Small-scale Simulation Research Articles

Thermodynamic analysis and atomistic modeling of subsurface cavitation in photomechanical spallation

The thermodynamic conditions defining the spatial extent of a subsurface region affected by the nucleation and growth of voids in photomechanical spallation of metals is investigated theoretically and in atomistic simulations. A theoretical analysis of cavitation in a surface region of a target melted by laser irradiation suggests a functional form for the temperature dependence of the cavitation threshold. A series of small-scale molecular dynamics simulations performed for Al, Ag, Cr, and Ni has revealed the minimum value of the free energy barrier that results in the onset of cavitation on the timescale of 10s of ps, which is a typical duration of transient tensile stresses produced by the unloading wave generated in the spallation regime. The predictive ability of the theoretical description is verified in large-scale simulations of photomechanical spallation of a Ni target and ablation of an Al target in the phase explosion regime. The temporal and spatial evolution of the free energy barrier for the onset of cavitation under conditions of short pulse laser processing is shown to univocally define the region where the nucleation of voids takes place.

On Mechanical Properties of New Railway Wheel Steels for Desert Environments and Sand Caused Wheel Damage Mechanisms

Worn railway wheels after service in the desert were investigated by means of failure analysis, hardness measurements, tensile tests, fracture toughness tests and low-cycle fatigue (LCF) tests. The results complete both previous small-scale laboratory experiments and FEM simulations performed by the authors on widely used steels and new steel grades allowing to better understand the main damage mechanisms due to the sand. These new steel grades were developed for forged-rolled solid wheels in alternative to AAR Class B/C/D steel grades and were designed to ensure high resistance to wear and rolling contact fatigue in the presence of sand, debris or gravel on the rails. This paper covers the in-depth mechanical characterization of wheels made of the new steels. Hardness measurements, tensile tests, fracture toughness tests and LCF tests were carried out. The results are compared with AAR Class B/C/D steels showing an excellent combination of fracture toughness, cyclic yield strength and ductility.

Direct Relative Edge Optimization, A Robust Alternative for Pose Graph Optimization

Pose graph optimization is a common problem in robotics and associated fields. Most commonly, pose graph optimization is performed by finding the set of pose estimates which are the most likely for a given set of measurements. In some situations, arbitrarily large errors in pose graph initialization are unavoidable and can cause these pose-based methods to diverge or fail especially in cases where global inputs become available after some time after initialization. This letter details the parameterization of the classic pose graph problem in a relative context, optimizing directly over relative edge constraints between vertices in the pose graph and not on the poses themselves. Unlike previous literature on relative optimization, this letter details relative optimization over an entire pose graph, instead of a subset of edges, resulting in greater robustness to arbitrarily large errors than the classic pose-based or prior relative edge-based methods. Several small-scale simulation comparison studies, along with single and multi-agent hardware experiments, are presented. Results point to relative edge optimization as a strong candidate for solving real-world pose graph optimization problems that contain large heading propagation or initialization errors.

Bulbous head formation in bidisperse shallow granular flow over an inclined plane

Rapid shallow granular flows over inclined planes are often seen in nature in the form of avalanches, landslides and pyroclastic flows. In these situations the flow develops an inversely graded (large at the top) particle-size distribution perpendicular to the plane. As the surface velocity of such flows is larger than the mean velocity, the larger material is transported to the flow front. This causes size segregation in the downstream direction, resulting in a flow front composed of large particles. Since the large particles are often more frictional than the small, the mobility of the flow front is reduced, resulting in a so-called bulbous head. This study focuses on the formation and evolution of this bulbous head, which we show to emerge in both a depth-averaged continuum framework and discrete particle simulations. Furthermore, our numerical solutions of the continuum model converge to a travelling wave solution, which allows for a very efficient computation of the long-time behaviour of the flow. We use small-scale periodic discrete particle simulations to calibrate (close) our continuum framework, and validate the simple one-dimensional (1-D) model with full-scale 3-D discrete particle simulations. The comparison shows that there are conditions under which the model works surprisingly well given the strong approximations made; for example, instantaneous vertical segregation.

