Two-dimensional Numerical Model Research Articles

Centrifuge model and numerical studies of strip footing on reinforced transparent soils

This paper presents the results of centrifuge model tests to investigate the deformation behaviour of unreinforced and reinforced transparent soil foundations under strip loading. A digital image analysis technique was employed to obtain the soil displacement field and strain distribution of reinforcements. Two-dimensional (2D) numerical models were developed and verified using the test results. The soil was modelled as a linearly-elastic perfectly-plastic material with Mohr–Coulomb failure criterion. The reinforcement was characterised using a linearly-elastic model considering rupture behaviour. Moreover, a parametric study was conducted to investigate the load–settlement response of foundations, distribution of reinforcement tension and failure sequence of reinforcements. The experimental and numerical studies show that the results obtained from the numerical simulations are in good agreement with the results of the centrifuge model tests. The 2D finite difference model developed using the user-defined functions coded into the FLAC programme can better simulate the progressive failure of the reinforcement layers in the tests. The failure sequence of reinforcement layers is not affected by the modulus and internal friction angle of soil and the reinforcement length, but is closely related to the combined effect of spacing and number of reinforcement layers and the combined effect of reinforcement stiffness and strength.

Analysis of the effect of pyrolytic coking on the flow and heat transfer performance of n-decane in cooling channels at supercritical pressure

• Coking deposition of n -decane on the inner surface of the cooling channel was simulated. • This model is capable of making reasonably accurate predictions in pyrolytic coking. • The effects of pyrolytic coking on flow, heat transfer, and pyrolysis performance were investigated. The formation and deposition of coke in cooling channels is the main factor that limits the application of regenerative cooling. To study the effects of coking deposition on regenerative cooling performance under heavy-cracking conditions, we established a two-dimensional axisymmetric numerical model based on the dynamic mesh model. A detailed pyrolytic chemical reaction model, which contains 16 species and 26 step reactions, and a modified coking deposition kinetic model are incorporated in the numerical model. The numerical model is validated by comparing the temperature, mass fraction, and coking thickness between simulation and experimental results. The transient numerical simulation of n -decane flow, heat transfer, pyrolysis, and pyrolytic coking within 20min was carried out. The results reveal that coking deposition has a significant effect on the flow and heat transfer performance of the cooling channel, while having minimal effect on the pyrolytic performance. The conductive heat transfer resistance will be greater than the convection when the coking layer thickness is more than 110 µm. The research methodology adopted in this study can be used to estimate and manage coke deposition in the regenerative cooling systems, and the quantitative results obtained can provide guidance for the design of cooling channels under heavy-cracking conditions.

Study on the spawning habitat suitability of four major Chinese carps in the fluctuating backwater area of the Three Gorges Reservoir

The upstream of the Yangtze River has abundant fish resources and is home to the four major Chinese carps (FMCCs), which are economically important fish in the Yangtze River that reside in the fluctuating backwater area (FBA) of the Three Gorges Reservoir (TGR). Their spawning habitat has been influenced by the Three Gorges Dam (TGD) operations. This study investigated the effects of dam operation on spawning habitat suitability and the potential spawning sites of FMCCs, using a physical habitat suitability model combined with a two-dimensional (2D) hydrodynamic numerical model. The habitat model was verified by field measurements of the fish egg density in the FBA, illustrating the capability of predicting the spawning availability of FMCC by comparing the habitat suitability index (HSI) distribution and the weighted usable area (WUA) with the simulation scenarios before and after the TGD construction. The habitat model was then applied to evaluate the effects of TGD operation on spawning habitat suitability and potential spawning grounds in the FBA. These results implied that the TGD operation had negative effects on spawning habitat suitability and produced temporal changes in the available WUA in the FBA. The predictions revealed that impoundment of the TGD decreased the suitability of the hydrodynamic factors. The main spawning period was delayed from April to May, and the variation in the water level induced by the reservoir operation scheme was likely the main factor. The HSI distribution in the FBA showed that the potential spawning grounds decreased to one site located in the Mudong-Luoqi River section. Furthermore, there was a rapid reduction in WUAs, accounting for approximately 18% of that seen in 2001 (before the TGD operation). The results of this study can aid in the management of the spawning grounds of FMCCs in the TGR and provide efficient methods for improving spawning habitat suitability, such as adjusting the pool level of the TGD during the falling stage.

