SEPARATION OF SMALL BUBBLE FROM GAS FLOW INJECTED INTO TURBULENT BOUNDARY LAYER

当大量的小气泡被注入到液体湍流边界层时，湍流阻力显著减小。最近，这种现象被应用于节能技术。气泡尺寸对减阻效果有显著影响。然而，在大多数实验中，通常是大量的气泡注入和有关于单个气泡的形成很少的报道。在本研究中，我们研究了小气泡的形成分离的气体流注入到湍流边界层从一个单一的淹没孔。利用高速数字摄像机对不同自由流速度和不同气速下的气泡分离过程进行了详细的观测。根据观测结果，考虑气泡的形阻、升力、虚质量惯性、表面张力、浮力和气体惯性，从椭球形气泡受力平衡的角度，从理论上得到了单个气泡的直径。计算结果与实验测量结果基本一致，当气体流速增大时，气体流动形成一个长的圆柱形射流，射流尾部向下。由于周围湍流的大阻力，射流在孔口处被撕裂。然而，从孔注入的气体立即冲击射流并聚结。这种现象的重复导致在孔口上的连续圆柱形射流。通过高速摄像机的观测，发现气泡在气液界面上的不稳定性使气泡从射流中部分离。根据考虑各种力作用的理论模型，计算了圆柱形射流的形态，数值计算的微分方程解与实际射流形态定性一致。

The turbulent drag is drastically reduced when a large quantity of small bubbles are injected into a liquid turbulent boundary layer. Recently this phenomenon is applied to a technique of energy conservation. It is known that the drag reduction is remarkably influenced by the bubble size. However, a large quantity of bubbles are generally injected in most experiments and there have been few reports concerning to the formation of individual bubble.In this research, we investigate the formation of small bubble separated from gas flow injecting to a turbulent boundary layer from a single submerged orifice. The behavior of gas flow and of bubble separation was observed in detail by high-speed digital video camera under various free stream velocities and gas velocities near the orifice with diameter from 0.2 to 0.4mm. Referring to the observation, the diameter of single bubble was theoretically obtained from the force balance on the ellipsoidal bubble by considering form drag, lift, inertia due to virtual mass of bubble, surface tension, buoyancy and gas inertia. The results roughly agree with those measured by experiment.When the gas velocity becomes large, the gas flow forms a long cylindrical jet with long tail in the downward direction. The jet is torn off on the orifice due to the large drag from the surrounding turbulent flow. However, the injected gas from the orifice immediately impacts the jet and coalesces. The repetition of this phenomena results in continuous cylindrical jet on the orifice. From the observation by the high-speed video camera, the bubble separates from middle part of the jet due to instability on gas-liquid interface, The configuration of cylindrical jet was calculated from a theoretical model considering various forces acting on the jet, The solution of differential equation calculated numerically qualitatively agrees with that of actual jet configuration.