An Introduction to Magnetohydrodynamics

Two circular cylinders in cross-flow: A review

The dynamics of swimming fish and flapping flags involves a complicated interaction of their deformable shapes with the surrounding fluid flow. Even in the passive case of a flag, the flag exerts forces on the fluid through its own inertia and elastic responses, and is likewise acted on by hydrodynamic pressure and drag. But such couplings are not well understood. Here we study these interactions experimentally, using an analogous system of flexible filaments in flowing soap films. We find that, for a single filament (or 'flag') held at its upstream end and otherwise unconstrained, there are two distinct, stable dynamical states. The first is a stretched-straight state: the filament is immobile and aligned in the flow direction. The existence of this state seems to refute the common belief that a flag is always unstable and will flap. The second is a flapping state: the filament executes a sinuous motion in a manner akin to the flapping of a flag in the wind. We study further the hydrodynamically coupled interaction between two such filaments, and demonstrate the existence of four different dynamical states.

Flexible filaments in a flowing soap film as a model for one-dimensional flags in a two-dimensional wind

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.

Friction Drag Reduction of External Flows with Bubble and Gas Injection

Hydrodynamic and hydromagnetic stability

Development of an ultrasound velocity profile monitor

Velocity profile measurement by ultrasound Doppler shift method

Velocity profile measurement by ultrasonic doppler method

The ultrasound velocity profile monitor has been developed. It utilizes the pulsed echo techniques of ultrasound, and it can measure the velocity profile quasi-instantaneously. Its applicability to...

Yasushi Takeda

Papers

Development of an ultrasound velocity profile monitor

Velocity profile measurement by ultrasound Doppler shift method

Velocity profile measurement by ultrasonic doppler method

Measurement of velocity profile of mercury flow by ultrasound doppler shift method

Ultrasonic detection of moving interfaces in gas–liquid two-phase flow