Bubble Motion Research Articles

Advancements in the Understanding of Single‐Bubble Dynamics: Integrated Experimental and Simulation Studies

Because of wake instability, bubbles always ascend along an unstable path, which is not considered when simulating their movement in steel refining and continuous casting systems. Utilizing the 3D shadow image method, a quantitative characterization of the bubble trajectories is conducted to establish a bubble path oscillation model considering the zigzag lateral movement of bubbles due to asymmetric wake shedding. This model is incorporated into a rectilinear bubble motion model within the Lagrangian framework and then used to simulate the free ascent of single bubbles within the 2.15–2.55 mm range. The predicted bubble ascent velocities and trajectories agree well with the experimental. The spatial position, velocity, acceleration, forces, volume swept by bubbles, and bubble residence time are discussed. The results shown that the dominant force in the horizontal direction is the lateral force, followed by the drag and virtual mass force. Compared to the rectilinear path model, the volume swept by bubbles with initial diameters of 2.15, 2.25, 2.34, 2.45, and 2.55 mm in this model increases by 30.8%, 32.0%, 34.0%, 31.5%, and 29.6%, respectively. These findings help accurately predict the path of bubbles and also may contribute to a better understanding of bubble dynamics in bubble metallurgy and clean steel production.

Heat and mass transfer performance analysis of a vertical tubular ammonia/water bubble absorber based on CFD modeling

Bubble absorbers are highly efficient in mass transfer processes, but their popularization is limited by the risks associated with too much ammonia charging. In order to reduce the ammonia charge, it is essential to optimize the heat and mass transfer performance in the absorber. Existing models provide in-depth analysis of bubble motion and morphology, but insufficient attention has been paid to the concentration distribution and interfacial heat and mass transfer resistance. In this study, a two-dimensional axisymmetric vertical tubular ammonia/water bubble absorber was modeled based on Fluent and phase equilibrium, absorption heat data based on Aspen was imported. The relative deviations were 9%, 0.45%, and 4.1% in terms of the completely absorbed height, the liquid phase ammonia mass fraction, and the outlet temperature, respectively. The results showed that the mass transfer deterioration point was at the gas column closure and the heat transfer deterioration point was at the gas column contraction. Variable parameter studies have shown that turbulence effects at the gas–liquid interface can lead to enhancement and deterioration of local heat and mass transfer, which in turn cause local oscillations in the concentration and temperature profiles. While increasing the interphase mass transfer driving force promotes the interphase mass transfer process, it hinders the diffusion mass transfer process within the liquid phase. This effect is particularly pronounced in the absence of intense cooling, leading to an increase in the complete absorbed height.

Round-trip motion of air-rich bubbles exhaled from a vapor-rich bubble generated at a local heating point

In this study, the round-trip motion of air-rich bubbles exhaled from a water vapor-rich bubble was investigated. The local heating of non-degassed water produced a vapor-rich bubble with a maximum diameter of 9 μm, which intermittently exhaled air-rich bubbles with a radius of less than 1 μm. The exhaled air-rich bubbles initially moved away from the heat source; however, as the air-rich bubbles fused and grew larger, they returned to the heat source and fused with the vapor-rich bubble. This round-trip motion of the air-rich bubbles is explained by the balance between the Marangoni and quasi-steady drag forces induced on the bubbles. As the quasi-steady drag force is approximately proportional to the bubble radius and the Marangoni force is proportional to the square of the bubble radius, a larger bubble correlates with a greater effect of the Marangoni force. To produce a quasi-steady drag force that can balance the increased Marangoni force, air-rich bubbles were attracted to the heat source against the flow created by the vapor-rich bubble. These results provide insight into the stabilization of water-vapor-rich microbubbles in non-degassed water, which can generate strong flows on the order of 1 m/s.

