Flow Characteristics Research Articles

A computational fluid dynamics-based girth welding model for large-diameter pipeline to reveal vertical weld formation mechanism

Welding quality significantly impacts the safety of large-diameter, long-distance pipelines. The molten pool flow, influenced by gravity, results in variations in the girth weld seam at different circumferential orientations. A three-dimensional transient computational fluid dynamics model was developed to study the multi-layer, multi-pass welding process of X80M pipeline, incorporating arc heat, droplet heat, gravity, and fluid flow in the molten pool. The model also simulates dual-torch automatic welding to examine heat transfer and flow characteristics. Notably, it reveals the morphological evolution of the girth weld seam from the 2 to 3 o'clock positions during continuous welding, an area lacking prior research. The results, validated by experiments, show good agreement between the simulation and thermal cycle curves. The weld pool solidifies from the sides toward the center, creating a concave shape due to downward metal flow, with dual-torch welding intensifying this effect. The second torch increases the concavity of filler layers, with depths rising by 21.81% and 32.20%. These findings offer new insights into pipeline girth welding.

Investigation of the flow characteristics and NOx formation mechanism of hydrogen micromix diffusion combustion with vortex generator

It is well established that hydrogen has become one of the most promising fuels for achieving zero-carbon emissions. However, hydrogen combustion still faces many challenges, particularly the flashback issue and high NOx emissions, which necessitate the exploration of hydrogen combustion technologies and pollution control measures. This paper proposes a micromix diffusion combustion scheme with a disturbance vortex generator (DVG). The flow characteristics of the micromix element were analyzed, including the velocity distribution, penetration depth, jet vortex length, and vortex structure. It was found that DVG would create an air streamwise vortex within the main airflow, thus accelerating the axial velocity decay and facilitating the penetration of hydrogen. The hydrogen jet vortex rolls up the mainstream air, forming a coupled vortex system, which further enhances mixing. Sensitivity analysis was carried out to study the influences of the key parameters of the micromix element. The hydrogen injection hole diameter and the DVG height were found to be the most influential factors. The interactions between vortex, flame, and NOx were investigated to obtain the mechanism of NOx formation. By conducting a comparative analysis of the effects of the hydrogen orifice diameter and the vortex generator height, it has been observed that augmenting the length of the jet vortex can significantly promote the mixing of hydrogen with the mainstream air, playing an essential role in reducing emissions. This study establishes a solid foundation for future research endeavors in the development of micromix diffusion combustors.

Effects of obstacle structures on the movement and deposition of debris flows in storage basins

Storage basins are among the main engineering measures used in debris flow mitigation engineering, but many storage basins lack engineering measures for decreasing the velocity and dissipating the energy of debris flows. Therefore, it is necessary to construct obstacles in storage basins to decrease the velocity and dissipate the energy of debris flows. However, the influence of obstacles on the movement and deposition characteristics of debris flows is not clear. Experiments were performed to investigate the effects of obstacles on the movement and deposition of debris flows. Obstacles caused debris flows to spread laterally, and obstacles with square cross sections were particularly conducive to the lateral spreading of debris flows. Compared with obstacles with round and square cross sections, diversion of debris flows by obstacles with triangular cross sections was relatively minor. Additionally, obstacles reduced the mud marks height of debris flows. The maximum mud marks height of debris flows decreased with decreasing spacing between obstacles, and the maximum height can be reduced by a large amount. The effect was greater for obstacles with square cross sections than other shapes. Thus, obstacles with square cross sections should be preferred for use in structures in storage basins for debris flows.

