.

Hydrodynamics of a compound drop in plane Poiseuille flow

We numerically investigate the hydrodynamics of a two-dimensional compound drop in a plane Poiseuille flow under Stokes regime. A neutrally buoyant, initially concentric compound drop is released into a fully developed flow, where it migrates to its equilibrium position. Based on the results, we find that the core–shell interaction affects the dynamics of both the core and the compound drop. During the initial transient period, the core revolves about the center of the compound drop due to the internal circulation inside the shell. At equilibrium, depending upon the nature of the flow field inside the shell, we identify two distinct core behaviors: stable state and limit-cycle state. In the stable state, the core stops revolving and moves outward very slowly. The core in the limit-cycle state continues to revolve in a nearly fixed orbit with no further inward motion. The presence of the core affects both deformation and migration dynamics of the compound drop. A comparison with the simple drop reveals that the core enhances the deformation of the compound drop. The outward moving core in the stable state pushes the compound drop toward the walls, while the revolving core in the limit-cycle state causes the compound drop to oscillate at its equilibrium position. The migration of the compound drop also affects the eccentricity of the core significantly. From the parametric study, we find that the core affects the compound drop dynamics only at intermediate sizes, and an increase in any parameter sufficiently causes a transition from the limit-cycle state to the stable state.

Finding acceptable parameter regions of stochastic Hill functions for multisite phosphorylation mechanism.

Multisite phosphorylation plays an important role in regulating switch-like protein activity and has been used widely in mathematical models. With the development of new experimental techniques and more molecular data, molecular phosphorylation processes emerge in many systems with increasing complexity and sizes. These developments call for simple yet valid stochastic models to describe various multisite phosphorylation processes, especially in large and complex biochemical networks. To reduce model complexity, this work aims at simplifying the multisite phosphorylation mechanism by a stochastic Hill function model. Furthermore, this work optimizes regions of parameter space to match simulation results from the stochastic Hill function with the distributive multisite phosphorylation process. While traditional parameter optimization methods have been focusing on finding the best parameter vector, in most circumstances, modelers would like to find a set of parameter vectors that generate similar system dynamics and results. This paper proposes a general α-β-γ rule to return an acceptable parameter region of the stochastic Hill function based on a quasi-Newton stochastic optimization algorithm. Different objective functions are investigated characterizing different features of the simulation-based empirical data, among which the approximate maximum log-likelihood method is recommended for general applications. Numerical results demonstrate that with an appropriate parameter vector value, the stochastic Hill function model depicts the multisite phosphorylation process well except the initial (transient) period.

Bottom-up Cu filling of annular through silicon vias: Microstructure and texture

Microstructural and morphological evolution during bottom-up Cu filling of annular through silicon vias (TSV) in a CuSO4–H2SO4–Cl-–poloxamine electrolyte is examined. Deposition proceeds in two distinct stages beginning with a passive-to-active state transition on the via sidewalls whose depth and ultimate thickness depends on the polymer flux. Growth is conformal or tapered with columnar grains whose width and texture differ between the outer and inner sidewalls of the annulus due to area reduction and expansion respectively. The outer sidewall and bottom surface have a preferred texture; ⟨111⟩//ND (via normal) and ⟨110⟩//CD (via circumferential). With time the sidewalls passivate while further deposition is localized to the via bottom. Bottom-up growth then fills the TSV with the formation and selective expansion of the ⟨110⟩//ND textured grains. At higher suppressor concentrations the initial onset and transient period of sidewall deposition is displaced to greater depth and the subsequent shift to bottom-up filling with large ⟨110⟩//ND texture grains occurs earlier. The dominant ⟨110⟩//ND texture during bottom-up filling is congruent with Cl− stabilized texture development. The absence of suppression indicates that the polymeric suppressor does not adsorb on the active upward propagating surface.

Novel Wood Resistance Measurement Method Reducing the Initial Transient Instabilities Arising in DC Methods Due to Polarization Effects

A novel method for measuring the electrical resistance in wood is presented. It is based on applying an Alternating Current (AC) to two electrodes rammed into the wood. The method reduces the transient time for value stabilization. In case of Direct Current (DC) resistance measurement methods, typically used in wood measurement, an initial transient exists, invalidating the measured value during an initial transient period. This measurement method uses an electronic circuit based on a relaxation oscillator where the wood automatically sets the oscillation frequency depending on its electrical resistance. Compared to other AC methods, this circuit greatly simplifies the measurement process, not requiring any previous analysis for wood AC frequency estimation. Experimental results for four different wood species showed that, in all cases, the transient response of the measured wood resistance is improved when compared to the DC resistance method, reducing the stabilization time from 10–12 min in DC measurements to almost zero for the novel measurement method. The deviation between the initial wood resistance measurement and the stable value resulted in a range between 0.2% and 5% with the proposed method, while a range between 6% and 10% was obtained when using the DC method. Moreover, the proposed circuit is able to detect changes in wood resistance produced by small variations in temperature and environmental relative humidity during continuous long term measurements. For repeatability analysis, it was verified that similar ambient conditions of temperature and relative humidity (variations lower than 1 °C and 1%, respectively) in different moments provided variations lower than 1.5%. The method can also be applied to other fiber materials affectd by polarization effects when an electrical current is applied into them.

