• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.1
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• pp.37-43
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• 1996

Breaking waves were generated in a 2-D flume. A piston-type wavemaker was operated in accordance with signals which consist of elementary harmonics with appropriate phase differences. These phase differences were estimated by using a linear wave theory so that wave crests were to be concentrated at the same position. The stroke of wavemaker was controlled to create plunging-type breaking waves. The signal with small amplitude could not generate breaking waves. In the case of moderate amplitudes, various breaking waves could be obtained. Most of breaking waves were spilling type. Only when the wavemaker was operated with appropriate amplitude, plunging-type breaking waves were generated. The parameters of breaking waves are the wave steepness and the frequency bandwidth. If the central frequency was low, breaking waves were not generated. Based on experimental data, we found that the wave height of breaking inception was H = 0.0113 gT$^2$. We also made computations by using a mixed Euler-Lagrangian scheme under the assumption of potential flow. The numerical results show good agreements with tank measurements.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.1B
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• pp.95-109
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• 2008

We numerically analyzed the nonlinear shoaling, a plunging breaker and its accompanying energetic suspension of sediment at a bed, and a redistribution of suspended sediments by a down rush of preceding waves and the following plunger using SPH with a Gaussian kernel function, Lagrangian Dynamic Smagorinsky model (LDS), Van Rijn's pick up function. In that process, we came to the conclusion that the conventional model for the tractive force at a bottom like a quadratic law can not accurately describe the rapidly accelerating flow over a swash zone, and propose new methodology to accurately estimate the bottom tractive force. Using newly proposed wave model in this study, we can successfully duplicate severely deformed water surface profile, free falling water particles, a queuing splash after the landing of water particles on the free surface and a wave finger due to the structured vortex on a rear side of wave crest (Narayanaswamy and Dalrymple, 2002), a circulation of suspended sediments over a swash zone, net transfer of sediments clouds suspended over a swash zone toward the offshore, which so far have been regarded very difficult features to mimic in the computational fluid mechanics.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.6
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• pp.606-613
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• 2007

This study investigates the behavior of a plunging wave and its associated runup and overtopping through velocity measurements and suggests an empirical formula for overtopping velocities on a structure. The plunging wave breaking in front of the structure generates very bubbly flow fields. For measurements of the two phase flow field of the breaking wave, particle image velocimetry and a modified optical method were employed. The obtained velocity fields were discussed in respect of the process of wave impinging, runup and overtopping. The overtopping velocity distribution is found to have a nonlinear profile showing a maximum magnitude at its front part. The relationship of self-similarity among dimensionless parameters is observed and used to obtain the regression formula to depict the overtopping velocity.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.7 no.4
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• pp.51-60
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• 1987

A two dimensional negatively buoyant flow from a shallow channel into rectangular ponds is analysed numerically by a finite difference scheme in a stretched coordinate system. In rectangular ponds, the confuguration of plunge flow largely depends on the densimetric Froude number, $Fr_e$, of the inflow. The velocity of front of the plunging flow increases with increasing negative buoyanty(decrease of $Fr_e$). The location of stable plunge point and maximun travel distance of plunge point are found to be a function of dimensionless time and densimetric Froude number. Large vortices develop on the both sides of negative buoyant flow or plunge flow.

• Journal of computational fluids engineering
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• v.8 no.2
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• pp.24-32
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• 2003

In this work, we propose a new idea of flapping airfoil design for optimal aerodynamic performance from detailed computational investigations of flow physics. Generally, flapping motion which is combined with pitching and plunging motion of airfoil, leads to complex flow features such as leading edge separation and vortex street. As it is well known, the mechanism of thrust generation of flapping airfoil is based on inverse Karman-vortex street. This vortex street induces jet-like flow field at the rear region of trailing edge and then generates thrust. The leading edge separation vortex can also play an important role with its aerodynamic performances. The flapping airfoil introduces an alternative propulsive way instead of the current inefficient propulsive system such as a propeller in the low Reynolds number flow. Thrust coefficient and propulsive efficiency are the two major parameters in the design of flapping airfoil as propulsive system. Through numerous computations, we found the specific physical flow phenomenon which governed the aerodynamic characteristics in flapping airfoil. Based on this physical insight, we could come up with a new kind of airfoil of tadpole-shaped and more enhanced aerodynamic performance.