Coupled effects of nonlinear internal gravity waves and seabed properties on underwater sound propagation

Underwater sound propagation can be affected by strong sound speed gradients induced by nonlinear internal waves in the ocean. Meanwhile, geoacoustic properties of the seabed control acoustic reflections. Experimental data collected at the shelfbreak on the northeast of the South China Sea were analyzed to investigate the joint effects. Both two-dimensional (2D) and three-dimensional (3D) numerical sound propagation models with realistic seafloor and oceanographic inputs have been established to study the nonlinear internal wave effects observed on hydrophone vertical line array moorings. Compared to the 3D model, the prediction errors and uncertainties of the 2D scheme neglecting the transversal/azimuth energy propagation are discussed. In addition, the impact of sediment properties on 3D propagation effects affecting underwater soundscapes in the area is investigated. [Work supported by the Office of Naval Research.]

Numerical Study of a Confined Vesicle in Shear Flow at Finite Temperature

The dynamics and rheology of a vesicle confined in a channel under shear flow are studied at finite temperature. The effect of finite temperature on vesicle motion and system viscosity is investigated. A two-dimensional numerical model, which includes thermal fluctuations and is based on a combination of molecular dynamics and mesoscopic hydrodynamics, is used to perform a detailed analysis in a wide range of the Peclet numbers (the ratio of the shear rate to the rotational diffusion coefficient). The suspension viscosity is found to be a monotonous increasing function of the viscosity contrast (the ratio of the viscosity of the encapsulated fluid to that of the surrounding fluid) both in the tank-treading and the tumbling regime due to the interplay of different temperature-depending mechanisms. Thermal effects induce shape and inclination fluctuations of the vesicle which also experiences Brownian diffusion across the channel increasing the viscosity. These effects reduce when increasing the Peclet number.

Flood inundation modelling by a machine learning classifier

ABSTRACT The present study proposes and evaluates a machine-learning classifier to simulate the flood inundation area in which adaptive neuro fuzzy inference system was applied to classify the simulated domain into flooded and non-flooded areas. Particle swarm optimization was utilized in the training process of the data-driven model. Moreover, the outputs of simulating floods by the two-dimensional numerical hydraulic model were used in the training and testing process. However, aerial images of observed floods could be used as well. Based on the results in the case study, the proposed data-driven classifier is able to reduce the computational complexities of the flood inundation modelling including runtime and CPU usage. The proposed model is highly reliable and robust for generating maximum flood inundation map in the major floods. The results indicated that the rate of incorrect assessment is less than 7% in all tests. It is recommendable to apply the proposed method in the future flood engineering projects in which numerous simulations of the maximum flooded area are required. The developed model considerably reduces the computational costs in the projects.

STABILITY AND RELIABILITY OF LONG-SPAN BRIDGE STRUCTURES

Introduction: Despite the fact that, in recent years, the construction of long-span bridges has been extensively developed, cases of bridge structures buckling in wind still occur, but the issue of their interaction with wind has not been sufficiently studied. Purpose of the study: We aimed to improve the structural integrity and operational safety of long-span bridge structures by conducting a computational and experimental study on the effect of various designs of aerodynamic dampers on the aerodynamic stability of such structures. Methods: The study was performed in two stages. At the first stage, preliminary two-dimensional numerical modeling was conducted to study the effect of various designs of aerodynamic dampers on the wind flow over selected bridge spans. Based on the results of the preliminary two-dimensional numerical modeling, we chose the most effective designs of aerodynamic dampers and made their models to conduct experimental studies on aerodynamic stability on a special test bench. Results: Based on the obtained computational and experimental results, we analyzed the effectiveness of various designs of aerodynamic deflectors and fairings used to improve the aerodynamic stability of the standard bridge structure under consideration. Discussion: For a span with one main girder, we determined the deflector design that reduces the vibration amplitude at high wind velocities.