Incompressible Fluids Simulation by Relaxing the Density-Invariant Condition in a Marine Simulator

To achieve small-scale ocean scene simulation in a marine simulator, we present an incompressible SPH algorithm by relaxing the density-invariant condition for incompressible fluids. As there are larger density errors of the fluid particles near or on the boundary, more iteration numbers are required. Taking boundary handling to modify the density of the fluid particles, the relaxation method is used to optimize the iterative method to solve the density-invariant condition, which can reduce the iteration numbers and average density deviation and improve the accuracy. Our proposed approach can achieve incompressible SPH and solve the problem of the particle deficiency. While ensuring the stability, the approach can allow large time steps and control the density deviation below 0.01% and improve the efficiency by reducing the iteration numbers and optimizing the calculating procedure and the initial value selection.

Numerical Simulation of Microemulsion Flooding in Low-Permeability Reservoir

Based on the features of microemulsion flooding in low-permeability reservoir, a three-dimension three-phase five-component mathematical model for microemulsion flooding is established in which the diffusion and adsorption characteristics of surfactant molecules are considered. The non-Darcy flow equation is used to describe the microemulsion flooding seepage law in which the changes of threshold pressure gradient can be taken into account, and the correlation coefficients in the non-Darcy flow equation are determined through the laboratory experiments. A new treatment for the changes of threshold pressure and the quantitative description of adsorption quantity of surfactant and relative permeability curves are presented, which enhance the coincidence between mathematical model and experiment results. The relative errors of main development indexes are within 4%. A software is programmed based on the model to execute a core-level small-scale numerical simulation in Chaoyanggou Oilfield. The fitting relative errors of the pressure, flow rate, and moisture content are 3.25%, 2.71%, and 2.54%, respectively. The results of laboratory experiments and numerical simulation showed that microemulsion system could reduce the threshold pressure gradient by 0.010 MPa/m and injection pressure by 0.6 MPa. The biggest decline in moisture content reaches 33%, and the oil recovery is enhanced by 10.8%.

Flow regimes and dynamic similarity of immersed granular collapse: A CFD-DEM investigation

Immersed granular collapses may encounter different flow regimes, such as free-fall (dry), fluid-inertial, and viscous regimes, depending on column geometry, particle size, particle density, fluid viscosity, and many other parameters. Understanding the controlling parameters of these regimes is important for both industrial and geological applications where grains and fluids coexist. It is also important to combine these parameters into dimensionless groups to guide down-scaled experiments and numerical simulations. In this work, we derive a set of dimensionless numbers (i.e., Stokes number, density ratio, and Reynolds number) based on typical time scales in the sedimentation of a sphere, and successfully verify the relevance of these numbers in determining flow regimes and maintaining dynamic similitude across length scales. The numerical method we use couples the computational fluid dynamics and discrete element method (CFD-DEM), which allows a wide variety of particle size and fluid viscosity to be chosen, keeping constant the Stokes number and density ratio. Quantitative data of front propagation and energy evolution are presented to characterize flow dynamics in different flow regimes. The collapse exhibits a transition from sliding-dominant to suspension-dominant behaviors as the Stokes number decreases, which gives rise to distinct deposit morphology in different regimes. Our findings enhance the understanding of inertial and viscous behaviors of immersed granular flows. The verified scaling rules and dimensionless parameters are of potential use in small-scale experiments and simulations where appropriate scaling is essential.

Building LEGO Towers

Global software engineering has changed the way software is developed today. To address the new challenges, many universities have launched specially tailored courses to train young professionals to work in globally distributed projects. However, a mere acknowledgment of the geographic, temporal, and cultural differences does not necessarily lead to a deep understanding of the underlying practical implications. Therefore, many universities developed alternative teaching and learning activities, such as multi-university collaborative projects and small-scale simulations or games. In this article, we present a small-scale exercise that uses LEGO bricks to teach skills necessary for global work. We describe the many different interventions that could be implemented in the execution of the exercise. We had seven runs of the exercises and report our findings from executing seven runs of the exercise with the total of 104 students from five different courses in two different universities. Our results suggest that the exercise can be a valuable tool to help students dealing with troublesome knowledge associated with global software engineering and a useful complement to the courses dedicated to this subject.