Experimental study of a single bubble's motion in a liquid metal under a horizontal magnetic field

An ultrasonic phased array system is introduced to study the three-dimensional (3-D) movement of a single bubble in a GaInSn alloy under a transverse magnetic field (MF), which is verified by bubble experiments in water. The 3-D motion trajectories of individual bubbles in the GaInSn are obtained under a horizontal MF. As the MF becomes stronger, the bubble successively oscillates in random directions (R mode), a direction perpendicular to the MF (V mode), a direction parallel to the MF (P mode) and finally it rises straight (S mode). The significant anisotropy of the oscillation directions at a moderate MF intensity may be due to the anisotropy of the vortex structure around the bubble. Furthermore, the oscillation amplitude gradually declines with increasing MF intensity until the bubble trajectory finally becomes a straight line. Our measurements allow us to specify the characteristic regions for the observed bubble modes in the $N-Eo-Re$ parameter space (N is the magnetic interaction parameter, Eo is the Eötvös number and Re is the Reynolds number). In addition, more detailed characteristics of bubble terminal velocity are revealed, showing that the bubble velocities are closely related to the motion modes. The increase in bubble velocity at a moderate MF intensity is caused by the weakening oscillation. At a high strength, the MF monotonically suppresses the rise velocity of the bubble with a fixed scaling law.

Statistical Analysis of Bubble Parameters from a Model Bubble Column with and without Counter-Current Flow

Bubble columns are widely used in numerous industrial processes because of their advantages in operation, design, and maintenance compared to other multiphase reactor types. In contrast to their simple design, the generated flow conditions inside a bubble column reactor are quite complex, especially in continuous mode with counter-current liquid flow. For the design and optimization of such reactors, precise numerical simulations and modelling are needed. These simulations and models have to be validated with experimental data. For this reason, experiments were carried out in a laboratory-scale bubble column using shadow imaging and particle image velocimetry (PIV) techniques with and without counter-current liquid flow. In the experiments, two types of gases—relatively poorly soluble air and well-soluble CO2—were used and the bubbles were generated with three different capillary diameters. With changing gas and liquid flow rates, overall, 108 different flow conditions were investigated. In addition to the liquid flow fields captured by PIV, shadow imaging data were also statistically evaluated in the measurement volume and bubble parameters such as bubble diameter, velocity, aspect ratio, bubble motion direction, and inclination. The bubble slip velocity was calculated from the measured liquid and bubble velocities. The analysis of these parameters shows that the counter-current liquid flow has a noticeable influence on the bubble parameters, especially on the bubble velocity and motion direction. In the case of CO2 bubbles, remarkable bubble shrinkage was observed with counter-current liquid flow due to the enhanced mass transfer. The results obtained for bubble aspect ratio are compared to known correlations from the literature. The comprehensive and extensive bubble data obtained in this study will now be used as a source for the development of correlations needed in the validation of numerical simulations and models. The data are available from the authors on request.

Effect of graphene coating on flow boiling in a minichannel at sub-atmospheric pressures

Flow boiling in a mini-channel with graphene-coated sintered porous copper surface was investigated at sub-atmospheric pressures by analyzing bubble motion and flow patterns. A method of surface modification based on graphene coating was presented for thermal management systems operating at lower temperatures. The experiments were carried out at mass fluxes of 60 kg.m–2.s–1 and 120 kg.m–2.s–1 and sub-atmospheric pressures of 48 kPa and 68 kPa in a mini-channel with dimensions of 50 mm × 15 mm × 2 mm using HFE-7100 coolant as the working fluid. At sub-atmospheric pressures, the bubble nucleation site density decreases, and the bubble diameter increases while the bubble release frequency decreases. The existence of porous surfaces and HFE-7100 coolant with lower surface tension leads to the inception of boiling at lower wall superheats. The graphene coating promotes microlayer evaporation and consequently increases the nucleation site density by enhancing the thermal conductivity and wicking rate. Additionally, it provides stable annular flow at high heat fluxes, whereas intermittent-annular flow is observed for the untreated surface under the same condition. This enhancement results in a more pronounced increase in the boiling heat transfer for the graphene-coated surface. As a result, this study provides the high potential of graphene coatings for phase change heat transfer enhancement at sub-atmospheric pressures.

Investigation on the effects of an elliptical wall on the dynamic behaviors of a bubble restricted by two parallel plates

The present paper investigates the dynamic behaviors of a bubble restricted by two parallel plates near an elliptical wall. The typical experimental phenomena of the bubble are recorded employing the high-speed photography and a theoretical Kelvin impulse model is established. The impacts of the spatial position and the curvature of the wall on the bubble collapse behaviors are quantitatively investigated through the theoretical model and verified against the experimental results. The Kelvin impulse intensity and the direction during the bubble collapse process are compared and discussed for different elliptical-shaped walls. The main conclusions include: (1) During the bubble collapse process, the phenomenon of the bubble uneven splitting is discovered. (2) At different spatial positions and wall curvatures, the bubble collapse jet angle, movement distance, and velocity are in good agreement with the theoretical Kelvin impulse predictions. (3) As the short-to-long axis ratio increases, the differences in the distributions of the Kelvin impulse intensity and the direction near the elliptical wall gradually become larger, and the range of the influence of the impulse intensity expands.