Computing efficiency of XBeach hydro- and wave dynamics on Graphics Processing Units (GPUs)

Numerical prediction of coastal inundation can be complex due to the multiple physical processes involved and typically requires two-dimensional numerical model extents, particularly in areas with complex along-shore morphology. Such model domains often incur relatively high computational expense. Recent extreme inundation studies for Wellington, New Zealand, were executed using the numerical tool, XBeach. Here, the two-dimensional physical dynamics associated with multiple small embayments and both reef and sandy beach substrates, require large, high spatial resolution numerical model extents, informed by a multi-source elevation surface. XBGPU, is a translation of key XBeach features into code that permits GPU-based acceleration. The present study presents a comparison between XBeach and XBGPU for the same numerical model configuration and extents. Three model resolutions were employed, ranging from a typical desktop CPU based XBeach model resolution, to the highest resolution model that will require High Performance Computing (HPC) scale resources. Two CPU HPC facilities were used and five GPUs to investigate the scalability of both XBeach and XBGPU. The latter ranged from desktop grade units to GPUs associated with professional computing facilities. XBeach scalability is investigated by mean of the speed-up ratio, the time saving ratio and the computational efficiency. These are in reference to the computational speed of a model running on one CPU core. The XBGPU speed-up ratio is presented as a function of the slowest GPU. Direct comparisons between XBeach and XBGPU were achieved by using computational capacity as a metric. The results indicate that even a desktop grade GPU can compete with the computational efficiency of HPC-scale CPU facilities. Small model resolutions presented inefficient scaling on both high-performance CPUs and GPUs while the high-resolution model presented near linear scalability for the high-performance GPUs. Nevertheless, HPCs can be efficient to solve large computational problems if enough CPUs are employed.

Numerical Experiments on Hydrodynamic Performance and the Wake of a Self-Starting Vertical Axis Tidal Turbine Array

In this paper, based on the CFD software ANSYS-Fluent, two-dimensional numerical models are established to investigate the hydrodynamic performance of a self-starting H-Darrius vertical axis tidal turbine (VATT) array of three turbines in a triangular layout with 3D in axial and radial distance. Three main aspects are explored in this study: (1) the self-starting performance, power coefficient, flow fields, and blade force of the double-row VATT array, which are compared with a stand-alone turbine, (2) the wake development of the front and rear displacement turbines, and (3) the feasibility of the double-row self-starting VATT array in practical applications. It is found that the power coefficients of the three turbines in the array all improved compared with that of the stand-alone turbine, and as the load increased, the difference between the averaged power coefficient of the array and a stand-alone turbine was more obvious, with a maximum difference of 3%. The main effects of the front turbines on the rear turbine are energy utilization and turbine vibration. Due to the beam effect between the front turbines, the incident flow rate of the rear turbine increased to approximately 1.2 times the free flow rate. However, the greater rotational fluctuations of the rear turbine mean that although it had a higher power factor, it was more susceptible to fatigue damage. The wake of the rear turbine in the array had a much larger area of influence on both the length and width, but the velocity deficit recovered more quickly to over 95% at a distance of 10D behind it. The rate of wake velocity recovery is load-dependent for a stand-alone self-starting turbine, but this was not evident in the arrays. The positive torque of the turbine is mainly generated when the blade rotates through an azimuth angle from 45° to 160° and mainly benefits from the inner side of the blade. For the double-row three-turbine array, the axial and radial spacing of 3D is reasonable in practical applications.

Efficient X-ray CT-based numerical computations of structural and mass transport properties of nickel foam-based GDLs for PEFCs

Nickel foams are excellent candidate materials for gas diffusion layers (GDLs) for polymer electrolyte fuel cells (PEFCs) and this is due to their superior structural and transport properties. A highly computationally-efficient framework has been developed to not only estimate the key structural and mass transport properties but also to examine the multi-dimensional uniformity and/or the isotropy of these properties. Specifically, multiple two-dimensional X-ray CT images and/or numerical models have been used to computationally determine the porosity, the tortuosity, the pore size distribution, the ligament thickness, the specific surface area, the gas permeability and the effective diffusivity of a typical nickel foam sample. The results show that, compared to the conventionally used carbon substrate, the nickel foam sample demonstrate a high degree of uniformity and isotropy and that it has superior structural and mass transport properties, thus underpinning its candidacy as a GDL material for PEFCs. All the computationally-estimated properties, which were found to be consistent with the corresponding literature data, have been presented and thoroughly discussed.

