Two-dimensional Numerical Simulations Research Articles

Enhancement of aerodynamic performance of Savonius wind turbine with airfoil-shaped blade for the urban application

The present study introduces a novel rotor configuration with an airfoil-shaped blade, based on the low-speed high-lift FX74-CL5-140 airfoil, for enhancing the performance of the Savonius wind turbine for a practical application in urban areas. The preferment of the present design over the original one with a semicircular shape is confirmed by the sequence of unsteady two-dimensional (2D) numerical simulation. The results demonstrate a new power coefficient peak (up to 16.5 % better than that of the original rotor) could be gained at the tip speed ratio (TSR) of 1.1. The effective working range is further extended to TSR > 0.7 compared to that at TSR > 1.0 in the literature. Insight into the flow dynamic, including pressure, velocity, streamline, and wake analysis, reveals that the thick blade near the overlap region, benefiting in terms of the turbine’s high performance. The airfoil blade increases the momentum energy around the rotor, changes the pressure on the concave side, and varies the origin of the pressure force, leading to an increase in the moment arm with more torque exhibited on the rotor at high TSR. In addition, the proper orthogonal decomposition (POD) analysis is also performed to better understand the flow phenomena behind the rotor. Overall, the present design is beneficial for urban applications because it can enhance the rotor performance without losing the omnidirectional and compactness features.

Kelvin-Helmholtz-induced mixing in multi-fluid partially ionized plasmas.

Turbulence is a fundamental process that drives mixing and energy redistribution across a wide range of astrophysical systems. For warm ([Formula: see text]) plasma, the material is partially ionized, consisting of both ionized and neutral species. The interactions between ionized and neutral species are thought to play a key role in heating (or cooling) of partially ionized plasmas. Here, mixing is studied in a two-fluid partially ionized plasma undergoing the shear-driven Kelvin-Helmholtz instability to evaluate the thermal processes within the mixing layer. Two-dimensional numerical simulations are performed using the open-source (PIP) code that solves for a two-fluid plasma consisting of a charge-neutral plasma and multiple excited states of neutral hydrogen. Both collisional and radiative ionization and recombination are included. In the mixing layer, a complex array of ionization and recombination processes occur as the cooler layer joins the hotter layer, and vice versa. In localized areas of the mixing layer, the temperature exceeds the initial temperatures of either layer with heating dominated by collisional recombinations over turbulent dissipation. The mixing layer is in approximate ionization-recombination equilibrium, however the obtained equilibrium is different to the Saha-Boltzmann local thermal equilibrium. The dynamic mixing processes may be important in determining the ionization states, and with that intensities of spectral lines, of observed mixing layers. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'.

Numerical investigation of vortex-induced vibrations (VIV) of a rotating cylinder in in-line and cross-flow directions subjected to oscillatory flow

This study investigates vortex-induced vibration (VIV) generated by a rotating circular cylinder across various rotation rates (α = 0, 0.25, 0.50, 0.75, and 1) subjected to oscillatory flow through two-dimensional numerical simulations. Numerical simulations are performed for two Keulegan-Carpenter (KC) numbers, 5 and 10, at Reynolds number (Re) of 150. A wide range of Reduced Velocity (Vr) is considered, ranging from 1 to 20. The vibration amplitude in both inline and cross-flow directions is significantly affected by the rotation of the cylinder, as well as the KC number and Vr. Notably, the impact of rotation is more pronounced on cross-flow direction vibrations than on in-line direction vibrations. For a non-rotating cylinder (α = 0), the cross-flow direction vibration for both KC numbers nearly ceases after surpassing the critical value of reduced velocity: Vr = 6 for KC = 5 and Vr = 11 for KC = 10. In rotating cylinders (α ≠ 0), vibration amplitude becomes zero only at the critical reduced velocity, beyond which it increases and eventually stabilizes. It is also observed that rotation rates significantly influence VIV behavior, notably widening the lock-in range of Vr for rotating cylinders compared to non-rotating ones. The vortex shedding is significantly affected by the rotation and becomes asymmetric.

