Two-dimensional Simulations Research Articles

The boundary layer instability beneath internal solitary waves and its sensitivity to vortex wakes

We investigated the stability of the bottom boundary layer (BBL) beneath periodic internal solitary waves (ISWs) of depression over a flat bottom through two-dimensional direct numerical simulations. We explored the convective versus absolute/global nature of the BBL instability in response to changes in Reynolds number, and the sensitivity of the instability to seeding noise in the front of the ISW – spanning laboratory to geophysical scales. The BBL was laminar at $Re_{ISW}=90$ and convectively unstable at $Re_{ISW}=300$ . At laboratory-scale $Re_{ISW}=300$ , the convective wave packet was periodically amplified by each successive ISW, until vortex shedding occurred. The associated noise-amplification behaviour potentially explains the discrepancies of the critical $Re_{ISW}$ between the lock–release laboratory experiments and our Dubreil–Jacotin–Long-initialized numerical simulations as the result of the difference in background noise. Instability energy decreased under the front shoulder of the ISW, analogous to flow relaminarization under a favourable pressure gradient. At geophysical-scale $Re_{ISW}=900$ , the BBL was initially convectively unstable, and then the instability tracked with the ISW, appearing phenomenologically similar to a global instability. The simulated initial convective instability at both $Re_{ISW}=300$ and $Re_{ISW}=900$ is in agreement with local linear stability analysis which predicts that the instability group speed is always lower than the ISW celerity. Increased free stream perturbations in front of the ISW and larger $Re_{ISW}$ shift the location of vortex shedding (and enhanced bed shear stress) beneath the wave, closer to the ISW trough, thereby potentially changing the location of maximum sediment resuspension, in agreement with field observations at higher $Re_{ISW}$ .

Numerical Investigation of Electrodeless Plasma Thruster with Rotating Electric Field

Two-dimensional simulations of various cross-sections under different magnetic field strengths in an electrodeless plasma thruster acceleration part using rotating electric field were conducted. The Particle-In-Cell and Monte Carlo Collision method was used to better understand plasma movement and find the ideal condition for the azimuthal current formation. The findings suggest that raising the voltage amplitude can improve the electric field’s penetration into the plasma, diminish the disparity between the azimuthal current’s maximum and minimum values, and consequently generate a more uniform size and distribution. Peak azimuthal current values are directly proportional to the ac frequency at frequencies above 100 MHz and inversely proportional to the frequency below 100 MHz. A rotating electric field that matches the background magnetic field could be produced by adjusting the phase difference among the two sets of antennas. And then the plasma is accelerated by the effect of E×B to achieve a more favorable azimuthal current. The results also demonstrate that an appropriately sized and distributed azimuthal current benefits more from a magnetic field with a strength between 200 and 1000 G and a value that declines from the cross-section center to its periphery.

Two-dimensional numerical simulations of mixing under ice keels

Two-dimensional numerical simulations of mixing under ice keels

Modeling two-dimensional ice shape based on fractal interpolation

The ice accretion data obtained from ice wind tunnel tests reveal a multiscale structure and rough surface. In the follow-up aerodynamic evaluation of icing airfoils, simplified two-dimensional ice shapes are generally used as substitutes, but this simplification changes the aerodynamic effects of the original ice shape. Therefore, finding a simple and effective two-dimensional ice shape simulation method is urgent. Due to the self-similarity characteristics of ice shape surfaces, fractal interpolation is proposed for ice shape simulation. First, the geometric characteristics of the ice shapes are analyzed to determine interpolation points, and an iterative function system is constructed for interpolation simulation. Considering the influence of various characteristics of ice shapes on aerodynamics, interpolation parameters are limited to simulating more realistic ice shapes. High-order numerical simulation methods were utilized to numerically simulate and analyze the aerodynamic characteristics of icing airfoil while also verifying the feasibility of fractal interpolation for simulating ice shapes. The analysis revealed that this method could effectively simulate ice profiles of various feature scales with minimal ice shape data. These simulated shapes closely resemble real ice formations and maintain the original aerodynamic characteristics of the icing airfoil. This method can be used to improve the computational accuracy of ice accretion codes and provide improvement strategies for complex ice shape prediction; thus, this method has great application prospects in engineering.

