• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.26 no.7
• /
• pp.1366-1375
• /
• 2002

In cold forging of piston-pin for automobile parts, the flow defect appears by the dead metal zone. This appearance evidently happens in products with a thin piercing thickness for the dimension accuracy and the decrease of material loss. The best method that can prevent flow defect is removing dead metal zone. The purpose of this study is to investigate the material flow behavior of forward-backward extruded piston-pin through the relative velocity ratio and the stroke control of upper moving punch & container using the flow control forming technique. The finite element simulations are applied to analyse the flow defect, then the results are compared with the plasticine model material experiments. The model experiment results are in good agreement with the FE simulation ones.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.25 no.6
• /
• pp.795-801
• /
• 2019

The aim of this study was to investigate the possibility of the skin-friction reduction by vortex control. A vortical system such as a horseshoe vortex, a hairpin vortex, and a wake region was induced around a hemisphere attached on a Perspex flat plate in the circulating water channel. Hairpin vortices were developed from the wake region and horseshoe vortices were formed by an adverse pressure gradient in front of the hemisphere. The horseshoe vortices located on the flank of the hemisphere induced a high momentum flow in the wake region by the direction of their vorticity. This process increased the frequency of the hairpin vortices as well as the frictional drag on the surface of the wake region. To reduce the skin-friction drag, suction control in front of the hemisphere was applied through a hole. Flow visualization was performed to optimize the free-stream velocity, size of the hemisphere, and size of the suction hole. Once the wall suction control mitigated the strength of the horseshoe vortex, the energy supplied to the wake region was reduced, causing the frequency of the hairpin vortex generation to decrease by 36.4 %. In addition, the change in the skin-friction drag, which was measured with a dynamometer connected to a plate in the wake region, also decreased by 2.3 %.

• Journal of the Society of Naval Architects of Korea
• /
• v.53 no.3
• /
• pp.180-187
• /
• 2016

As distinct from a slender body, the separation of the boundary layer on a bluff body give rise to complex wakes in which various kinds of vortices form, develop and interact with each other. In this paper, we investigate cavitation wake field behind wedge shaped two-dimensional fin models. Eight different models are tested at the Chungnam National University Cavitation Tunnel (CNU-CT). First, we measure wake cavity shapes and compare with numerical results, which shows the good agreement with each other. In addition, we demonstrate that wake flow characteristics of the control fin are clearly identified by the correlation analysis of high-speed camera images and pressure fluctuation measurements.

• Journal of the Society of Naval Architects of Korea
• /
• v.49 no.2
• /
• pp.174-181
• /
• 2012

In this paper, the Flow Control Fin(FCF) optimization has been carried out using computational fluid dynamics(CFD) techniques. This study focused on evaluation for the performance of the FCF attached in the stern part of the ship. The main advantage of FCF is to enhance the resistance performance through the lift generation with a forward force component on the foil section, and the propulsive performance by the uniformity of velocity distribution on the propeller plane. This study intended to evaluate these functions and to find optimized FCF form for minimizing viscous resistance and equalizing wake distribution. Four parameters of FCF are used in the study, which were angle and position of FCF, longitudinal location, transverse location, and span length in the optimization process. KRISO 300K VLCC2(KVLCC2) was chosen for an example ship to demonstrate FCF for optimization. The optimization procedure utilized genetic algorithms (GAs), a gradient-based optimizer for the refinement of the solution, and Non-dominated Sorting GA-II(NSGA-II) for Multiobjective Optimization. The results showed that the optimized FCF could enhance the uniformity of wake distribution at the expense of viscous resistance.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.24 no.1
• /
• pp.92-100
• /
• 2018

Vortical systems are considered a main feature to sustain turbulence in a boundary layer through interaction. Such turbulent structures result in frictional drag and erosion or vibration in engineering applications. Research for controlling turbulent flow has been actively carried out, but in order to show the effect of vortices in a turbulent boundary layer, it is necessary to clarify the mechanism by which turbulent energy is transferred. For this purpose, it is convenient to demonstrate and capture phenomena in a laminar boundary layer. Therefore, in this study, the interactions of disturbed flow around a hemisphere on a flat plate in laminar flow were analyzed. In other words, a street of hairpin vortices was generated following a wake region formed after flow separation occurred over a hemisphere. Necklace vortices surrounding the hemisphere also appeared due to a strong adverse pressure gradient that brought high momentum fluid into the wake region thereby leading to an increase in the frequency of hairpin vortices. To mitigate the effect of these necklace vortices, local suction control was applied through a hole in front of the hemisphere. Flow visualization was recorded to qualitatively determine flow modifications, and hot-film measurements quantitatively supported conclusions on how much the power of the hairpin vortices was reduced by local wall suction.

