• 대한기계학회논문집A
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• 제25권5호
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• pp.779-787
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• 2001

The multi degree of freedom system using magnetic levitation has been implemented successfully. Differently from another noncontact systems, the developed system was focused on the maximization of the system stiffness under the constraint of a limited input. The variation of a relative adopting point between the magnetic pair, its location on the fixed base, and the selection of optimal specifications for the main active magnetic elements give us another chance to realize the increased robustness against external disturbances with the less control inputs. In this paper, the overall development procedures are given including the optimal design, the dynamic modeling, the various control tests, and the main issues to be solved.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2001년도 제33회 춘계학술대회 개최
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• pp.212-217
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• 2001

This paper presents the stability characteristics of a rotor-bearing system supported by actively controlled hydrodynamic journal bearing. The proportional and derivative controls including coupled motion are adopted for the control algorithm to control the hydrodynamic journal bearing with a circumferentially groove. Also, the cavitation algorithm implementing the Jakobsson-Floberg-Olsson boundary condition is adopted to predict cavitation regions in the fluid film more accurately than conventional analysis which uses the Reynolds condition. The stability characteristics are investigated with the Routh-Hurwitz criteria using the linear dynamic coefficients which are obtained from the perturbation method. The stability characteristics of the rotor-bearing system supported by active controlled hydrodynamic journal bearing are investigated for various control gain. It is found that the speed at onset of instability is increased for both proportional and derivative control of the bearing, and the proportional and derivative control of coupled motion is more effective than proportional and derivative control of uncoupled motion.

• Tribology and Lubricants
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• 제17권4호
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• pp.283-289
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• 2001

The paper presents the stability characteristics of a rotor-bearing system supported by actively controlled hydrodynamic journal bearing. The proportional and derivative controls including coupled motion are adopted for the control algorithm to control the hydrodynamic journal bearing with a circumferentially groove. Also, the cavitation algorithm implementing the Jakobsson-Floberg-Olsson boundary condition is adopted to predict cavitation regions in the fluid film more accurately than a conventional analysis which uses the Reynolds condition. The stability characteristics of a rotor-bearing system supported by actively controlled hydrodynamic journal bearing are investigated for various control gains with the Routh-Hurwitz criteria using the linear dynamic coefficients which are obtained from the perturbation method. It is found that the speed at onset of the instability is increased for both proportional and derivative control of the bearing. It is also found that the proportional and derivative control of the coupled motion is more effective than that of the uncoupled motion.

• 한국정밀공학회지
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• 제15권9호
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• pp.33-42
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• 1998

If a ship diesel engine is operated by consolidated control with Controllable Pitch Propeller(CPP), the minimum fuel consumption is achieved together with the demanded ship speed. For this, it is necessary that the ship is operated on the ideal operating line which satisfies the minimum fuel consumption and that the pitch angle of CPP and throtle valve angle are controlled simultaneously. In this point of view, this paper presents a controller design method for a ship propulsion system with CPP based on the decoupling control theory. To do this, Linear Matrix Inequality(LMI) approach is introduced for the control system to satisfy the given $H_\infty$ control performance and robust stability in the presence of physical parameter perturbations. The validity and applicability of this approach are illustrated by simulation in the all operating ranges.

• Kyungpook Mathematical Journal
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• 제61권3호
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• pp.591-612
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• 2021

In this paper, a uniformly convergent numerical scheme is designed for solving singularly perturbed reaction-diffusion problems. The problem is converted to an equivalent weak form and then a Galerkin finite element method is used on a piecewise uniform Shishkin mesh with linear basis functions. The convergence of the developed scheme is proved and it is shown to be almost fourth order uniformly convergent in the maximum norm. To exhibit the applicability of the scheme, model examples are considered and solved for different values of a singular perturbation parameter ε and mesh elements. The proposed scheme approximates the exact solution very well.

• Advances in nano research
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• 제16권6호
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• pp.579-591
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• 2024

This study explained the effects of radiation, magnetic field, and nanoparticle shape on the peristaltic flow of an Upper-Convected Maxwell nanofluid through a porous medium in an asymmetric channel for a better understanding of cooling and heating mechanisms in the presence of magnetic fields. These phenomena are modeled mathematically as a system of non-linear differential equations, that are solved under long-wavelength approximation and low Reynolds number conditions using the perturbation method. The results for nanofluid and temperature described the behavior of the pumping characteristics during their interaction with (the vertical position, thermal radiation, the shape of the nanoparticle, and the magnetic field) analytically and explained graphically. Also, the combined effects of thermal radiation parameters and some physical parameters on pressure rise, pressure gradient, velocity, and heat distribution are pointed out. Qualitatively, a reverse velocity appears with combined high radiation and Grashof number or combined high radiation and low volume flow rate. At high radiation, the spherical nanoparticle shape has the greatest effect on heat distribution.

