Two-step Simulation Research Articles

A Numerical Study of Topography and Roughness of Sloped Surfaces Using Process Simulation Data for Laser Powder Bed Fusion.

The simulation of additive manufacturing has become a prominent research area in the past decade. Process physics simulations are employed to replicate laser powder bed fusion (L-PBF) manufacturing processes, aiming to predict potential issues through simulated data. This study focuses on calculating surface roughness by utilizing 3D surface topology extracted from simulated data, as surface roughness significantly influences part quality. Accurately predicting surface roughness using a simulation remains a persistent challenge. To address this challenge, the L-PBF technique with two different cases (pre- and post-contouring) was simulated using two-step process physics simulations. The discrete element method was utilized to simulate powder spreading, followed by the Flow-3D melting simulation. Ten layers were simulated at three different linear energy density (LED) combinations for both cases, with samples positioned at a 30-degree angle to accommodate upskin and downskin effects. Furthermore, a three-dimensional representation of the melted region for each layer was generated using the thermal gradient output from the simulated data. All generated 3D layers were stacked and merged to consolidate a 3D representation of the overall sample. The surfaces (upskin, downskin, and side skins) were extracted from this merged sample. Subsequently, these surfaces were analyzed, and surface roughness (Sa values) was calculated using MATLAB. The obtained values were then compared with experimental results. The downskin surface roughness results from the simulation were found to be within the range of the experimental results. This alignment is attributed to the fact that the physics simulation primarily focuses on melt pool depth and width. These promising findings indicate the potential for accurately predicting surface roughness through simulation.

Advanced Monte Carlo simulations and benchmark of residual dose rate assessments in the ATLAS detector at CERN LHC

Background The European Organization for Nuclear Research (CERN) pushes the frontiers of physics through the Large Hadron Collider (LHC), a 27-kilometre hadron accelerator capable of producing proton-proton collisions at a center-of-mass energy of up to 13.6 TeV. Its four main detectors (ALICE, ATLAS, CMS, LHCb) are unique examples of advanced particle detection technology and complexity. Ensuring radiological safety throughout the LHC’s environments and lifecycle is a critical task managed by the CERN Radiation Protection group. In this context, assessing residual dose rates is crucial for planning exposure situations, such as maintenance and upgrade projects during LHC shutdowns. Methods This work presents advanced Monte Carlo techniques developed at CERN to predict residual dose rates in the LHC experimental caverns, focusing on the ATLAS detector. Predictions are made using the FLUKA Monte Carlo code and the SESAME toolkit for two-step simulations. The simulation results are benchmarked against experimental residual dose rate measurements collected during LHC Long Shutdown 2 at ATLAS. Additionally, forecasts for Long Shutdown 3 are also presented. Results The FLUKA benchmark for the ATLAS detector demonstrates a general agreement within a factor of 1.5. This consistency validates both the FLUKA geometry model of the ATLAS detector and the reliability of the Monte Carlo techniques used to predict residual dose rates in such complex environments. Conclusions The study highlights the predictive capabilities of the FLUKA and SESAME coupling for simulating residual dose rates in large LHC experiments. This method plays a crucial role in ensuring radiological safety, primarily for dose prediction and optimization. The reliability of this approach is significantly supported by the benchmark results obtained at ATLAS and presented in this paper.

CFD—ANN study on axial dispersion in helical and serpentine millimetric coils

CFD simulations are reported to compare hydrodynamic/axial dispersion in a classical helical-shaped and planar serpentine-shaped tubular reactor at a millimetric scale (1-5 mm) for the first time. A two-step simulation approach is validated against experimental data on residence time distribution in serpentine coils. The method is then used for parametric analysis to explore the impact of geometric parameters on the coiling factor and axial dispersion coefficient. Dean vortices in the coiling region are visualized. The findings from the parametric analysis are utilized to develop empirical correlations and Artificial Neural Network (ANN) models for estimating coiling factors and axial dispersion coefficients in both serpentine and helical coils. The absolute average relative error (AERR) in predicting the coiling factor is 3.5% for helical coils and 4.6% for serpentine coils. The AERR of the ANN model for predicting the axial dispersion coefficient is 14% for helical coils and 10% for serpentine coils. The study reveals that both axial dispersion coefficient and pressure drop are higher in serpentine coils compared to helical coils. Axial dispersion is found to be relatively unaffected by pitch (helical) or bend diameter (serpentine) and tube diameter but shows significant reduction with a decrease in pitch circle diameter (helical) or gap between bends (serpentine). The developed correlations and ANN models can aid in the precise sizing of tubular reactors designed as helical or serpentine coils. This is exemplified through a case study involving the synthesis of an ionic liquid, where the helical configuration demonstrated a performance advantage of approximately 6% over the serpentine configuration.