Irradiance characteristic of a small-scale solar simulator for testing thermal collectors

This paper describes the development of a small-scale solar simulator for research and educational purposes. The main goal is to provide a uniform and stable distribution of irradiation from a set of four floodlight halogen lamps, over a targeted area of 40x40 cm2. The floodlights are placed perpendicularly to each other, on the top end of a light tube. Two types of halogen lamps were characterised and the total irradiation of the solar simulator was measured over a 5x5 grid. The results showed that the two arrangements of lamps are able to achieve a level of 1000 W/m2 on the 40x40 cm2 targeted area, but the non-uniformity is unsatisfactory. In order to reach a non-uniformity level of 10% using the proposed lamps arrangements, the targeted area should be reduced to 31x31 cm2.

Boiling heat transfer modelling: A review and future prospectus

This paper reviews the current status of boiling heat transfer modelling, discusses the need for its improvement due to unresolved intriguing experimental findings and emergence of novel technical applications and outlines the directions for an advanced modelling approach. The state-of-the-art of computational boiling heat transfer studies is given for: macro-scale boiling models applied in two-fluid liquid-vapour interpenetrating media approach, micro-, meso-scale boiling computations by interface capturing methods, and nano-scale boiling simulations by molecular dynamics tools. Advantages, limitations and shortcomings of each approach, which originate from its grounding formulations, are discussed and illustrated on results obtained by the boiling model developed in our research group. Based on these issues, we stress the importance of adaptation of a multi-scale approach for development of an advanced boiling predictive methodology. A general road-map is outlined for achieving this challenging goal, which should include: improvement of existing methods for computation of boiling on different scales and development of conceptually new algorithms for linking of individual scale methods. As dramatically different time steps of integration for different boiling scales hinder the application of full multi-scale methodology on boiling problems of practical significance, we emphasise the importance of development of another algorithm for the determination of sub-domains within a macro-scale boiling region, which are relevant for conductance of small-scale simulations.

Evaluating small-scale simulation for training firearm safety skills.

There is limited research using small-scale simulation in applied behavior analysis. We used small-scale simulation to train firearm safety skills to 3- to 5-year-old children and assessed whether the skills generalized to the natural environment through in situ assessment. Three participants completed the training, and all participants learned the safety skills from simulation training. Two of the participants acquired the safety skills after the first simulation training, and the third participant required one booster training before demonstrating the safety skills in the natural environment.

MVBatch: A matlab toolbox for batch process modeling and monitoring

A novel user-friendly graphical interface for process understanding, monitoring and troubleshooting has been developed as a freely available MATLAB toolbox, called the MultiVariate Batch (MVBatch) Toolbox. The main contribution of this software package is the integration of recent developments in Principal Component Analysis (PCA) based Batch Multivariate Statistical Process Monitoring (BMSPM) that overcome modeling problems such as missing data, different speed of process evolution and length of batch trajectories, and multiple stages. An interactive user interface is provided, which aims to guide users in handling batch data through the main BMSPM steps: data alignment, data modeling, and the development of monitoring schemes. In addition, a small-scale non-linear dynamic simulator of the fermentation process of the Saccharomyces cerevisiae cultivation is available to generate realistic batch data under normal and abnormal operating conditions. This generator of synthetic data can be used for teaching purposes or as a benchmark to illustrate and compare the performance of new methods with sound techniques published in the field of BMSPM.

