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

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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.