Optimal oversampling ratio in two-step simulation

Abstract This paper analyses a novel two-step Monte Carlo simulation algorithm to estimate the weighted volume of a polytope of the form A ⁢ z ≤ T {Az\leq T} . The essential idea is to partition the columns of A into two categories – a lightweight category and a heavyweight category. Simulation is done in a two-step manner where, for every sample of the lightweight category variables we use multiple samples of the heavyweight category variables. Thus, the heavyweight category variables are oversampled with respect to the lightweight category variables and increasing samples of the heavyweight variables at the expense of the lightweight variables will lead to a more efficient Monte Carlo method. In this paper we present a fast heuristic approximate for estimating the optimal oversampling ratio and substantiate with experimental results which confirm the effectiveness of the method.

Simulation of Two-Step Block Approach for Solving Oscillatory Differential Equations

This study demonstrates the derivation of a two-step block scheme simulation through a linear block approach. The scheme's fundamental properties were thoroughly analyzed and found to fulfill all necessary conditions. The research focused on examining specific classes of oscillatory differential equations and comparing them to established methods. The findings indicate that the newly proposed methods exhibit superior accuracy and faster convergence compared to the existing methods investigated in this research. Consequently, the results highlight the improved precision and quicker convergence achieved with the new method. All computations were executed using Maple 18 software

Unveiling the pathways of ethanol decomposition to carbon radicals by nanosecond pulse bubble discharge in liquid: a two-step hybrid model

In recent years, bubble discharge in liquid has become a novel approach for the synthesis of carbon nanomaterials; however, the fundamental discharge process and synthesis mechanism are still not well understood. In this work, we build a two-step simulation model (combining 2D fluid dynamics and zero-dimensional plasma kinetics) to investigate nanosecond pulse discharge in an Ar bubble immersed in liquid ethanol and chemical reaction processes inside. The 2D simulation results show that discharge develops along the gas‒liquid interface where ethanol decomposes, resulting in much higher densities of active species (CH3, C2H5 and OH). The electric field of the selected reference point near the interface obtained by the 2D model is transmitted into the 0D model. The numerical results show that the decomposition of ethanol mainly occurs at the discharge stage, in which electron impact dissociation (e.g. C2H5OH + e → CH2OH + CH3 + e) and Penning dissociation (e.g. C2H5OH + Ar* → CH2OH + CH3 + Ar) dominate. The density of all carbonaceous species rapidly increases during discharge, while that of some carbon radicals (CH and C) continues to increase due to neutral species reactions when discharge ceases. By quantitative analysis of the reaction contributions, the dominant pathways of C2, CH and C are revealed, i.e. C2H5OH → C2H5 → [C2H, C2H2, C2H3] → C2 and C2H5OH → CH3 → CH2 → CH → C. In addition, the formation pathways of H and OH radicals, which are indispensable for the transformation of carbonaceous intermediates, are also analysed.

Considerations on the relaxation time in shear-driven jamming.

We study the jamming transition in a model of elastic particles under shear at zero temperature, with a focus on the relaxation time τ_{1}. This relaxation time is from two-step simulations where the first step is the ordinary shearing simulation and the second step is the relaxation of the energy after stopping the shearing. τ_{1} is determined from the final exponential decay of the energy. Such relaxations are done with many different starting configurations generated by a long shearing simulation in which the shear variable γ slowly increases. We study the correlations of both τ_{1}, determined from the decay, and the pressure, p_{1}, from the starting configurations as a function of the difference in γ. We find that the correlations of p_{1} are longer lived than the ones of τ_{1} and find that the reason for this is that the individual τ_{1} is controlled both by p_{1} of the starting configuration and a random contribution which depends on the relaxation path length-the average distance moved by the particles during the relaxation. We further conclude that it is γ_{τ}, determined from the correlations of τ_{1}, which is the relevant one when the aim is to generate data that may be used for determining the critical exponent that characterizes the jamming transition.

