• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2002.10a
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• pp.245-250
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• 2002

This report describes the effects of following flaws due to ship's propeller on the fish farm structure when the ship's propeller is operated in full power. This study is applied an incompressible newtonian fluid theory, which is governed the Navier-Stokes equation. For the numerical solution, Neumann equation are applied as the boundary conditions. The result shows that the flow velocity near the fish farm is 1.0 m/sec. The actual measurement carries out by using propeller type velocimeter in order to measure the velocity of following flows and currents around the fish farm area. The result shows that the maximum velocity near the fish farm structure is 1.2 m/sec in depth of 1.5 m. This velocity is used for calculation of external force on the fish farm structure. The results of structural strength of the fish farm structures show that the actual maximum bending moment and bending stress are less than the damage strength of material. So the fish farm structure is not affected by the following flows and currents of ship's propeller.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• v.1
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• pp.45-49
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• 2006

The flow around a foil with waterjet was investigated using the two-frame PIV(CACTUS 3.1) system. After separation, unsteady recirculation & reattachment region was shown as a result at reading edge. Separation area was decreased to 1/3 more by waterjet system with coanda effect. Angle of attack and waterjet velocity was a variable in the experiment. Each parameters was controlled to $0^{\circ}{\sim}35^{\circ}$ and 0[m/s]${\sim}$9.2[m/s]. The separation of flow appearanced at first when the angle of attack is $17^{\circ}{\sim}18^{\circ}$. However, according to grew up of velocity, beginning of the separation was delayed. In this experiment, vortex and separation region was disappeared by blown when each parameters are low level, and separation controlled more certainly.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.3
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• pp.1-9
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• 2005

The interaction between hypersonic free stream and side jet flow at high altitudes is investigated by using the direct simulation Monte Carlo (DSMC) method. In order to alleviate the difficulty associated with the large density difference between the free stream and the side jet flow and to simulate the two flows simultaneously, a weighting factor technique is applied. For validation, the corner flow over a pair of plates perpendicularly attached is calculated with and without a side jet, and the results are compared with experiment. For a more realistic configuration, the flow past a blunted cone cylinder shape is solved. The leeward or windward jet is injected into the free stream and the effect on the aerodynamic force and moment is observed at various flow angles. The lambda shock effect and the wake structure are studied in terms of the surface pressure differential. A higher interaction between the free stream and the side jet flow is observed when the side jet is injected in the windward direction.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.2
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• pp.139-146
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• 2009

In this study, an assessment is made for the helicopter vibration reduction of composite rotor blades using an active twist control concept. The piezoceramic shear actuation mechanism along with elastic couplings of composite blades is used for vibration reduction. The rotor blades are modeled as composite box-beams with actuator layers bonded on the outer surfaces of the thin-walled section. The governing equations of motion for helicopter blades are obtained using Hamilton's principle. A time domain unsteady aerodynamic theory with free wake model is used to obtain the airloads. Various rotor configurations with different elastic couplings with appropriate actuator placement are used to investigate the hub vibration characteristics. Numerical results show that a substantial reduction of $N_b$/rev hub vibration can be achieved using the optimal control algorithm.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.8
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• pp.729-736
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• 2009

In order to understand flow characteristics and flight mechanism of hummingbird's flapping flight, two-dimensional numerical analysis is carried out on the flapping motion of hummingbird, Selasphorus rufus. Hummingbird's flapping wing motion is realistically modeled from wind tunnel experimental data to perform numerical analysis. Numerical simulation shows that, as freestream velocity changes, wing trajectory is also adjusted and it substantially affects lift and thrust generation mechanism. According to this tendency, flight domain is separated as "low speed" and "high speed" regime, and each flight domain is studied for physical understanding. As a result, the lift generation during downstroke can be explained by the well-known effects, such as leading edge vortex effect, delayed stall, wake capture and so on. In addition, the lift generation during upstroke, the unique character of hummingbird, is also examined by detailed flow analysis. The thrust generation mechanism is investigated by examining the hummingbird's wing bone structure, vortex generation pattern and the resulting pressure gradient.

