The injection of gas bubbles into a turbulent boundary layer of a liquid phase has multiple different impacts on the original flow structure. Frictional drag reduction is a phenomenon resulting from their combined effects. This explains why a number of different void–drag reduction relationships have been reported to date, while early works pursued a simple universal mechanism. In the last 15 years, a series of precisely designed experimentations has led to the conclusion that the frictional drag reduction by bubble injection has multiple manifestations dependent on bubble size and flow speed. The phenomena are classified into several regimes of two-phase interaction mechanisms. Each regime has inherent physics of bubbly liquid, highlighted by keywords such as bubbly mixture rheology, the spectral response of bubbles in turbulence, buoyancy-dominated bubble behavior, and gas cavity breakup. Among the regimes, bubbles in some selected situations lose the drag reduction effect owing to extra momentum transfer promoted by their active motions. This separates engineers into two communities: those studying small bubbles for high-speed flow applications and those studying large bubbles for low-speed flow applications. This article reviews the roles of bubbles in drag reduction, which have been revealed from fundamental studies of simplified flow geometries and from development of measurement techniques that resolve the inner layer structure of bubble-mixed turbulent boundary layers.

Frictional drag reduction by bubble injection

Ultrasonic Doppler Technique for Application to Multiphase Flows: A Review

Drag reduction promoted by repetitive bubble injection in turbulent channel flows

Measuring uncertainty of ultrasonic longitudinal phase velocity estimation using different time-delay estimation methods based on cross-correlation: Computational simulation and experiments

We have proposed a novel methodology using ultrasonic velocity profiling to estimate the effective viscosity of bubble suspensions that are accompanied by non-equilibrium bubble deformations in periodic shear flows. The methodology was termed “ultrasonic spinning rheometry” and validated on measurement of the effective viscosity of particle suspensions that has a semi-empirical formula giving good estimation of the actual viscosity. The results indicated that the proposed technique is valid for particle volume fractions below 3.0 %. Applying this to bubble suspensions suggested that the effective value of temporal variations in the capillary number, $$\hbox{Ca}_{\rm rms}$$
 , is an important indicator to distinguish regimes in estimating the effective viscosity: Unsteady flows having larger $$\hbox{Ca}_{\rm rms}$$
 number than the critical capillary number for the deformation of bubbles are categorized into Regime 2 that includes both highly unsteady conditions and large steady deformation of bubbles.

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Yasushi Takeda

Papers

Ultrasonic Doppler Velocity Profiler for Fluid Flow