Masatoshi Yuhi

Slow flow of a Bingham fluid in a shallow channel of finite width

We present a theory for the three-dimensional flow of a Bingham-plastic fluid in a shallow and wide channel. Focusing attention on slow flows appropriate for gentle slopes, low discharge rates or the final stage of deposition, we ignore inertia and apply the long-wave approximation. For steady flows, the velocity distribution, total discharge, and section-averaged flux are obtained analytically in terms of the fluid property and the geometry of the channel cross-section. Nonlinear stationary waves, which connect a uniform depth upstream to another uniform depth downstream, are then investigated, for both wet and dry beds. A numerical scheme is applied to calculate the transient flow evolution. The final development of the stationary wave due to steady discharge upstream is obtained numerically and the relation between the tongue-like shape of the wave front and the fluid property is discussed. The phase speed of the stationary wave is also derived analytically. Finally, the transient spreading of a finite fluid mass released from a reservoir after a dam break is simulated numerically. The transient development of the front and the final extent of deposition are examined.

Numerical analysis of gas-particle two-phase flows

This paper is concerned with a numerical analysis of axisymmetric gas-particle twophase flows. Underexpanded supersonic free-jet flows and supersonic flows around a truncated cylinder of gas-particle mixtures are solved numerically on the super computer Fujitsu VP-400. The gas phase is treated as a continuum medium, and the particle phase is treated partly as a discrete one. The particle cloud is divided into a large number of small clouds. In each cloud, the particles are approximated to have the same velocity and temperature. The particle flow field is obtained by following these individual clouds separately in the whole computational domain. In estimating the momentum and heat transfer rates from the particle phase to the gas phase, the contributions from these clouds are averaged over some volume whose characteristic length is small compared with the characteristic length of the flow field but large compared with that of the clouds. The results so obtained reveal that the flow characteristics of the gas-particle mixtures are widely different from those of the dust-free gas at many points.

Slow spreading of fluid mud over a conical surface

We investigate the slow spreading of fluid mud over a gently sloped conical surface which may simulate a shallow basin or a hill. The mud is assumed to behave as a Bingham plastic possessing a finite yield stress and the lubrication approximation is used. Because of the finite yield stress, a variety of non-trivial equilibrium profiles can exist, corresponding to the state of deposit at the end of upward or downward motion. Analytical solutions are derived for axially symmetric deposits. It is shown that the front of the final profile in axisymmetric spreading is of a common form in dimensionless variables, independent of the total mud volume. Transient evolutions are then studied numerically by employing a finite-volume scheme for both axially symmetric and asymmetric spreading from a localized source. The characteristic features of the mud pile at different stages of spreading are examined. The final shape in asymmetric spreading is strongly affected by the total volume released and by the rate of discharge.

ANALYTICAL SOLUTION FOR WAVE-INDUCED SEABED RESPONSE IN A SOIL-WATER TWO-PHASE MIXTURE

The dynamic response of the seabed to ocean surface waves is treated analytically on the basis of an elastic wave theory in a soil-water mixture. The seabed is modeled as the aggregate of a poro-el...

Supersonic gas-particle two-phase flow around a sphere

This paper describes supersonic flows of a gas-particle mixture around a sphere. The Euler equations for a gas-phase interacting with a particle one are solved by using a TVD (Total Variation Diminishing) scheme developed by Chakravarthy & Osher, and the particle phase is solved by applying a discrete particle-cloud model. First, steady two-phase flows with a finite loading ratio are simulated. By comparing in detail the dusty results with the dust-free ones, the effects of the presence of particles on the flow field in the shock layer are clarified. Also an attempt to correlate the particle behaviours is made with universal parameters such as the Stokes number and the particle loading ratio. Next, non-steady two-phase flows are treated. Impingement of a large particle-cloud on a shock layer of a dust-free gas in front of a sphere is numerically simulated. The effect of particles rebounded from the sphere is taken into account. It is shown that a temporal reverse flow region of the gas is induced near the body axis in the shock layer, which is responsible for the appearance of the gas flow region where the pressure gradient becomes negative along the body surface. These phenomena are consistent with the previous experimental observations. It will be shown that the present results support a flow model for the particle-induced flow field postulated in connection with ‘heating augmentation ’ found in the heat transfer measurement in hypersonic particle erosion environments. The particle behaviour in such flows is so complicated that it is almost impossible to treat the particle phase as an ordinary continuum medium.

Stem waves along vertical wall due to random wave incidence

Abstract This study investigates stem waves, propagating along a vertical wall, due to obliquely incident random waves through laboratory experiments and numerical simulations. Attention is paid to the difference or similarity between the stem waves due to periodic waves and random waves, the nonlinear and linear characteristics, and the effect of wave breaking on the evolution of stem waves. The following were found from this study: as the incident angle of waves become large or the nonlinearity of the incident waves become small, the significant stem wave height, normalized by the incident significant wave height, becomes large. This tendency is the same as that generated by the Stokes waves or cnoidal waves. However, regardless of the nonlinearity of incident waves, the width of stem waves is almost the same. This is a different point between the stem waves due to periodic and random waves. The wave breaking suppresses the growth of the stem waves.

