Numerical Simulation Of Dynamic Behavior And Induced Wake For Single Bubble Rising Near The Wall

Single bubbles rising in the close vicinity of a vertical wall is one of the focuses in the field of gas-liquid two-phase flow.Due to the combined action of gravity,inertial force,viscous force and surface tension,and the influence of wall effect,the migration law of near-wall bubbles is very complicated.In this work,three-dimensional direct numerical simulation is utilized to explore in detail the bubble rising behavior and wake structures.In this paper,the migration behavior and wake structure of near-wall buoyancy-driven single bubbles are studied by threedimensional direct numerical simulation to reveal the interaction between bubble migration,wake structure and solid wall surface.Aiming at the near-wall bubbles of three different types of migration paths: linear,zigzag and spiral,this paper uses the VOF method to capture the small-scale bubble phase interface,and combines the adaptive meshing technology to encrypt the gas-liquid interface and the high-velocity gradient region in the flow field,and further analyzes the influence of the initial distance away from the wall center and the initial shape of the bubble on the ascending trajectory,deformation,velocity and wake structure of the bubble.The main work and innovation points completed are as follows:(1)The low accuracy of the curvature of the continuous surface tension model leads to spurious current near the bubble interface.To overcome this problem,this study compares the improvement effect of Level Set function and height function on the accuracy of gas-liquid interface curvature solution.Since the height function is easy to implement and reduces the spurious current in the gravity-free field by three orders of magnitude at least,the VOF model coupled with the height function is finally chosen to realize the bubble interface reconstruction.(2)A mathematical model of the bubble rising by buoyancy near the wall is established based on the open-source software Basilisk.Adaptive Mesh Refine method is used to improve the computational efficiency and mesh independence analysis is performed.The accuracy of the numerical model used in this paper is verified by comparing with the experimental data obtained from the high-speed photography experimental equipment.(3)The differences in migration behavior,shape deformation and wake characteristics of linear,zigzag,and spiral bubbles were investigated rising in the unbounded region and near the wall.It is found that the wall has an obstructive effect on the diffusion of the vortex boundary layer on the surface of the linear bubble,and the counter force drives the bubble to move away from the wall.And the wall has the effect of inducing the vortex to fall off.So,the bubble oscillates earlier in the x-y plane,resulting in the bubble path changing from linear to zigzag.In contrast,the wall has a stable effect on the bubble motion in the z-y plane.Therefore,the spiral bubble takes longer to rise along the zigzag path.(4)The three-dimensional numerical simulation of the bubble rising near the wall under different parameters was conducted to analyze the effects of the initial distance of the bubble away from the wall and the initial shape of the bubble on the bubble dynamics.It is found that the direction of the transverse lift force is closely related to the initial distance of the bubble away from the wall.And in the process of the initial position away from the wall,the transverse lift force will change from an attractive force pointing to the wall to a repulsive force driving the bubble away from the wall.In addition,the near-wall bubble is less affected by the initial shape of the bubble during the rising process,which exists in the initial ascent stage.Figure 61,Table 7,References 77...