Numerical simulation of electric field-induced phase transition evolution and boiling characteristics in the evaporative cooling medium C6F12O

Phase change cooling technology offers high cooling efficiency, safety, and reliability, representing a novel approach to achieving efficient heat dissipation for high-power and large-capacity electrical equipment. The formulation of the cooling medium is pivotal to phase change cooling technology. However, current media exhibit compatibility, stability, economy, and environmental friendliness deficiencies. Consideration could be given to implementing the C6F12O medium due to its superior overall performance and ability to meet the latent heat requirements in phase change cooling equipment. This paper employs a numerical simulation approach that combines the phase field method based on the Cahn-Hilliard equation with the theory of electrohydrodynamics. It investigates the impact of temperature, electric field intensity, and electric field direction on the evolution of bubble motion and the boiling state of the C6F12O medium, considering the interaction of electric-fluid-heat-phase fields. Numerical results indicate that the system undergoes initial nucleate boiling, nucleate boiling, and film boiling stages at T = 330–335 K, T = 335–350 K, and T ⩾ 355 K, respectively. The introduction of an appropriate electric field can enhance the motion evolution of C6F12O bubbles. However, attention must be paid to the formation of bubble channels under high field strength to prevent potential decreases in insulation performance. An inhomogeneous electric field in the vertical direction proves more effective in improving the bubble release rate compared to a uniform electric field. To some extent, an inhomogeneous electric field in the horizontal direction can prevent the mass accumulation of bubbles in regions of high field intensity. This research has the potential to offer theoretical guidance for the engineering application of the C6F12O phase change cooling medium.

Experimental study on the effect of circulating liquid velocity on bubble size distribution and turbulence characteristic in an external loop airlift reactor

AbstractThe influence of circulating liquid velocity on bubble size distribution (BSD) and turbulence characteristics of an external loop airlift reactor was studied in this paper. The instantaneous and time‐averaged velocity in the riser were studied using particle image velocimetry (PIV), and BSDs were measured by digital image analysis technique based on machine learning. Then turbulence kinetic energy (TKE) and energy dissipation rate (EDR) were calculated through the velocity field. The results indicate that as the circulating liquid velocity increased, the peak value of BSD rapidly decreased from nearly 6 mm to approximately 2 mm. The radial velocity of the liquid gradually decreased and changed direction, eventually increasing again. TKE first decreased and then increased. Compared with bubble flow with a BSD peak of 2–6 mm, bubble flow with a BSD peak of 2 mm had larger TKE. The radial movement of bubbles had great influence on the turbulence characteristics. This study demonstrates that selecting an appropriate circulating liquid velocity can reduce the diameter of bubbles while obtaining greater TKE, thereby improving the mass transfer and reaction efficiency in external loop airlift reactors (EL‐ALR).

Investigation on laser-induced bubble collapse among triple particles based on high-frame-rate photography and the Kelvin impulse model

In fluid machinery, the concurrent presence of cavitation bubbles and particle clusters leads to considerably damage to material surfaces. This study investigates the dynamics of a bubble situated among triple particles based on the Kelvin impulse model and high-frame-rate photography, focusing on the impact of the dimensionless distance of particles and the bubble size. Specifically, the jet, bubble motion, and bubble interface evolution characteristics are quantitatively evaluated. The following conclusions are obtained: (1) The collapse shapes of the bubble can be divided into three typical cases: equilateral triangle shape, isosceles triangle shape, and arcuate shape. (2) Among the triple particles, four zero-Kelvin-impulse locations are present, around which the jet direction is extremely sensitive to the bubble initial position. As the bubble initial position moves along the central line, the bubble motion direction dramatically changes during its collapse. (3) The relative position of bubble and particles is the key parameter that affects the bubble dynamics. As the bubble–particle distance decreases, the non-uniformity of bubble collapse morphology and the bubble motion distance will become more significant.

