Characteristics of the momentum and scalar transfer processes of the airflow over a train of waves moving at a constant speed are investigated using a direct numerical simulation. We simulate the flow for various wave ages (c/u_* = 0, 2, 4, 5, 6, 8, 10, 12, 16 and 20), where c is the phase speed of the wind wave and u_* is the friction velocity on the wave surface. The results show that the vertical gradients of both the mean velocity and the mean scalar depend on the wave age at kz < 1.5 due to the effects of the wave-induced momentum flux and wave-induced scalar flux. Correspondingly, the drag coefficient and scalar exchange coefficient strongly depend on the wave age. The wave age dependence of the scalar transfer coefficient is approximately the same strength as that of the drag coefficient.

View full abstract

The concept of the equilibrium slope which was first studied by Bagnold was reconsidered from the fundamental point of view. Given the measured seabed slope of the real coasts as the equilibrium slope, simple equations for evaluating sediment transport flux corresponding to the wave field were derived without dividing cross-shore and longshore sand transports. Applying the energetics model by Bagnold, the equations of sediment transport flux using only wave conditions at the breaking point as the external force were derived, and a numerical model for predicting beach changes were developed based on these equations. Present model was applied to investigate the refilling mechanism of an offshore dredging hole as well as the formation of scarp, beach changes around groins built on a coast with predominant longshore sand transport and the meandering of the offshore contours, and beach changes around the headlands built using a pair of offshore breakwaters on the Shimizu coast in Shizuoka Prefecture. Finally the model was applied to reproduce beach changes on the Toban coast, where beach changes between groins were measured. The appropriateness of the model was verified by these calculations.

View full abstract

In Hiroshima Bay connected to the Seto Inland Sea, the fluid mud layer is generated at the sea bottom by the overload of nutrient salt. It is assumed that the fluid mud layer is the cause of water quality degradation of Hiroshima Bay. The pore water movement in the sediment at the sea bottom is studied by local observations, laboratory experiment, and numerical simulation. The turbidity increasing does not only depend on the velocity above the bottom but also on the movement of pore water in the consolidated sediment. According to the experiment, the fluid mud doesn’t well up when the velocity of under 6cm/s is occurred above the bed. In these results, the bottom flow, observed at the head of Hiroshima Bay, cannot cause the upwelling of the bottom sediment. Together with the seasonal variations of fiscal and chemical states in the consolidate sediment, the pore water movement could have its influence on the sea bottom environment.

View full abstract

Hysteresis is known to arise in stage-discharge relations of flow in streams with movable beds. In certain cases, the hysteresis is caused by the transition of riverbed forms between dune and flat bed regimes. From a large number of field and experimental data, it is commonly said that the resistance relation is a multivalued function of hydraulic parameters because two different bed forms can appear alternatively under a single hydraulic condition. The authors have proposed a mathematical model to provide a physical explanation for the hysteresis arising in stage-discharge relations due to the dune-flat bed transition, and two different bed forms appearing alternatively under a single hydraulic condition. They performed a weakly nonlinear stability analysis with the use of the growth rate expansion method incorporated with a multiple-scale perturbation technique, and showed that the dune-flat bed transition is characterized by subcritical bifurcation. In this paper, a similar analysis is performed without approximations employed in the previous analysis. We also take into account an appropriate dependency between the Froude and Shields numbers. In addition, we redefine the Froude number as a bifurcation parameter, and reevaluate experimental data in order to validate the analysis. The analysis reveals that the dune-flat bed transition is characterized by subcritical bifurcation in the range of small average bed slopes and large velocity coefficients. A comparison between experimental data and the result of the analysis show a good agreement in that two different bed forms can appear alternatively under a single hydraulic condition at least in the range of large velocity coefficients.

