• 한국해양공학회지
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• 제32권2호
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• pp.106-115
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• 2018

The forward and afterward parts of ships and offshore structures are designed to improve the fuel consumption performance. These are made of curved plates with a large thickness. If a fabricated curved plate has some dimensional errors, a lot of additional cost is incurred in the assembly process. Thus, an accurate dimensional assessment is very important for fabrication. In this paper, we propose an assessment method for the dimensional quality management of curved plates. This can be applied to data measured using a variety of three-dimensional instruments, with boundary measurement points automatically classified and sorted to create a measurement surface. The assessment is evaluated after matching the CAD surface and the measured surface considering constrained conditions. The fabrication assessment is evaluated as a probability of how much the tolerance is satisfied.

• Journal of Mechanical Science and Technology
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• 제18권2호
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• pp.302-312
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• 2004

This work developed improved slip factor model and correction method to predict flow through impeller in forward-curved centrifugal fan. Both steady and unsteady three-dimensional CFD analyses were performed to validate the slip factor model and the correction method. The results show that the improved slip factor model presented in this paper could provide more accurate predictions for forward-curved centrifugal impeller than the other slip factor models since the present model takes into account the effect of blade curvature. The correction method is provided to predict mass-averaged absolute circumferential velocity at the exit of impeller by taking account of blockage effects induced by the large-scale backflow near the front plate and flow separation within blade passage. The comparison with CFD results also shows that the improved slip factor model coupled with the present correction method provides accurate predictions for mass-averaged absolute circumferential velocity at the exit of impeller near and above the flow rate of peak total pressure coefficient.

• 소성∙가공
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• 제25권6호
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• pp.379-389
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• 2016

As a stator for steam turbine diaphragm, hollow-type nozzle stator to substitute for conventional solid one is introduced in this study. This hollowed stator can be separated into two parts such as upper and lower plates with large and curved surface area. This study focuses on thick plate forming process for the upper plate of the hollow-partitioned nozzle stator. First, to reduce forming defects such as under-cut and localized thinning of the deformed plate, and to avoid tool interruption between forming punch and lower die, tool design including the position determination of forming surfaces is performed. Uni-axial tensile tests are carried out using SUS409L steel plate with initial thickness of 5.00mm, and plastic strain ratio (r-value) is also obtained. Due to the asymmetric curved configuration of the upper plate, it is hard to adopt a series of blank holder or draw-bead, so the initial plate during this thick plate forming experiences unstable and non-uniform contact. To easy this forming difficulty and find suitable tool geometry without sliding behavior of the workpiece in the die cavity, two geometric parameters with respect to each shoulder angle of the lower die and the upper punch are adopted. FE models with consideration of 21 combinations for the geometric parameters are built-up, and numerical simulations are performed. From the simulated and predicted results, it is shown that the geometric parameter combinations with ($30^{\circ}$, $90^{\circ}$) and ($45^{\circ}$, $90^{\circ}$) for the shoulder angle of the lower die and the upper punch are suitably applied to this upper plate forming of the hollow-partitioned nozzle stator used for the turbine diaphragm.

• International Journal of Naval Architecture and Ocean Engineering
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• 제13권1호
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• pp.236-245
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• 2021

In this study, direct numerical and large eddy simulations of transitional flows around studs were conducted to investigate the effectiveness of turbulence stimulators at very low speeds for the minimum propulsion power condition of four knots. For simplicity, the studs were assumed to be installed on a flat plate, while the wake was observed up to 0.23 m downstream behind the second stud. For applicability to a model ship, we also studied the flow characteristics behind the first and second studs installed on a curved plate, which was designed to describe the geometry of a bulbous bow. A laminar-to-turbulent transition was observed in the wake at Re_D ≥ 921 (U_∞≥0.290 m/s), and the wall shear stress at Re_D = 1162 (U_∞ = 0.366 m/s) in the second wake was similar to that of the fully developed turbulent boundary layer after a laminar-to-turbulent transition in the first wake. At Re_D = 581 (U_∞ = 0.183 m/s), no turbulence was stimulated in the wake behind the first and second studs on the flat plate, while a cluster of vortical structures was observed in the first wake over the curved plate. However, a cluster of vortical structures was revealed to be generated by the reattachment process of the separated shear layer, which was disturbed by the first stud rather than directly initiated by the first stud. It was quite different from a typical process of transition, which was observed at relatively high Re_D that the spanwise scope of the turbulent vortical structures expanded gradually as it went downstream.