Open-jet testing: Investigating turbulence and geometric scale effects on surface pressures in the atmospheric boundary layer

Bluff body aerodynamics is essential for the design and safety of structures exposed to wind forces. Traditional atmospheric boundary layer wind testing often fails to replicate the complex turbulence characteristics of real-world flows, necessitating innovative testing methodologies. We developed an open-jet testing approach and conducted experiments on scaled models (1:7.6 and 1:10) at Reynolds numbers ranging from 0.5 × 106 to 1 × 106, significantly higher than those typically achieved in conventional testing. This methodology produced integral length scales approximately ten times larger than those observed in traditional methods, resulting in 25%–300% higher peak pressures than those from small-scale tests, closely aligning with full-scale field data. Our findings emphasize the necessity of testing under complete atmospheric boundary layer turbulence to ensure accurate wind pressure predictions. Insights into the effects of advanced flow on separation, reattachment, and pressure distribution inform new experimental protocols and have significant implications for the design and safety of structures in wind-prone regions. By establishing a robust benchmark for future experimental and computational simulations in wind engineering, this approach promotes the development of safer, more resilient, and economically viable building designs capable of withstanding extreme wind events exacerbated by climate change, contributing to sustainable infrastructure advancement.

The blockage and erosion characteristics of woody debris flow on an erodible gully bed: Insight from a small-scale model experiment

During the transportation process of debris flow with large wood (LW), phenomena such as channel blockage and collapse frequently occurs, resulting in increased discharge surges, heightened erosion intensity, and amplified damage. Accurately predicted the blockage performance is the basis of evaluating the damage and disaster mitigation of woody debris flow. In this study, we conducted a series of laboratory experiments of woody debris flow on erodible gully bed. The experiment results show that the final blockage types can be divided into three types: non-blockage, blockage, and semi-blockage. Temporary blockage will cause abundant sediment deposited temporarily and then released instantaneously, resulting in destructive surges and eventually lead to semi-blockage and non-blockage. The blockage degree is positively correlated with the relative length, relative content of LW, and bulk density of debris flow, but negatively correlated with slope. Channel blockage is often accompanied by significant local erosion effect, and the erosion depth of downstream channel increases with the increase in blockage degree. The blockage and collapse mechanism of woody debris flow was analyzed, and the results emphasized that channel erosion promoted the outbreak of blockage collapse. Based on the analysis of blockage performance, we propose an improved blockage criterion F to evaluate the blockage degree, and the high probability range of temporary blockage is determined as 1.5–5.0. The results can provide reference for the risk assessment and mitigation of woody debris flow.

Temporal asynchrony analysis for dynamic operation of hydraulic-thermal-electricity multiple energy networks based on holomorphic embedding method

Analyzing the operational states of multiple energy networks (MEN) in multi-energy systems is crucial for ensuring system stability. The dynamic operational characteristics of different energy flows pose challenges for computational analysis. Traditional steady-state methods are inadequate for addressing the dynamics of MEN, especially when dealing with temporal discrepancies between hydraulic and thermal flows in thermal networks (TN) and the heterogeneity between TN and electrical networks. Therefore, this paper proposes a novel holomorphic embedding method (HEM) based on multi-stage decomposition method. The developed HEM constructs a time coefficient matrix and utilize inner-outer loop recursion to handle the time lag between thermal flow and hydraulic flow in the TN. Additionally, we reconstruct a holomorphic matrix, integrating hydraulic flow to bridge thermal and electric power flows, thereby improving the operational heterogeneity among different networks. Real-case simulations show that when the Taylor expansion order in HEM is equal to 4, the proposed method achieves a mere 1% discrepancy from actual operational data, enhancing computational efficiency by 60% compared to the Newton-Raphson method. Moreover, in this real-case, the TN exhibits a maximum delay response time of 180s compared to electrical networks. Exploiting this delay time effectively increases renewable energy generation within multi-energy systems by 961.58kW per day.

Improve neural representations with general exponential activation function for high-speed flows

Characterizing flow fields with neural networks has witnessed a considerable surge in recent years. However, the efficacy of these techniques is typically constrained when applied to high-speed compressible flows, due to the susceptibility of nonphysical oscillations near shock waves. In this work, we focus on a crucial fundamental component of neural networks, the activation functions, to improve the physics-informed neural representations of high-speed compressible flows. We present a novel activation function, namely, the generalized exponential activation function, which has been specifically designed based on the intrinsic characteristics of high-speed compressible flows. Subsequently, the performance of the proposed method is subjected to a comprehensive analysis, encompassing training stability, initialization strategy, and the influence of ancillary components. Finally, a series of representative experiments were conducted to validate the efficacy of the proposed method, including the contact-discontinuity problem, the Sod shock-tube problem, and the converging–diverging nozzle flow problem.

