Faculty Profiles - NAKAMURA Tomoaki

Papers - NAKAMURA Tomoaki

Division display 181 - 200 of about 201 ／ All the affair displays >>

Local scour in front of quay wall caused by bow thruster and its countermeasure using filter units Reviewed

Mizutani, N., Nakamura, T., Shinoda, Y., and Koyama, H.

Proc., 19th International Offshore and Polar Engineering Conference, ISOPE page： 1280-1287 2009.6

Language：English

This study is intended to investigate local scour in front of a quay wall and its generation mechanism. Hydraulic model experiments have been conducted to investigate the local scour in front of quay wall. Also, countermeasures against local scour using armor stones and filter units are investigated based on the experiment. To consider the mechanism of local scour, numerical simulation has been conducted. The numerical computation method employed in this study includes a flow simulation inside seabed and dynamic response of seabed like sediment transport. Time variation of pore pressure obtained in the experiments indicates the generation of local liquefaction due to strong flow and its response of seabed, and this may encourage the local scour. Experimental results clearly show that armoring by filter units is effective to reduce the local scour.

Mechanism of backfilling sand discharge from a gap under a vertical revetment Reviewed

Nakamura, T., Hur, D.-S., and Mizutani, N.

Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE Vol. 134 ( 3 ) page： 178-186 2008.11

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Coastal disasters such as sliding, settlement, and overturning of coastal structures as well as subsidence of reclaimed areas occur due to toe scouring and backfilling sand discharge. In this study, the sand discharge mechanism from a gap under a vertical revetment without wave absorption and foot protection is investigated using hydraulic model experiments and newly developed numerical simulations. In the hydraulic model experiments, sand discharge and sinkholes similar to those observed in the field surveys are reproduced, and it is found that sand discharge is very sensitive to wave steepness and relative gap height. In numerical simulations, a comparison with the experimental data demonstrates the validity of the numerical methods. It is revealed that sand discharge tends to occur with an increase of the maximum seaward velocity just outside the gap under the revetment as well as the maximum volumetric strain inside the most offshore side of the sand inside the gap.

Study on estimation of collision force of a drifted container using fluid-structure interaction analysis Reviewed

Yeom, G.-S., Nakamura, T., Usami, A., and Mizutani, N.

Annual Journal of Coastal Engineering, JSCE Vol. 55 page： 281-285 2008.11

Language：Japanese Publishing type：Research paper (scientific journal)

This study proposes a calculation scheme which enables to estimate the collision time and force of drifted container by run-up tsunami. Simulated behaviors of the drifted container by a run-up wave are found to be in good agreement with the laboratory experimental results. A set of this calculated values are used as the initial values of the fluid-structure interaction analysis which simulates the collision of the drifted container. It is confirmed from these simulations that the added mass (water mass at the rear of the container) plays an important role on the collision and the collision force and time become constant when the adequate volume of the water at the rear of the drifting container is considered. Moreover, the effect of the added mass for the collision force and time is verified in full scale collision simulation.

Simulation on tsunami-induced topographic change using sediment transport formula with dynamic effective stress inside sand bed Reviewed

Nakamura, T.,and Mizutani, N.

Annual Journal of Civil Engineering in the Ocean, JSCE Vol. 24 page： 57-62 2008.7

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

A sediment transport formula coupled with dynamic effective stress inside a sand bed was proposed and incorporated into an existing topographic change model so that local scour could be reproduced under severe wave action such as run--up tsunamis, and topographic change simulation on tsunami--induced local scour around a square structure was performed to verify the applicability of the proposed model. Compared with hydraulic model experiments, the present model improved the computational accuracy of not only the initial stage of local scour at the seaward corner of the structure but also the maximum depth of a scour hole after tsunami wave action; hence, the present model was very useful in predicting deposition and erosion patterns under a strong influence of effective stress inside a sand bed.

Tsunami scour around a square structure Reviewed

Nakamura, T., Kuramitsu, Y., and Mizutani, N.

Coastal Engineering Journal, JSCE Vol. 50 ( 2 ) page： 209-246 2008.6

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Tsunami-induced local scour around a land-based square structure on a sand foundation was investigated with laboratory experiments and numerical simulations. The laboratory experiments revealed that a scour hole at the seaward corner of the structure could be reproduced and the maximum scour depth was affected by the relative tsunami wave height and embedded depth of the structure. It was also found with the numerical simulations that a scour hole around the seaward corner of the structure could be evaluated with not only flow velocities but also effective stresses because restricting force between sand particles decreased with the effective stresses.

Tsunami-induced local scour around a square structure Reviewed

Nakamura, T., Kuramitsu, Y., and Mizutani, N.

Proc., Solutions to Coastal Disasters 2008, ASCE page： 106-117 2008.4

Authorship：Lead author Language：English

Tsunami-induced local scour around a land-based structure with a square cross-section on a sand foundation was investigated with both laboratory experiments and numerical simulations. An isolated long wave was adopted as an incident tsunami wave. In the hydraulic model experiments, it was found that the maximum scour depth at the seaward corner of the structure was sensitive to the relative overtopping height of the tsunami wave. In the numerical simulations, it was revealed that the local scour around the seaward corner of the structure could be evaluated with not only flow velocities on the foundation but also effective stresses near the surface of the foundation because restricting force between sand particles declined due to a decrease in the effective stresses.