Review paper on applications of the HEC-RAS model for flooding, agriculture, and water quality simulation

ABSTRACT The Hydrologic Engineering Center (HEC) of the United States Army Corps of Engineers (USACE) has developed the HEC-River Analysis System (RAS) hydraulic channel flow model as part of its portfolio of hydrologic and hydraulic modeling tools. HEC-RAS is a new version with capabilities to model flooding, modeling for agriculture production computations for water quality, and many more related to hydraulic modeling. The analysis components of the HEC-RAS system include one-dimensional steady flow water surface profile computations, one-dimensional and or two-dimensional unsteady flow simulation, one-dimensional and two-dimensional computations of quasi-unsteady or fully unsteady flow movable boundary sediment transport, and one-dimensional water quality analysis. The main objective of this paper is to review the use of HEC-RAS on flooding, agriculture, and water quality. The three primary components of the data input are plan data, geometry data, and flow data. Three of these scenarios applied to HEC-RAS and were proven by previous reviewers.

Minimum time search using ant colony optimization for multiple fixed-wing UAVs in dynamic environments

The use of multiple fixed-wing unmanned aerial vehicles in search and rescue missions after natural disasters has become of great interest as they can search large areas and find survivors as quickly as possible. This paper discusses a minimum time cooperative search scheme that utilizes ant colony optimization and new heuristic functions to tackle various constraints in a dynamic environment. The study makes novel use of Dubins curves in the heuristic functions to consider the kinematic limitations of fixed-wing UAVs when planning tangent continuity paths. Furthermore, a novel probabilistic approach is introduced to model the uncertainties induced by dynamic obstacles and determine optimal search paths that are safe and practical in a grid search environment. The performance of the proposed search algorithm is tested through two-dimensional and three-dimensional simulations, statistical analysis, and comparison with other well-known optimization algorithms. To randomize the simulated cooperative search, different search scenarios with static and dynamic obstacles are run several times.

The synergistic design of 5 V ESD protection applications using two holding voltage improving methods

In this paper, a series of Low Voltage Triggering Silicon-Controlled Rectifier (LVTSCR)-based devices were designed and fabricated in a 0.25 μm Bipolar-CMOS-DMOS (BCD) process. Two distinct methods, the integration of additional doping regions and current paths, are investigated to improve the proposed devices’ holding voltage (Vh). Two-dimensional device simulation is employed to elucidate the working mechanism of these ESD protection devices, complemented by the introduction of a transmission line pulse (TLP) measuring system to assess their ESD protection capabilities. Comparative analysis of TLP results reveals that both methods contribute significantly to the augmentation of holding voltage in LVTSCR ESD protection devices. The refined structure, LVTSCR_BN, with two additional current paths, surface and buried, demonstrated a noticeable increment in holding voltage. With its holding voltage of 7.619 V and trigger voltage of 10.61 V, LVTSCR_BN is proved suitable for the ESD protection of circuits operating at the voltage of 5 V.

Optimization of Pin Fins Using Computational Fluid Dynamics and Machine Learning

This paper presents a two-part study focusing on the optimization of pin-fin arrays for gas turbine blade cooling. The first part of the study examines the thermal performance of various pin-fin shapes and sizes using computational fluid dynamics. The study investigates circular, elliptical, hexagonal, and rectangular cross sections, with emphasis on the hydraulic diameter and Reynolds number. Two-dimensional simulations mimicked a confined, staggered array of uniform-sized pins under different inlet conditions. For a hydraulic diameter of 0.053 m and a Reynolds number of 5500, the rectangular pins showed the highest Nusselt number 8.98, 16.01, and 20.62% larger than circular, elliptical, and hexagonal pins, respectively, but also a pressure increase 68.30% higher on average. For a hydraulic diameter of 0.046 m and a Reynolds number of 1000–15,000, it was shown that all shapes have an exponential increase in pressure and a logarithmic decay in the Nusselt number. Standing out was the rectangular shape, which, at a maximum Reynolds number of 15,000, had a pressure increase of 108% larger than the next in line, the hexagonal pin. In the same range of inlet conditions, the Gee–Webb coefficient was the highest for the circular pin, which was selected as the main shape for the following segment. In the second part of the study, a machine learning framework based on a neural network architecture is presented. The neural network predicts the thermal performance of different pin-fin array configurations. At the end, this framework is used in an optimization process that explores the advantages of nonuniform arrays consisting of differently sized circular pins as opposed to the standard uniform structures of the first part. The model proves to be capable of accurately predicting the characteristic thermal performance coefficients across a wide range of input parameters. The nonuniform array managed a 23% reduction in pressure rise at a 0.2% exit temperature increase compared to a reference standard array of the same hydraulic diameter.