Dynamic reaction of slag-based geopolymer with brick powder: insights from acoustic emission, heat flow, and relaxation characteristics

Fractal dimension, heat, and NMR relaxation characterizations were employed to monitor the dynamic geopolymerization process of slag-based geopolymer (SBG) with brick powder (BP) in this research. Introducing 30% BP to SBG resulted in a delay of 6minutes in the occurrence of polymerization peak, with a 24.12% decrease in heat peak value. Furthermore, during the rapid polymerization phase, the time of the gel formation in SBG containing 30% BP was reduced by 34.18%, accompanied by a 20.91% decrease in gel water content. The sequence of AE parameters associated with early-age geopolymerization reaction has good fractal characteristics, with fitting coefficient greater than 0.95. Importantly, the heat flow curve, water evolution, and corresponding fractal dimension curves have good correspondence at 0-3h. Especially, the dissolution peak and polymerization peak of the heat flow curve basically coincide with the fractal dimension curve. The fractal dimension not only serves as an effective indicator for evaluating the geopolymerization process but also compensates for the limitations of calorimetry in characterizing reaction processes with relatively small particle migration and local heat release.

The influence of pin fin structure on the flow and heat transfer characteristics of the manifold microchannel heat sink

The influence of pin fin structure on the flow and heat transfer characteristics of the manifold microchannel heat sink

Flow structure and characteristics of fluid undergoing a sudden contraction to an annular gap under dynamic boundary

Pipeline transport serves as an effective means to alleviate traffic congestion and reduce carbon emissions from transportation. The hydraulic delivery system, which employs pipeline cars as carriers, addresses the limitations of existing systems. However, its transportation efficiency is affected by variations in the flow structure within the pipelines. During the acceleration of the pipeline car, the sudden contraction flow field from circular to annular gap formed in the vicinity of the end face under dynamic boundary conditions. This study utilized particle image velocimetry (PIV) to visualize and measure the sudden contraction flow field. Based on the obtained experimental results, it investigated the impact of dynamic boundary velocity on the flow structure, velocity characteristics, and energy dissipation of the annular gap. The acceleration process of the dynamic boundary is the conversion of flow energy into the kinetic energy of the annular gap flow and the kinetic energy of the pipeline car. This process is accompanied by phenomena of velocity slip and velocity overshoot. As the velocity of the pipeline car increases, the recirculating vortex within the annular gap dissipates and eventually disappears. The velocity slip gradually decreases, the location of the overshoot point shifts radially, and the magnitude of the overshoot diminishes before ultimately vanishing. From static to steady, the probability density distribution of the slipstream face transitions from a distribution with high skewness and low peak value to a normal distribution with high peak value and low skewness. The irreversible losses that arise in a sudden contraction flow field can be quantified by the increase in entropy. Due to the similarity of the solving processes of large Eddy simulation and PIV, a combined sub-grid stress model is used to solve the flow losses in the flow field. The turbulent dissipation occurs mainly in the recirculation region, shear layer, and high-speed shear regions near the wall.

Magnetohydrodynamics (MHD) flow of ternary nanofluid and heat transfer past a permeable cylinder with velocity slip

The increasing demand for energy-efficient and environmentally sustainable solutions has driven the exploration of ternary nanofluids for enhanced heat transfer applications. This numerical study investigates the magnetohydrodynamics (MHD) flow and heat transfer characteristics of a ternary hybrid nanofluid (copper-alumina-titania/water) over a shrinking/stretching cylinder, considering the effects of velocity slip, suction, and curvature parameters. The variable wall temperature is also included in the physical model. The model in PDEs is transformed into a simplified version of differential equations (ODEs) which can be solved using the bvp4c solver. Validation against previously published data confirms the accuracy of the model. Two solutions are possible if the shrinking surface is permeable (with suction effect) while only unique solution for the stretching case. Surprisingly, the copper-alumina-titania/water exhibits a lower heat transfer rate but higher skin friction as compared to the copper-alumina/water. Specifically, the heat transfer coefficient decreases by 5-10% when incorporating the titania nanoparticle, while an increase of 8-12% for the skin friction coefficient. Additionally, the velocity slip and curvature parameters accelerate the boundary layer separation, whereas increased suction delays this process. These findings provide insights into optimizing thermal management systems using ternary hybrid nanofluids, particularly under MHD effects.