Motion of a small bubble in forced vibrating sessile drop

In this letter, the motion of small gas bubbles within sessile water drops on a vibrating substrate is investigated numerically using a two-phase diffuse interface method. Depending on the amplitude of the plate vibration, the motion of the gas bubbles falls into three distinct regimes: oscillating within the drop, sticking to the substrate, or escaping from the drop. In particular, the motion of oscillating bubbles follows a harmonic function, and is found to be closely related to a combined effect of the deformation of the sessile drop and the vibration of the plate. To interpret the underlying mechanism, we analyze the dominant forces acting on the bubbles in the non-inertial framework fixed to the plate, and take account of the periodic deformation of the drop, which effectively induces flow acceleration inside the drop. As a result, we establish a theoretical model to predict the bubble motion, and correlate the amplitude and phase difference of the bubble with the Bond and Strouhal numbers. The theoretical prediction agrees with our numerical results. These findings and theoretical analysis provide new insights into controlling bubble motion in sessile drops.

Bubble dynamics in high-pressure flow boiling conditions

High resolution measurements of key parameters relevant to bubble departure processes in high-pressure water systems has been a major challenge over the past several decades due to the small size and fast motion of vapor bubbles, making conventional optical measurements unreliable. Addressing this difficulty is critical, as accurate bubble departure data and computationally inexpensive models are needed for future two-phase heat transfer models. To close this gap, we use high-speed imaging to track bubble size and velocity in high-pressure flow boiling conditions (10 – 40 bar). It is observed that bubbles are almost always perfectly spherical in shape and slide along the boiling surface immediately after nucleating. To predict sliding, we conduct a force balance on the bubble and derive an equation of motion in the direction of the bulk flow. Interestingly, we find that at high pressure conditions, the equation of motion can be significantly simplified, resulting in an analytical expression for the bubble motion, growth time and departure diameter, allowing us to develop simple and computationally efficient equations. The applicability of the model and departure criterion across a broader range of high-pressure conditions is discussed, and recommendations for further characterization of bubble departure processes are made.

Numerical study of bubble rise in plunging breaking waves

During the occurrence of plunging wave breaking, a substantial number of multi-scale bubbles are generated. These submerged bubbles persist for extended periods and contribute to the distinct acoustic and optical characteristics of the wake. In this study, we utilize high-fidelity simulations, combined with the adaptive refinement strategy to accurately track bubbles of multi-scales during the entire rising stage. Unlike previous studies, our emphasis is specifically on investigating the process of bubble rising during plunging wave breaking. Comprehensive statistical analyses are performed and characteristics of bubbles across various scales are also provided. Our findings reveal that most bubbles are concentrated in small scales, while larger bubbles rapidly ascend to the surface or undergo fragmentation into smaller bubbles through breaking cascades eventually. A distinct stratification of bubble size distribution along the depth direction is observed. Bubble velocity distributions are also important characteristics that are frequently neglected in studies of plunging wave breaking. Bubbles primarily spread along the spanwise direction, with a uniform distribution of velocity in this dimension. The velocity distribution of bubbles displays asymmetric tails that extend to higher velocities, and within this high-velocity regime, a power law behavior is observed, similar to the size distributions. Ultimately, the flow field is left with only a few small bubbles, moving at an exceedingly low speed. Furthermore, dynamical evolution of bubble rise in plunging wave breaking is described in detail and we analyze the intricate interactions between bubbles and turbulent flows. We observe that vortices are predominantly generated in close proximity to the bubbles, and bubble motion plays a crucial role in initiating turbulent flows. Simultaneously, these vortices contribute to the fragmentation of large-scale bubbles, transforming them into smaller counterparts due to turbulent fluctuations.

Gas-driven triboelectric nanogenerator for mechanical energy harvesting and displacement monitoring

Gas as an energy carrier has been widely studied, ranging from small bubble motions to steam strike turbines. Despite the remarkable results, fewer studies have investigated the gas-driven liquid column movement commonly used in hydraulic systems, oil extraction, and other fields. Herein, we proposed a gas-driven triboelectric nanogenerator (GD-TENG) with liquid column motion for mechanical energy harvesting and displacement monitoring. When the gas drives a single liquid column to move inside a polytetrafluoroethylene (PTFE) tube, the solid-liquid interface of the GD-TENG can achieve efficient sliding friction and generate an open-circuit voltage of 15.9 V. Meanwhile, the influence of some critical parameters such as liquid column length, PTFE tube wall thickness and liquid column motion speed on the output performance of the GD-TENG were systematically investigated. Furthermore, the GD-TENG can successfully harvest mechanical energy to power microelectronic devices. A sensing system was designed based on the GD-TENG device for displacement monitoring of moving objects. This work provides a new research path in the field of mechanical energy harvesting and self-powered displacement monitoring.