• 소성∙가공
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• 제17권8호
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• pp.570-578
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• 2008

A flexible forming apparatus is composed a number of punches which have spherical pin tip shape instead of conventional solid die. The flexible forming tool consisted of punch array in a matrix form was proposed as an alternative forming method to substitute the conventional line heating method which use heat source to induce residual stress along specified heating lines. In this study, application of the flexible forming process to the small scale curved plate forming was conducted. Numerical simulations for both solid and flexible die forming process were carried out to compare the shape of the products between flexible and conventional die forming process. In addition, spring-back analysis was conducted to figure out the feasibility of the flexible forming process comparing with the die forming process in view of final configuration of the specimens. Moreover, experiment was also carried out to confirm the formability of the process. Consequently, it was confirmed that the flexible die forming method has capability and feasibility to manufacture the curved plates for shipbuilding.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 춘계학술대회 논문집
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• pp.220-223
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• 2008

Large curved plate blocks are widely used to construct hull structure in shipbuilding industry. Most curved plates are manufactured by using manual method called as line heating that use deformation caused by residual stress after local heating along a line which is perpendicular to the curvature direction. However, its working environment is poor and its formability is totally dependent on an experienced technician. In view of that, multi-point dieless forming (MDF) technology that use reconfigurable punch arrays instead of one piece die is proposed in this study. The MDF process is based on a concept of equivalent die surface made by numbers of punches which has round tip at the end of it. In this study, numerical simulation for common curvature type such as saddle shape was carried out. In addition, experiments in the plate forming process were also conducted to compare with the numerical results in view of final configuration. Consequently, it was noted that the proposed dieless forming method has considerable feasibility to substitute the new process for conventional manual method.

• 소성∙가공
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• 제27권1호
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• pp.18-27
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• 2018

As one of the functional metal parts in steam turbine diaphragm assembly, the hollow-partitioned turbine nozzle (stator) has large and thick geometries, as well as an asymmetric configuration. Therefore it is hard to support a metal blank in the die cavity. To ease this situation and control posture and position of metal blank (workpiece), a blank support structure is newly introduced. The blank support structure is basically composed of enlarged arms from the blank, guide pins and linear bearings. It can help to control the intermediate blank without a critical sliding phenomenon. The operation mechanism of this blank support structure, during thick plate forming for the hollow-partitioned turbine nozzle stator, is first evaluated. A series of FEM-based numerical simulations, with respect to the width of the guide arm as geometric design parameters, are carried out to investigate its applicable range. As the results, it is observed the blank support structure for this thick plate forming can guide the workpiece to have stable posture during the plate forming process.

• 한국CDE학회논문집
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• 제17권3호
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• pp.188-197
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• 2012

Recently, aluminum ships are constructed more than ever because of the environmental pollution generated by FRP (Fiber Reinforced Plastic) ships. In particular, FRP ships have been replaced by the Aluminum ships. The forming process of the curved aluminum plate has been performed only by labor works without systematic technique. Therefore, it is difficult to construct the aluminum ship that the design satisfies both required propulsion performance and hull design. Present study introduces a MPSF (Multi Point Stretching Forming) that is a flexible manufacturing technique to form large sheet panels of doubly curvature. The hull pieces are stretch-formed over the MPSD (multi-point stretching die) generated by the punch element matrix. In this study, MPSF is applied to deform the doubly curved surfaces of aluminum ship. The forming system including FEA (finite element analysis) of the processes for stretching the plate were carried out by static implicit analysis is suggested. Residual deformation of the surface is modeled by an elasto-plastic contact phenomena while the forming process is simulated by FEA. Finally, the proposed system is also validated, comparing the deformed shape by MPSF with that of object surfaces.

• 해양환경안전학회지
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• 제17권3호
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• pp.295-300
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• 2011

선박은 상자형태 구조로 구성되어 있으며 선박의 선 수미, 선저만곡부, 갑판 등에 주로 사용하고 있다. 이런 구조는 박판구조이며 1차 지지부재로 사용된다. 평판구조와 비교하였을 때 상이한 거동을 보이며 일반적으로 압축하중을 받을 경우 곡률변화에 따라 다른 좌굴 및 최종 강도 경향을 나타냈다. 따라서 본 논문에서는 압축하중을 받고 있는 보강곡판인 1/2+1+1/2 bay 모델에 대하여 비선형유한요소해석을 수행하였으며 매개변수 영향은 곡률변화뿐만 아니라 세장비, 웨브높이/두께 등을 고려하여 해석모델에 대한 붕괴모드에 대해 검토하였다.

• 대한조선학회논문집
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• 제48권6호
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• pp.528-538
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• 2011

As a forming method for curved hull plates more efficient than the flame bending, mechanical bending using multi point press forming and die-less forming is discussed in this paper. the mechanical forming is a flexible manufacturing system for automatically forming of hull parts. It is especially suited to varied curved parts. This paper discusses a multiple point pressing machine composed of a pair of reconfigurable punches in order to achieve the rapid forming of curved hull plates using division forming and presents how forming information is obtained from the given design surface. Although the mechanical forming can be efficient in the metal forming, spring back after pressing is a phenomenon which must be carefully considered when quantifying the process variables. If the spring back is not accurately controlled, the fabricated shell plate cannot meet assembly tolerance. This paper describes the principles to calculate the proper stroke of each punch at the divided areas. the strokes are determined by an iterative process of sequential pressing and spring back compensation from an unfolded flat shape to its given design surface. FEA(finite element analysis) is used to simulate the spring back of the plate and the IDA(iterative displacement adjustment) method adjusts the offset of pressing punches from the deformation results and the design surface. The shape deviations of two surfaces due to spring back are compensated by integrated system using FEA and IDA method. For the practical application, It is aimed to develop an integrated system that can automatically perform the compensation process and calculate strokes of punches of the double sides' reconfigurable multiple-press machine and some experimental results obtained with mechanical bending are presented.