• Smart Media Journal
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• v.8 no.4
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• pp.38-45
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• 2019

When there is a spray flow such as from a pesticide nozzle, winds affect the droplet flow of a rotary-wing drone accompanied by a strong wake, with a severe oscillation. Especially, during forwarding flights or when winds come from the side, compare to a simple hovering flight as the droplet is in the effect of aerodynamic drag force, the effect of spraying region becomes even larger. For this reason, the spraying of pesticides using drones may cause a greater risk of scattering or a difference in droplet dispersion between locations, resulting in a decrease in efficiency. Therefore, through proper numerical modeling and its applied simulation, an indication tool is required applicable for the various flight and atmospheric conditions. In this research, we completed both experiment and numerical analysis for the strong downwash from the rotor and flight velocity of the drone by comparing the probability density function of droplet distribution to build a spraying system that can improve the efficiency when spraying droplets in the pesticide spray drone.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.9
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• pp.6305-6314
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• 2015

This study presents numerical simulations of mean flow and turbulence structure of an open channel with submerged vegetation. Filtered Navier-Stokes equations are solved using large-eddy simulation (LES). The immersed boundary method (IBM) is employed based on a Cartesian grid. The numerical result is compared with experimental data of Liu et al. (2008) and shows that simulated results coincided reasonably with experimental data within the average error of 10%. Strong vortices are generated at the interface between vegetated and non-vegetated regions with spanwise extent. The generation of turbulence induced by shear at the interface is interfered with wake turbulence, resulting turbulence intensity maximum. Turbulence produced by shear affects the flow in vegetated region and the penetration depth increases with an increase in the submergence ratio. This result can be used to understand sediment transport mechanisms in the vegetated region.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.4
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• pp.50-56
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• 2012

Vent mixer can provide main flow directly into a recirculation region downstream of the mixer to enhance fuel-air mixing efficiency. Based on experimental results of three-dimensional velocity, vorticity and turbulent kinetic energy obtained by a stereoscopic PIV method, the performance of the vent mixer was compared with that of the step mixer which was used as a basic model. Thick shear layers of the vent mixer induced the increase of the penetration height. The turbulent kinetic energy mainly distributed along a boundary layer between the main flow and the jet plume. This turbulent field activates mass transfer in a mixing region, leading to the mixing enhancement.

• Journal of the Society of Naval Architects of Korea
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• v.31 no.1
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• pp.84-93
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• 1994

The purpose of present study is to investigate the surface roughness on tip-vortex cavitation and its induced noise, emanating from an elliptic wing of NACA 0012 section. Roughness elements of $200{\mu}m$ are applied to the 10% portion of wing tip, and then, the wing tip as well as the leading edge. It is shown from cavitation observation that the cavitation inception is first visible at about half chord downstream of wing tip for most experimental conditions, and developed into the tip-vortex cavitation and finally the fully developed cavitation as cavitation number is decreased. Acoustic noise generated by a tip-vortex cavitation has its frequency range of 3 kHz to 50 kHz, while the fully-developed cavitation at lower cavitation number induces a broad band spectrum. It is also shown that, when the roughness elements are applied to the wing tip and the leading edge, the cavitation characteristics and its induced noise are improved. Moreover, it is appeared that the condition at which the rough surface is at pressure side gives a better result. although its lift-drag ratio is reduced.

• Journal of the Society of Naval Architects of Korea
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• v.31 no.2
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• pp.45-56
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• 1994

A computationally efficient numerical method based on potential flow is developed for time-domain simulation of the nonlinear free-surface flows around a 2-dimensional hydrofoil. This numerical method, namely, spectral/boundary-element method, is a mixed one of the high-order spectral method and the boundary-element method in time-domain. The high-order spectral method is used to calculate the nonlinear evolution of free-surface, and the boundary-element method is used to calculate the effects of the hydrofoil and the shed vortex. As application examples, nonlinear free-surface flows around a 2-dimensional hydrofoil which starts from the rest and translates near the free-surface with or without harmonic oscillations are calculated. Nonlinear/unsteady results of free-surface waves and hydrodynamic farces are shown and discussed. Particularly, the results of steady-state which are obtained as a special case of the present unsteady solution are compared with others' calculated and experimental results, and good agreements are observed.