Shock-induced deflagration-enhanced characteristics of Cu-PTFE/Al tandem EFPs impacting multi-layer spaced plates

Polytetrafluoroethylene/aluminum (PTFE/Al) reactive material is a pivotal research object in the aerospace, military, and mechanical engineering fields and can release chemical energy (CE) under shock or impact. However, its relatively low mechanical strength limits its applications. The present paper proposes a Cu-PTFE/Al (73.5wt. %/26.5wt. %) double-layer liner that can form tandem explosive formed projectiles (EFPs) under the shock of shaped charges, which not only retains the strong penetration ability but also shows a more significant lateral enhancement effect through the deflagration reaction. Here, the preparation process of the PTFE/Al liner is given, and an analytical model for the Cu-PTFE/Al tandem EFP of the damage process against multi-spaced plates is established, revealing the penetration and deflagration-enhanced mechanisms. Subsequently, a two-step segmented numerical simulation for the penetration–deflagration coupling effects is conducted, and the time-space interaction process and damage results between kinetic energy penetration and CE deflagration are obtained. A series of experiments of tandem EFPs against spaced plates are conducted, including the different materials, thickness ratio, and standoff. Experimental results show that compared with Cu–Cu tandem EFP with the same condition, the penetration ability of Cu-PTFE/Al composite EFP is reduced, but the damage enhancement effect is greatly improved; the maximum damage area of a single plate is increased by 220.1%, and the average damage area of a single plate is increased by 76.2%. This study provides important reference data and a theoretical basis for the design of metal-reactive tandem EFPs.

Integrating core physics and machine learning for improved parameter prediction in boiling water reactor operations

This study introduces a novel method for enhancing Boiling Water Reactor (BWR) operation simulations by integrating machine learning (ML) models with conventional simulation techniques. The ML model is trained to identify and correct errors in low-fidelity simulation outputs, traditionally derived from core physics computations. These corrections aim to align the low-fidelity results closely with high-fidelity data. Precise predictions of nuclear reactor parameters like core eigenvalue and power distribution are crucial for efficient fuel management and adherence to technical specifications. Current high-fidelity transport calculations, while accurate, are impractical for real-time predictions due to extensive computational demands. Our approach, therefore, utilizes the standard two-step simulation process-assembly-level lattice physics calculations followed by whole-core nodal diffusion computations-to generate initial results, which are then refined using the ML-based error correction model. The methodology focuses on improving simulation accuracy in regular BWR operations rather than developing a universal ML predictor for reactor physics. By training an advanced neural network model on the difference in high-fidelity and low-fidelity simulations, the model can reduce the nodal power error from low-fidelity simulations to around 1% on average and the core eigenvalue down to under 100 pcm. This result is under the condition of the normal variations of control rod pattern and core flow rate changes in standard BWR operations used in the training and evaluation of the machine learning model. This work suggests a promising approach for achieving more accurate, computationally feasible simulation solutions in nuclear reactor operation and management.

Pre- versus Post-synaptic Forms of LTP in Two Branches of the Same Hippocampal Afferent.

There has been considerable controversy about pre- versus postsynaptic expression of memory-related long-term potentiation (LTP), with corresponding disputes about underlying mechanisms. We report here an instance in male mice, in which both types of potentiation are expressed but in separate branches of the same hippocampal afferent. Induction of LTP in the dentate gyrus (DG) branch of the lateral perforant path (LPP) reduces paired-pulse facilitation, is blocked by antagonism of cannabinoid receptor type 1, and is not affected by suppression of postsynaptic actin polymerization. These observations are consistent with presynaptic expression. The opposite pattern of results was obtained in the LPP branch that innervates the distal dendrites of CA3: LTP did not reduce paired-pulse facilitation, was unaffected by the cannabinoid receptor blocker, and required postsynaptic actin filament assembly. Differences in the two LPP termination sites were also noted for frequency facilitation of synaptic responses, an effect that was reproduced in a two-step simulation by small adjustments to vesicle release dynamics. These results indicate that different types of glutamatergic neurons impose different forms of filtering and synaptic plasticity on their afferents. They also suggest that inputs are routed to, and encoded by, different sites within the hippocampus depending upon the pattern of activity arriving over the parent axon.