Investigation of flow interference and heat transfer characteristics of two staggered circular cylinders in cross flow

The flow and heat transfer characteristics around staggered cylinders are critical for optimizing thermal performance in engineering applications like heat exchangers. This study investigates the flow and heat transfer properties around two staggered cylinders at Re = 100 using a double-distributed lattice Boltzmann method. Eight typical flow patterns are recognized based on vortex and temperature fields, phase diagram, and St. The effects of gap flow, shear layer, vortex shedding, and wake evolution behaviors on heat transfer characteristic are discussed. Results demonstrate that when the wake is a single vortex street, the shear layer reattachment and the weaker gap flow both lead to a high temperature in the gap between the two cylinders, which subsequently adversely affects the heat transfer performance. Inversely, the vortex impingement and the stronger gap flow enhance the heat transfer performance. The deflected gap flow leads to a narrow and wide street in the wake, and the cylinder located on the opposite side of the gap flow deflection has better heat transfer performance. Furthermore, two coupled vortex streets in the wake can further improve the heat transfer performance of the cylinders. These findings provide valuable insights for enhancing thermal management strategies in engineering applications.

Numerical analysis of thermal and hydraulic characteristics of flow boiling in oblique interconnected microchannel heat sinks

The rising demand for efficient cooling solutions in high-performance electronics underscores the importance of microchannel heat sinks (MCHS) with interconnected channels for thermal management. Despite advancements, limited research has explored the influence of oblique interconnectors on flow distribution and their impact on optimizing thermal and hydraulic performance. This study addressed this gap by numerically investigating the hydrothermal behavior of interconnected MCHS with oblique interconnectors, using HFE-7100 as the working fluid. Six geometric configurations were compared to a baseline parallel microchannel design to determine the optimal interconnector size and orientation. The study employed the Lee mass transfer model and the Volume of Fluid (VOF) approach to capture phase-change dynamics and flow behavior under varying mass fluxes (280.4–1121.6 kg/m2s) and heat fluxes (40–60 W/cm2). The results revealed two distinct flow regimes: suction-induced flows that enhanced thermal performance and disruptive flows that impaired it. One particular design (Case 1) demonstrated superior performance with a ∼26 % higher heat transfer coefficient, ∼23 % lower thermal resistance, and a 12.45 K reduction in wall superheat relative to the baseline, though at the expense of a ∼14 % increase in pressure drop. These findings highlighted the critical role of interconnector orientation in balancing thermal and hydraulic performance, offering a design guideline for future innovations in microchannel heat sinks.

Flow characteristics induced by a multiform windbreak in complex terrains with and without a train: A simplified method for calculating aerodynamic loads

This study aims to investigate common multiform windbreaks, aligned parallel to railway tracks and perpendicular to incoming wind, in complex terrains. Using unsteady simulations, the study analyzes airflow downstream of these windbreaks and the aerodynamic characteristics during train passage. It evaluates the wind-protection performance of various windbreak types and transitions and identifies factors that influence performance. Results indicate that the vertical surface walls offer stronger wind protection compared to slope walls or viaduct barriers. Flow patterns near transitions reveal that upstream airflow shifts longitudinally from high-performance windbreaks to lower-performance ones, reentering the railway line space from the latter. This suggests a design approach in which neighboring windbreaks exhibit similar performance to optimize protection. On aerodynamic characteristics of the train, the maximum side force on the leading vehicle is found proportional to wind speed and train speed to the powers of 1.6 and 0.5, respectively; train speed affects the pressure on the streamlined head and the vortices around the leeward side. A simplified calculation for aerodynamic loads on a vehicle is proposed and explored with a consideration of wind speed above the railway line. An error margin of the maximum side force by this simplified method is 8.4%, and the saving is at least 88.2% of the computational resources when assessing the crosswind stability of a vehicle. The proposed design for the multiform windbreak, along with the simplified calculation method, can improve the performance of a multiform windbreak and increase the efficiency of assessing crosswind safety for railway operations downstream of the windbreak.