• 한국전자파학회논문지
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• 제19권5호
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• pp.533-540
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• 2008

등간격 안테나 소자들로 구성된 모노펄스 추적용 선형 배열 안테나의 합 및 차 패턴 합성 문제에서 원하는 패턴을 만족시키도록 하는 개별 안테나 급전 여기 전류의 상대적인 가중치들을 도출하는 효율적인 수치적 방법을 제안한다. 이 방법은 패턴 배열 인자를 표현하는 Schelkunoff 다항식에 내재된 패턴 null 점들의 최적 섭동에 기본을 둔다. 따라서 여기 전류 가중치들을 직접 최적화하는 기존의 방식과는 달리, 이 방법은 null 점 제어에 의해 원하는 개별 sidelobe 레벨(SLL)들을 갖는 패턴과 해당 여기 전류 가중치들을 쉽게 합성할 수 있는 장점을 가진다. 또한, 최적화 과정에서 null 점 초기값에 따라 두 가지 형태의 차 패턴들을 합성할 수 있음을 보인다. 임의의 SLL들을 갖는 패턴들을 합성하고, 도출된 결과들을 배열 인자 식에 대입함으로써 제안한 방법의 타당성을 수치적으로 검증한다.

• Steel and Composite Structures
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• 제29권4호
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• pp.557-567
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• 2018

Cylindrical shell structures buckle at service loads which are much lower than their associated theoretical buckling loads. The main source of this discrepancy is the presence of various imperfections which are created on the cylinder body during different processes as manufacturing, handling, assembling and machining. Many cylindrical shell structures are still designed against buckling based on the experimental data introduced by NASA SP-8007 as conservative lower bound curves. This study employed the numerical based Linear Buckling mode shape Imperfection (LBMI) method and modified it using a stochastic method to assess the effect of geometrical imperfections in more details on the buckling of cylindrical shells with and without the cutout. The comparison of results with those obtained from the numerical Simcple Perturbation Load Imperfection (SPLI) method for cylinders with and without cutout revealed a good correlation. The effect of two parameters of size and number of cutouts on the buckling load was investigated using the linear buckling and Modified LBMI methods. Results confirmed that in cylinders with a small cutout inserting geometrical imperfection using either SPLI or modified LBMI methods significantly reduced the value of the predicted buckling load. However, in cylinders with larger cutouts, the effect of the cutout is dominant, thus considering geometrical imperfection had a minor effect on the buckling loads predicted by both SPLI and modified LBMI methods. Furthermore, the modified LBMI method was employed to evaluate the combination effect of cutout numbers and size on the buckling load. It is shown that in small cutouts, an increasing in the cutout size up to a certain value resulted in a remarkable reduction of the buckling load, and beyond that limit, the buckling loads were constant against D/R ratios. In addition, the cutout number shows a more significant effect on decreasing the buckling load at small D/R ratios than large D/R ratios.

• 한국습지학회지
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• 제7권3호
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• pp.57-66
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• 2005

본 연구는 최적화모형을 이용하여 관측된 단일 강우-유출 사상으로부터 최적단위도와 침투율 공식의 매개변수를 결정하였다. 단일 강우-유출사상으로부터 절대오차누계를 최소로 하는 선형계획모형을 정립하고, 황구지천 수직교 지점에서의 최적침투율과 단위도를 결정하였다. 침투율 공식의 섭동 단계에서는 시산법을 적용하여 Kostiakov, Philip, Horton 및 Green-Ampt 공식의 매개변수를 결정하였다. ${\Phi}$지표법에 의한 단위도 종거의 값은 유일하게 결정되지만 침투율 공식의 매개변수에 따라 단위도의 종거는 변한다. 본 연구에서는 관측된 단일 강우-유출사상에 관하여 제안된 모형을 적용하였으며 종래의 방법에 의한 오차보다 작은 단위도를 구할 수 있었다.

• 한국수자원학회논문집
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• 제30권3호
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• pp.225-233
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• 1997

부체의 장주기동요, 연안구조물과 계류선의 파괴 등과 같은 역기능으로 인해 파군현상의 정확한 해석은 시급한 과제라 하겠다. 본고에서는 연안계에서 발생하는 파랑과 가장 근접한 비선형불규칙 파랑계를 산정하여 해안구조물의 피로거동에 지대한 영향을 미치는 파군당 파랑의 수와 해안구조물의 first excursion failure mode를 결정하는 high run에서의 파랑수를 중심으로 파군현상을 해석하였다. 해석과정에 mapping technique과 유의파경사를 perturbation parameter로 섭동이론이 사용되었다. 해석결과 Gaussian wave계에서 균등분포하는 것으로 알려진 위상함수는 평균값 주위에 집중분포하였고, 그 정도는 비선형성이 증가할수록 심화되었다. threshold crossing rate의 경우 비선형성이 심화될수록 평균해수위보다 큰 쪽으로 분포형이 이동하였으며 파군당 파랑수와 high run에서의 파랑수도 비슷한 경향을 보여 최근 설치범위가 심해쪽으로 확대되는 해안구조믈의 경우 피로거동에 대한 보강이 요만된다.