SPATIAL MATHEMATICAL MODELING OF STATIC COMPOST PILES WITH HEAT RECOVERY

Composting experiments with heat recovery reveal spatial non-uniformity in parameters such as temperature, oxygen concentration and substrate degradation. In order to recover heat from static compost piles via integrated heat exchanger there is the need to investigate the temperature distribution for placing the heat exchangers and the interaction between heat recovery, substrate degradation and oxygen concentration to ensure quality of composting process. This study introduces a spatial model to predict the variation in controlling parameters such as temperature, oxygen concentration, substrate degradation and airflow patterns in static compost piles with heat recovery using Finite element method (FEM) in COMSOL Multiphysics ® Version 5.3. The developed two-dimensional axisymmetric numerical model considers the compaction effects and is validated to real case pilot-scale compost pile experiments with passive aeration. Strong matching with the real case experiment was achieved. The spatial model demonstrated that the compaction effect is extremely important for realistic modeling because it affects airflow, temperature distribution, oxygen consumption and substrate degradation in a compost pile. Heat recovery did not disrupt the composting process. Case studies revealed strong influence of convective heat loss through the edges and a 10 % improvement of heat recovery rate with ground insulation. The simulation indicates that an optimized placing of heat recovery pipes could increase the average heat extraction by 10-40 %.

Structural, electronic, and thermal properties of hydrogenated silicon carbide with thermal heat transfer application in nanoscale metal oxide semiconductor field-effect transistor

In this paper, studies of thestructural electronic and thermal properties of hydrogenated silicon carbide (6H-SiC) have been carried out applying first-principles calculations based on the linearized augmented plane wave method. A two-dimensional numerical model is proposed for 6H-SiC Metal oxide semiconductor field-effect transistor (MOSFET) to investigate the heat transfer on the transistor characteristics using a commercial software application. Structural and electronic results show 6H-SiC lattice parameters (a) and (c) of 3.0817Å and 15.1179Å respectively and an indirect band gap about 2.015 eV. The thermodynamic calculations show that the heat capacity of 6H-SiC is stable at higher temperature. Moreover, the single-phase-lag heat conduction model is used to calculate the temporal temperature distribution in centerline of 6H-SiC MOSFET. The temperature values obtained are 300K-360K at time 7 ps and 12 ps respectively.

3D Numerical Simulation and Structural Optimization for a MEMS Skin Friction Sensor in Hypersonic Flow.

The skin friction of a hypersonic vehicle surface can account for up to 50% of the total resistance, directly affecting the vehicle’s effective range and load. A wind tunnel experiment is an important and effective method to optimize the aerodynamic shape of aircraft, and Micro-Electromechanical System (MEMS) skin friction sensors are considered the promising sensors in hypersonic wind tunnel experiments, owing to their miniature size, high sensitivity, and stability. However, the sensitive structure including structural appearance, a gap with the package shell, and flatness of the sensor will change the measured flow field and cause the accurate measurement of friction resistance. Aiming at the influence of sensor-sensitive structure on wall-flow characteristics and friction measurement accuracy, the two-dimensional and three-dimensional numerical models of the sensor in the hypersonic flow field based on Computational Fluid Dynamics (CFD) are presented respectively in this work. The model of the sensor is verified by using the Blathius solution of two-dimensional laminar flow on a flat plate. The results show that the sensor model is in good agreement with the Blathius solution, and the error is less than 0.4%. Then, the influence rules of the sensitive structure of the sensor on friction measurement accuracy under turbulent flow and laminar flow conditions are systematically analyzed using 3D numerical models of the sensor, respectively. Finally, the sensor-sensitive unit structure’s design criterion is obtained to improve skin friction’s measurement accuracy.

Numerical simulation of the thermal state and selecting the shape of air channels in heat-storage cells of electric-thermal storage

Application of electric-thermal storage (ETS) in combination with energy generating equipment on the basis of renewable energy sources shall be considered as an effective concept, as well. This article deals with a numerical and experimental study of the thermal processes that take place in the heat charging and heat emission operation modes in ETS units. A two-dimensional numerical model for heat exchange processes adopted to ETS operation in the charging and heat emission modes has been developed. The role of the air channels’ geometric parameters of heat-storage cells, air velocity in the channels and temperature difference on the temperature distribution in heat-storage cells and on the ETS operation efficiency during the charging and heat emission modes has been studied. The temperature distribution in heat-storage ETS cells during the charging and emission heat modes of ETS has been obtained, for air velocity in the channels of heat-storage cells in the range of 2 m/s to 3 m/s. Temperature values of air channels wall, that of heated air in the channels and the heat-transfer coefficient in the air channels of the heat-storage cells have been obtained. The energy-efficient design of the heat-storage cell having two round-shaped air channels has been proposed insuring a higher heat transfer in the air channels and reduces the ETS unit electricity consumption by 28%.