Computational Study on Flow Characteristics of Shocked Light Backward-Triangular Bubbles in Polyatomic Gas

This study computationally examined the Richtmyer–Meshkov instability (RMI) evolution in a helium backward-triangular bubble immersed in monatomic argon, diatomic nitrogen, and polyatomic methane under planar shock wave interactions. Using high-fidelity numerical simulations based on the compressible Navier–Fourier equations based on the Boltzmann–Curtiss kinetic framework and simulated via a modal discontinuous Galerkin scheme, we analyze the complex interplay of shock-bubble dynamics. Key findings reveal distinct thermal non-equilibrium effects, vorticity generation, enstrophy evolution, kinetic energy dissipation, and interface deformation across gases. Methane, with its molecular complexity and higher viscosity, exhibits the highest levels of vorticity production, enstrophy, and kinetic energy, leading to pronounced Kelvin–Helmholtz instabilities and enhanced mixing. Conversely, argon, due to its simpler atomic structure, shows weaker deformation and mixing. Thermal non-equilibrium effects, quantified by the Rayleigh–Onsager dissipation function, are most significant in methane, indicating delayed energy relaxation and intense turbulence. This study highlights the pivotal role of molecular properties, specific heat ratio, and bulk viscosity in shaping RMI dynamics in polyatomic gases, offering insights on uses such as high-speed aerodynamics, inertial confinement fusion, and supersonic mixing.

Numerical Study of Supersonic Cavity Flows with Different Turbulent Models

A numerical study of supersonic cavity flows is conducted with different turbulent models, including RANS, LES, DES, DDES, and IDDES. Firstly, a supersonic cavity-ramp flow with Ma∞ = 2.92 is simulated numerically. It shows that the results of DDES and IDDES are nearly equivalent. The wall skin-friction coefficients obtained by these two models are in better agreement with the experimental measurement than those of DES. The reason is that the RANS region of DDES and IDDES is larger than that of DES, so it produces more turbulence in the near-wall region. In comparison, RANS cannot accurately predict large separation flows. The shear layer’s reattachment point on the ramp predicted by RANS is biased downstream compared to the experimental measurement, making its overall prediction accuracy worse than other models. The results obtained by LES are comparable to those obtained by DDES and IDDES except for the wall skin-friction coefficient, which is much smaller because the grid resolution is not high enough to accurately resolve the near-wall turbulent flow. Secondly, the effect of different aft-wall angles (θ) on the flow characteristics of the supersonic cavity is investigated numerically with IDDES. We find that as θ decreases, the oscillation intensity of the cavity flow continuously decreases. However, the change in cavity drag with θ is non-monotonic, which means that there might be a critical θ at which the drag penalties of a cavity are minimal. Therefore, an optimal design should be achieved when changing θ to control the oscillation intensity of supersonic cavity flows.

CFD analysis of turbulent flow characteristics of charge inside the combustion chamber of a dual–fuel homogeneous charge compression ignition engine under varying swirl ratios

This paper discusses the effects of the swirl ratio on the distribution of turbulent kinetic energy (TKE), combustion and emissions characteristics of a dual-fuel HCCI (Homogeneous charge compression ignition) engine fed with n-dodecane (premixed) and ethanol (injection). The performance of the Internal combustion (IC) engine is investigated using STAR-CD. ECFM-3Z (Extended Coherent Flame Combustion Model-3Zones) model is considered for the computational fluid dynamics (CFD) simulations. This study also includes the influence of swirl ratio on pressure and temperature of the combustion and hence the NOx (Nitrogen oxides), soot and CO2 (Carbon dioxide) emissions. The pressure, temperature and emissions of the combustion have been dropped significantly by increasing the swirl ratio. There is an increase in the distribution of air-fuel and hence the corresponding combustion flame by increasing the swirl ratio. It is observed that the reduced emissions of the combustion were achieved in the case of swirl ratio 3 and the values of the NOx emission, soot and CO2 emission are 0.0024 g/kg fuel, 0.0022 g/kg fuel and 5.12 g/kg fuel respectively. Eventually, the paper deals with the significance of the swirl ratio on NOx and CO2 emissions with limited loss of the piston work.