Islanding detection scheme for converter‐based DGs with nearly zero non‐detectable zone

This study presents a new method for detecting an islanding event in a microgrid with embedded converter-based distributed generation (DG). Unlike other schemes, the proposed scheme injects negligible perturbations into the microgrid after the generation of an alert signal. The proposed scheme then uses another index called superimposed impedance, ΔZ. The ΔZ is characterised by a low steady-state magnitude during the grid-connected mode and a high magnitude during the islanded mode. Furthermore, for a fault at the point of common coupling (PCC), the magnitudes of ΔZ and PCC voltage are both very low – except during the initial transient period, where |ΔZ| momentarily crosses the threshold. Therefore, an islanding event can be detected if the magnitude of ΔZ is high for some specified time. Moreover, a fault event will not be misdirected as an islanding event because the steady-state magnitudes of both the ΔZ and PCC voltage are very low in the case of a fault. The robustness of the proposed detection scheme is evaluated against different islanding conditions and also, for a fault at the PCC, first by using MATLAB-based simulations and later by using a laboratory-based experimental setup. A comparison with a recently published detection scheme shows the superiority of the proposed schemes.

Evolution of similarity lengths in anisotropic magnetohydrodynamic turbulence

In an earlier paper (Wan et al., J. Fluid Mech., vol. 697, 2012, pp. 296–315), the authors showed that a similarity solution for anisotropic incompressible three-dimensional magnetohydrodynamic (MHD) turbulence, in the presence of a uniform mean magnetic field $\boldsymbol{B}_{0}$, exists if the ratio of parallel to perpendicular (with respect to $\boldsymbol{B}_{0}$) similarity length scales remains constant in time. This conjecture appears to be a rather stringent constraint on the dynamics of decay of the energy-containing eddies in MHD turbulence. However, we show here, using direct numerical simulations, that this hypothesis is indeed satisfied in incompressible MHD turbulence. After an initial transient period, the ratio of parallel to perpendicular length scales fluctuates around a steady value during the decay of the eddies. We show further that a Taylor–Kármán-like similarity decay holds for MHD turbulence in the presence of a mean magnetic field. The effect of different parameters, including Reynolds number, mean field strength, and cross-helicity, on the nature of similarity decay is discussed.

Selection of quasi-stationary states in the Navier–Stokes equation on the torus

The two dimensional incompressible Navier–Stokes equation on with , periodic boundary conditions, and viscosity is considered. Bars and dipoles, two explicitly given quasi-stationary states of the system, evolve on the time scale and have been shown to play a key role in its long-time evolution. Of particular interest is the role that δ plays in selecting which of these two states is observed. Recent numerical studies suggest that, after a transient period of rapid decay of the high Fourier modes, the bar state will be selected if , while the dipole will be selected if . Our results support this claim and seek to mathematically formalize it. We consider the system in Fourier space, project it onto a center manifold consisting of the lowest eight Fourier modes, and use this as a model to study the selection of bars and dipoles. It is shown for this ODE model that the value of δ controls the behavior of the asymptotic ratio of the low modes, thus determining the likelihood of observing a bar state or dipole after an initial transient period. Moreover, in our model, for all , there is an initial time period in which the high modes decay at the rapid rate , while the low modes evolve at the slower rate. The results for the ODE model are proven using energy estimates and invariant manifolds and further supported by formal asymptotic expansions and numerics.

Fatigue life prediction via the time‐dependent evolution of linear and nonlinear mechanical parameters determined via Fourier transform of the stress

ABSTRACTThis study proposes a new concept for the lifetime prediction of solid polymers under mechanical fatigue. The time evolution of linear and nonlinear mechanical parameters was studied using polystyrene (PS), polymethylmethacrylate (PMMA), and styrene‐acrylonitrile (SAN) samples in strain controlled (large amplitude) oscillatory shear (torsion). Higher harmonics of the stress were detected and quantified via Fourier transform (FT). Besides storage and loss modulus (G′, G″), the ratio of the third (I3) harmonic over the fundamental one, I1 (I3/1), was analyzed. After an initial transient period (regime I), the three parameters (G′, G″, and I3/1) linearly de/increase with the cycle number (N) (regime II) before failure onset (regime III) occurs. In regime II, the absolute values of the rates of change (derivative) of G′, G″, and I3/1 (dG′/dN, dG″/dN, and dI3/1/dN) were correlated with the lifetime (Nf). An inverse proportionality between the rate of change and lifetime was found allowing better prediction while testing. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46634.

Steady-state and transient transport studies of gas permeation through dense membranes using on-line mass spectrometry

Abstract Polydimethylsiloxane PDMS, polyethylene PE (the most used polymer in food packaging) and pectin (biopolymer potentially used as wound dressing material and in food packaging) were characterised in terms of their gas transport properties. This characterisation was performed by on-line mass-spectrometry, MS, with the upstream and downstream compartments of the membrane unit at atmospheric pressure, in order to mimic the operating conditions of the applications addressed. A simple, direct restriction was used for allowing the downstream gas mixture to reach the mass spectrometer detector. Monitoring of gas permeation by on-line mass spectrometry proved to be a highly precise and reproducible technique, which makes possible the study of multicomponent gas mixtures in dry and humidified gas conditions, without requiring sampling and additional off-line procedures and analysis. Data acquisition, with time intervals as short as one second, makes possible the comparative study of permeation processes of each gas present in different feed streams (pure gases, gas mixtures under dry and humidified conditions) during the initial transient period, allowing for inferring about solute-membrane interactions. Information about steady-state transport may also be acquired, and are in agreement with values reported in literature.