Dynamic thermal performance of cryogenic liquid hydrogen heating from gas-liquid stratification to supercritical under a constant heat flux condition: A two-step simulation

The dynamic thermal performance was significant for safety storage of cryogenic liquid hydrogen in tank. In this work, a numerical simulation was presented to investigate the thermal performance of cryogenic liquid hydrogen in a C-type tank heating from gas-liquid stratification to supercritical state. A two-step simulation using separate model was applied. The results show that a leading rise of absolute pressure was observed due to the uneven temperature distribution in the gas-liquid stratification state. Accordingly, the rise of heat flux will enhance the non-uniformity of temperature distribution and reduce the safety storage time. Meanwhile, the effects of heat flux and thermodynamic state on the safety storage time were discussed. Further, the effects of identification of critical state boundary on the accuracy of the two-step simulation were analyzed. It was found that the boundary condition when local supercritical state occurs was better than other two conditions in this work.

Experimental and numerical investigations on the microstructural features and mechanical properties of explosively welded aluminum/titanium/steel trimetallic plate

In this paper, 3A21Al/TA1/CCSB composite plate was fabricated successfully by explosive welding to improve the bond strength. The microstructure characteristics of aluminum/titanium and titanium/steel bonding interfaces were systematically investigated by two-step numerical simulation and SEM/EBSD characterizations. Firstly, the SPH (Smoothed Particle Hydrodynamics) and Lagrange methodology were used to simulate and calculate the collision velocity and collision angle, establishing an accurate impact condition. Secondly, the SPH model was used to simulate the high-speed oblique impact welding process. The simulation results reproduce the formation of wavy interface, vortex zone and jet, which are consistent with the experimental results. Combined with SEM, EBSD, and EDS analysis, continuous and uniform wavy structures were found at different interfaces of the composite plate, showing significantly different wave lengths and amplitudes. Dynamic recovery and recrystallization took place at the bonding interfaces which showed refined grain with different preferred orientations. The observed structural characteristics and the thermodynamic state predicted by SPH simulation inferred the evolutions of microstructures (including morphologies, micro defects, grain structures). The tensile strength and fracture elongation of the trimetallic plate reach 448.2 MPa and 14.15%, respectively. In addition, the shear strengths of each interface far exceed the corresponding acceptable standard values.

Lateral enhancement effect of reactive PELE: Two-step segmented simulation and analytical modeling

Comparing with traditional PELE, the reactive PELE shows a more significant lateral enhancement effect due to the deflagration reaction of the filling by impact. Here an analytical model for thin-walled jacket fragmentation and dispersion is presented based on the wave interaction and the reaction of reactive filling. Subsequently, a two-step segmented numerical simulation for the impact deflagration of reactive PELE is developed. Based on the numerical simulation, the wave interaction process and the internal stress distribution of the filling were given firstly, then the deflagration reaction of reactive filling, jacket fragmentation and dispersion were simulated. By the theory and simulation, the experiment results of reactive PELE impacting steel plate are analyzed. In the experiment, the thickness of the front steel target is 6mm-20 mm, and the multi-layer witness aluminum plate is set behind the steel target to obtain the lateral effect of the reactive PELE. The analysis results show that the released energy of the reactive filling increases the maximum radial velocity of the jacket fragments by more than 40 %, and the perforation dispersion on the witness plate significantly depends on the filling reaction level and the front steel plate thickness, the lateral enhancement mechanism of reactive PELE impacting plate is explained.

Numerical and experiment study on ventilation performance of the equipment compartment of Alpine high-speed train

Alpine High-Speed Train serves on the Lanzhou-Urumqi Line in northwest China, where the terrain is mainly the Gobi Desert. To adapt to this complex environment, the independent-air duct is mainly used for the electrical facilities inside the equipment compartment to prevent the spread of sand particles. This isolated air duct makes ventilation characteristics of the equipment susceptible to the external environment. For this reason, this work aims to clarify and investigate the ventilation characteristic of electrical facilities. A two-step simulation method using IDDES (Improved Delayed Detached Eddy Simulation) and a real-vehicle tracking test using the T-typed pitot tubes were conducted. In the simulation, it is found that the ventilation performance can be influenced by the location of the equipment compartment, facilities and the fan mounted inside. By comparing the results of the test and simulation, they share the same characteristic that the air outlet volume of the converter near the head car is being promoted while it near the tail car is being inhibited. The maximum deviation ratio between the test and simulation is 9%. Therefore, the measurement method in this study is relatively reliable.

