• Journal of the Korean Society of Propulsion Engineers
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• v.25 no.1
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• pp.1-11
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• 2021

POGO is a dynamic axial instability phenomenon that occurs in liquid-propelled rockets. As the natural frequencies of the fuselage and those of the propellant supply system become closer, the entire system will become unstable. To predict POGO, the propellant (oxidant and fuel) tank in the first stage is modeled as a shell element, and the remaining components, the engine and the upper part, are modeled as mass-spring, and structural analysis is performed. The transmission line model is used to predict the pressure and flow perturbation of the propellant supply system. In this paper, the closed-loop transfer function is constructed by integrating the fuselage structure and fluid modeling as described above. The pogo suppressor consists of a branch pipe and an accumulator that absorbs pressure fluctuations in a passive manner and is located in the middle of the propellant supply system. The design parameters for its design optimization to suppress the decay phenomenon are set as the diameter, length of the branch pipe, and accumulator. Multiple-objective function optimization is performed by setting the energy minimization of the closed loop transfer function in terms of to the mass of the pogo suppressor and that of the propellant as the objective function.

• Journal of the Korea Society for Simulation
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• v.28 no.2
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• pp.35-48
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• 2019

Domestic hydraulic drifters till now have been developed by benchmarking products from overseas leading companies. However, they do not have excellent impact performance as they are not suitable for characteristics (large flow rate and low pressure) of Korean hydraulic drill power pack, and therefore, research on the optimum design has not made much headway. This study performs multi-objective function optimization for hydraulic drifters whose capacity has been redesigned to deal with the large flow rate, and also with the help of this function, it aims to improve impact power and reduce supply and surge pressure. A summary of the research study is as follows: First, we set goals for improving impact power, supply pressure, and surge pressure, and then perform multi-objective function optimization on them. After that, we secure the reliability of the optimized analytical model by comparing the test results of the prototype built by the optimized design with the analysis results of the analytical model. This study used SimulationX, that is the hydraulic system analysis software, and EasyDesign, which is a multi-objective function optimization program. Through this research, we have achieved the results that satisfy the goal of developing high power drifters suitable for Korean type hydraulic drills.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.12
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• pp.1793-1803
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• 2010

The weighting method and goal programming require weighting factors or target values to obtain a Pareto optimal solution. However, it is difficult to define these parameters, and a Pareto solution is not guaranteed when the choice of the parameters is incorrect. Recently, the Mahalanobis Taguchi System (MTS) has been introduced to minimize the Mahalanobis distance (MD). However, the MTS method cannot obtain a Pareto optimal solution. We propose a function called the skewed Mahalanobis distance (SMD) to obtain a Pareto optimal solution while retaining the advantages of the MD. The SMD is a new distance scale that multiplies the skewed value of a design point by the MD. The weighting factors are automatically reflected when the SMD is calculated. The SMD always gives a unique Pareto optimal solution. To verify the efficiency of the SMD, we present two numerical examples and show that the SMD can obtain a unique Pareto optimal solution without any additional information.

• Journal of Korean Society of Steel Construction
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• v.12 no.5 s.48
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• pp.503-513
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• 2000

This study treats the criteria, considering the fuzziness occurred by optimization design. And we applied two weighting methods to show the relative importance of criteria. This study develops multi-objective optimization programs implementing plain stress analysis by FEM and discrete optimization design uniformaly. The developed program performs a sample design of 10-member steel truss. This study can carry over the multi-objective optimization based on total system fuzzy-genetic algorithms while performing the stress analysis and optimization design. Especially, when general optimization with unreliable constraints is cannot be solve this study can make optimization design closed to realistic with fuzzy theory.

• Proceedings of the Korea Water Resources Association Conference
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• 2006.05a
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• pp.1405-1409
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• 2006

본 연구는 다목적 유전자알고리즘을 이용해서 상수도관망의 펌프 스케쥴링을 최적화하는 방안을 도출하기 위한 연구이다. 상수도관망의 시스템을 모의하기 위한 방법으로는 EPANET을 이용해서 각각의 결정변수에 관한 펌프운영비용을 산정하였으며, 이렇게 계산된 편익함수의 값을 다목적 유전자알고리즘의 목적함수로 이용해서, 펌프스케쥴링에 관한 여러 가지 대안을 제시하였다. 상수도관망의 운영을 위해서는 안정적인 용수공급이 가능하면서도 최소한의 에너지비용을 필요로 하는 펌프운영방식을 필요로 하게 되는데, 이때 용수수요량의 변화 등 운영여건이 바뀌게 되더라도, 수리적 안정성이 확보되는 범위내에서의 최적 운영방안이 필요하게 된다. 이와 같은 펌프운영방안의 최적화는 상당히 많은 수의 결정변수를 요구하게 되고, 결정변수의 탐색범위 또한 상당히 넓은 경우가 대부분이기 때문에, 국부최적해에 수렴하는 현상이 나타나지 않고, 넓은 범위의 결정변수 영역을 빠르게 탐색할 수 있으며, 여러 가지 대안을 효율적으로 제시할 수 있는 다목적 유전자알고리즘을 사용하였다. 본 연구를 통하여 개발된 상수도 관망 펌프 스케쥴링 기법을 이용하면, 운영여건의 변화에 따른 펌프운영비용의 최소화 여부를 검토할 수 있으며, 이를 바탕으로 각 단계별 수도시설을 종합적으로 고려한 경제적 운영방안의 검토가 가능할 것으로 판단된다.

