Masaru Miyanaga

Effect of microbubbles on the structure of turbulence in a turbulent boundary layer

Abstract: The time-averaged velocity and turbulence intensity distributions were measured by a laser Doppler velocimeter in a turbulent boundary layer filled with microbubbles. The void fraction distribution was also measured using a fiber-optic probe. The velocity decreased in the region below 100 wall units with an increase in bubble density. This led to a decrease in the velocity gradient at the wall, which was consistent with a decrease in shearing stress on the wall. The turbulence intensity in the buffer layer increased at a low microbubble density, and then began to decrease with an increasing microbubble density. Based on the present measurements, the mechanism of turbulence reduction by microbubbles is discussed and a model is proposed.

Reduction of skin friction by microbubbles and its relation with near-wall bubble concentration in a channel

Determination of the flow structure near the wall is essential for a clear insight into the phenomenon of skin friction reduction by microbubbles in a turbulent boundary layer. An important parameter, is the bubble concentration or void fraction in the wall region in drag-reducing conditions. The purpose of this paper is to show drag-reducing effects due to microbubbles in a water channel and, more importantly, to show the dependence of the drag-reduction values on the near-wall void fraction. A two-dimensional channel with an aspect ratio of 10 was specially built for this purpose with provisions for air injection through porous plates. Skin friction was directly measured by a miniature floating element transducer with a 5-mm circular sensing disk mounted flush on the top wall 67 channel-heights downstream of the injector. The wall friction in the presence of air bubbles was found to be reduced under the same bulk velocity when compared with the value without air. Detailed void fraction profiles across the channel were obtained by a sampling probe and a fiber-optic probe. Better collapse of the drag reduction data, independent of different profile shapes, was found when plotted against the near-wall void fraction than against a cross-sectional mean void fraction. While this dependence reconfirms that the phenomena are essentially inner-region dependent, the lack of influence of the bubble distribution patterns away from the wall implies lack of outer region influence.

Experimental study on microbubble ejection method for frictional drag reduction

The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2–10 m/s It was confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. According to previous experiments, the size of the bubbles is an important factor in frictional drag reduction by microbubble ejection. Usually bubbles larger than a certain diameter (for example 1 mm) have no effect on frictional drag reduction. Three different methods were proposed and tested to generate smaller bubbles. Among them, a 2D convex (half body of an NACA 64-021 section) with ejection holes at the top was the best and most promising. The diameter of the bubbles became about one-third the size of the reference ejection on a flat plate. Moreover, the bubble size did not increase with increasing flow rate. This is a favorable characteristic for practical purposes. The skin friction force was measured directly with a miniature floating element transducer, and decreased drastically by microbubble ejection from the top of the 2D convex shape.

Frictional Drag Reduction by Injecting High-Viscosity Fluid Into Turbulent Boundary Layer

This paper presents a new method to reduce turbulent frictional drag by injecting high-viscosity fluid into the boundary layer. When the turbulent region of the boundary layer is filled with high-viscosity fluid, and the viscosity of the viscous sublayer is kept low, the Reynolds stress in the turbulent region is reduced and therefore requires a greater velocity gradient to transfer the momentum. The greater velocity gradient in the turbulent region resulsts in a reduction of the velocity gradient at the viscous sublayer, which causes a drop in shear stress at the wall. Such a boundary-layer structure could be created by injecting two different fluids from double slits on a wall. Sugar syrup and water were used as the high-viscosity fluid and the low-viscosity fluid, respectively. The shear stress was directly measure by shear stress pick-ups mounted flush on the wall. The shearing stress was reduced by more than 50% at the optimum injection condition. A water/water injection experiment was also performed to show the effect of injection itself.

Effect of Microbubble Cluster on Turbulent Flow Structure

Wall shearing stress is decreased by the existence of microbubbles in a turbulent boundary layer as discovered by McCormick and Bhattacharyya [1] in 1973. Since that time experimental research has been conducted on air-water bubble flow [2]-[7]. It has been demonstrated that shearing stress is decreased to a value one order of magnitude lower than the original value under optimum conditions [2].

Frictional Drag Reduction by Injecting Bubbly Water into Turbulent Boundary Layer and The Effect of Plate Orientation

This paper presents experimental results showing frictional drag reduction by injecting microbubblemixed- water through a slit into a turbulent boundary layer. The drag became one tenth of the value without injection at optimum conditions.