• Water for future
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• v.27 no.2
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• pp.63-72
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• 1994

In this study, hydraulic characteristics for upstream migration of fish in a pool-and-weir fishway were analyzed through experiment. The results showed that streaming flow was preferable to plunging flow for upstream migration of fish and it was not good to make an orifice beneath the septum since it generates turbulent jet and eddies. Protrusions on the side wall of fishway were preferable to grooves since they decelerate flow velocity and make upstream migration easy. A vertically movable septum was necessary for a flushing of deposited bed material, and net installing over fishway was also needed for birds not to approach the fishway and eat fish.

• Journal of Korea Water Resources Association
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• v.41 no.7
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• pp.669-679
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• 2008

The present study examines similarity of behavior between an overtopping wave generated by a plunging wave and a dam-break flow through hydraulic model tests. The dam-break flow has been employed to estimate the overtopping effect on the basis of the dam-break flow's behavior similar to the overtopping. In this study, the overtopping velocity was measured by a modified image technique using bubble and bubble texture images called bubble image velocitmetry. From the measurements, the vertical profiles of horizontal overtopping velocity at cross-sections along the deck were presented and discussed. Maximum velocity and depth-averaged velocity at each cross-section were compared with an analytical solution solving the dam-break flow, Ritter's solution. The initial water depth of importance for the solution was determined from the tested wave condition and the overtopping measurements. The comparison shows that the solution with the initial water depth estimated using the front velocity of the overtopping wave is in good agreements with the measurements.

• 한국전산유체공학회:학술대회논문집
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• 2002.10a
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• pp.35-40
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• 2002

This paper deals with a thrust generation on flapping-airfoil by dynamic stall. Dynamic stall refers to a series of complicated aerodynamic phenomena accompanied by a stall delay in unsteady motion. In most cases, once it occurs, the dynamic stall may lead to an abrupt fluctuation of aerodynamic forces. An inverse $k\acute{a}rm\acute{a}n$ vortex has been considered as a main reason for a thrust generation. In this paper, however, we have found out that a thrust is closely related to reduced frequency and leading edge vortex in addition to inverse Karman vortex. In order to certify our opinion, picking and plunging motions were calculated with the parameter of amplitude and frequency by using the unsteady, incompressible Navier-Stokes flow solver with a two-equation turbulence model. For more efficient computation, it is parallelized by MPI programming method.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.6
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• pp.873-879
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• 2009

The steady breakers at an infant stage are investigated through the numerical simulation. The appearing condition and characteristics of the sub-breaking waves are reviewed by analysing bow waves. The instability analysis is possibly done through the relationship between the free-surface curvature and circumferential force, which is obtained from the momentum equations. Navier-Stokes equations are solved by a finite difference method where the body-fitted coordinate system, the wall function and the advanced mesh system are invoked. The numerical result shows that the gradient of M/$U_s$ is greatly influenced by the Froude number and the decrease of M/$U_s$ indicates that the flows are unstable. Additionally flows with plunging or spilling are simulated successfully, but the application of breakers to the severely broken wave still remains to be settled in the future.

• Journal of Advanced Research in Ocean Engineering
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• v.4 no.4
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• pp.163-173
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• 2018

There are many issues in fluid structure interactions when dealing with the free surface flows in a sloshing tank. For example the problem of how yielding a highly nonlinear wave with a simple forced motion over a short duration is of concern here. Nonlinear waves are generated in a rectangular tank which is forced horizontally; its motion consists of a single cycle of oscillation. One of the objectives is to end up with a shape of the free surface yielding a wide range of critical flows by tuning few parameters. The configuration that is studied here concerns a plunging breaker accompanied with a critical jet where great kinematics are simulated. The numerical simulations are performed with a twodimensional code which solves the fully nonlinear free surface boundary conditions in Potential Theory.