Friction Drag Reduction of External Flows with Bubble and Gas Injection

Abstract

The lubrication of external liquid flow with a bubbly mixture or gas layer has been the goal of engineers for many years, and this article presents the underlying principles and recent advances of this technology. It reviews the use of partial and supercavities for drag reduction of axisymmetric objects moving within a liquid. Partial cavity flows can also be used to reduce the friction drag on the nominally two-dimensional portions of a horizontal surface, and the basic flow features of two-dimensional cavities are presented. Injection of gas can lead to the creation of a bubbly mixture near the flow surface that can significantly modify the flow within the turbulent boundary layer, and there have been significant advances in the understanding of the underlying physical process of drag reduction. Moreover, with sufficient gas flux, the bubbles flowing beneath a solid surface can coalesce to form a thin drag-reducing air layer. The current applications of these techniques to underwater vehicles and surface ships are discussed.

Underwater Restoration and Retention of Gases on Superhydrophobic Surfaces for Drag Reduction

“…Drag reduction for turbulent flows has also been reported [8]. SHPo surfaces are considered a superior alternative to the existing bubble injection method [10] for drag reduction, because the stable gas upon the surfaces makes the SHPo method passive (i.e., energy efficient) and simple (i.e., easy implementation). Moreover, it has been shown that the minimized solid-liquid contact on SHPo surfaces can resist surface fouling [11].…”

Effect of bubble’s arrangement on the viscous torque in bubbly Taylor-Couette flow

“…However, despite several attempts in this field, the physical mechanisms implied into the bubbly drag reduction are to the best of our knowledge, not completely understood 1 . Consequently, it is still non straightforward to extrapolate results obtained for small scale models to large scale ship's hull model.…”

Development of a Miniature Shear Sensor for Direct Comparison of Skin-Friction Drags

“…Reducing the drag of marine vehicles via gas lubrication has long been practiced by injecting gas bubbles or creating a cavitation gas pocket (Ceccio 2010) and reached a significant drag reduction (~ 95%) at a high Reynolds number (Re x ~ 10 7 ) flow (Lay et al 2010). Since the gas film (or bubbles) does not stay on the solid surface by nature, however, these methods should continue supplying the gas with additional energy, overshadowing the benefit of drag reduction and limiting applications.…”