Synergistic influence of gyrotactic microorganisms and bimolecular reaction on bidirectional tangent hyperbolic fluid with Nield boundary conditions: A biomathematical model

In biomedical engineering, the behaviour of gyrotactic microorganisms with non-Newtonian fluids such as tangent hyperbolic fluids improve the design of targeted drug delivery systems. In this system control over microorganism movement is essential. The present study deals with the synergistic influence of gyrotactic microorganisms and bimolecular reactions on the bidirectional flow of tangent hyperbolic fluids under Nield boundary conditions. Further, the flow characteristic of the non-Newtonian fluid is enhanced by incorporating the impact of thermal radiation, heat sources, Brownian motion, and thermophoresis. The presentation of these phenomena is vital for an extensive range of applications, including industrial processes, biomedical engineering, and environmental management. The analysis employs advanced mathematical modelling which needs suitable transformation rules to get the non-dimensional form and further numerical simulation is presented with the assistance of the “shooting-based fourth-order Runge-Kutta technique”. The results are depicted for the several contributing factors via the built-in in-house function bvp4c in “MATLAB”. The authentication of the study with the prior research is a benchmark to precede further research in this direction. However, the outstanding results are; the fluid velocity is controlled by increasing non-Newtonian Weissenberg number whereas the velocity slip shows dual characteristics on the axial velocity distribution. Further, the motile microorganism profile is controlled by the enhanced bioconvection Lewis number.

Transient flow characteristics of single and twin circular impinging jets using particle image velocimetry and proper orthogonal decomposition

This study experimentally investigated the transient flow characteristics of single and twin parallel circular impinging jets using Particle Image Velocimetry (PIV) and Proper Orthogonal Decomposition (POD). High spatiotemporal resolution data were collected to analyze and visualize the distribution and interactions of multi-scale vortex structures. The experimental data were evaluated for PIV uncertainty and statistical convergence. The flow field distribution of the fountain that may occur in the twin impinging jets was also discussed. The results revealed that, for the oblique single jet, high-energy large-scale structures primarily concentrate in downhill regions and gradually decompose into smaller-scale structures. In the combination of twin jets, vortices in the shear layers merge in an alternating pattern. The frequencies of the inner and outer shear layers are sensitive to jet spacing with different characteristic interaction patterns repeatedly observed based on varying spacings. Three distinct cases are defined based on the interaction of the twin jets with the impingement plate. Additionally, three distinct flow field structures of fountain are defined to facilitate, indicating the degree of combination as well. The velocity profiles of the fountain are similar and adhere to a Gaussian distribution. Furthermore, a semi-empirical equation was developed to describe the centerline velocity of the fountain.

Transient behavior of thermocapillary convection in thin liquid film exposed to step laser heating

Temporally developing thermocapillary convection induced by step laser heating of a thin liquid film has been studied numerically. Computations are performed using the commercial software STAR CCM+ version 2022.1. The liquid film of silicone oil (high Prandtl number fluid) is 60 mm in diameter and 3 mm in thickness. Flow characteristics related to surface velocity and surface temperature have been studied. Validation of the computations is achieved for the surface velocity, the velocity along thickness and the surface temperature through comparison with PIV and IR camera measurements. The laser-beam with a carbon dioxide gas laser (10.4 μm in wavelength) is used for heating. It is found that the temporally developing profile of surface velocity shows two local velocity peaks (uS1 and uS2) at two radial locations (rS1 and rS2) respectively. The first peak, uS1, appearing due to the steep temperature gradient generated by laser-beam heating and its radial position, rS1, do not change noticeably with time. On the other hand, the second peak, uS2, travels radially outwards with decreasing magnitude in a self-propelling manner until its radial position, rS2, approaches an asymptotic maximum. Detailed analysis of the coupling among radial temperature gradient, local pressure variation and local convective acceleration near the second peak reveals that hydrothermal mechanisms are responsible for self-propelling travel of uS2. The transient behaviors of both primary and secondary velocity peaks are found to depend on the fluid viscosity and the laser-beam settings