Development of Measurement and Analysis System for 3-Dimensional Instantaneous Image of Turbulent Flows, and Application to Real River Flows.

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1. Aims : Most flows encountered in our environments are turbulent. Turbulence augments the ability of momentum and material exchanges. In river flows, it contributes to increases in the transport capasity of sediment material, the action of scoring river beds and banks and the diffusivity of pollutants. In order to understand fundamentally these processes, the investigation of the coherent structure or large eddy structure of turbulence are indispendable. For more than two decades, the intensive researches have been performed on the problem. However, most of the knowledge on coherent turbulence has been obtained from small scale laboratory experiments and in the range of low Reynolds numbers. One of other defects of these researches is that the results have been deduced by the technique of conditional sampling and ensemble averages, which as a result of averaging distorts the real image of turbulence.Consequently, this research aims at (1) to establish the measurement and analysis sys … More tem of 3-dimensional instantaneous images of turbulence, and (2) apply them to a real river flow in flood periods.2. Method : (1) Multi-point velocity measurement instruments (hot-wires in wind tunnel and small scale electro-magnetic velocimeters (probe : 5 mmphi and 5cm length) in use for rivers) have been developed.(2) Inverse estimation of flow fields from sparse data has been established by application of an efficient inter- and extrapolation method and the MASCON method, which improves through the variational principle the raw inter- and extrapolated data to satisfy the continuity equation.3. Results : (1) From velosity data obtained simultaneously at sparselly positioned measuring points, the instantaneous image of large-scale turbulence can be reconstructed by the MASCON method.(2) The system and method has been applied to the turbulence research in a wind tunnel as well as a river flow in flood periods (Himuma-gawa).(3) The coherent structures of turbulence observed in low Reynolds number ranges have been confirmed in real river flows with high Reynolds numbers. However, it lacks a symmetric feature in terms of a mean flow direction.(4) The phenomenon of "Boil" which is a nearly circular violent motion of water surface observed frequently in flood periods in shown to be caused by a strong upward turbulence. Less