Analysis of the Effect of Clogging of Road Grate Inlets Due to Poor Construction and Lack of Strength on Drainage Efficiency and Momentum

As urban flooding damage increases due to the increases in heavy rain owing to climate change, the need for research to prevent urban flooding damage is increasing as well. In this connection, because of poor grate construction, the positions of the upper and lower grate inlet spheres are not fixed, or the entrance is narrowed owing to damage caused by a lack of strength, which impedes drainage efficiency. To investigate this effect, we quantitatively analyzed the drainage efficiency and momentum according to the degree of clogging of the grate inlet using a two-dimensional numerical simulation, finding that the clogging of the grate inlet due to the two flow rate cases was increased by 25% p each. Furthermore, the drainage efficiency and momentum by water depth and flow velocity after clogging were quantitatively analyzed through a two-dimensional numerical simulation. As the clogging increased by 25% p, the drainage efficiency decreased to approximately 25–26% p, while for momentum, all cases with low flow rates were clogged, or the momentum tended to continue to increase along the longitudinal slope. However, at high flow rates, the maximum momentum was obtained when the longitudinal slope was 2%. These results are expected to contribute to the prevention of damage from urban flooding through the efficient management of runoff in urban areas by providing quantitative evidence on the necessity and efficiency of grate inlet management to prevent urban flooding.

Effect of Nb and B on the Precipitation Behaviors in Al-Ti-Nb Balanced-Ratio Ni-Based Superalloy: A Phase-Field Study

In this paper, quantitative two-dimensional (2-D) phase-field simulations were performed to gain insight into the effects of B and Nb for Al-Ti-Nb balanced-ratio GH4742 alloys. The microstructure evolution during the precipitation process was simulated using the MICRESS (MICRostructure Evolution Simulation Software) package developed in the formalism of the multi-phase field model. The coupling to CALPHAD (CALculation of PHAse Diagram) thermodynamic databases was realized via the TQ interface. The morphological evolution, concentration distribution, and thermodynamic properties were extensively analyzed. It is indicated that a higher Nb content contributes to a faster precipitation rate and higher amounts and the smaller precipitate size of the γ′ phase, contributing to better mechanical properties. The segregation of the W element in γ′ precipitate due to its sluggish diffusion effect has also been observed. Higher temperatures and lower B contents accelerate the dissolution of boride and reduce the precipitation of borides. With the increased addition of B, the formation of borides may have a pinning effect on the grain boundary to hinder the kinetic process. In addition, borides are prone to precipitate around the interface rather than in the bulk phase. Once the M3B2 borides nucleate, they grow in the consumption of γ′ phases.

Numerical study of hydrogen ratio effect on detonation initiation characteristics of aviation kerosene with hydrogen addition

