• 한국정밀공학회지
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• 제21권7호
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• pp.107-114
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• 2004

This paper presents a two-time scale approach for vibration reduction of a high speed Cartesian manipulator. High speed manipulators would be subject to mechanical vibration due to high inertia forces acting on linkages. To achieve high throughput capability, such motion induced vibration would have to be damped quickly, to reduce settling time of the manipulator end-effector. This paper develops a two-time scale model fer a structurally-flexible Cartesian manipulator. Based on the two-time scale model, a composite controller consisting of a computed torque method for the slow time-scale rigid body subsystem, and a linear quadratic state-feedback regulator for the fast time-scale flexible subsystem, is designed. Simulation results show that the proposed two time-scale controller yields good performance in attenuating structural vibration arising due to excitation from inertial forces.

• Journal of the Korean Data and Information Science Society
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• 제21권3호
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• pp.555-560
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• 2010

One-dimensional approach to two-dimensional warranty data involves modeling us- age as a function of time. Iskandar (1993) suggests a simple linear model for usage. However, simple linear form of intensity function is of limited value to model the situa-tion where the intensity varies over time. In this study Weibull intensity is considered where the scale parameter is expressed in terms of different models. We will nd out how each parameter in the model a ects the warranty cost and which model gives a bigger number of failures within the two-dimensional warranty region.

• Journal of applied mathematics & informatics
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• 제11권1_2호
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• pp.215-241
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• 2003

In this paper, solute transport in heterogeneous aquifers using a modified Fokker-Planck equation (MFPE) is investigated. This newly developed mathematical model is characterised with a time-, scale-dependent dispersivity. A two-dimensional finite volume quadrilateral mesh method (FVQMM) based on a quadrilateral background interpolation mesh is developed for analysing the model. The FVQMM transforms the coupled non-linear partial differential equations into a system of differential equations, which is solved using backward differentiation formulae of order one through five in order to advance the solution in time. Three examples are presented to demonstrate the model verification and utility. Henry's classic benchmark problem is used to show that the MFPE captures significant features of transport phenomena in heterogeneous porous media including enhanced transport of salt in the upper layer due to its parameters that represent the dependence of transport processes on scale and time. The time and scale effects are investigated. Numerical results are compared with published results on the some problems.

• Journal of Positioning, Navigation, and Timing
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• 제10권3호
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• pp.197-206
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• 2021

We compared two typical ensemble time scale algorithms; AT1 and Kalman filter. Four commercial atomic clocks composed of two hydrogen masers and two cesium atomic clocks provided measurement data to the algorithms. The allocation of relative weights to the clocks is important to generate a stable ensemble time. A 30 day-average-weight model, which was obtained from the average Allan variance of each clock, was applied to the AT1 algorithm. For the reduced Kalman filter (Kred) algorithm, we gave the same weights to the two hydrogen masers. We also compared the frequency stabilities of the outcome from the algorithms when the frequency offsets and/or the frequency drift offsets estimated by the algorithms were corrected or not corrected by the KRISS-made primary frequency standard, KRISS-F1. We found that the Kred algorithm is more effective to generate a stable ensemble time scale in the long-term, and the algorithm also generates much enhanced short-term stability when the frequency offset is used for the calculation of the Allan deviation instead of the phase offset.

• 한국연소학회:학술대회논문집
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• 한국연소학회 1997년도 제15회 KOSCO SYMPOSIUM 논문집
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• pp.163-173
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• 1997

This article presents an application of a large-scale structural mixing model (Broadwell et al. 1984) to the blowout of turbulent reacting jets discharging perpendicularly into an unconfined cross air-flow. In an analysis of a common stability curve, a plausible explanation can be made that the phenomenon of blowout is related only to the mixing time scale of the two flows. The most notable observation is that the blowout distance is traced at fixed positions at all times according to the velocity ratio R. Measurements of the lower blowout limits in the liftable flame agree qualitatively with the blowout parameter ${\varepsilon}$, proposed by Broadwell et al. Good agreement between the results calculated by a modified blowout parameter ${\varepsilon}^'$ and experimental results confirms the important effect of a large-scale structure in specifying the stabilization feature of blowouts.

