Search Results (144)

This study experimentally investigates the dynamic behavior and energy conversion characteristics of a single contaminated bubble (d_eq = 2.49–3.54 mm, Re_b = 470–830) rising near a vertical wall (S* = 1.41–2.02) in a linear shear flow (the conditions of average flow rate 0.1 m/s and shear rate 0.5 s⁻¹) using a vertical water tunnel and varying sodium dodecyl sulfate (SDS) concentrations (0–50 ppm) and bubble sizes (via needle nozzles). High-speed imaging with orthogonal shadowgraphy captures bubble trajectories, rotation, deformation, and oscillation modes (2, 0) and (2, 2), revealing that an increasing SDS concentration suppresses deformation and the inclination amplitude while enhancing the oscillation frequency, particularly for smaller bubbles. Velocity analysis shows that vertical components remain steady, whereas wall-normal and spanwise fluctuations diminish with surfactant concentration, indicating stabilized trajectories. Additional mass force coefficients are larger for bigger bubbles and decrease with contamination level. Energy analysis demonstrates that surface energy dominates the total energy budget, with vertical kinetic energy comprising over 70% of the total kinetic energy under high SDS concentrations. The results highlight strong scale dependence and Marangoni effects in controlling near-wall bubble motion and energy transfer, providing insights for optimizing gas–liquid two-phase flow processes in chemical and environmental engineering applications. Full article

Scouring is an erosional process driven by the water motion over a sediment bed. Scour can lead to structural safety risks of built structures and to riverbanks’ instabilities and collapse. In particular, scouring in river bends is a known phenomenon caused by secondary flow currents. This scouring can result in negative impacts on the economic and social activities that occur on the riverbanks. On the other hand, the erosion and scouring processes of riverbeds are often addressed by means of heavy civil engineering construction works. Aiming at looking for different solutions for the scour in river bends, this research investigates the use of an air bubble screen system to minimize the scouring in river bends by providing detailed measurements of sedimentation patterns and velocity fields in a mild 90-degree bend where an air screen bubble was installed. The air bubble screen is generated by injecting compressed air through a perforated pipe placed on the bed along the outer bend. Different parameters were tested, including the water flow rate in the channel, the air flow rate, the angle of attack between the air bubble screen and the secondary flow, and flow direction. The air bubble screen opposes the direction of the bend’s induced secondary flows, altering the velocity pattern such that the maximum velocity at cross-sections of 45°, 65°, 80°, and 90° were displaced from the outer wall as much as 53%, 68%, 89%, and 84% of the width, respectively. The air bubble screen system also reduced the secondary flow power in the maximum scour zone by 35%. Hence, the maximum scour depth was reduced by 59% to 79.8% for the maximum flow rate by increasing the air bubbles’ angle of attack relative to the primary flow from 0° to 90°. Finally, the limitations of this study and its applicability to real cases is discussed. Full article

The interfacial Dzyaloshinskii–Moriya interaction (DMI) plays a pivotal role in stabilising and controlling the motion of chiral spin textures, such as Néel-type bubble domains, in ultrathin magnetic films—an essential feature for next-generation spintronic devices. In this work, we investigate domain wall (DW) dynamics in magnetron-sputtered Ta(3 nm)/Pt(3 nm)/Co(1 nm)/RuO₂(1 nm) [Ru(1 nm)]/Pt(3 nm) multilayers, benchmarking their behaviour against control stacks. Vibrating sample magnetometry (VSM) was employed to determine saturation magnetisation and perpendicular magnetic anisotropy (PMA), while polar magneto-optical Kerr effect (P-MOKE) measurements provided coercivity data. Kerr microscopy visualised the expansion of bubble-shaped domains under combined perpendicular and in-plane magnetic fields, enabling the extraction of effective DMI fields. Brillouin light scattering (BLS) spectroscopy quantified the asymmetric propagation of spin waves, and micromagnetic simulations corroborated the experimental findings. The Pt/Co/RuO₂ system exhibits a Dzyaloshinskii–Moriya interaction (DMI) constant of ≈1.08 mJ/m², slightly higher than the Pt/Co/Ru system (≈1.03 mJ/m²) and much higher than the Pt/Co control (≈0.23 mJ/m²). Correspondingly, domain walls in the RuO₂-capped films show pronounced velocity asymmetry under in-plane fields, whereas the symmetric Pt/Co/Pt shows negligible asymmetry. Despite lower depinning fields in the Ru-capped sample, its domain walls move faster than those in the RuO₂-capped sample, indicating reduced pinning. Our results demonstrate that integrating RuO₂ significantly alters interfacial spin–orbit interactions. Full article