Detonation initiation is one of the most important problems in the research of pulse detonation engine (PDE). It is difficult for aviation kerosene to meet the performance requirement of short-distance detonation initiation in PDE. Hydrogen can improve the detonability of fuels. Aviation kerosene with hydrogen addition provides a new method to shorten the detonation initiation distance in PDE. Therefore, hydrogen ratio effect on detonation initiation characteristics of hydrogen-doped aviation kerosene was studied by two-dimensional numerical simulation with the adoption of standard k-ε model, finite rate reaction model and simplified aviation kerosene/hydrogen/air reaction kinetics mechanism. The results indicated that hydrogen-doped aviation kerosene/air mixtures could not be successfully detonated with the minimum hydrogen ratio of 9.73 %, while the detonation was eventually initiated with higher hydrogen ratios, suggesting that the detonability of hydrogen-doped aviation kerosene was improved by increasing hydrogen ratio. The critical hydrogen ratio of 11 % was determined by further research. As the hydrogen ratio increased, the initiation time and distance of the plane detonation wave first increased slowly and then decreased rapidly, while the detonation velocity raised monotonically. The degree of spherical wave decoupling had an influence on the detonation initiation. Specifically, less spherical wave decoupling induced two Mach stems to propagate at the same time, which was conducive to accelerating the initiation of plane detonation.

Simulation study of heat transfer enhancement for all-solid-state room temperature magnetic refrigeration system

To address issues relating to large thermal resistance and low heat transfer efficiency between and inside magnetocaloric materials (MCM) in all-solid-state room-temperature magnetic refrigeration technology, this study adopted the method of inserting high thermal conductivity materials (HTCM) into MCM and coupling Peltier elements between MCMs to enhance the heat transfer inside MCM and between microcells, thereby improving the refrigeration performance of the system. In doing so, a two-dimensional simulation model was developed using COMSOL Multiphysics software. The cooling performance enhancement effect of the system resulting from heat transfer improvement within MCM and between MCMs was investigated separately. Furthermore, the impacts of various system operating parameters (such as the number of micro-cell divisions, initial operating temperature, heat transfer time, magnetic field strength, Peltier input voltage), different HTCMs and insertion volume ratios on the system’s cooling performance were explored. The simulation results reveal that, in comparison with the original magnetic refrigeration system, the optimized heat transfer time of the system following enhanced heat transfer within the MCM is reduced from 180s to 8s, accompanied by a 115% increase in the maximum temperature span. Furthermore, coupling the Peltier elements elevates the maximum temperature span by 274%.

Interactions and pattern formation in a macroscopic magnetocapillary SALR system of mermaid cereal

When particles are deposited at a fluid interface they tend to aggregate by capillary attraction to minimize the overall potential energy of the system. In this work, we embed floating millimetric disks with permanent magnets to introduce a competing repulsion effect and study their pattern formation in equilibrium. The pairwise energy landscape of two disks is described by a short-range attraction and long-range repulsion (SALR) interaction potential, previously documented in a number of microscopic condensed matter systems. Such competing interactions enable a variety of pairwise equilibrium states, including the possibility of a local minimum energy corresponding to a finite disk spacing. Two-dimensional (2D) experiments and simulations in confined geometries demonstrate that as the areal packing fraction is increased, the dilute repulsion-dominated lattice state becomes unstable to the spontaneous formation of localized clusters, which eventually merge into a system-spanning striped pattern. Finally, we demonstrate that the equilibrium pattern can be externally manipulated by the application of a supplemental vertical magnetic force that remotely enhances the effective capillary attraction.

Convection driven by a nonuniform radiative internal heat source in a cavity: Example of medical isotope production in liquid targets

In the present study we use two-dimensional direct numerical simulations (DNS) to understand the coupled heat transfer to fluid flow in a liquid target for the production of nuclear medicines. Fluid motion is driven by buoyancy created by heat generated by a proton beam. The internal heat source has a gaussian distribution in the vertical direction and a rapidly growing intensity in the horizontal direction until it reaches a range at the Bragg peak where the heating drops to zero. The structure of the heating imposes two convective cells, separated at the location of the range. We solve the governing fluid flow and energy equations in a square cavity subject to highly nonuniform internal heating generated by the energy deposition of a proton beam. While most studies of convection driven by an internal heat source in a fluid layer have been focused on a uniform heating of the fluid, our study shows that the nonuniformity in the heat source has important implications for the temperature and flow fields, the boundary heat fluxes, and the growth of convective instabilities in the flow. Interestingly, the scalings of the maximum and averaged temperatures with the Rayleigh number compare similarly to previously found power laws for uniformly heated fluid layers. At higher power levels, the layer of fluid near the top cold boundary becomes convectively unstable via Rayleigh–Taylor instabilities. By comparing the rate of growth of these instabilities to their rate of advection to the boundaries of the cavity, a model is developed that predicts the instability of the convective cells for different values of the range of the beam and the Rayleigh number. Crucially, we demonstrate that the disturbances in the production of isotopes due to convective instabilities and the design of the cooling system is dependent on the location of the Bragg peak and must be considered in design of future generation of this class of target.