A Comparative Study on Shear Size Effect of Steel-And BFRP-RC Shear Walls Under Cyclic Lateral Loads

ABSTRACT This paper presents a comparative study on the size effect behaviors of shear performances of concrete shear walls reinforced by steel bars and Basalt Fiber Reinforced Polymer (BFRP) bars. All the works are conducted through a two-dimensional meso-scale numerical simulation model representing the concrete shear wall. The horizontal reinforcement ratio and the sectional size are the main parameters concerned by the present study. Both the seismic behaviors under cyclic loading and the size effect behaviors of the shear performances were simulated and investigated. It can be concluded from the simulation results, (1) the use of steel bars or BFRP bars as horizontal reinforcement does not change the shear failure modes of concrete shear walls, (2) the size effect on the nominal shear strength of BFRP reinforced concrete (BFRP-RC) shear wall is more significant compared to that for steel-RC shear wall, (3) the increase of horizontal reinforcement ratio improves the shear capacity of the concrete shear wall, while weakens the size effect on the nominal shear strength, and (4) the size effect law for the nominal shear strength of steel-RC shear wall is applicable to BFRP-RC shear wall. Finally, the importance of considering size effect in the evaluation of shear performance of steel-/BFRP-RC shear wall is manifested by comparing the predictive results of several popular design codes with the simulation results.

Evaluation of charging process performance of phase change material enhanced with a partially filled metal matrix: A numerical study

Phase change materials (PCMs) may be employed in thermal energy storage to bridge the gap between supply and demand for energy. The main obstacle to PCMs' broad adoption is their limited heat conductivity, leading to their rates of energy recovery and storage being sluggish. This work focuses on tackling this difficulty by adopting a honeycomb structure to improve the performance of a phase-change material using a two-dimensional numerical simulation of melting time. The thermal behavior of a two-dimensional rectangular thermal storage container with phase change material and a honeycomb structure HCS partly filled in the enclosure was the focus of the studies. The current work compares the thermal behavior during melting in a rectangular cavity filled with (Pure PCM alone, Bottom, and Top HCS configurations) that are exposed to a continuous heat source, both with and without natural convection. The solidification/melting module of the commercial CFD software ‘ANSYS Fluent’, which is based on the enthalpy-porosity model, permits modeling based on a transient calculation, making it feasible to examine the charging process of eicosane. For all three configurations, the evolution of the liquid fraction is represented. The storage unit's thermal performance is improved by the use of partially filled honeycomb structures (HCS), according to numerical findings. The optimal reduction in total melting time is attained through the implementation of a top-HCS arrangement, yielding a significant 11% enhancement when natural convection is neglected, while the inclusion of a honeycomb matrix enables only a 3% improvement for the bottom-HCS configuration. Moreover, outcomes reveal that natural convection minimizes melting time differences between the three studied configurations and expedites the process, notably benefiting the reference configuration compared to the two other scenarios. Furthermore, the outcomes demonstrate that natural convection reduces discrepancies in melting times among the three studied configurations and accelerates the charging process. The influence of natural convection is notably more significant for the reference case.

Progressive collapse analysis of the corner strut subsystem in a propped excavation with inadequate system safety performance

Frequent collapse accidents in propped excavations stem from inadequate attention to the system safety performance of the retaining structures. This paper examines a propped excavation accident case, where the corner section of the retaining system, with two levels of struts failed, contrasting with the intact section with only one level of strut. The failure causes and the performances of different excavation designs were analyzed and compared by two-dimensional (2D) and three-dimensional (3D) numerical simulations. The actual retaining system design incorporated two levels of struts in the corner section to protect the building outside, presenting a disparity in strut elevation between the corner sections and those with only one level of strut. Despite the 2D analysis deeming the design safe, the 3D analysis reveals a high risk of structural failure due to significant bending moments in the capping beam between different sections. The absence of axial support from the fractured capping beam resulted in the overturning of the corner strut subsystem of the retaining structures into the excavation. This study reveals the progressive collapse mechanism of the corner strut subsystem and highlights the necessity of 3D analysis for excavation design to ensure the system safety performance of the retaining structures. Ensuring the integrity of retaining structures, particularly the connecting structural components between different sections, and preventing local structural failures to enhance system safety performance are critical in complex excavation designs.

