Effect of Microbubble Diameter on Frictional Drag Reduction
Details
When microbubbles are injected into turbulent boundary layer, the frictional resistance is reduced. The effect of bubble size is one of the important parameters. The bubble size is changed by adding surfactant, 3-pentanol, to water. The reduction is greater on the upper wall than on the lower wall. But the bubble size is smaller, the reduction on the upper wall is smaller than that without 3-pentanol and the reduction on the lower wall is larger. In other words, the difference in the reduction between upper and lower walls becomes smaller. This phenomenon is explained by the buoyancy force and turbulence diffusion. Further, the local velocity and turbulence intensity distributions were measured using a laser doppler velocimeter. The turbulence intensity increased when microbubbles existed in the turbulent core, whereas, the turbulence intensity decreased when microbubbles existed in the near-wall region.
