垂直壁面附近上升单气泡的弹跳动力学研究

采用高速摄影技术结合阴影法,对静止水中垂直壁面附近上升单气泡运动进行实验研究,对比气泡尺度及气泡喷嘴与壁面之间的初始无量纲距离 ($S^{\ast}$)对气泡上升运动特性的影响,分析气泡与壁面碰撞前后,壁面效应与气泡动力学机制及能量变化规律.结果表明,对于雷诺数$Re \approx 580 \sim 1100$,无量纲距离$S^{\ast } <2 \sim3$时,气泡与壁面碰撞且气泡轨迹由无约束条件下的三维螺旋转变成二维之字形周期运动;当$S^{\ast } >2 \sim3$时,壁面效应减弱,有壁面约束的气泡运动与无约束气泡运动特性趋于一致.气泡与壁面碰撞前后,壁面效应导致横向速度峰值下降为原峰值的70%,垂直速度下降50%;气泡与壁面碰撞前,通过气泡中心与壁面距离($x/R$)和修正的斯托克斯数相关式可预测垂直速度的变化规律.上升气泡与壁面碰撞过程中,气泡表面变形能量单向传输给气泡横向动能,使得可变形气泡能够保持相对恒定的弹跳运动.提出了气泡在与壁面反复弹跳时的平均阻力系数的预测模型,能够很好地描述实验数据反映出的对雷诺数${Re}$、韦伯数${We}$和奥特沃斯数${Eo}$等各无量纲参数的标度规律. 

THE BOUNCE DYNAMICS OF A RISING SINGLE BUBBLE NEAR A VERTICAL WALL 1)

By using high speed photography technology combined with shadow method, the motion of a single rising bubble near a vertical wall in stationary water is experimentally studied. The effects of bubble size and the initial dimensionless distance between the nozzle and the wall ($S^{\ast})$ on the rising motion characteristics of bubbles were compared. The wall effect, bubble dynamic mechanism and energy variation rule before and after the collision between bubbles and the walls are analyzed. The results show that for the Reynolds number $Re \approx 580\sim 1100$, and the initial dimensionless distance between the nozzle and the wall $S^{\ast } < 2\sim 3$, the bubbles collide with the wall surface and the bubble trajectory changes from three-dimensional spiral under unconstrained conditions to two-dimensional zigzag periodic motion. However, when $S^{\ast } > 2\sim 3$, the wall effect weakens, and the movement characteristics of the bubble with wall constraint tends to be consistent with that without constraint. Before and after the bubble collides with the wall, the wall effect causes the peak value of transverse velocity to drop to 70% of the original peak value, and vertical velocity drop to 50%. Before the bubble collides with the wall, the vertical velocity variation rule can be predicted by the distance between the bubble center and the wall ($x/R)$ and the modified Stokes number correlation formula. In the process of collision between the rising bubble and the wall surface, the deformation energy of the bubble surface is transmitted to the transverse kinetic energy of the bubble in one direction, so that the deforming bubbles can maintain a relatively constant bouncing motion. The prediction model of the average resistance coefficient of bubbles in the repeated bouncing with the wall surface is proposed, which can describe the dimensionless parameters of Reynolds number, Weber number and Eo number reflected by the experimental data.