The present paper reviews a history of turbulence research in Hydraulics and Fluid Mechanics. In particular, the recent research of coherent structures are addressed, which are the most essential and important phenomena in wall turbulence such as boundary layers and open channels, as well as in free turbulence such as mixing layers, jets and wakes, because the coherent vortices generate turbulence from mean-flow energy and because they govern various transport phenomena such as momentum, energy, heat and mass. Although these researches have been intensively conducted in only the last twenty years, it is emphasized here that Leonardo da Vinci (1452-1519) was the first researcher in the world to observe coherent vortices, i. e. hairpin vortex, in water flows. Very recent refined experiments and numerical supercomputer simulations of coherent structures such as bursting phenomena are reviewed. Lastly, the prospective research toward the 21st century is proposed. The necessity of international research project of “Geophysical Large-scale Turbulence (GLT)” is stressed to clarify the global changes of environment in water and atmospheric spheres, such as various natural disasters, warmings due to CO₂ greenhouse effect and acid rains.

View full abstract

A short review is given on the remote sensing by means of earth observation satellites, and its application to the elucidation of geophysical fluid phenomena of the ocean and the atmosphere. Examples of applications are given of infrared radiometer data of GMS and NOAA satellites to the studies of some characteristic phenomena observed in the ocean near Japan.

View full abstract

A two-dimensional laminar wake is formed behind a strip of metal screen. Due to the flow through the screen no dead water region is formed. The wake is excited by sound from a loudspeaker. The natural transition is simulated by the introduction of band noise of which frequency lies in the region of high linear growth. After the linear growth random low-frequency components are generated by the nonlinear interaction. Since the components have maximum intensity on the center line, the randomization seems to start there. The energy diffuses in the spectral space following frequency combination law. The spectrum changes gradually into continuous and smooth form. By the introduction of sound with two frequencies large-scale ordered motions are produced in the wake.

View full abstract

The random choice method successfully simulates one-dimensional inviscid compressible flow using relatively coarse meshes. If we apply this method to multidimensional flow, however, strongly unstable noises near rigid walls propagate into all over the flow field and destroy the solutions. In order to suppress the noises, this paper introduces a stochastic numerical viscosity which enables us to control the effect of the viscosity and to obtain reasonable numerical results.

View full abstract

The stable posture of a triangular or a square cylinder is experimentally studied when it is free to rotate about its central axis fixed perpendicular to a uniform flow. Both cylinders come to rest facing a side to the oncoming flow.

View full abstract

Subharmonic resonance of surface waves in a container subject to vertical periodic oscillation is reviewed theoretically from a standpoint of a low-dimensional dynamical system. Bifurcations of solutions of the system are shown in connection with the shape of a container. It is stressed that although the system with the symmetry and the degeneracy has only stable regular states, it becomes to have a chaotic state when the degeneracy breaks.

View full abstract