Hiroji Nakagawa

Prediction of the contributions to the Reynolds stress from bursting events in open-channel flows

In this paper we intend to predict the magnitude of the contribution to the Reynolds stress of bursting events: ‘ejections’, ‘sweeps’, ‘inward interactions’ and ‘outward interactions’. We shall do this by making use of the conditional probability distribution of the Reynolds stress − uv , which can be derived by applying the cumulant-discard method to the Gram-Charlier probability distribution of the two variables u and v . The Reynolds-stress fluctuations in openchannel flows over smooth and rough beds are measured by dual-sensor hot-film anemometers, whose signals are conditionally sampled and sorted into the four quadrants of the u, v plane by using a high-speed digital data processing system. We shall verify that even the third-order conditional probability distribution of the Reynolds stress shows fairly good agreement with the experimental results and that the sequence of events in the bursting process, i.e. ejections, sweeps and interactions, is directly related to the turbulent energy budget in the form of turbulent diffusion. Also, we shall show that the roughness effect is marked in the area from the wall to the middle of the equilibrium region, and that sweeps appear to be more important than ejections as the roughness increases and as the distance from the wall decreases.

Structure of space-time correlations of bursting phenomena in an open-channel flow

The present study is to investigate the structure of space-time correlations of bursting motions, such as ejections and sweeps in an open-channel flow, by a new conditional sampling analysis of the instantaneous velocity and Reynolds-stress signals measured simultaneously by two dual-sensor hot-film probes. One probe was fixed near the edge of the buffer layer, while the other probe was moved in the streamwise, vertical and spanwise directions. The sorted instantaneous Reynolds-stress signals obtained from the fixed probe were used as a detecting information of the occurrences of ejection or sweep events. The streamwise and vertical spatial characteristics of the ejection-sweep motions, and their convection process are investigated in detail. Also, the spanwise spatial properties of the high- and low-speed streaks in the bursting motions are examined experimentally by the present conditional sampling method. Next, a qualitative model is proposed which attempts to explain the space-time structures of the bursting phenomenon, on the basis of the above anemometry information and other visual observations.

Three-dimensional turbulent structure in straight open channel flows

Secondary currents are generated and modified as a result of the anisotropy of turbulence, which is caused by the boundary conditions of the bed, the side-wall and the free surface, as well as the aspect ratio of the channel and the channel geometry. Secondary currents affect the primary mean flow, producing threedimensional structures. Such a generation mechanism for secondary currents in closed and open channel flows can be explained well by using the longitudinal vorticity equation. The secondary motions in open channel flows are quite different from those in closed channel flows.

Experimental investigation on turbulent structure of backward-facing step flow in an open channel

Turbulence measurements of the backward-facing step flow, including the reverse flow region, in an open channel were conducted using a two-component Laser Doppler anemometer. The mean velocity distribution and turbulence characteristics were revealed in and around the separated shear layer and recirculation region (separation bubble). Reattachment properties and the dynamic pressure distribution were analyzed varying the Reynolds and Froude numbers. The structure of the shear stress was also analyzed on the basis of the momentum equation. The separated step flow and its recirculation in an open channel are similar to that in boundary layer and duct flows. But, the former is more complicated than the latter, because the pressure is relaxed by the existence of the free surface. Finally, the exchange processes of mean-flow and turbulent energies as well as momentum were examined downstream of the step.

Secondary Currents in a Straight Channel Flow and the Relation to Its Aspect Ratio

The initiation and maintenance mechanism of multi-cellular secondary currents in a straight wide-river is quite unknown at present. This study investigated experimentally the existence of secondary currents over a flat bed by varying the aspect ratio of channel. It was suggested strongly that an initiation of cellular secondary currents may be motivated by the mutual interaction between the secondary currents and the sand bed.

Turbulent Structure of Backward-Facing Step Flow and Coherent Vortex Shedding from Reattachment in Open-Channel Flows

The turbulent structure of the backward-facing step flow, including the reverse flow region, in open-channel flows was investigated by making use of two sets of two-component Laser Doppler anemometers (LDA). The turbulence characteristics and reattachment properties were revealed. The time-averaged reattachment length was dependent on both the Reynolds number and Froude number in open-channel flows. In particular, it should be noted that the reattachment length became smaller in supercritical flows than in subcritical flows. The instantaneous reattachment point moved over a distance of the time-averaged reattachment length. This suggested strongly that a coherent structure of the kolk-boil vortex might be generated due to the unsteady and low-frequency motions of the reattachment point.

Unsteady Behaviour of Dunes

In alluvial streams, several kinds of sand waves are formed, and their scales and behaviours severely affect sediment transport rate and hydraulic resistance. Hence, it is very important on designing of hydraulic structures to estimate the behaviours of sand waves. The resistance law of flow in alluvial streams and the geometrical properties of sand waves have been investigated by many hydraulic researchers, and several prediction formulas have been proposed. For example, Chezy coefficient and dune length or height have been expressed by several hydraulic parameters such as flow depth and bed shear stress. These are, however, unknown parameters. Phenomenologically, hydraulic parameters are to be known according to the resistance law, and it may be affected by the scales of sand waves. In fact, known parameters in alluvial streams are only the properties of bed materials and fluid, the bed slope, and the flow discharge.

SEDIMENT-CLOUD BASED MODEL OF SUSPENSION OVER RIPPLE BED DUE TO WAVE ACTION

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Momentum Exchange between Fluid and Particles and Mutual Collisions among Particles in Saltation Layer

In order to make a general model for granular material transport in saltation motion by fluid, (i) collision and rebounding of saltating particles at the bed; (ii) interaction between fluid and particles; and (iii) mutual collisions among saltating particles are investigated.

Experimental and Numerical Study on Turbulent Structure and Secondary Currents in Compound Open-Channel Flows

It is very important in Hydraulic Engineering as well as in River Environment Engineering to investigate turbulent structures in compound open-channel f ows with flood plains. The present study is to conduct both the accurate turbulence mesurements with two-component fiber-optic laser Doppler anemometer ( LDA) and the numerical simulations using three-dimensional algebraic stress model (3-D ASM). Both of them were compared with each other. As the results, numerical calculations including secondary currents were in a good agreement with accurate LDA database of compound open-channel flows. Of particular significance is the anisotropy of turbulence and the interaction between main channel and flood plain.