View full abstract

We developed a new fluid solver that combines strengths of both particles and grids. Massless lagrangian marker particles are put into the Eulerian grid and advected according to the velocity field to capture the interface. In the particle framework, the density function that describes the interface between the different phases is corrected using suitable kernel functions of a SPH method. The present method coupling a CIP method with a SPH method (a Particle CIP method) is therefore able to simulate fluid pouring and splashing. The outline of the paper is as follows: First the governing equations and Particle CIP method are described on the basis of the CIP method. Next, the effectiveness and usefulness of the present method are evaluated qualitatively and quantitatively in two and three dimensional benchmark problems and free surface flow problems: Layleigh Taylor instability and collapse of a water column.

View full abstract

Numerical computations of tsunami-generation processes due to seabed deformation were performed using a 3D model for incompressible fluids, resulting in accurate “initial profiles” of tsunamis. The seabed displacement is described by the temporal change of porosity around the sea bottom, while the water surface elevation is calculated through a 3D VOF method. The initial profile of tsunamis, whose generation is dependent on the initial water depth, is not always same as the permanent shift of seabed. The dynamic pressure of fluid appears due to the vertical acceleration of seabed uplift. Tsunami phenomena including both generation and propagation were simulated, as well as runups in V2D cases.

View full abstract

It is needed to reveal coherent horizontal vortices in meandering compound open-channel flows for predicting the transport and distribution of suspended sediment in actual rivers. However, in previous studies, flow-visualization techniques such as PIV and PTV, have not sufficiently been applied to such flows, and thus, there is almost no detailed information on coherent turbulence in meandering compound open-channel flows. Therefore, in this study, we conducted innovative flow-visualization measurements by using a multi-layer scanning PIV system, and analyzed instantaneous velocity vectors on horizontal planes at several different elevations simultaneously. As the results, the time-space correlations between the different elevations, the vortex-core motion and POD properties were revealed in the meandering compound open-channel flows.

View full abstract

A gas-liquid two-phase approach is proposed in which a free surface is treated not as a boundary but as a strongly-stable density interface. In this framework, the unresolved surface-turbulence interaction can be treated readily as the buoyancy effects. This new approach is based on clear physics, needs no ad hoc assumptions, and is hence widely applicable. Here the approach is incorporated in a finite-difference method along with RANS turbulence closures and a level-set interface-capturing method. Applications to open-channel flows including a flow over a trench with large surface deformation show favorable accuracy of the method and also indicate the need of employing more sophisticated RANS models for better performance.

View full abstract

Owing to the characteristics of Taiwan’s watersheds includes erodible solid, uneven rainfall, steep slopes and high mountains, only 30 percent of terrestrial surface is not occupied by mountains. Traditional river engineering work has always emphasized safe and practical, but not necessarily ecologically or environmentally sound designs. In recent years, however, environmental awareness and the demands of outdoor recreation have risen dramatically. Large scaled river "beautification" projects have received considerable attentions and many have been implemented. On the other hand, still relatively few engineering projects are designed for maintaining ecological functions or habitat protection and restoration. The lack of close communication among biologists, ecologists, and civil and environmental engineers has been recognized as an important factor. Another significant factor is the scarcity of data on the quantitative relationship between hydraulic patterns, streamflow structure and habitat requirement for specific target species. In this paper, a brief description of the status of activities regarding "ecological engineering methods" in Taiwan is given. A case example of recent collaborative efforts including the hatchery center, streambank treatment, and backwater area made for saving the Formosan landlocked salmon population is described, with emphasis on the quantification of the relationship between refuges and habitat requirements of the endangered fish. A two-dimensional computational fluid dynamic model is used to help the design of backwater area. It shows that the hydraulic model can be considered as a suitable technique for habitat restoration.

View full abstract

A roughness and time dependent mixing length equation is developed from an extension of von Kármán’s similarity hypothesis, based on local equilibrium (turbulent energy production balanced by the dissipation), with an algebraic equation for the shape of the turbulent kinetic energy. Our vertical mixing length profile and the related mean velocities approach the experimental data well. We show that our equation follows Prandtl’s mixing length equation only near a smooth wall. The use of the proposed time-dependent mixing length equation in a turbulent oscillatory boundary layer shows, like the k-ε model, an increase in the mixing length and the eddy viscosity near flow reversal.

View full abstract