Hydraulic conductivity tensor of anisotropic soils: the impact on seepage flow

Natural or artificial changes in soil stratigraphic-plane (i.e., the deposition of soil and sediments into distinct layers) orientation can cause variations in hydraulic conductivity in different direction. Hydraulic conductivity must, therefore, be considered – in this case – as a nondiagonal, full tensor to appropriately represent the effect of the orientation of soil stratigraphic planes on the seepage flow pattern. This paper introduces the derivation of the formula for the three-dimensional nondiagonal hydraulic conductivity full tensor calculation in terms of azimuth and vertical angles. Furthermore, two-dimensional numerical simulations of the seepage flow beneath a concrete dam are presented to demonstrate the need to account for the nondiagonal elements of the hydraulic conductivity tensor in anisotropic soils of varying degrees of stratigraphic tilt with respect to the coordinate system.

Generative-Adversarial-Network-Based Image Reconstruction for the Capacitively Coupled Electrical Impedance Tomography of Stroke.

This study investigated the potential of machine-learning-based stroke image reconstruction in capacitively coupled electrical impedance tomography. The quality of brain images reconstructed using the adversarial neural network (cGAN) was examined. The big data required for supervised network training were generated using a two-dimensional numerical simulation. The phantom of an axial cross-section of the head without and with impact lesions was an average of a three-centimeter-thick layer corresponding to the height of the sensing electrodes. Stroke was modeled using regions with characteristic electrical parameters for tissues with reduced perfusion. The head phantom included skin, skull bone, white matter, gray matter, and cerebrospinal fluid. The coupling capacitance was taken into account in the 16-electrode capacitive sensor model. A dedicated ECTsim toolkit for Matlab was used to solve the forward problem and simulate measurements. A conditional generative adversarial network (cGAN) was trained using a numerically generated dataset containing samples corresponding to healthy patients and patients affected by either hemorrhagic or ischemic stroke. The validation showed that the quality of images obtained using supervised learning and cGAN was promising. It is possible to visually distinguish when the image corresponds to the patient affected by stroke, and changes caused by hemorrhagic stroke are the most visible. The continuation of work towards image reconstruction for measurements of physical phantoms is justified.

Influence of slit asymmetry on blow-off and flashback in methane/hydrogen laminar premixed burners

One way to decarbonize the heat production sector is to gradually replace natural gas with hydrogen. The design of laminar premixed burners capable of operating with both methane and hydrogen is however challenging as these fuels have drastically different burning properties, which narrows the operating range. While methane flames face limitations in stabilization due to blow-off at high power, hydrogen flames tend to be susceptible to flashback at low power. This study investigates the effects of slits symmetry breaking on the blow-off of methane flames and flashback of hydrogen flames through two-dimensional direct numerical simulations of canonical asymmetrical slit configurations, revealing uneven interactions between the main openings and smaller auxiliary slits. The equations governing the reactive flow dynamics are coupled to a heat transfer solver in the solid phase to elucidate the thermal and hydrodynamic mechanisms determining the operability limits. Viscous dissipation in the small auxiliary slits together with a substantial preheating of the fresh gases by heat redistribution through the solid phase is found to govern flame stabilization in asymmetrical geometries, improving blow-off resistance of methane-air flames. Then, flashback of hydrogen-air flame is found to be driven by the competition between (i) the mass flow rate distribution between the main and auxiliary slits, (ii) preheating of the gas through the auxiliary slits and (iii) the ability of the main and auxiliary slits to quench the flame. The interplay of these phenomena gives rise to complex behaviors, wherein asymmetrical configurations could exhibit significantly enhanced resistance to flashback compared to symmetrical geometries. This conclusion, verified for different burner thicknesses and slit spacing, may be used to guide the design of fuel-flexible laminar burners.Novelty and significance statement1. Analyzing the effect of slits symmetry breaking on blow-off of methane flames and flashback of hydrogen flames for multi-perforated laminar burners.2. Defining a strategy for wall heat flux redistribution to assess the asymmetrical interaction between the main and auxiliary flames through the flame-holder.3. Demonstrating and explaining the positive impact of symmetry breaking on the blow-off resistance of methane flames.4. Highlighting the substantial and non-monotonous impact of symmetry breaking on hydrogen flame flashback.5. Identifying the interplay of thermal and hydrodynamic mechanisms in asymmetrical slits leading to improvement of flashback resistance for hydrogen-air flames compared to symmetrical configurations.