Multiscale modeling of cutting processes for TiBw/TA15 composites based on the interface model parameters identification method by microcolumn compression

TiBw/TA15 composites are featured by the network-structured distribution of the reinforced phase quasi-continuously. The multiphase microstructure including the TiB ceramic whisker reinforcement, the titanium alloy matrix and the interface plays a significant role in cutting processes. In this paper, multiscale finite element (FE) cutting models in accordance with the structural characteristics of TiBw/TA15 composites are established. Then, the interfacial mechanical parameters of TiBw/TA15 composites are recognized by the microcolumn compression approach, and the thermoplastic equivalent constitutive parameters (ECP) of the TiBw-rich region in the model are obtained by a two-step FE simulation. The cutting model is validated by orthogonal turning experiments. Finally, the formation of segmented chips can be visualized by the cutting models, which is caused by the compressive stress and the TiB whiskers failure. In addition, the mapping relationship between the critical equivalent strain (CES) and the hardened layer depth (HLD) is established. The results indicate that the HLD reduces with the decrease of cutting speed (VC) and depth of cutting (h). The findings in this paper facilitate a further understanding of the TiBw/TA15 composites cutting mechanism.

Phase-field simulation of the formation of new grains by fragmentation during melting of an ABD900 superalloy

Laser powder bed fusion (L-PBF) is an additive manufacturing method which involves local laser melting of powder particles, a partial remelting of previously deposited layers, and subsequent re-solidification under high thermal gradients and cooling rates. The transition between melting and re-solidification becomes visible as melt pool boundaries in optical micrographs and plays a crucial role: Apart from creating a strong segregation zone, the transition determines whether the microstructure is inherited and carried over to the next layer, or whether new grains with new orientations are formed. While heterogeneous nucleation is suppressed due to the lack of seeding particles at the small length scales inherent to L-PBF, alternatively, new grains can form via dendrite fragmentation, as demonstrated in this paper by phase-field simulations using the software MICRESS®. By strong coupling between the phase-field equation and a thermal 1D-cylinder approach for the long-range temperature field, consistency between latent heat and microstructure is ensured. To allow for a systematic variation of the orientation relationship between the dendrite growth direction and the respective temperature gradient, a two-step simulation procedure for two overlapping tracks with variable gradient directions is developed. Growth conditions which promote fragmentation and formation of new grains are analyzed and discussed.

Numerically efficient two-step CFD simulation model of flow regime and heat transfer for hot water shower sterilizers

This work was carried out to create a numerically efficient validated simulation model for Hot Water Shower (SWS) sterilizers. The goal of the work was to achieve a more homogeneous temperature and water distribution inside the sterilizers developed in the future. A lab-scale test bench of an SWS sterilizer with a configurable water volume flow rate and temperature was designed to validate the Computational Fluid Dynamics (CFD) model that was created. The heating and cooling phases at water volume flow rates of 20 and 40m3/h per m2 spraying surface were analyzed. Additionally, the flow and film regime were studied. The CFD model was split into two simulation steps to significantly improve the numerical efficiency. The novel two-step approach developed is computationally 1800 times faster as compared with a complete sterilization cycle than a state-of-the-art approach. The simulated temperature of the test load was in good accordance with the experimental measurements. The flow regime was largely accurately replicated. Overall, the created validated CFD model is suitable for upscaling to an industry-sized SWS sterilizer due to its numerical efficiency.

Simulation of the response of a diamond-based radiation detector to ultra-short and intense high-energy electron pulses

Single-crystal synthetic diamond sensors have been widely used in radiation dosimetry and beam diagnostics. The foreseen harsh radiation environment in electron-positron colliders at the luminosity frontier requires a thorough investigation of diamond’s response to large radiation bursts, in particular, to intense high-energy electron pulses. In this article, a two-step numerical simulation approach (Sentaurus + LTspice) is proposed to explore this topic. The time response of the diamond detector is simulated using TCAD-Sentaurus while the transmission effect of the electronic circuit is taken into account using LTspice. Good agreement is observed between results of the numerical simulation and preliminary experimental data from detector’s exposure to high-energy sub-picosecond electron pulses, on both the amplitude and the shape of the induced signals. This simulation combination is a novel approach to designing and optimizing diamond detectors for radiation and beam loss monitoring in particle physics experiments.

