The present paper briefly illustrates several issues of flood disaster mitigation and environmental resources in rivers, focusing on sediment transportations and associated topographic changes caused by debris flows in steep areas as well as by bedload and suspended load in alluvial reaches. In addition, several methods and corresponding results are shown for analyzing these sediment problems. Finally, the author will review the mechanics employed in existing methods in order to suggest that a stiff wall has formed, and to suggest the possibility for a new start.

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A prominent feature of turbulence is the enhancement of mixing or diffusion effects, generally contributing to the loss of macroscopic steeply-varying structures in flows. In a variety of flows, however, those structures may persist against diffusion effects. This fact suggests the existence of some action balancing with diffusion effects. In the present talk, such balancing process is sought in light of the concept of diffusion and counterdiffusion in turbulent flows related to combustion, the transport barrier in fusion-plasma confinement, swirling effects in engineering and astronomical jets, etc

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Bubble jet printer is a drop-on-demand type ink jet printer, which uses boiling of liquids to eject ink drops from nozzles. A merit of the bubble jet printer is that small pitched multiple nozzle arrangement is possible by using thin-film resistors to cause boiling. In this paper, the basic mechanism and development path of the bubble jet printer is reviewed. In these 20 years, the drop volume is reduced to 1/50, and photographic printing quality is realized. The experimental observation of drop ejection process, compared with fluid dynamic calculation, is also presented.

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The evolution of passive material lines in turbulent channel flow is studied by the direct numerical simulation. The Reynolds numbers based on the friction velocity, the channel half width and the kinematic viscosity are 180 and 395. The deformation of a line for the higher Re_τ is more rapid especially at y⁺ = 15. The line length increases exponentially in time as exp[γt], with the stretching rateγ=0.10-0.18τ_η^-1 in the range of the present Reynolds numbers. This is reasonable because the lines are deformed efficiently by the Kolmogorov-scale eddies with the Kolmogorov time-scale (τ_η). The obtained value of γ is close to that of homogeneous isotropic turbulence.

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We investigate the jammed states and phase transitions of traffic in the optimal velocity model with the optimal velocity function of multi-stage. The dynamic state of real traffic is too complex to explain with the conventional optimal velocity model. We use the multi-stage velocity function to take into account the real force on a car, and study the traffic state by use of numerical simulations. The neutral stability line and coexisting curve have some local maxima in phase diagram. The traffic states change by the complex jamming transition. We find that the traffic states have different characteristics from the conventional optimal velocity model.

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A series of experimental investigations have been carried out to examine the influence of free-stream turbulence and acoustic noise on laminar-to-turbulent transition in boundary layers on an NACA0012 airfoil. The quantitative evaluation of the influence on the total growth rate N of the most amplified disturbance obtained from the e^N method at the transition location is made in an empirical relation between the total growth rate N and both the residual turbulence Tu and the sound pressure level SPL. Consequently, the transition location is detected as a linear equation N=C_Tu×ln (Tu) +C_SPL×SPL+C for not only 2-D case with zero angles of both sweep and attack but also 3-D case with non-zero angles of both sweep and attack, though each coefficient in N depends on geometrical configuration of airfoil such as cross-sectional shape and swept angle. Surface temperature measurement technique by use of infrared camera is shown to be efficient and useful for transition detection in low subsonic flow. Usefulness is also shown in visualizing cross-flow stationary vortices growing in 3-D boundary layer.

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This is a live voice of the late Professor Imai who spoke of “Miscellanea on Fluid Dynamics” in the first meeting of the Fluid Dynamicist Circle (over West Tokyo) at University of Electro-Communications in November, 1995. He began with how difficult it is to understand pressure in flow, mentioning Cisotti's paradox, and stressed that the force on an object in flow should be considered as the momentum flux of the flow absorbed by it. In connection with this, various interesting stories he experienced were interwoven.

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