A NUMERICAL STUDY OF SINUSOIDAL OSCILLATORY FLOWS AROUND A VERTICAL WALL-MOUNTED CIRCULAR CYLINDER

This study concerns the vortex motions around the base of a vertical wall-mounted circular cylinder exposed to sinusoidal oscillatory flows. The flow field is simulated using the full three-dimensional unsteady Navier-Stokes equations. First, preceding three-dimensional simulations, a series of calculations have been carried out for two-dimensional oscillatory flows around a cylinder. Several patterns of vortex shedding are identified within particular ranges of flow amplitude. Fairly good agreements have been obtained between numerical and experimental results for vortex motions and flow induced forces. Next, three-dimensional flow fields around the base of a vertical wall-mounted cylinder have been investigated. The range of Keulegan-Carpentar number (KC) is from 1 to 20. The influence of KC on the development of the horseshoe vortex is reproduced quite well. The predicted lee-wake flow regimes above the bed boundary layer are similar to the two-dimensional results, while the vortex shedding in the bed ...

Numerical Study of Three-Dimensional Flow Fields Around the Base of a Vertical Cylinder in Oscillatory plus Mean Flow

This paper presents the results of a numerical investigation on the flow fields around a cylinder exposed to oscillatory plus mean flow. The flow fields were simulated using the Navier-Stokes equations expressed in a generalized curvilinear coordinate system. Descriptions are given of the three-dimensional flow structures around the base of the cylinder and examinations are made of the effects of the coexisting current ratio V (sub r), and the Keulegan-Carpenter number, KC, on the vortex motion. It is shown that the horseshoe vortex can be correlated well with a single dimensionless parameter. The bed shear stress under the vortices is also investigated in detail. The amplification of the bed shear stress is heavily influenced by V (sub r) and KC. The results show that the maximum shear stress beneath the horseshoe vortex is sufficiently larger than that of the pure oscillatory flow and the value is less than that of the pure current case.

Comparison of accelerated erosion in riverbed and downstream coast by EOF analysis

ABSTRACT Yuhi, M., Dang, M.H., Umeda, S., 2013. Comparison of accelerated erosion in riverbed and downstream coast by EOF analysis. This study investigates the long-term variations of sediment volume in an integrated watershed composed of the Tedori River and the Ishikawa Coast, Japan, where both the riverbed and the coast have experienced rapid and serious erosion due to the combined effects of natural and anthropogenic factors. Contemporary data for coastal and riverbed evolution were compiled and compared, and the decadal variations of the seabed and riverbed elevation were examined using a set of field surveys collected over several decades. These topographic data have been analyzed using empirical orthogonal functions (EOFs) to determine the coastal responses to the reduction of sediment supply related to the antecedent riverbed erosion. It was shown that the characteristics of volumetric variation could be captured well by the temporal coefficient of the first mode of EOFs. The study area was divided into several sub-regions, and the variations of sediment volume were examined on decadal scales. The correlation between the variations has been discussed in relation to the anthropogenic modification such as the material mining in the river basin and the construction of coastal structures. On the regional scale, the correlation between the river and coastal evolution showed that the anthropogenic modifications in the fluvial basin have led to an accelerated imbalance of the coastal sediment budget, while the modified river regulations and construction of coastal structures have recently mitigated the river and coastal erosion. Several years of time lag were recognized between the riverbed and the coastal erosion.

SHOCK WAVES IN NONUNIFORM GAS

Shock waves propagating through a stratified gas are investigated numerically and analytically. A shock wave is produced by a piston that begins to move from rest abruptly at some constant velocity in a two-dimensional horizontal duct. Initially the gas in the duct has a temperature or density distribution only along the vertical axis at a constant pressure. The initial density distribution, which is assumed to change monotonically, has a zero spatial gradient at the upper and lower walls and then has a single inflection point. It is confirmed that at least three types of shock patterns can be realized asymptotically. The first is a single curved shock, the second is a shock with a Mach stem (Mach reflection), and the last is a shock with a reflected branch (regular reflection). The first is substantially steady but the latter two are essentially unsteady. The time evolution of the induced flow field is investigated in detail. Based on this information, an analytical solution for the substantially steady curved shock is obtained in the coordinate system fixed on the shock. The shock profile as well as the induced flow field is investigated in detail with this solution. It is shown that the analytical results can predict quite well the numerical results. Finally, the flow instability of this shock-induced flow is investigated, because the induced flow has a nonuniform horizontal velocity distribution along the vertical axis at a constant pressure.