Shinya Umeda

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 ...

An experimental study of compressed air generation using a pendulum wave energy converter

This study describes a novel system of compressed air generation using a pendulum wave energy converter installed in a coastal defense structure. The objective of this study is to understand how much energy from the incoming waves can be captured for use by the system. Laboratory experiments were carried out to determine the effects of wave and system load conditions on energy conversion efficiency and wave reflection. The test results show that the system can operate with a high degree of efficiency under standing wave and high-load conditions. A reduction of wave reflection can be achieved by the system under a wide range of wave conditions.

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.

Seasonal to Decadal Variability of Shoreline Position on a Multiple Sandbar Beach

ABSTRACT Umeda, S.; Yuhi, M., and Karunarathana, H., 2018. Seasonal to Decadal Variability of Shoreline Position on a Multiple Sandbar Beach. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 261–265. Coconut Creek (Florida), ISSN 0749-0208. Seasonal to decadal variability of shoreline position at a long multi-barred beach with gentle slopes was investigated using a dataset of biannual shoreline surveys of 23 years. An empirical orthogonal function (EOF) and spectral analysis on 15.2 km long stretch were conducted to describe characteristic patterns of shoreline variation at multiple scales in time and space. A relatively alongshore uniform shoreline migration toward offshore in summer and onshore in winter was dominant in the first EOF mode which secondarily exhibited a long-term trend of shoreline recession. Mid-term shoreline variations related to shoreline sand wave and longshore sandbars were represented by the combinations of the second to sixth EOF modes filtered through two bands of dominant frequency. The shoreline pattern reconstructed on a longer time scale represented a large scale shoreline undulation whose scale and migration speed were comparable to shoreline sand waves observed in various coasts. On the otherhand, the shoreline pattern reconstructed on a shorter time scale represented alongshore uniform shoreline migration toward onshore and offshore in about five-year cycles, which corresponded to the average cycle of net offshore sandbar migration observed at the site. The periodic shoreline recession-advance related to sandbar migration was estimated to be comparable to seasonal scale shoreline variation. A correlation analysis between seasonal scale shoreline variation and wave statistics exhibited that shoreline advance in summer increased with low wave heights. It was suggested that a moderate waves were responsible for seasonal scale shoreline advance in this coast.

Scour process around monopiles during various phases of sea storms

ABSTRACT Umeda, S., 2013. Scour process around monopiles during various phases of sea storms An understanding of scour processes during the passage of sea storms is particularly important for the design of structures and the development of effective scour protection works. This study investigates the scour process at a slender monopile during various phases of a storm to give a better understanding of scour and backfilling mechanisms. Laboratory experiments were carried out to simulate the scouring effect at a pile by a storm consisting of a succession of seven phases of irregular waves. Detailed examinations were conducted to explore variations in the scour depth and bed topography. It turns out that scour patterns resulting from irregular waves can be characterized using the Keulegan-Carpenter number and the Shields parameter (KC1/3, θ1/3), which require significant near-bottom velocities. The scour process during a heavy storm consists of four main phases: (1) no bed deformation, (2) accretion, (3) erosion, and (4) backfilling. The amount of scouring increases as the order of the phase increased during a storm. The scour during phase (2), accretion, is in the transient condition between clear-water scour and live-bed scour. The scour in phase (3), erosion, is distinguished by live-bed scour, horseshoe vortices, vortex shedding and cone-shaped scour holes with vortex ripples. The influence of backfilling, phase (4), as a storm decreases in intensity, has a significant effect on the scour depth around the perimeter of the pile. The scour depth at the end of the event is much lower than the maximum scour depth during the event. However, backfilling of the scour hole makes only a weak recovery in total sediment volume around the pile, because the backfilling is confined almost exclusively to the neighborhood of the pile.

NUMERICAL SIMULATION OF COUPLING MOTION BETWEEN BLADE AND FLUID FOR THE WATERWHEEL WITH YAWING BLADES

This paper presents the results of a numerical investigation on the flow field and the fluid force acting on a yawing blade of a new type waterwheel in unidirectional flow. Because the performance of the waterwheel heavily depends on the behavior of the spring-supported blades, we examined the effects of the spring stiffness coefficient and the rotational speed of waterwheel on the torque magnitude. Flow was simulated by solving the unsteady Navier-Stokes equations, in the Arbitrary Lagrangian-Eulerian formulation. Descriptions are given of the interaction between the viscous fluid and the blade. The results indicate that properly controlling of the spring coefficient is important o get a larger net force during one rotation. The net force becomes large when the wheel rotates at a speed of about 0.4 times as fast as the velocity of the main flow. Fairly good agreements have been obtained between the numerical results and the experimental for the relationship between the net force and the rotational speed.