When a circular cylinder oscillates transversely in water at rest, a three-dimensional streaming flow streaked with the chains of separated dye sheets is produced over a certain range of amplitude of the oscillation. This paper reports observations of this three-dimensional flow instability.

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The steady streaming induced by oscillatory viscous flow of small amplitudes over a wavy wall has been analysed, and the computed flow patterns have been found to agree well with the flow patterns visualized experimentally in a tube. When the first parameter, α/δ (the ratio of the wavelength of the wavy wall to the thickness of Stokes layer), becomes larger than about 26, the streaming has a double structure consisting of regions of upper and lower pairs of recirculations. As the second parameter, L/δ (the ratio of the amplitude of the wavy wall to the thickness of Stokes layer), is increased, the upper pair of recirculations squeezes in a gap between the lower recirculations above the troughs of the wall. A similar double structure of steady streamings was also observed above ripple marks formed under oscillatory viscous flow. A bearing is suggested of this phenomenon on the determination of stationary profiles of ripple marks.

Water tank experiments were conducted on the internal symmetric solitary waves generated by gravity flow intrusion at the horizontal interface of a two-layer fluid. Velocity distributions in the waves were first measured using an image processor. Flow visualization studies revealed some new features of the solitary waves, including a marked decrease in the phase velocities of two large-amplitude, oppositely propagating waves after their head-on collision.

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