Deposition of shear‐thinning viscoelastic fluids by an elongated bubble in a circular channel regarding the weakly elastic regime

AbstractThin‐film deposition of fluids is ubiquitous in a wide range of engineering and biological applications, such as surface coating, polymer processing, and biomedical device fabrication. While the thin viscous film deposition in Newtonian fluids has been extensively investigated, the deposition dynamics in frequently encountered non‐Newtonian complex fluids remain elusive, with respect to predictive scaling laws for the film thickness. Here, we investigate the deposition of thin films of shear‐thinning viscoelastic fluids by the motion of a long bubble translating in a circular capillary tube. Considering the weakly elastic regime with a shear‐thinning viscosity, we provide a quantitative measurement of the film thickness with systematic experiments. We further harness the recently developed hydrodynamic lubrication theory to quantitatively rationalize our experimental observations considering the effective capillary number and the effective Weissenberg number , which describe the shear‐thinning and the viscoelastic effects on the film formation, respectively. The obtained scaling law agrees reasonably well with the experimentally measured film thickness for all test fluids. Our work may potentially advance the fundamental understanding of the thin‐film deposition in a confined geometry and provide valuable engineering guidance for processes that incorporate thin‐film flows and non‐Newtonian fluids.

Unravelling the existence of asymmetric bubbles in viscoelastic fluids

We study the motion and deformation of a single bubble rising inside a cylindrical container filled with a viscoelastic material. We solve numerically the mass and momentum balances along with the constitutive equation for the viscoelastic stresses, without assuming axial symmetry to allow the growth of three-dimensional disturbances. Hence, we may predict the emergence of the notorious knife-edge shape of the bubble, which is a result of a purely elastic instability triggered in the locality of the trailing edge. Our results compare well with existing experiments. We visualize, for the first time to the best of our knowledge, the flow kinematics and dynamics that arise downstream of the bubble. We propose two quantities, one kinematical and one geometrical, for the determination of the onset of the instability. We demonstrate that extension-rate thinning in the constitutive law is necessary for the emergence of the instability. Moreover, our results indicate that increasing (a) the deformability of the bubble and (b) the initial extension rate hardening of the viscoelastic material, prior to thinning, triggers the instability earlier. These novel findings help us formulate and propose a mechanism that controls the onset of the instability and explain why the knife-edge shape is not encountered as frequently.

Study on the Dynamic Characteristics of Single Cavitation Bubble Motion near the Wall Based on the Keller–Miksis Model

The dynamic model of cavitation bubbles serves as the foundation for the study of all cavitation phenomena. Solving the cavitation bubble dynamics equation can better elucidate the physical principles of bubble dynamics, assisting with the design of hydraulic machinery and fluid control. This paper employs a fourth-order explicit Runge–Kutta numerical method to solve the translational Keller–Miksis model for cavitation bubbles. It analyzes the collapse time, velocity, as well as the motion and force characteristics of bubbles under different wall distances γ values. The results indicate that as the distance between the cavitation bubble and the wall decreases, the cavitation bubble collapse time increases, the displacement of the center of mass and the amplitude of translational velocity of the cavitation bubble increase, and the minimum radius of the cavitation bubble gradually decreases linearly. During the stage when the cavitation bubble collapses to its minimum radius, the Bjerknes force and resistance experienced by the bubble also increase as the distance to the wall decreases. Especially in the cases where γ = 1.3 and 1.5, during the rebound stage of the bubble, the Bjerknes force and resistance increase, causing the bubble to move away from the wall. Meanwhile, this study proposes a radiation pressure coefficient to characterize the radial vibration behavior of cavitation bubbles when analyzing the radiation sound pressure. It is found that the wall distance has a relatively minor effect on the radiation pressure coefficient, providing an important basis for future research on the effects of different scale bubbles and multiple bubbles. The overall idea of this paper is to numerically solve the bubble dynamics equation, explore the characteristics of bubble dynamics, and elucidate the specific manifestations of physical quantities that affect bubble motion. This provides theoretical references for further engineering applications and flow analysis.