• Structural Engineering and Mechanics
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• 제41권4호
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• pp.559-573
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• 2012

An adaptive modeling and simulation technique is introduced for the effective and reliable fluid-structure interaction analysis using MSC/Dytran for large-scale complex pressurized liquid containment. The proposed method is composed of a series of the global rigid sloshing analysis and the locally detailed fluid-structure analysis. The critical time at which the system exhibits the severe liquid sloshing response is sought through the former analysis, while the fluid-structure interaction in the local region of interest at the critical time is analyzed by the latter analysis. Differing from the global coarse model, the local fine model considers not only the complex geometry and flexibility of structure but the effect of internal pressure. The locally detailed FSI problem is solved in terms of multi-material volume fractions and the flow and pressure fields obtained by the global analysis at the critical time are specified as the initial conditions. An in-house program for mapping the global analysis results onto the fine-scale local FSI model is developed. The validity and effectiveness of the proposed method are verified through an illustrative numerical experiment.

• 대한기계학회논문집
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• 제7권4호
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• pp.451-460
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• 1983

Computational four-equation turbulence model is developed and is applied to predict twodimensional unsteady thermal surface discharge into a reservoir. Turbulent stresses and heat fluxes in the momentum and energy equations are determined from transport equations for the turbulent kinetic energy (R), isotropic rate of kinetic energy dissipation (.epsilon.), mean square temperature variance (theta. over bar $^{2}$), and rate of destruction of the temperature variance (.epsilon. $_{\theta}$). Computational results by four-equation model are favorably compared with those obtained by an extended two-equation model. Added advantage of the four-equation model is that it yields quantitative information about the ratio between the velocity time scale and the thermal time scale and more detailed information about turbulent structure. Predicted time scale ratio is within experimental observations by others. Although the mean velocity and temperature fields are similarly predicted by both models, it is found that the four-equation model is preferably candidate for prediction of highly buoyant turbulent flows.

• 대한기계학회논문집
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• 제11권3호
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• pp.444-453
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• 1987

본 연구에서는 난류구조에 대한 유선곡률의 영향을 명확히 반영하는 적절한 곡률수정 2-방정식모델을 개발하고자 한다. 이 연구에서 제안된 모델의 타당성은 다 음의 2차원 재순환유동에 대한 실험결과와 계산결과의 비교를 통해서 입증될 것이다. (1) Moss와 Bake에 의하여 맥동열선 풍속계로 측정된 두꺼운 수직벽주위의 유동` (2) 레이저 도플러 속도계로 Fraser와 Siddig에 의해 측정된 얇은 수직벽유동` (3)맥동열 선 풍속계로 Eaton이 실험한 후면벽유동` (4)맥동열선 풍속계로 Moss와 Baker가 측정 한 전면벽유동. 새로운 곡률수정 2-방정식모델은 2장에서 설명되고 있으며, 3장에서 는 경계조건과 수치계산 과정이 간단이 기술되어 있다. 그 뒤에 4장에는 계산결과와 실험치에대한 비교검토가 설명되어 있고 마지막으로 5장에서는 본 연구에 대한 결론을 맺고 있다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제8권6호
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• pp.589-601
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• 2016

The hybrid grid was adopted and numerical prediction analysis of propeller unsteady bearing force considering free surface was performed for mode and full-scale KCS hull-propeller-rudder system by employing RANS method and VOF model. In order to obtain the propeller velocity under self-propulsion point, firstly, the numerical simulation for self-propulsion test of full-scale ship is carried out. The results show that the scale effect of velocity at self-propulsion point and wake fraction is obvious. Then, the transient two-phase flow calculations are performed for model and full-scale KCS hull-propeller-rudder systems. According to the monitoring data, it is found that the propeller unsteady bearing force is fluctuating periodically over time and full-scale propeller's time-average value is smaller than model-scale's. The frequency spectrum curves are also provided after fast Fourier transform. By analyzing the frequency spectrum data, it is easy to summarize that each component of the propeller bearing force have the same fluctuation frequency and the peak in BFP is maximum. What's more, each component of full-scale bearing force's fluctuation value is bigger than model-scale's except the bending moment coefficient about the Y-axis.

• 대한기계학회논문집B
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• 제26권1호
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• pp.133-140
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• 2002

This article presents an application of a large-scale structural mixing model(Broadwell et at. 1984) to the blowout of turbulent reacting cross flow jets. Experimental observations, therefore, aim to identify the existence of large-scale vortical structure exerting an important effect upon the flame stabilization. In the analysis of common stability curve, it is seen that the phenomenon of blowout are only related to the mixing time scale of the two flows. The most notable observation is that the blowout distance is traced at a fixed positions according to the velocity ratio at all times. Measurements of the lower blowout limits in the liftable flame are qualitatively in agreement with the blowout parameter $\xi$, proposed by Broadwell et al. Good agrement between the results calculated by a modified blowout parameter $\xi$'and the present experimental results confirms the important effect of large-scale structure in the stabilization feature of blowout.