Effects of pore water pressure on scour - in case of local scour due to runup tsunami -

Nakamura, T., Mizutani, N., and Kuramitsu, Y.

Proc., Geomorphological Comparative Research on Natural Disaster Mitigation in the Coastal Regions of Tropical Asia page： 165-172 2007.11

Authorship：Lead author Language：English

Large-scale sediment transport due to the 2004 Indian Ocean Tsunami resulted in substantial erosion and scour around many structures. In this paper, we investigated tsunami-induced local scour around a land-based structure with a square cross-section on a sand foundation with both hydraulic model experiments and numerical simulations capable of computing interactions between tsunami waves and sand beds composed of nonlinear materials. In the hydraulic model experiments, we found that local scour holes around the seaward corner of the structure could be reproduced, and the maximum final scour depth at the seaward corner of the structure was sensitive to the relative incident tsunami wave height and the relative embedded depth of the structure. In the numerical simulations, we revealed that the tsunami-induced local scour around the seaward corner of the structure could be evaluated with not only flow velocity on the sand foundation but also effective stress inside the surface layer of the foundation because the effective stress acting on the foundation significantly affected restricting force between sand particles.

Local scour around a square structure due to a runup tsunami Reviewed

Nakamura, T., Kuramitsu, Y., and Mizutani, N.

Annual Journal of Coastal Engineering, JSCE Vol. 54 page： 856-860 2007.11

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

The 2004 Indian Ocean tsunami caused large-scale sediment transport, resulting in substantial scour around a huge number of structures. In this study, tsunami-induced local scour around a land-based structure with a rectangular cross-section on a sand foundation was investigated with a series of hydraulic model experiments and numerical simulations. It was, as a result, revealed that local scour around the seaward corner of the structure could be evaluated with not only flow velocities on the sand foundation but effective stresses near the surface of the sand substrate, that is, flow velocities with a large Shields number resulted in a scour hole around the seaward corner of the structure during restricting force between sand particles fell down due to a decrease in effective stresses inside the sand substrate.

Three-dimensional numerical analysis on runup tsunami deformation around containers on an apron and tsunami-induced wave pressure acting on the containers Reviewed

Nakamura, T., Shiraishi, K., Usami, A., Mizutani, N., Miyajima, S., and Tomita, T.

Proc., Asian and Pacific Coasts 2007 page： 865-879 2007.9

Authorship：Lead author Language：English

Many drifting debris resulted from a series of devastating tsunamis triggered by the Sumatra-Andaman earthquake. In recent years, a huge demand for container ships leads to the increase in containers piled up on an apron. Once such a huge tsunami strikes a port area, it is possible that drifting containers cause severe destructions of port facilities. In this study, runup tsunami deformation around containers fixed on an apron is investigated with a three-dimensional numerical simulation. It is consequently revealed that an upper limit of maximum wave pressure acting on the containers due to a runup tsunami wave is evaluated with maximum runup height in front of the containers, assuming a hydrostatic condition.

Local scour around a square structure due to runup tsunami Reviewed

Nakamura, T., Mizutani, N., and Kuramitsu, Y.

Proc., Coastal Structures 2007, ASCE page： 1891-1902 2007.7

Authorship：Lead author Language：English

On December 26, 2004, a series of lethal tsunamis caused by the Sumatra-Andaman earthquake struck the coastal areas of many countries along the Indian Ocean, resulting in substantial erosion and scour around coastal structures due to large-scale sediment transport. In this study, local scour due to a runup tsunami wave around a land-based square structure was investigated with both hydraulic model experiments and numerical simulations. As a result, the hydraulic model experiments revealed that the deepest scour hole was located near the seaward corner of the structure, and the maximum scour depth was affected by the relative incident tsunami wave height and the relative embedded depth of the structure. From the numerical simulations, it was also found that the local scour around the seaward corner of the structure resulted from flow velocities on a reclaimed beach during restricting force between sand particles fell down due to a decrease in effective stresses inside the sand at the seaward corner of the structure.

Tsunami wave force and its estimation method - forces on a rectangular body -

Mizutani, N., Nakamura, T., and Usami, A.

Proc., INDO-JAPAN Workshop on Coastal Problems and Mitigation Measures - Including the Effect of Tsunami page： 135-145 2007.7

Sand suction mechanism in artificial beach composed of rubble mound breakwater and reclaimed sand area Reviewed

Hur, D.-S., Nakamura, T., and Mizutani, N.