The steady plane radial diverging flow of a viscous or inviscid particle-fluid suspension is studied using a novel two-fluid model. For the initial flow field with a uniform particle distribution, our results show that the relative velocity of particles with respect to the fluid depends on their inlet velocity ratio at the entrance, the mass density ratio and the Stokes number of particles, and the particles heavier (or lighter) than the fluid will move faster (or slower) than the fluid when their inlet velocities are equal (then Stokes drag vanishes at the entrance). The relative motion of particles with respect to the fluid leads to particle migration and the non-uniform distribution of particles. An explicit expression is obtained for the steady particle distribution eventually attained due to particle migration. Our results demonstrated and confirmed that, for both light particles (gas bubbles) and heavy particles, depending on the particle-to-fluid mass density ratio, the volume fraction of particles attains its maximum or minimum value near the entrance of the radial flow and after then monotonically decreases or increases with the radial coordinate and converges to an asymptotic value determined by the particle-to-fluid inlet velocity ratio. Explicit solutions given here could help quantify the steady particle distribution in the decelerating radial flow of a particle-fluid suspension. Full article

To improve environmental sustainability and operational safety in maritime industries, the development of efficient methods for removing biofouling from submerged surfaces is critical. This study investigates the erosion mechanisms of cavitation jets as a non-contact, high-efficiency method for detaching marine organisms, including bacteria and larvae, from ship hulls and underwater infrastructure. Through erosion experiments on coated specimens, variations in jet morphology, and flow visualization using the Schlieren method, we examined how factors such as jet incident angle and nozzle configuration influence removal performance. The results reveal that erosion occurs not only at the direct jet impact zone but also in regions where cavitation bubbles exhibit intense motion, driven by pressure fluctuations and shock waves. Notably, single-hole jets with longer potential cores produced more concentrated erosion, while multi-jet interference enhanced bubble activity. These findings underscore the importance of understanding bubble distribution dynamics in the flow field and provide insight into optimizing cavitation jet configurations to expand the effective cleaning area while minimizing material damage. This study contributes to advancing biofouling removal technologies that promote safer and more sustainable maritime operations. Full article

This study investigates the aerodynamic performance of a symmetric NACA 0018 airfoil under harmonic pitching motions at low Reynolds numbers, a regime characterized by the presence of laminar separation bubbles and their impact on aerodynamic forces. The analysis encompasses oscillation frequencies of 1 Hz, 2 Hz, and 13.3 Hz, with amplitudes of 4° and 8°, along with steady-state simulations conducted for angles of attack up to 20° to validate the numerical model. The results reveal that the γ-Re_θ turbulence model provides improved predictions of aerodynamic forces at higher Reynolds numbers but struggles at lower Reynolds numbers, where laminar flow effects dominate. The inclusion of the 13.3 Hz frequency, relevant to Darrieus vertical-axis wind turbines, demonstrates the effectiveness of the model in capturing dynamic hysteresis loops and reduced oscillations, in contrast to the k-ω SST model. Comparisons with XFOIL further highlight the challenges in accurately modeling laminar-to-turbulent transitions and dynamic flow phenomena. These findings offer valuable insights into the aerodynamic behavior of thick airfoils under low Reynolds number conditions and contribute to the advancement of turbulence modeling, particularly in applications involving vertical-axis wind turbines. Full article

Aerodynamic characteristics of a pitching NACA 0012 airfoil, including the load performance and flow field features, are studied using numerical simulations in this paper. Large Eddy Simulations (LESs) have been performed, and the chord-based Reynolds number is set to $6.6\times {10}^4$. Pitching frequency varies from 3 to 20 Hz, corresponding to a reduced frequency of 0.094–0.628 ($k=\pi f_{p}c/U_{\infty }$, where $f_p$ is the pitching frequency, c is the chord length, and $U_{\infty }$ refers to the incident flow speed). As the pitching frequency increases, the maximum lift coefficient achieved in one pitching cycle decreases, and the direction of the lift hysteresis loop changes as the pitching frequency exceeds a certain value, leading to a change in the lift of the sign at the zero-incidence moment, which is a result of the instantaneous flow patterns on the airfoil surface. As the pitching frequency increases, flow unsteadiness develops less in one pitching cycle, and the time duration in which the turbulence boundary layer can be detected in one pitching cycle shrinks. Additionally, for the pitching airfoil, combinations of the flow patterns on the upper and lower sides, such as laminar separation and the turbulent boundary layer, or laminar separation and the laminar separation bubble, were observed on the airfoil surface, and these were not detected on a static airfoil at the corresponding Reynolds number. This is considered an effect of the pitching motion that is in addition to the phase-lag effect. Full article