Small satellite formations and constellations for observing sub-daily mass changes in the Earth system

SUMMARY Satellite gravity missions so far are medium size satellites consisting of one or a pair of satellites flying in near-polar or sun-synchronous orbital planes. Due to the limited observation geometry and the related space–time sampling, high-frequency non-tidal mass variation signals from atmosphere and ocean cannot be observed and cause temporal aliasing. For current single-pair satellite gravimetry missions as GRACE and GRACE Follow-On (GRACE-FO) temporal aliasing is the limiting factor and represents the major error source in the gravity field time-series. Adding a second inclined satellite pair to a GRACE-like polar pair (Bender constellation) currently is the most promising solution to increase the spatio-temporal resolution and to significantly reduce the temporal aliasing error. This shall be implemented with the MAGIC mission in future. With the ongoing developments in miniaturization of satellites and gravity-relevant instruments (accelerometers and intersatellite ranging), in future constellations of multiple small satellite pairs may solve this problem even beyond the capabilities of a Bender constellation. Therefore, in this study the capabilities of such constellations flying in specific formations are investigated in order to enable a retrieval of the temporal gravity field on short time scales. We assess the performance of up to 18 satellite pairs. The satellite configurations cover satellite pairs in polar and inclined orbits flying in pair-wise or pearl-string formation with varying mean anomalies and right ascensions of the ascending node (RAAN). As future potential miniaturized instruments optomechanical accelerometers with similar performance as those flying on GRACE-FO are a candidate, while for the intersatellite ranging instrument still some technological development is required. Therefore, in this study a microwave ranging system equivalent to the GRACE and GRACE-FO instruments performance is taken as baseline assuming that such instruments can be miniaturized in future as well. In numerical closed-loop simulations, up to nine different satellite configurations with up to 18 satellite pairs are evaluated based on the retrieval of the non-tidal temporal gravity field on a monthly basis. From the simulation results it is concluded that the best-performing satellite constellation of 18 polar satellite pairs already is outperformed by a typical Bender-like constellation of 1 polar and 1 inclined pair. In general, we identify that increasing the number of satellite pairs leads to an improved gravity field retrieval, either at low spherical harmonic degree and order (d/o) by the shift in RAAN or at high d/o by the shift in mean anomaly. By a two-step simulation approach, co-estimating also (sub-)daily gravity fields for selected configurations with a large number of satellite pairs distributed equally over the globe, it is possible to retrieve stand-alone gravity fields at 24, 12 and 6 hr temporal resolution. Ultimately it is concluded that a network of miniaturized satellites with instrument performances similar to GRACE-FO and flying in a well-defined constellation has the potential to observe (sub-)daily mass variations and therefore could drastically reduce the problem of temporal aliasing due to high frequency mass variations in the Earth system.

Nano-structuring of a high entropy alloy by severe plastic deformation: Experiments and crystal plasticity simulations

The evolution of microstructure, texture, and mechanical properties of an induction melted non-equiatomic Al6Co23.5Fe23.5Mn23.5Ni23.5 (at.%) high entropy alloy subjected to severe plastic deformation was investigated experimentally and by simulations. Analyses using electron backscatter diffraction and transmission Kikuchi diffraction images revealed the evolution of the microstructures. The coarse-grained initial structure was deformed down to an average grain size of 50 nm after a shear strain of 11 by employing the high-pressure compressive shearing process, at room temperature. Transmission electron microscopy analysis showed the presence of nano-twins. The high deformation led to a significant increase in the strength, up to ≈ 1.0 GPa. X-ray diffraction macro-texture analysis revealed a shear texture with the dominance of the B/B¯{112}110-type component whose intensity varied with strain. A two-step Taylor-type polycrystal plasticity simulation approach reproduced the texture by a correlation value of 91%. In the first part of the modeling, grain fragmentation was simulated, while in the second part, grain boundary shearing and deformation twinning were considered together with the operation of {111}112-type partial slip. The effect of twinning was also examined in the texture modeling and the simulations estimated that its fraction was less than 2%.