Ocean Engineering Vol. 34 ( 8-9 ) page： 1104-1119 2007.6

Language：English Publishing type：Research paper (scientific journal)

An artificial beach has been constructed compensating for losing of the natural one caused by the development of coastal area. In this paper, the hydraulic model tests are carried out to investigate the suction phenomenon on the artificial beach constituted of rubble mound breakwater with gravel and the reclaimed sand area. In addition, the numerical model for waves, structures and seabed interaction as well as the numerical method based on the u-p approximation of the Biot equations is developed for investigation of suction mechanism. After verification of the numerical models by comparing numerical results with experimental data, the numerical models are further used to clarify the detailed suction mechanism of the reclaimed sand. The factors that affect the suction phenomenon are examined experimentally and their critical values are presented. Also, it can be pointed out that the vertical discharge velocity as well as the volumetric strain around the still water level of the boundary between the breakwater and the beach gets up to the critical value, the reclaimed sand starts to flow out to the offshore, and it finally leads to caves and cave-ins in the reclaimed zone.

Study on dynamic stress inside a reclaimed land and its effect on local scour around a rectangular structure due to runup tsunami Reviewed

Nakamura, T., Kuramitsu, Y., and Mizutani, N.

Annual Journal of Coastal Engineering, JSCE Vol. 53 page： 521-525 2006.11

Study on mechanism of backfilling sand outflow and subsidence of the reclaimed beach behind a vertical seawall Reviewed

Nakamura, T., Hur, D.-S., and Mizutani, N.

Annual Journal of Coastal Engineering, JSCE Vol. 53 page： 836-840 2006.11

Tsunami wave force and its estimation method - forces on a rectangular body -

Mizutani, N. and Nakamura, T.

Proc., Geomorphological Comparative Research on Natural Disaster Mitigation in the Coastal Regions of Tropical Asia page： 25-31 2006.8

Language：English

Wave force due to run-up tsunami is investigated in this paper. Laboratory experiments have been conducted to measure wave pressure and forces on a rectangular body. Numerical simulation has also performed to compute deformation of tsunami on apron and resultant wave pressure and wave forces on the body. Present numerical simulation method has been shown to have excellent performance in simulating wave deformation and wave forces. Moreover, a simple estimation method of the wave force has been proposed in this study.

Three-dimensional numerical analysis on tsunami deformation due to container on apron and tsunami-induced wave pressure acting on container

Nakamura, T., Shiraishi, K., Usami, A., and Mizutani, N.

Proc., 8th International Summer Symposium, JSCE page： 107-110 2006.7

Three-dimensional numerical analysis on tsunami waves around containers on apron and tsunami-induced wave pressure and tsunami force acting on containers Reviewed

Nakamura, T., Shiraishi, K., Usami, A., Mizutani, N., Miyajima, S., and Tomita, T.

Annual Journal of Civil Engineering in the Ocean, JSCE Vol. 22 page： 517-522 2006.7

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

Tsunami-induced wave pressure and tsunami force acting on containers on an apron are investigated with a three-dimensional numerical simulation based on MARS (Multi-interface Advection and Reconstruction Solver) as well as hydraulic model experiments for the measurements of runup height on the apron and tsunami force acting on the containers. It is consequently found that the numerical method allows the good estimations of both the runup height on the apron and tsunami force acting on the containers. It is also revealed that the maximum runup height in front of the containers gives the maximum wave pressure on the containers with hydrostatic pressure approximation, however impact pressure on the lower part of the containers may exceed this hydrostatic water pressure.

Suction mechanism of reclaimed sand behind a rubble seawall Reviewed

Nakamura, T., Hur, D.-S., and Mizutani, N.

Journal of Hydraulic, Coastal and Environmental Engineering, JSCE Vol. 62 ( 1 ) page： 150-162 2006.3

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

Suction mechanism of reclaimed sand behind a rubble seawall has been investigated with hydraulic model experiments and numerical methods considering wave, structure and seabed interaction. It is consequently revealed that the Ursell parameter and the relative seawall width, namely, the ratio of the seawall width and the wavelength, have significant effects on the suction phenomenon regardless of the inclination of the seawall, and the sand suction easily occurs as vertical velocity in front of the beach and volumetric strain inside the sand increase. In addition, an increase of the sand/rubble diameter ratio results in a decrease of the aforementioned vertical velocity and the volumetric strain; hence efficiency of filter layers for the prevention of the sand suction is demonstrated.

Study on suction mechanism of reclaimed sand based on a numerical simulation of wave, structure and seabed interaction Reviewed

Nakamura, T., Hur, D.-S., and Mizutani, N.

Annual Journal of Coastal Engineering, JSCE Vol. 52 page： 836-840 2005.11

Experimental study on suction mechanism of reclaimed area behind a rubble mound breakwater Reviewed

Hur, D.-S., Nakamura, T., Mizutani, N., and Iwata, K.

Proc., Asian and Pacific Coasts 2005 page： 847-856 2005.9

Language：English

Suction mechanism of reclaimed sand behind a rubble mound breakwater, which has never been studied so far, is investigated with two-dimensional hydraulic model experiments. Two types of breakwater are adopted: an inclined rubble mound breakwater and an upright rubble mound breakwater. It is consequently revealed in the laboratory experiment that cave-ins and caves, which are quite similar to ones observed in the field, can be reproduced, and the Ursell parameter and the relative breakwater width have significant effects on sand suction. In addition, it is necessary that filters and/or geotextile sheets are installed for the prevention of sand suction accidents.

　 PREV - NEXT 　

<NAKAMURA Tomoaki>

Papers - NAKAMURA Tomoaki