Fundamental Experiment and Numerical Simulation of Ne/Xe Plasma Magnetohydrodynamic Electrical Power Generation

We conducted experiments in a shock-tube facility to test magnetohydrodynamic (MHD) electrical power generation with Xe-seeded Ne plasma. The performance and plasma behavior of the experimental generator were examined using time-dependent r-θ two-dimensional numerical simulations. In the experiment, an enthalpy extraction ratio of about 5.0% was obtained using a disk-shaped MHD generator with radio frequency pre-ionization. The numerical simulations show that the enthalpy extraction ratio is improved by adding small amounts of Xe to Ne, but also that adding excessive amounts of Xe deteriorates the generator performance. When an appropriate inlet electron temperature is assumed in the simulations, 5900–6450 K (inlet ionization degree of 0.25×10−4–0.72×10−4) for a seed fraction (mole % of Xe) of 0.1%, 6050–6280 K (1.08×10−4–1.70×10−4) for a seed fraction of 1.0%, and 5750–5950 K (1.27×10−4–1.97×10−4) for a seed fraction of 5.0%, the plasma structure almost becomes similar to the nonuniform plasma structure observed in the experiment, as too does the enthalpy extraction ratio. The simulations suggest that achieving an inlet ionization degree above 5–6×10−4 when using Ne/Xe with a seed fraction of 0.1–1.0% as the working gas should enable an enthalpy extraction ratio above 20%, which is expected to surpass that for pure Ar.

Viscoplastic elliptical objects impacting a solid surface

This theoretical and numerical study focuses on the physical mechanism driving the spreading of viscoplastic elliptical millimetric/centimetric objects after they impact a solid surface under no-slip conditions. The two-dimensional impacting objects are described as Bingham fluids. The two-dimensional numerical simulations are based on a variational multi-scale approach devoted to multiphase non-Newtonian fluid flows. The obtained results are analyzed considering the spreading dynamics, energy budgets and scaling laws. They show that, under negligible capillary effects, the impacting kinetic energy of the elliptical objects is dissipated through viscoplastic effects during the spreading process, giving rise to three flow regimes: inertio-viscous, inertio-plastic, and mixed inertio-visco-plastic. These regimes are strongly affected by the initial aspect ratio of the impacting objects, which reveals the possibility of using morphology to control spreading. Finally, the results are summarized in a diagram linking the object's maximum spreading and spreading time with different spreading regimes through a single dimensionless parameter called impact number.

A numerical study of natural convection through a vertical heated channel with a confined circular cylinder

This study examines thermal flow structures and heat transfer through a vertical heated channel with an adiabatic circular cylinder symmetrically positioned between lateral walls. A two-dimensional numerical simulation is conducted covering a range of parameters including Rayleigh numbers Ra = 8.9 × 106–8.9 × 108, cylinder positions relative to the channel height h = 0–0.50, and blockage ratios (the ratio of cylinder diameter to channel width) β = 0.25–0.75. Three distinct flow regimes are observed at different Rayleigh numbers and blockage ratios, including steady symmetric, unsteady periodic, and unsteady asymmetric flow regimes. The steady symmetric and unsteady periodic flows are observed at all blockage ratios, while the unsteady asymmetric flow is only observed at the highest blockage ratio of β = 0.75 for Rayleigh numbers above 8.9 × 107. It is found that the presence of the cylinder significantly enhances mixing and turbulence in the channel, which in turn enhances heat transfer through the channel. A 64.3% enhancement of heat transfer is achieved at β = 0.50 and h = 0.05 for Ra = 8.9 × 108.

An analytical subthreshold I–V model of SiC MOSFETs

In this article, an analytical I–V model for calculating subthreshold current of SiC MOSFETs is presented. This model starts with planar MOSFETs and utilizes the one-dimensional Poisson’s equation to derive an analytical expression for the surface potential. Subsequently, it employs this expression as a foundation for subthreshold current calculations. Then the model is extended to DMOSFETs based on the fact that channel current formation mechanism of DMOSFETs is similar to that of planar MOSFETs. Comparative analysis of our model calculations with two-dimensional numerical simulation software reveals that our model exhibits a high degree of agreement in the case of planar MOSFETs and DMOSFETs. Interface traps were also considered in our analytical model, which agrees well with experimental data published elsewhere. Furthermore, the model retains a certain degree of accuracy and predictive capability even in the presence of short-channel effect. Our subthreshold model becomes complementary to existing models which only describe the I–V characteristics of SiC MOSFETs when the transistors operate above the threshold voltages.

