• Journal of Ocean Engineering and Technology
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• v.32 no.4
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• pp.244-252
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• 2018

This paper provides quantification of the effects of the turbulence model and grid refinement on the analysis of tip vortex flows by using the RANS(Reynolds averaged Navier-Stokes) method. Numerical simulations of the tip vortex flows of the NACA $66_2$-415 elliptic hydrofoil were conducted, and two turbulence models for RANS closure were tested, i.e., the Realizable $k-{\varepsilon}$ model and the Reynolds stress transport model. Numerical results were compared with available experimental data, and it was shown that the data for the Reynolds stress transport model that were computed on the finest grid system had better agreement in reproducing the development and propagation of the tip vortex. The Realizable $k-{\varepsilon}$ model overestimated the turbulence level in the vortex core and showed a diffusive behavior of the tip vortex. The tip vortex cavitation on the hydrofoil and its trajectory also showed good agreement between the current numerical results that were obtained using the Reynolds stress transport model and the results observed in the experiment.

• Journal of the Society of Naval Architects of Korea
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• v.54 no.2
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• pp.96-101
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• 2017

Amphibious assault vehicles have been used in the Marine Corps. In recent years, their ability to move faster is becoming one of the most important considerations. At high speeds, the vehicle tends to sink at the stern and sometimes the opposite occurs. Such dynamic trim plays a significant role in determining the vehicle's hydrodynamic performance. Furthermore, an excessive trim by stern upsets the viewing angle. We have thus considered a stern hydrofoil to reduce the dynamic trim of the amphibious assault vehicle. Laboratory-scale resistance tests were conducted in a towing tank at the Seoul National University (SNU). This study aims to make a preliminary assessment of the hydrodynamic performance of the vehicle with the stern hydrofoil and to investigate permissible speed range of the vehicle. The experimental results show that the stern hydrofoil can successfully achieve a reduction of both the dynamic trim and the hydrodynamic resistance at running speeds above 20 km/h.

• Journal of Korean Port Research
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• v.13 no.1
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• pp.167-174
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• 1999

Numerical analysis of two-fluid flows for both water and air is carried out. Free-Surface flows with an arbitrary deformation have been simulated around two dimensional submerged hydrofoil. The computation is performed using a finite volume method with unstructured meshes and an interface capturing scheme to determine the shape of the free surface. The method uses control volumes with an arbitrary number of faces and allows cell-wise local mesh refinement. the integration in space is of second order based on midpoint rule integration and linear interpolation. The method is fully implicit and uses quadratic interpolation in time through three time levels The linear equation systems are solved by conjugate gradient type solvers and the non-linearity of equations is accounted for through picard iterations. The solution method is of pressure-correction type and solves sequentially the linearized momentum equations the continuity equation the conservation equation of one species and the equations or two turbulence quantities.

• Journal of the Society of Naval Architects of Korea
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• v.30 no.4
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• pp.31-38
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• 1993

Horn rudders or flap rudders with a gap between forward and after part are used for effective steering of a ship or a submerged body. It is necessary to analyze the effect of a gap since it affects the performance of rudders. In this paper, an equation to calculate the lift acting on a two-dimensional flap rudder in uniform flow is derived by using the thin hydrofoil theory and the analytic solution of viscous flow in a channel formed by two coaxial cylindrical walls to which a pressure gradient is applied.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.2
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• pp.33-46
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• 2004

The cavitating noise of underwater propeller is considered numerically in this study. The main purpose of this research is to analyze these noise sources from marine propeller. The approach for investigation is a potential based panel method coupled with acoustic analogy. To predict propeller sheet cavitation noise, the blade surface cavity is considered as a single valued pulsating volume of vapor attached to the blade surface. The time dependent cavity volume data are used for noise prediction. Furthermore, we analyze hydrofoil cavitation bubble behavior and noise using Eulerian/Lagrangian approach. Through this study, we can analyze dominant noise source of marine propeller and provide a basis for proper noise control strategies.