Multiphysics modeling femtosecond laser ablation of Ti6Al4V with material transient properties

Femtosecond laser ablation associates with a series of multiphysics phenomena (electron thermalization, electron thermal conduction, electron-phonon coupled energy transport, melting, vaporization and phase explosion) and the instant variation of material transient properties, which impacts the laser energy absorption and the subsequent material removal. In this work, a multiphysics model with material transient properties is developed to study the femtosecond laser ablation of Ti6Al4V. A two-dimensional axisymmetric simulation is performed. The predicted ablation depth and ablation diameter with material transient properties show good agreements with the experimental data. The necessity of material transient properties for femtosecond laser ablation is demonstrated. Besides analyzing the roles of material transient properties on femtosecond laser ablation, the ablation thresholds of Ti6Al4V are probed for different pulse durations. In addition, the impact of pulse duration on femtosecond laser heat affected zone is studied. This work contributes to understand roles of material transient properties in femtosecond laser ablation.

CFD modelling of a chemical looping combustion system − Exploring the potential of natural oxygen carriers

Research advancements in Carbon Capture technology has allowed for Chemical Looping Combustion (CLC) to emerge as a promising technique which can be feasibly implemented into existing power plant infrastructures. In this study, a 120 kWth interconnected Dual Circulating Fluidized Bed (DCFB) reactor has been studied, which uses methane fuel and NiO/Al2O3 particles as oxygen carrier. Two-dimensional transient CFD simulations have been conducted for the DCFB – CLC system using an Eulerian – Eulerian model coupled with heterogeneous reactions and the results have been compared to existing literature. Using the same methodology, two natural ore potential oxygen carriers in Pakistan: Hematite (Fe-based) and Hausmannite (Mn-based) have been evaluated for the same pilot scale apparatus. The fuel conversion and combustion efficiencies have been evaluated for the oxygen carrier particles and have been compared with the original system. The results show that Fe ore provides lesser combustion efficiency as compared to NiO, whereas Mn ore exhibits extremely high reactivity. In retrospect, the use of a mixed (Fe-Mn) oxide oxygen carrier can be considered as a viable option.

CFD Calculation of Natural Convection Heat Transmission in a Three-Dimensional Pool with Hemispherical Lower Head

The integrity of a pressure vessel (PRV) is affected by the decay heat of the molten pool in the lower head, so it is very important to study the natural convection heat transmission in the lower head. Scholars from all over the world have carried out a lot of experimental studies and calculations to determine the convection mechanism and heat transmission characteristics of the molten pool. Most of them were based on the empirical formula of convective heat transmission on the wall of a hemispherical molten pool based on two-dimensional slice experiments and simulation calculations. In this study, FLUENT 2021R1 software was used to simulate the three-dimensional convective heat transmission process of the hemispherical molten pool, and the temperature, velocity and wall heat transmission coefficients of the flow field were analyzed. The results were in good agreement with experimental data from UCLA (University of California, Los Angeles). Through research on the heat transmission of the lower head, the results showed that in the region where the wall angle was θ between approximately 72 and 90 degrees, heat transmission coefficients had larger fluctuations, and a more reasonable empirical relation was proposed. Comparing between the simulation results of CFD and the two-dimensional empirical formula, it was found that the latter one was smaller under the same θ angle condition. Finally, the convection phenomena of different external temperatures were simulated, and the main factors affecting the flow field by temperature were analyzed.