An operational coastal forecasting tool for performing ensemble modeling

An automated coastal estuarine modeling system, termed Multistage, is created in the ADCIRC hydrodynamic model. The software development was motivated by the need for reducing the simulation wall-clock time to increase the scalability of ADCIRC for ensemble modeling. Multistage makes use of a one-way nesting technique as an alternative to the continuous single-domain approach. Hydrodynamic predictions, such as storm surge and waves, are made for a targeted coastal region in a two-stage scheme consisting of a large-scale coarse mesh simulation and a small-scale fine mesh simulation. The nesting technique is adjusted for accurate and efficient coastal estuarine modeling with careful considerations. We propose and successfully test a method for the coarse mesh extending onshore rather than the common practice of extending the fine mesh offshore. Repetitive ensemble computations are allowed over both the coarse mesh and the fine mesh in an efficient time frame. Modeling preparations and processes are scripted to eliminate the additional efforts needed for applying this model nesting technique. The tool is tested on two coastal estuarine systems, and its performance is compared with the conventional single-domain method and observational data. By implementing Multistage, forecast runtimes are reduced by 50% to more than 80% using the same model parameters and computational resources as used in the conventional single-domain technique. Depending on the domain and model type, it is possible to perform 3-to-5 ensemble simulations on both coarse and fine meshes by applying the nesting technique and using the same amount of CPU-hours needed to complete one single-domain simulation. This research details the concept, preliminary results and offline applications of the tool. Future work is focused on enhancing the ease of use and on development and implementation of a real-time version.

Using Small-scale Blast Tests and Numerical Modelling to Trace the Origin of Fines Generated in Blasting

Waste fines from rock breakage often negatively influence economics and environment. The Austrian Science Fund (FWF) sponsors a project to investigate the cause of the fines by studying blast fragmentation throughout small-scale blast tests and numerical simulations. The tests include blast-loading confined granite and mortar cylinders by detonating cord with 6, 12, and 20 g/m of PETN. The blast-driven dynamic cracking at the end face of the cylinder opposite to the initiation point is filmed with a high-speed camera. The filming is followed up by an analysis of surface and internal crack systems and sieving of the blasted cylinders to quantify the amount of fine material created. The numerical simulations cover the blast fragmentation of a mortar cylinder. These simulations use Finite and Discrete Element Methods (FEM, DEM) with explicit time integration. The model cylinders are loaded by a pressure evolution acting on the borehole wall. Both methods produce realistic crack patterns, consisting of through-going radial cracks with crack intersections around a crushed zone at the borehole. Furthermore, the DEM models have also yielded realistic fragment size distributions (FSD). The paper covers the present progress of the ongoing project and related future work.

A modified method for estimating inherent strains from detailed process simulation for fast residual distortion prediction of single-walled structures fabricated by directed energy deposition

Predicting residual distortion in metal additive manufacturing (AM) is important to ensure quality of the fabricated component. The inherent strain method is ideal for this purpose, but has not been well developed for AM parts yet. In this paper, a modified inherent strain model is proposed to estimate the inherent strains from detailed AM process simulation of single line depositions on top of each other. The obtained inherent strains are employed in a layer-by-layer static equilibrium analysis to simulate residual distortion of the AM part efficiently. To validate the model, depositions of a single wall and a rectangular contour wall models with different number of layers deposited by a representative directed energy deposition (DED) process are studied. The proposed model is demonstrated to be accurate by comparing with full-scale detailed process simulation and experimental results. To make the method practical, a small-scale detailed simulation model is proposed to extract the mean inherent strains. Based on this approach, simulation results applied to the rectangular contour wall structures of different heights show that the modified inherent strain method is quite efficient, while the residual distortion of AM parts can be accurately computed within a short time. The improvement of the computational efficiency can be up to 80 times in some specific cases.

Rejection sampling for tempered Lévy processes

We extend the idea of tempering stable Levy processes to tempering more general classes of Levy processes. We show that the original process can be decomposed into the sum of the tempered process and an independent point process of large jumps. We then use this to set up a rejection sampling algorithm for sampling from the tempered process. A small scale simulation study is given to help understand the performance of this algorithm.

Modeling Nonlinear Synaptic Dynamics: A Laguerre-Volterra Network Framework for Improved Computational Efficiency in Large Scale Simulations.