• Bulletin of the Society of Naval Architects of Korea
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• v.32 no.1
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• pp.32-35
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• 1995

고속선의 발전방향은 초고속화와 더불어 대형화에 초점이 맞추어져 있으며, 고속선의 대형화는 새로운 복합 지지방식을 필요로 한다. 복합 지지방식은 종래의 지지방식을 두 가지 이상 병행 이용하는 것인데, 현재 개발되고 있는 복합 비비방식들을 보면 내항성이 우수한 전몰 수중동 체(submerged lower hull)나 수중익등이 많이 응용되고 있다. 이는 초고속선에서 내항성능이 차지하는 비중이 매우 큼을 보여주는 것이라 하겠다. 또한, 선박의 대형화 그 자체도 내항성능을 제고시키는 한 방법이 된다. 이상에서 초고속선의 내항성능에 대해 개략적으로 살펴보았다. 현재 각국에서 대형 초고속선의 개발에 경쟁적으로 나서고 있음은 주지의 사실이다. 국내에서도 이에 대한 활발한 연구가 이루어지기를 바라고, 머지 않은 장래에 연구성과가 결실을 맺어 웬만한 황천에서도 안락한 항해가 가능한 우수한 초고속선이 개발되기를 기대해 본다.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2016.10a
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• pp.579-582
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• 2016

Recently, wireless communication system has been widely used in a variety of fields along with the remarkable advancement of communication technology. Sound communication system in underwater wireless communication is utilized in underwater communication since it has better transmission capacity than the existing RF. However, sound communication system has the problems such as low speed communication, transmission delay and limited bandwidth, and the studies have been actively conducted on the visible light communication underwater to improve these problems. Therefore, the visible light communication underwater using ASK modulation is suggested for the effective communication in this article.

• Journal of Ocean Engineering and Technology
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• v.15 no.4
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• pp.120-125
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• 2001

This paper describes the effect of fly ash replacement on the setting time of antiwashout underwater concrete, where cement was replaced by 0% to 50%. Experimental work was performed on the condition of sea water and in air to find out the characteristics of setting time between the concretes that were cast in air and cast in 15$^{\circ}C$ of sea water. The experimental results show that the setting time of underwater concrete with 50% replacement was delayed about 10 hours than normal concrete. And it can be concluded that, at the case of underseawater concrete addicted with fly ash, the delayed final setting times are shown as the function Tf=0.069F+7.69, where Tf is the delayed final setting time and F is quantity of fly ash, respectively. These results confirm that the setting time underseawater concrete could be prolonged.

• 한국기계연구소 소보
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• v.4 no.2
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• pp.49-63
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• 1982

In this paper, a classical gust problem is treated by using the numerical lifting¬-surface theory to verify the effect of gust-a sudden fluctuating fluid velocity around an object, which is normal to the main stream direction-on the hydrody¬namic forces, especially the mean thrust in upstream direction, acting on the two¬-dimensional flat plate. In this case, the mean thrust wholly resorts to the leading edge suction, and it is the same situation to the case of the heaving plate in uniform flow. The ph¬enomenon of leading edge suction is very important for the flapping propulsion of animals, typical to fish and birds, and can be related to the prediction of the hydrodynamic forces acting on marine propellers operating in gustlike wakes of ships. The results of this paper can be easily superposed to those of the reference [1J in order to solve the problem of the two-dimensional oscillacting plate in gust

• Journal of the Korean Society for Precision Engineering
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• v.22 no.8 s.173
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• pp.34-41
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• 2005

Generally, it is not easy to get a suitable controller for multi variable systems. If the modeling equation of the system can be found, it is possible to get LQR control as an optimal solution. This paper suggests an LQR learning method to design LQR controller without the modeling equation. The proposed algorithm uses the same cost function with error and input energy as LQR is used, and the LQR controller is trained to reduce the function. In this training process, the Jacobian matrix that informs the converging direction of the controller Is used. Jacobian means the relationship of output variations for input variations and can be approximately found by the simple experiments. In the simulations of a hydrofoil catamaran with multi variables, it can be confirmed that the training of LQR controller is possible by using the approximate Jacobian matrix instead of the modeling equation and this controller is not worse than the traditional LQR controller.