Effect of surfactant on dynamics and gas-liquid mass transfer for single carbon dioxide bubbles

In carbonation reaction, the impact of surfactants on the movement and mass transfer behavior of carbon dioxide bubbles were crucial for the production of high-quality silicon dioxide products. The study employed the Matlab image processing system to investigate the shapes, motion velocities, and rise path of single carbon dioxide bubbles within varying concentrations of cetyltrimethyl ammonium bromide (CTAB) solutions. This exploration aimed to elucidate the effect of CTAB on the movement and mass transfer coefficient (MTRC) of single carbon dioxide bubbles. Alterations in the CTAB solution concentration and bubble sizes significantly influenced the mass transfer and movement characteristics of single carbon dioxide bubbles. The conclusion was drawn through the computation of the MTRC of single carbon dioxide bubbles in deionized water and CTAB solutions: the addition of CTAB significantly reduced the mass transfer efficiency of carbon dioxide in deionized water, with the MTRC of CTAB solutions decreasing as the solution concentration increased. Utilizing the stagnant cap model, it can be concluded that CTAB's reduction of the MTRC of carbon dioxide was attributed to the high viscosity of the medium, low bubble velocity, stable flow conditions, and intensified adsorption degree of CTAB molecules at the gas-liquid interface. According to the significant periodic trajectories of single carbon dioxide bubbles in the stable range, the correlation between the trajectories of compressible bubbles in surfactant solution and mass transfer in liquid phase was established, and the previous method based on rigid sphere model was modified. The paper proposed the mechanism of CTAB's influence on single carbon dioxide bubbles, offering crucial theoretical insights into the industrial application of surfactants in the carbonation process for producing silicon dioxide.

Study on dynamic characteristics of cavitation in underwater explosion with large charge

Underwater explosions (UNDEX) generate shock waves that interact with the air–water interface and structures, leading to the occurrence of rarefaction waves and inducing cavitation phenomena. In deep-water explosions, complex coupling relationships exist between shock wave propagation, bubble motion, and cavitation evolution. The shock wave initiates the formation of cavitation, and their growth and collapse are influenced by the pressure field. The collapsing bubbles generate additional shock waves and fluid motion, affecting subsequent shock wave propagation and bubble behavior. This intricate interaction significantly impacts the hydrodynamic characteristics of deep-water explosions, including pressure distribution, density, and phase changes in the surrounding fluid. In this paper, we utilize a two-fluid phase transition model to capture the evolution of cavitation in deep-water explosions. Our numerical results demonstrate that the introduction of a two-phase vapor–liquid phase change model is necessary to accurately capture scenarios involving prominent evaporation or condensation phenomena. Furthermore, we find that the cavitation produced by the same charge under different explosion depths exhibits significant differences, as does the peak value of cavitation collapse pressure. Similarly, the cavitation produced by different charge quantities under the same explosion depth varies, and the relationship between cavitation volume and charge quantity is not a simple linear increase. The research methods and results presented in this paper provide an important reference for studying the dynamic characteristics of deep-water explosions.

Analysis of mechanisms of bubble characteristics and molten pool dynamics in EDM based on a three-phase flow model

Bubble characteristics and bubble volume fraction have significant effects on crater morphology and machining efficiency during electrical discharge machining (EDM) in liquid. Therefore, there is a need to elucidate the mechanism of bubble characteristics in EDM and analyze the mechanism of crater formation based on these bubble characteristics. However, since EDM is a transient multiphysics process, it is extremely difficult to understand bubble characteristics using experimental methods. Therefore, the generation, expansion, explosion, and disappearance of bubbles during EDM were simulated based on a three-phase flow model in this study. The model used the phase field method to track the boundary motion, judged the existence mode of each phase according to the order parameter gradient function, and analyzed the formation of crater morphology by the gradient of surface tension in the molten pool. The motion process of bubbles, the material removal mode, and the formation mechanism of crater morphology during the pulsed discharge were analyzed using the model. The results revealed that the periodic motion of the plasma channel would drive the molten pool to produce the corresponding periodic motion, and the bubbles would grow following the periodic motion of the molten pool. The material was removed in different ways in different periods, among which the removal mode of molten splash was predominant. The surface tension gradient inside the molten pool, the periodic motion of the molten pool, and the hydrostatic pressure in the molten region had a combined effect on bubble motion, the flow of the molten pool, and the formation of the crater morphology.