Multistability of elasto-inertial two-dimensional channel flow

Elasto-inertial turbulence (EIT) is a recently discovered two-dimensional chaotic flow state observed in dilute polymer solutions. Two possibilities are currently hypothesized to be linked to the dynamical origins of EIT: (i) viscoelastic Tollmien–Schlichting waves and (ii) a centre-mode instability. The nonlinear evolution of the centre mode leads to a travelling wave with an ‘arrowhead’ structure in the polymer conformation, a structure also observed instantaneously in simulations of EIT. In this work we conduct a suite of two-dimensional direct numerical simulations spanning a wide range of polymeric flow parameters to examine the possible dynamical connection between the arrowhead and EIT. Our calculations reveal (up to) four coexistent attractors: the laminar state and a steady arrowhead regime (SAR), along with EIT and a ‘chaotic arrowhead regime’ (CAR). The SAR is stable for all parameters considered here, while the final pair of (chaotic) flow states are visually very similar and can be distinguished only by the presence of a weak polymer arrowhead structure in the CAR regime. Analysis of energy transfers between the flow and the polymer indicates that both chaotic regimes are maintained by an identical near-wall mechanism and that the weak arrowhead does not play a role. Our results suggest that the arrowhead is a benign flow structure that is disconnected from the self-sustaining mechanics of EIT.

Numerical investigations of AC arcs’ thermal characteristics in the short gap of copper-cored wires

Excessive alternating current (AC) arcs generated in electric systems will accumulate heat and easily cause fire. This paper studies the thermal characteristics of different numbers of AC arc plasma generated in a short gap of copper-cored wires in the air. The number of AC arcs is controlled in the AC arc experiment and an infrared thermal imager measures the temperature change at the specified position. Based on magnetohydrodynamics (MHD), a two-dimensional axisymmetric AC arc discharge numerical simulation model is established. The volt-ampere characteristic of the AC arc is used to solve the MHD simulation model to obtain the same 'zero current' characteristics as the real AC arc in the experiment. A large amount of heat accumulates in the electrode gaps when the arc generation, and then the heat dissipates in the 'zero current' stage. The continuously generated arc makes the temperature higher. The volume of the space area with a temperature higher than 10,000 K increases with the arc current, but is unrelated to the number of arcs. The volume of the space area with a temperature higher than 524.15 K and the temperature on the electrode are both positively correlated with the number of AC arcs and arc current. The results of this study can provide a reference for the detection standard of AC arc faults and the prevention of electrical fire.

Unsteady Numerical Simulation of Two-Dimensional Airflow over a Square Cross-Section at High Reynolds Numbers as a Reduced Model of Wind Actions on Buildings

Airflow over a square cross-section at high Reynolds numbers and different angles of incidence is investigated with the aim of providing deeper insight into wind actions on elongated structures and, in particular, tall buildings. The flow around bluff bodies is characterized by separation at sharp corners, as well as possible flow reattachment at side surfaces. The alternate shedding of vortices is also generated in the wake of bluff bodies due to the unsteady nature of flow separation. Two-dimensional (2D) URANS numerical simulations were conducted in order to model transient flow and examine wind actions on a square used as a model of a typical cross-section of a tall building far from its roof and the ground. For validation purposes, the study’s numerical results on drag and lift coefficients, Strouhal numbers, as well as pressure coefficient distribution were found to be in good agreement with available experimental and numerical results in the literature for relatively low Reynolds numbers. The numerical study was then extended to higher Reynolds numbers, approaching values that are pertinent for wind flow around buildings, thus addressing the lack of such results in the literature. On the basis of these results, the impact of Reynolds numbers and angles of incidence on drag and lift coefficients, as well as the pressure coefficient distribution along the walls of the cross-section, is highlighted.