Responses of tidal duration asymmetry to morphological changes in Lingding Bay of the Pearl River Estuary

Tidal asymmetry is one of the main factors for generating net transport for waterborne materials in tidal estuaries, and thus, this phenomenon has significant influences on controlling morphological development and the ecological environment. Tidal propagation is sensitive to changes in the coastline and geometry of estuarine regions. Moreover, tidal waveforms vary with various factors, such as coastline changes and bathymetry evolution due to local anthropogenic activities. The topography of Lingding Bay (LDB) of the Pearl River Estuary (PRE) has greatly changed since the 1960s because of human interventions, but the response of tidal duration asymmetry (TDA) to morphological changes is still poorly understood. Utilizing the two-dimensional Delft-3D flexible mesh numerical model, the spatial pattern of TDA and its primary contributors in LDB of the PRE were reproduced for 1964, 1989, and 2016, accounting for the changes in both shoreline and bathymetry owing to human interventions. The results reveal that as the tidal wave propagates upstream, the tidal skewness increases from negative values to positive values longitudinally, indicating the transition from a shorter ebb-duration state to a shorter flood-duration state. Additionally, a prominent shift in TDA and its primary contributors takes place approximately in the period of 1989. In 1964-1989, the tidal skewness increased by at least 0.1 throughout the LDB, indicating that the flood duration of the entire bay was shortened significantly. However, in 1989-2016, the tidal skewness decreased by at most 0.15 throughout the LDB, representing a longer flood duration in the entire LDB. The scenario simulations reveal that reclamation-induced shoreline changes control the increase in TDA and its primary contributors by enhancing width convergence of estuary in the period of 1964-1989. Conversely, the increase in water depth plays a vital role in the decrease of TDA in the period of 1989-2016. The results obtained from this study are particularly useful for understanding the controlled factors contributing to net sediment transport and the associated long-term morphological evolution in estuaries heavily impacted by human interventions.

Successive solidification and constrained remelting of n-octadecance in a rectangular enclosure: Experimental and numerical study

The present research reports experimental and numerical analyses on successive solidification and constrained melting of phase change material (PCM) in a rectangular aluminium enclosure cooled and heated at two vertical and the bottom walls by flowing water. Detailed and quantitative analysis of temperature field, solid front evolution, and shrinkage void formation during successive solidification and remelting of n-octadecane has been studied. After complete solidification, a shrinkage void forms at the top of the cavity. During remelting, liquid PCM formed at the sidewalls flows towards the centre of the enclosure and accumulates in the void. The liquid PCM accumulated in the shrinkage void causes a local temperature rise of 4 °C during the remelting phase, which results in faster melting of PCM from the top. Further, a two-dimensional numerical model is used to predict volumetric shrinkage and expansion during solidification and remelting. The numerically predicted shrinkage void profile is compared with the experiments with the help of real-time images and thermocouple readings and found to be in reasonably good agreement. Moreover, numerical analysis shows oscillations in PCM temperatures at the bottom zone of the enclosure during the remelting process, which indicates the presence of Kelvin–Helmholtz instability formed due to the shear flow between hot liquid close to the bottom wall and cold melting PCM.