• Journal of Navigation and Port Research
• /
• v.37 no.3
• /
• pp.257-262
• /
• 2013

A ship operating in rough sea may suffer from an undesirable motion which may severely degrade the performance of equipment onboard and give a person an uncomfortable feeling. Hence, roll stabilization received a considerable attention and various devices including bilge keels, stabilizing fins, gyroscopic, anti-rolling tanks, rudders and flaps have been conceived and utilized for the purpose. The Coanda effect is evident when a jet stream is applied tangential to a curved surface of a hydrofoil since then the jet increases the circulation around the foil and consequently the lift. Model tests and numerical simulation have been conducted to examine the practicality of a fixed type fin stabilizer augmented by the Coanda jet. The results show that the lift coefficient of the modified Coanda fin at the zero angle of attack identically coincides with that of the original fin at ${\alpha}=\26^{\circ}$ when Coanda jet is supplied at the rate of $C_j$ = 0.25. It is also shown that fixed type fin stabilizers for active control of the motions of ships and the other mobile units without rotation can be put to practical use if the Coanda effect is applied.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.27 no.7
• /
• pp.1106-1115
• /
• 2021

In this study, a fin that controls ship stern flow was attached on stern hull of a 80k bulk carrier to improve resistance performance. The rectangular cross-sectional fin was attached at several locations on the hull, and angle to streamline was changed with constant length, breadth, and thickness. The resistance performance and wake on propeller plane of the hull with and without the fin were analyzed using model-scale computational fluid dynamics simulation. The analysis results were extrapolated to full-scale to compare the performance and wake of the full-scale ship. First, the fin changed path of bilge vortex that flowed into the propeller along the stern hull without the fin to transom stern. This change increased pressure of the stern hull and upper region of the propeller, so pressure resistance and total resistance of the hull were reduced - the nearer the fin location to after perpendicular (AP) and base line of the hull, the larger the reduction of the resistances. Second, nominal wake fraction of the hull with the fin was lower than that without the fin. This dif erence was in proportion to the angle of the fin, but the total resistance reduction was in proportion until a certain angle at which the reduction was maximum. The largest total resistance reduction was approximately 2.1% at 12.5% of length between perpendiculars from the AP, 10% of draft from the base line, and 14° with respect to the streamline.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.46 no.12
• /
• pp.986-993
• /
• 2018

Lateral thrust jet has better maneuverability performance than the control surface like the conventional fin for attitude control or orbital transition of guided weapons. However, in the supersonic region, a jet interaction flow occurs due to the lateral thrust jet during flight, and a complicated flow structure is exhibited by the interaction of the shock wave, boundary layer flow, and the vortex flow. Especially, hit-to-kill interceptors require precise control and maneuvering, so it is necessary to analyze the effect of jet interaction flow. Conventional jet interaction analyses were performed under low altitude conditions, but there are not many cases in the case of medium altitude condition, which has different flow characteristics. In this study, jet interaction flow analysis is performed on the lateral jet controlled interceptor operating at medium altitude. Based on the results, the structural characteristics of the flow field and the changes of aerodynamic coefficient are analyzed.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.11 no.3
• /
• pp.802-807
• /
• 2010

The high efficient and cooling system is very important to the control panels of electrical distributors, and Industrial automated system including computer. Also, it can be used widely in various industrial systems such as industrial robots, numerically controlled machining center, and so on. The cooling method which flowing gasses were forced to circulate by compulsion was adapted in this study. then development of counter flow type of cooling system for effective panel cooling. In the present study, fin assembly was developed for this cooling system. As results, the developed system has the improvements of cooling performances and radiant heat ratio. Its increasing of airflow mass is about 20%, and radiation rate of heating is twice or more as high as the conventional system.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2001.11a
• /
• pp.1-12
• /
• 2001

In cold forging of piston-pin for automobile parts, the flow defect appears by the dead metal zone. This appearance evidently happens in products with a thin piercing thickness for the dimension accuracy and the decrease of material loss. The best method that can prevent flow defect is removing dead metal zone. The purpose of this study is to investigate the material flow behavior of forward-backward extruded piston-pin through the relative velocity ratio and the stroke control of upper moving punch & container using the flow control forming technique. The finite element simulations are applied to analyse the flow defect, then the results are compared with the plasticine model material experiments. Finally, the model experiment results are in good agreement with the FE simulation ones.