Switching-on Delay Jitter Caused by Lateral Distribution of Current Channel of Avalanche Transistor

The stability of the avalanche transistor’s (AT’s) switching-on process is essential for its extensive application in power semiconductors. The switching-on process was typically described in one-dimensional terms, overlooking the effects of multi-dimensional structural variations on stability. This paper investigated the influence of the lateral distribution of current channels on the switching-on delay jitter in the AT. The lateral size of the current channel affects the transit time by changing the electron path in the base region, resulting in the switching-on delay jitter of the AT. An analytical formula for the lateral size of the current channel and the switching-on delay jitter has been proposed. The two-dimensional simulation model of the AT gave the distribution of current channels. The model’s accuracy was verified by comparing experimental and simulation data. The experimental data proved that the base transit time was the main component of the switching-on delay. The results show that the switching-on delay jitter can be significantly reduced by adjusting the current channel’s lateral size. In addition, the trigger signal’s characteristics also change the current channel’s lateral distribution and then affect the stability of the switching-on delay, which provides a new perspective for the design and application of ATs.

Programmable order by disorder effect and underlying phases through dipolar quantum simulators

In this work, we study two different quantum simulators composed of molecules with dipole-dipole interaction through various theoretical and numerical tools. Our first result provides knowledge of the quantum order by disorder effect of the S=1/2 system, which is programmable in a quantum simulator composed of circular Rydberg atoms in the triangular optical lattice with a controllable diagonal anisotropy. When the numbers of up spins and down spins are equal, a set of subextensive degenerate ground states is present in the classical limit, composed of continuous strings whose configuration enjoys a large degree of freedom. Among all possible configurations, we focus on the stripe (up and down spins aligning straightly) and kinked (up and down spins forming zigzag spin chains) patterns. Adopting the the real space perturbation theory, we estimate the leading order energy correction when the nearest-neighbor spin exchange coupling, J, is considered, and the overall model becomes an effective XXZ model with a spatial anisotropy. Our calculation demonstrates a lifting of the degeneracy, favoring the stripe configuration. When J becomes larger, we adopt the infinite projected entangled-pair state (iPEPS) and numerically check the effect of degeneracy lifting. The iPEPS results show that even when the spin exchange coupling is strong the stripe pattern is still favored. Next, we study the dipolar bosonic model with tilted polar angle which can be realized through a quantum simulator composed of cold atomic gas with dipole-dipole interaction in an optical lattice. By placing the atoms in a triangular lattice and tilting the polar angle, the diagonal anisotropy can also be realized in the bosonic system. With our cluster mean-field theory calculation, we provide various phase diagrams with different tilted angles, showing the abundant underlying phases including the supersolid. Our proposal indicates realizable scenarios through quantum simulators in studying the quantum effect as well as extraordinary phases. We believe that our results indicated here can also become a good benchmark for two-dimensional quantum simulators. Published by the American Physical Society 2024

Modelling two-dimensional driving behaviours at unsignalised intersection using multi-agent imitation learning

Traffic at unsignalised intersection usually involves complex interactions. It is critical to explicitly model the two-dimensional driving behaviours and capture the interactions among vehicles. However, little effort has been made to develop microscopic traffic simulation models at unsignalised intersections, which can generate realistic vehicle trajectories. To fill this gap, this study aims to develop a two-dimensional data-driven simulation model at an unsignalised intersection based on multi-agent imitation learning for generating realistic trajectories of vehicles crossing the intersection and macroscopic traffic characteristics. We propose a multi-agent adversarial inverse reinforcement learning model with four policies (MA-AIRL-4) to separately learn the driving behaviours with different directions (East-bound, West-bound, South-bound, and North–bound) by capturing the interactions between the vehicles. Using the vehicle trajectories data extracted from an unsignalised intersection of the open-source Interaction Dataset, we evaluate the performance of the proposed model by comparing it with several bench-marking models, i.e., MA-AIRL-2 model with two policies (i.e., one policy for West-East and North-South direction, respectively), MA-AIRL-1 model with one policy for all vehicles, three corresponding multi-agent generative adversarial imitation learning (MA-GAIL) models with four policies, two policies and one policy, respectively, and long short-term memory (LSTM) model. The results demonstrate the superiority of the proposed MA-AIRL-4 model in generating accurate vehicle trajectories and reproducing realistic speed distributions. The interpretation of the recovered reward function also indicates the effective learning of the proposed model and provides insights into the learned behaviours.