Effects of Confining Pressure and Hydrostatic Pressure on the Fracturing of Rock under Cyclic Electrohydraulic Shock Waves

For an array of applications of the high voltage pulse discharge technology in reservoir stimulations and to gain a deeper understanding of the fractures mechanism of deep well rock under cyclic electrohydraulic shock waves (EHSWs), the effect of confining pressure and hydrostatic pressure on the fracturing of rock under EHSWs are investigated in this paper. Firstly, a two-dimensional (2D) water-explosive numerical model is built to match the computed peak pressure of the EHSW with that obtained by the empirical formula by tuning the relevant parameters, based on the equivalent method of EHSWs. Then, a rock model is established to obtain the stress distribution under static loads. Subsequently, the water-explosive model is coupled with the rock model to obtain the stress distribution under static and dynamic loads. In addition, based on this coupling model, the influences of confining pressure and hydrostatic pressure on circumferential stress, radial stress in the rock and the fracturing of rock around the wellbore are discussed. Finally, two improvement measures (increasing discharge energy and changing loading mode) are proposed to acquire greater fracture density based on intensive numerical simulations. The results show that the increase in hydrostatic pressure is beneficial to the crack formation and development, whereas confining pressure is harmful. Moreover, the inhibitory effect of confining pressure on crack formation is greater than the promotion effect of hydrostatic pressure on crack formation. Increasing the discharge energy can effectively promote the development of the number and length of main cracks. Under four repetitive loading modes with the same total discharge energy (1.36 × 15 kJ), the greatest fracture density can be obtained by using repetitive loading mode with a gradually decreasing mode of discharge energy (first level: 2 times (1.36 × 5 kJ); second level: 5 times (1.36 × 1 kJ)).

Two-dimensional MHD equilibria of diamagnetic bubble in gas-dynamic trap

This article presents a magnetohydrodynamic (MHD) two-dimensional numerical model of diamagnetic bubble equilibria in an axisymmetric open trap. The theoretical model consists of the Grad–Shafranov equilibrium equation and the transport equation obtained within the resistive single-fluid MHDs with isotropic pressure. Found are the numerical solutions corresponding to the diamagnetic confinement mode. In particular, the equilibria of the diamagnetic bubble in the gas-dynamic multimirror trap are calculated. We investigate the effect of magnetic field corrugation on the equilibrium; the corrugation of the vacuum field is shown to lead to a rather moderate corrugation of the bubble boundary if the period of corrugation is sufficiently small. A valuable numerical result is the distribution of the diamagnetic field, which would be useful for optimizing the position of the wall-stabilization plates.

Numerical simulation of effectively driving the trajectory of magnetic particles in a Newtonian fluid using a uniform magnetic field

Herein, the interaction and relative motion of two circular magnetic particles in a static flow and planar Poiseuille flow is investigated via numerical simulation. A two-dimensional numerical model is constructed based on Maxwell’s finite element method, fully considering the interactions between particles and particles, particles and magnetic fields, and particles and flow fields. First, the motion state and action mechanism of the magnetic particles in contact state in the static fluid are analyzed under a vertical magnetic field; then, the simulation results are verified via experiments. Based on the motion state of the magnetic particles in the planar Poiseuille flow, the feasibility of effectively controlling the trajectory of magnetic particles in the planar Poiseuille flow using a magnetic field is discussed. In the static flow, the vertical magnetic field was unable to separate the contacting magnetic particles; thus, the magnetic field cannot effectively control magnetic particles in static flows. In the planar Poiseuille flow, the free contact and separation of magnetic particles was effectively controlled by the combined action of the magnetic field and the fluid. This study provides insights into the interactions among magnetic particles in static flows and summarizes a set of methods for effectively controlling two circular magnetic particles.

Advanced geothermal energy storage systems by repurposing existing oil and gas wells: A full-scale experimental and numerical investigation

Advanced Geothermal Energy Storage systems provides an innovative approach that can help supply energy demand at-large scales. They operate by injection of heat collected from various sources into an existing well in low temperature subsurface to create an artificial and sustainable geothermal reservoir to enable electricity generation. Very few studies investigated this approach, but those relied on hypothetical scenarios. Therefore, this study focuses on the field-scale investigation of an Advanced Geothermal Energy Storage in the low-temperature Illinois basin. A systematic preliminary data analysis was conducted to probe thermal energy storage properties of the subsurface. A full-scale field test was performed to delineate the subsurface thermal processes. Results from the field test were used to calibrate a two-dimensional axisymmetric numerical model for a single push/pull well. Various operational schemes were simulated for using the validated numerical model. The results indicated that the energy storage efficiency might reach up to 82% and the extracted fluids could generate an electrical power of 5.74 MW in five years for a simultaneous monthly injection and production scheme with an initial 90 days charging period. The proposed system which benefits from existing infrastructure, offers an economical and environmental approach for thermal storage with high storage efficiency.