The identification of leakage sources in underwater natural gas pipelines (UNGPs) remains a critical challenge due to complex environmental conditions. In this study, we propose a novel simulation–optimization method, integrating numerical bubble plume dynamics models with surrogate models to enable accurate leakage parameter inversion. First, a bubble plume underwater motion simulation model was developed based on the actual conditions of the study area to predict the future spatial and temporal variation characteristics of the bubble plumes in certain wave fields. Then, the simulation–optimization method was applied to determine the leakage velocity and offset distance of the underwater gas pipeline leakage source via inversion. To reduce the computational load of the optimization model by repeatedly invoking the simulation model, the Kriging method and a backpropagation (BP) neural network were used to build surrogate models for the numerical model. Finally, the optimized surrogate model was solved using the simulated annealing method, and the inverse identification results were obtained. The experimental results show that both methods can achieve a high inversion accuracy. The relative error of the Kriging model is no more than 12%, and the running time is 13 min. Meanwhile, based on the BP neural network surrogate model, the relative error of the BP neural network model is about 14%, and the running time is 2.5 min. Full article

This study presents a novel unresolved SPH-DEM coupling framework to investigate the complex interactions between rising gas bubbles and sinking solid particles in multiphase systems. Traditional numerical methods often struggle with large deformations, multiphase interfaces, and computational efficiency when simulating dense particle-laden flows. To address these challenges, the proposed model leverages SPH’s Lagrangian nature to resolve fluid motion and bubble dynamics, while the DEM captures particle–particle and particle–bubble interactions. An unresolved coupling strategy is introduced to bridge the scales between fluid and particle phases, enabling efficient simulations of large-scale systems with discrete bubbles/particles. The model is validated against benchmark cases, including single bubbles rising and single particle’s sedimentation. Simulation studies reveal the effects of particle/bubble number and initial distance on phase interaction patterns and clustering behaviors. Results further illustrate the model’s capability to capture complex phenomena such as particle entrainment by bubble wakes and hindered settling in dense suspensions. The framework offers a robust and efficient tool for optimizing industrial processes like mineral flotation, where bubble–particle dynamics play a critical role. Full article

The shape of a bubble changes near an elastic boundary, and this alteration also influences the boundary itself. This study investigates bubble shape and boundary displacement near an elastic cylindrical boundary through an electric spark bubble experiment. Three parameters—dimensionless distance, elastic cylinder tension, and dimensionless size—are discussed and analyzed in relation to bubble shape. For studying elastic cylinder boundary displacement, a displacement formula is proposed by establishing a motion model, and impulse is used for verification. Furthermore, the elastic cylinder tension employed in this study has negligible impact on boundary displacement. Understanding how bubble shape changes near an elastic boundary, along with the corresponding boundary displacement, provides valuable insights into the stability and durability of materials and structures under similar conditions. The elasticity of the cylinder and its displacement response to external forces can help predict long-term behavior, contributing to the reliability assessment of engineering systems involving elastic boundaries and fluid dynamics. Full article

Bubbles in pipes are widely present in marine engineering, transmission, and fluid systems with complex environments. This paper divides tubes into short, longer, and long tubes due to different lengths. In short tubes, the formation, development, and stability of spark bubbles are deeply analyzed through numerical simulation and experimental measurement, and the morphology and period of vortex rings generated in the surrounding fluid are studied. The results show that bubbles in tubes are significantly elongated compared with those in free fields. Changing the parameters of tubes can affect the size and oscillation speed of vortex rings. Secondary cavitation is found in asymmetric positions in longer tubes. The conditions, positions, and periods of multiple secondary cavitations are summarized in a series of experiments on long tubes. It is found that bubbles in tubes are related to the ${\gamma }_t$ and ${\gamma }_L$ tube parameters. More secondary cavitation is easily generated in thinner and longer tubes. In addition, the pumping effect brought about by the movement of bubbles in tubes is studied. By designing reasonable tube parameters, the life cycle of bubbles can be changed, and the pumping efficiency can be improved. This study provides important theoretical support for the reliability of the movement of bubbles and surrounding fluid in tubes and lays a foundation for the optimization and promotion of this technology in practical applications. Full article

The numerical solution of flow and temperature fields in and around a hot metal component being immersed into a cooling fluid offers powerful insights into investigating industrial quenching processes. The calculation requires a simultaneous solution of the Navier Stokes and the according energy equation. Difficulties arise at the boundaries where high heat transfer rates are forced from the solid surface to the fluid due to high metal temperatures. Heat transfer rates are determined based on the similarity theory, but reliable heat transfer equations valid for the high temperature typical of quenching processes are rare. This paper presents a two-fluid VOF (volume-of-fluid method) approach, giving an insight into the transient heat transfer and its oscillations. Unlike our previous publications, this paper uses the lumped heat conduction model to obtain the heat transfer coefficient in the film boiling heat transfer mode. Its application leads to an estimation of an average heat transfer coefficient. Furthermore, the unsteady distribution of the heat transfer coefficient values, shown in our previous paper, is now supplemented with the corresponding flow behavior obtained using the numerical simulation. In our approach, the vapor bubble formation during the film boiling phase is tracked directly (DNS of interface motion, not turbulence), and the unsteady heat transfer coefficient distribution is obeyed. Full article