• Journal of Korean Association for Spatial Structures
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• v.8 no.1
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• pp.49-56
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• 2008

A multi-objective optimization approach is presented for force design of tensegrity structures. The geometry of the structure is given a priori. The design variables are the member forces, and the objective functions are the lowest eigenvalue of the tangent stiffness matrix that is to be maximized, and the deviation of the member forces from the target values that is to be minimized. The multi-objective programming problem is converted to a series of single-objective programming problems by using the constraint approach. A set of Pareto optimal solutions are generated for a tensegrity grid to demonstrate the validity of the proposed method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.6
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• pp.987-992
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• 2002

In an attempt to solve multiobjective optimization problems, many traditional methods scalarize the objective vector into a single objective. In those cases, the obtained solution is highly sensitive to the weight vector used in the scalarization process and demands the user to have knowledge about the underlying problem. Moreover, in solving multiobjective problems, designers may be interested in a set of Pareto-optimal points, instead of a single point. In this paper, pareto-based Continuous Evolutionary Algorithms for Multiobjective Optimization problems having continuous search space are introduced. This algorithm is based on Continuous Evolutionary Algorithms to solve single objective optimization problems with a continuous function and continuous search space efficiently. For multiobjective optimization, a progressive reproduction operator and a niche-formation method fur fitness sharing and a storing process for elitism are implemented in the algorithm. The operator and the niche formulation allow the solution set to be distributed widely over the Pareto-optimal tradeoff surface. Finally, the validity of this method has been demonstrated through a numerical example.

• Journal of Korean Society of Steel Construction
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• v.9 no.3 s.32
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• pp.457-465
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• 1997

The Shape optimization makes it possible to reduce the weight of structure and cost then member sizing optimization. A vast amount of imprecise information is existed in constraints of the optimum design. It is very difficult and sometimes confusing to describe and to deal with the several criteria which contain fuzzy degrees of relatives importance. This paper proposed weighting strategies in the multiobjective shape optimization of fuzzy structural system by ${\alpha}$-cut approach. The algorithm in this research is numerically tested for 2-bar truss structure. The result show that. the user can choose the one optimum solution in practices as obtaining the optimum solutions according to the ${\alpha}$-cut approach, weight of volume and displacement.

• Proceedings of the Korea Water Resources Association Conference
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• 2018.05a
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• pp.34-34
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• 2018

본 연구는 다목적 유전자 알고리즘 Non-Dominated Sorting Genetic Algorithm II (NSGA-II)를 활용하여 자동 검보정 알고리즘을 개발하고, 이를 준분포형 수문모형인 SWAT (Soil and Water Assessment Tool) 모형에 적용하여 평가하고자 한다. 집중형 모형과 달리, 분포형 모형은 유역 내 다양한 물리적 변수와 공간 이질성(spatial heterogeneity)을 표현하기 위한 많은 매개변수를 포함하고 있고, 최근에는 기후 변화와 장기 가뭄과 같은 이상 기후에 따른 물 부족, 수질 오염 및 녹조 현상 등을 고려하기 위해 매개변수의 시간적인 변동성을 고려하기 위한 연구도 수행되고 있다. 이에 따라 본 연구에서 개발한 다목적 알고리즘은 다양한 매개변수의 시공간적 특성을 고려할 수 있도록 작성되었으며, Python으로 개발하여 타 모형으로의 확장성 및 범용성을 고려하였다. SWAT 모형의 유출 해석은 결정계수(Coefficient of determination, $R^2$), RMSE(Root mean square error), 모형 효율성 계수(Nash-Sutcliffe efficiency, NSE) 및 IOA(Index of agreement) 등을 활용해 기존 연구 결과와 비교분석할 수 있도록 하였으며, 사용자의 선택에 따라 다른 목적함수 또한 활용할 수 있도록 하였다. NSGA-II를 활용한 SWAT 모형의 유출 해석은 다목적 함수를 고려함에 따라 실측값과 높은 상관성을 보여줄 것으로 판단되며, 이상 기후 기간 설정에 따른 유동적인 매개변수 변화를 적용할 수 있을 것으로 기대된다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.3
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• pp.123-129
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• 2002

본 논문에서는 다목적실용위성 1호 개발 과정 중에 수행했던 연성 하중 해석에 대하여 소개하였다. 먼저 일반적인 연성 하중 해석 과정과 다목적실용위성 개발시 수행한 실제의 과정을 비교하였다. 그리고, 다목적실용위성 1호의 유한요소모델과 본 해석에 관련된 각 기관들의 역할을 설명하였다. 준비행모델의 진동 시험 결과를 이용한 유한요소모델의 조정에 대하여 설명하고, 또한, 해석에 입력으로 사용되는 외력함수에 해당하는 하중들에 대하여 설명하였다. 대표적인 해석 결과들을 제시하여 위성이 발사하중에 대하여 안전하다는 것이 예측되었음을 보여 주었다.