On the maximum velocity profile of wave boundary layer flows in the very rough turbulent regime

The maximum velocity profile in turbulent wave boundary layer flows has been experimentally investigated under both regular and irregular wave conditions. Four types of seabed models are adopted, i.e., smooth, sand-covered, uniform-sphere-covered and nonuniform-stone covered. The results show that the maximum overshoot increases with the decreasing A/ks (A is the semi-excursion of fluid particles in the free stream and ks is the bottom roughness), but it is not notably influenced by the irregularity of the waves nor the nonuniformity of bottom roughness elements. Two-dimensional numerical simulations are carried out to reveal the physics behind the large overshoot behavior. It is found that the gap flow accelerates around roughness elements and subsequently contributes to the overshoot. A three-parameter defect function is proposed that well describes the maximum velocity profile, and all three model parameters are correlated with the governing flow parameter A/ks.

High-fidelity numerical simulations in Eulerian/Lagrangian framework for liquid fuel jets in crossflow with atomization and evaporation: Effect of aerodynamic Weber number

The effect of the aerodynamic Weber number Weg on the atomization-evaporation process of liquid fuel jets in crossflow is investigated by detailed numerical simulations accounting for evaporation from both the gas–liquid interface and dispersed particles in the Eulerian/Lagrangian framework. The accuracy of the evaporation model on the gas–liquid interface treated in the Eulerian manner is verified by applying two-dimensional (2D) numerical simulations to the evaporation of a single droplet and comparing the computational results with the theoretical solution and psychrometric data. The results show that the present detailed numerical simulations reasonably reproduce the primary and secondary atomization characteristics of the liquid jets in crossflow under different breakup modes. Additionally, considering both evaporations from the Eulerian gas–liquid interface and Lagrangian dispersed particles is essential for precisely predicting the fuel mixing with the oxidizer and the ignition. As Weg increases, the Sauter mean diameter (SMD) of generated fuel droplets decreases in the whole region. Furthermore, as Weg increases, the tendency that the fuel vapor mass fraction is high in the upstream region and decreases downstream becomes more evident. However, the temperature behind the liquid column do not decrease despite the increasing heat loss due to evaporation. These behaviors can be well explained in terms of the differences in the atomization, evaporation, and heat transfer between the liquid jet and hot air crossflow against Weg.

An Improved Adaptive Iterative Extended Kalman Filter Based on Variational Bayesian

The presence of unknown heavy-tailed noise can lead to inaccuracies in measurements and processes, resulting in instability in nonlinear systems. Various estimation methods for heavy-tailed noise exist. However, these methods often trade estimation accuracy for algorithm complexity and parameter sensitivity. To tackle this challenge, we introduced an improved variational Bayesian (VB)-based adaptive iterative extended Kalman filter. In this VB framework, the inverse Wishart distributionis used as the prior for the state prediction covariance matrix. The system state and noise parameter posterior distributions are then iteratively updated for adaptive estimation. Furthermore, we make adaptive adjustments to the IEKF filter parameters to enhance sensitivity and filtering accuracy, thus ensuring robust prediction estimation. A two-dimensional target tracking and nonlinear numerical UNGM simulation validated our algorithm. Compared to existing algorithms RKF-ML and GA-VB, our method showed significant improvements in RMSEpos and RMSEvel, with increases of 21.81% and 22.11% respectively, and a 49.04% faster convergence speed. These results highlight the method’s reliability and adaptability.

Numerical analysis of the flow over four side-by-side square cylinders with different gaps

This study conducts two-dimensional numerical simulations of the flow over four square cylinders arranged side by side at a low Reynolds number (Re) of 100. The investigation primarily centers on the influence of the gap to a square cylinder width ratio (g*) on the flow. The range of g* spans from 0.1 to 7.0. Within this parameter range, three distinct flow regimes emerge based on the inherent flow characteristics. These regimes are defined as follows: (1) single bluff body flow (g* ≤ 0.3), (2) flip-flopping flow (0.3 < g* < 2.0), and (3) modulated periodic flow (g* ≥ 2.0). Additionally, the modulated periodic flow is further categorized into three distinct flow patterns. Various aspects of these different flow regimes are examined, including vortex contours, velocity fields, and liquid force coefficients around the cylinders. Moreover, detailed illustrations are provided for the modulation behaviors in vortex structures and liquid force coefficients. Finally, the proper orthogonal decomposition technique is employed to identify and analyze the underlying spatial coherent structures in the flow field, offering further insights into the dynamic features of wakes.