Synapses are key components in signal transmission in the brain, often exhibiting complex non-linear dynamics. Yet, they are often crudely modelled as linear exponential equations in large-scale neuron network simulations. Mechanistic models that use detailed channel receptor kinetics more closely replicate the nonlinear dynamics observed at synapses, but use of such models are generally restricted to small scale simulations due to their computational complexity. Previously, we have developed an ``input-output'' (IO) synapse model using the Volterra functional series to estimate nonlinear synaptic dynamics. Here, we present an improvement on the IO synapse model using the extbf{Laguerre-Volterra network (LVN) framework. We demonstrate that utilization of the LVN framework helps reduce memory requirements and improves the simulation speed in comparison to the previous iteration of the IO synapse model. We present results that demonstrate the accuracy, memory efficiency, and speed of the LVN model that can be extended to simulations with large numbers of synapses. Our efforts enable complex nonlinear synaptic dynamics to be modelled in large-scale network models, allowing us to explore how synaptic activity may influence network behavior and affects memory, learning, and neurodegenerative diseases.

A demonstration on the similarity of pitch response between MW wind turbine and small-scale simulator

This study demonstrates the similarity in the pitch responses of a target multi-MW wind turbine and a small-scale simulator by reflecting the nonlinearity of the aerodynamic torque. A 3.5 kW simulator is used to emulate a target 2 MW wind turbine with similar dynamic characteristics of the pitch controller. The pitch controller, consisting of a proportional-integral (PI) controller and a gain-scheduler for the 3.5 kW simulator, is designed to provide similar pitch responses to those of the target 2 MW wind turbine for the entire wind speed region above the rated. The highly nonlinear aerodynamic properties with varying wind speed and their effects on the two poles of the closed-loop pitch control system are investigated. The pitch PI gain values and gain-scheduler are designed such that the natural frequencies and damping ratios of the closed-loop pitch control systems for the target 2 MW wind turbine and the 3.5 kW simulator will be almost the same at both the design and cut-out wind speeds. The overspeed, overpower, and settling time for pitch responses to step wind speeds are investigated, and their values for the target 2 MW wind turbine and the 3.5 kW simulator are compared quantitatively. Through both numerical simulations and experiments, it is demonstrated that the 3.5 kW simulator has similar dynamic characteristics to the target 2 MW wind turbine in terms of pitch response.

Using weighting algorithms to refine source direction determinations in all-sky gravitational wave burst searches with two-detector networks

I explore the possibility of resurrecting an old, non-Bayesian computational approach for inferring the source direction of a gravitational wave from the output of a two-detector network. The method gives the beam pattern response functions and time delay, and performs well even in the presence of noise and unexpected signal forms. I further suggest an improvement to this method in the form of a weighting algorithm that usefully improves its accuracy beyond what can be achieved with simple best-fit methods, validating the new procedure with several small-scale simulations. The approach is identified as complementary to --rather than in competition with-- the now-standard Bayesian approach typically used by the LIGO network in parameter determination. Finally, I briefly discuss the possible applications of this method in the world of three-or-more-detector networks and some directions for future work.

Small-scale Dynamo Simulations: Magnetic Field Amplification in Exploding Granules and the Role of Deep and Shallow Recirculation

Abstract We analyze recent high-resolution photospheric small-scale dynamo simulations that were computed with the MURaM radiative MHD code. We focus our analysis on newly forming downflow lanes in exploding granules, as they show how weakly magnetized regions in the photosphere (the center of granules) evolve into strongly magnetized regions (downflow lanes). We find that newly formed downflow lanes initially exhibit mostly a laminar converging flow that amplifies the vertical magnetic field embedded in the granule from a few 10 G to field strengths exceeding 800 G. This results in extended magnetic sheets that have a length comparable to granular scales. Field amplification by turbulent shear first happens a few 100 km beneath the visible layers of the photosphere. Shallow recirculation transports the resulting turbulent field into the photosphere within minutes, after which the newly formed downflow lane shows a mix of strong magnetic sheets and turbulent field components. We stress in particular the role of shallow and deep recirculation for the organization and strength of magnetic field in the photosphere and discuss the photospheric and sub-photospheric energy conversion associated with the small-scale dynamo process. While the energy conversion through the Lorentz force depends only weakly on the saturation field strength (and therefore deep or shallow recirculation), it is strongly dependent on the magnetic Prandtl number. We discuss the potential of these findings for further constraining small-scale dynamo models through high-resolution observations.