Microreactors have the advantages of high heat and mass transfer efficiency, strict control of reaction parameters, easy amplification, and good safety performance, and have been widely used in various fields such as chip manufacturing, fine chemicals, and biomanufacturing. However, narrow microchannels in microreactors often become filled with catalyst particles, leading to blockages. To address this challenge, this study proposes a multiphase flow heat transfer model based on the lattice Boltzmann method (LBM) to investigate the dynamic changes during the bubble collapse process and temperature distribution regularities. Based on the developed three-phase flow dynamics model, this study delves into the shock dynamic evolution process of bubble collapse and analyzes the temperature distribution regularities. Then, the flow patterns under different particle density conditions are explored. The study found that under the action of shock wave, the stable structure of the liquid film of the bubble is destroyed, and the bubble deforms and collapses. At the moment of bubble collapse, energy is rapidly transferred from the potential energy of the bubble to the kinetic energy of the flow field. Subsequently, the kinetic energy is converted into pressure waves. This results in the rapid generation of extremely high pressure in the flow field, creating high-velocity jets and intense turbulent vortices, which can enhance the mass transfer effects of the multiphase flows. At the moment of bubble collapse, a certain high temperature phenomenon will be formed at the collapse, and the high temperature phenomenon in this region is relatively chaotic and random. The pressure waves generated during bubble collapse have a significant impact on the motion trajectories of particles, while the influence on high-density particles is relatively small. The results offer a theoretical basis for understanding mass transfer mechanisms and particle flow patterns in three-phase flow. Moreover, these findings have significant practical implications for advancing technologies in industrial applications, including chip manufacturing and chemical process transport. Full article

The inward vortex–swirl-type motion of convective, rectilinear water flow has been studied vis-à-vis its propensity for bubble formation, with a particular focus on the microbubble region. It has been found that a large population of smaller microbubbles, around 1 μm in diameter, is created in the process of these types of motions, and the time-dependent behaviour of this “micro-bubbly” water is analysed as Stokes’ law for microbubble dissipation occurs, such as bubble population, dissolved oxygen, pH, etc. Exponential decay analysis on the DLS-measured microbubble populations gave relaxation times τ of ~2.4 h and 3.6 h in exp(−t/τ) fits for DI and filtered tap water, respectively. The downward shift in pH was about 0.08 ± 0.016 and 0.11 ± 0.018 for DI and filtered tap water, respectively. For DI water, the level of dissolved oxygen (DO) at room temperature of 19 °C was ~102% at “t = 0”, and it declined to ~87% within 3 h (with the unprocessed background sample being about 84 ± 1.1%). The respective DO decay results in the case of the filtered tap water (at 19 °C) were ~105% at “t = 0”, declining to 91% within 3 h (background = 86 ± 1.2%). This allows for the dynamic properties to be understood in the context of how microbubbles determine the observed properties of post-flow water, including rationalising the observations of its time-transient properties. Naturally, this may well be of interest in gas transfer optimisation in the growing field of “fine-bubble engineering”. Full article

The RELAP5 code is a computational tool designed for transient simulations of light water reactor coolant systems under hypothesized accident conditions. The original wall drag partition model in the RELAP5 code has a problem in that the bubble velocity is predicted to be faster than the water velocity in the fully developed flow in a constant-area channel. The wall drag partition model, based on the wetted perimeter concept, proves insufficient for accurately modeling bubbly flows. In this study, the wall drag partition model was modified to account for the physical motion of fluid particles. After that, the modified RELAP5 code was applied to predict the SBLOCA of a full-scale nuclear power plant. Considering the SBLOCA scenario, the behavior change in the loop seal clearing phenomenon was clearly shown in the analysis by the model change. Upon the termination of natural circulation, the loop seals were cleared, allowing the steam trapped within the system to discharge through the break. The modified model was confirmed to have an impact at this time. It mainly affected the timing and shape of the loop seal clearing and delayed the overall progress of the accident. It was observed that the flow rate of the bubbly phase decreased as the modified model accounted for wall friction during dispersed flow in the horizontal section, impacting the two-phase flow behavior at the conclusion of the natural circulation phase. Full article