• Nuclear Engineering and Technology
• /
• 제54권11호
• /
• pp.4322-4328
• /
• 2022

The two-fluid model is widely used to describe two-phase flows in complex systems such as nuclear reactors. Although the two-phase flow was successfully simulated, the standard two-fluid model suffers from an ill-posed nature. There are several remedies for the ill-posedness of the one-dimensional (1D) two-fluid model; among those, artificial viscosity is the focus of this study. Some previous works added artificial diffusion terms to both mass and momentum equations to render the two-fluid model well-posed and demonstrated that this method provided a numerically converging model. However, they did not consider mass conservation, which is crucial for analyzing a closed reactor system. In fact, the total mass is not conserved in the previous models. This study improves the artificial viscosity model such that the 1D incompressible two-fluid model is well-posed, and the total mass is conserved. The water faucet and Kelvin-Helmholtz instability flows were simulated to test the effect of the proposed artificial viscosity model. The results indicate that the proposed artificial viscosity model effectively remedies the ill-posedness of the two-fluid model while maintaining a negligible total mass error.

• 한국통신학회논문지
• /
• 제21권5호
• /
• pp.1317-1324
• /
• 1996

An angular spectral inverse technique, applying the moment method procedure with a series expansion for the induced field in each enlarged cell, is suggested to reconstruct permittivity profiles of inhomogeneous dielectric objects and to reduce the ill-posedness inherent to inverse scattering problems. The angular spectral inverse scattering using the pulse basis function in enlarging the scatterer has the ill-posedness due to the input data of higher spectra. To reduce the number of higher spectra, enlarging the cell size and averaging over the cell with a suitable weighting function are found to play improtant roles for the reduction of ill-posedness of the angular spectral inverse scattering problems.

• 제어로봇시스템학회:학술대회논문집
• /
• 제어로봇시스템학회 2003년도 ICCAS
• /
• pp.134-137
• /
• 2003

Newton-Raphson is most used algorithm in EIT(electrical impedance tomography), cross-sectional distribution of resistivity is reconstructed by mean of both generating and sensing electrodes attached onto the surface of the object. EIT has been suffered from the severe ill-posedness which is caused by the inherent low sensitivity of boundary measurements to any changes of internal resistivity values. In this paper, we propose modified cost function and weighting factor that compensate for low sensitivity between boundary measurements and internal resistivity and improve performance of Newton-Raphson for EIT in region of interest.

• 전기전자학회논문지
• /
• 제20권2호
• /
• pp.152-162
• /
• 2016

전기 저항률 단층촬영법(ERT)은 표면 전극으로부터 측정된 전압을 사용하여 물체 내부의 임피던스 분포를 영상화하는 기술이다. ERT 역문제는 비정치성(ill-posedness)이 매우 심하여 영상복원의 수렴성을 확보하기 위해 조정방법이 사용된다. 사용된 조정방법에 따라 영상복원 성능이 달라지므로 상황에 따라 보다 강건한 영상 복원 성능을 얻기 위해, 서로 다른 영상복원 특성을 나타내는 L1-norm 조정방법과 Total Variation (TV) 조정방법의 두 개의 모드가 상호작용하는 상호작용 이중-모드 조정방법을 제안하였다. 제안한 이중-모드 조정방법은 실제 상황에 따라 달라지는 모드 확률을 계산하고 이에 근거하여 적합한 모드를 선택하거나 두 개의 모드를 결합한다. 모의실험을 수행하여 제안된 기법의 영상 복원 성능을 평가한 결과 비교적 양호한 성능을 나타내었다.

• 대한수학회논문집
• /
• 제12권1호
• /
• pp.165-176
• /
• 1997

In order to overcome computational ill-posedness which arises when we solve the least square problems, nonlinear smoothing splines are used. The existence and the convergence on nonlinear smoothing spline are shown with numerical experiments.

• 전기전자학회논문지
• /
• 제18권2호
• /
• pp.272-281
• /
• 2014

전기 임피던스 단층촬영법을 이용한 정적 영상 복원에서 대표적으로 사용되고 있는 복원 알고리즘은 modified Newton-Raphson(mNR) 알고리즘으로 수렴 속도 및 추정 정확도 측면에서 비교적 다른 알고리즘들에 비해 좋은 성능을 나타낸다. mNR 알고리즘에서는 측정 전압과 계산 전압과의 차이, 즉 잔류오차를 최소화하도록 목적함수를 설정하고 이를 반복 연산하여 내부의 저항률 분포를 추정한다. 이때 EIT 역문제의 비정치성을 완화시키기 위해 조정방법을 사용하며 조정인자에 따라 서로 다른 영상 복원 성능을 나타낸다. 기존 기법에서는 반복 연산마다 일정한 상수 값의 조정인자를 사용하기 때문에 대상 물체의 내부 상태가 변하거나 측정 잡음 등이 있는 경우 때때로 조정인자에 따라 영상 복원이 수렴되지 않는다. 따라서 본 논문에서는 영상 복원 수렴 및 성능을 개선하기 위하여 잔류오차에 기반하여 반복 연산마다 자동적으로 조정인자를 수정하는 기법을 제안하였다. 시뮬레이션과 실험을 수행하여 제안된 기법의 영상 복원성능을 평가한 결과 비교적 양호한 성능을 나타내었다.

• 전자공학회논문지B
• /
• 제29B권9호
• /
• pp.37-49
• /
• 1992

The standard definition of computational vision is a set of inverse problems of recovering surfaces from images. Thus the common characteristics of the most early vision problems are ill-posed. The main idea for solving ill-posed problems is to restrict the class of admissible solutions by introducing suitable a priori knowledge. Standard regurarization methods lead to satisfactory solutions of early vision problems but cannot deal effectively and directly with a few general problems, such as discontinuity and fusion of information from multiple modules. In this paper, we discuss limitations of standard regularization theory and present new stochastic method. We will outline a rigorous approach to overcome part of ill-posedness of image restoration, edge detection, and stereo vision problems, based on Bayes estimation and MRF(Markov random field) model, that effectively deals with the problems. This result makes one hope that this framework could be useful in the solution of other vision problems.

• 한국해양공학회지
• /
• 제17권5호
• /
• pp.19-24
• /
• 2003

The question of wave source identification in a wave maker problem is the primary objective of the this paper. With the observed wave elevation, the existence of the wave maker velocity is discussed with the help of the mathematical theory of inverse problems. Utilizing the property of the Strum-Liouville system and compactness, the uniqueness and the ill-posedness(in the sense of stability) for the identification are proved.

• 에너지공학
• /
• 제10권3호
• /
• pp.183-187
• /
• 2001

Impedance imaging for binary mixture is a kind of nonlinear inverse problem, which is usually solved iteratively by the Newton-Raphson method. Then, the ill-posedness of Hessian matrix often requires the use of a regularization method to stabilize the solution. In this study, the Levenberg-Marquredt regularization method is introduced for the binary-mixture system with various resistivity contrasts (1:2∼1:1000). Several mixture distribution are tested and the results show that the Newton-Raphson iteration combined with the Levenberg-Marquardt regularization can reconstruct reasonably good images.

• 한국광학회:학술대회논문집
• /
• 한국광학회 1989년도 제4회 파동 및 레이저 학술발표회 4th Conference on Waves and lasers 논문집 - 한국광학회
• /
• pp.147-151
• /
• 1989

A spectral inversion scheme, based on the moment-method procudure with series-expanded field within each cell by dividing the dielectric object into a small number os rectangular cells with large area, is developed to regularize the ill-posedness inherent in inverse scattering problems. One of the interesting features on the presented scheme is that the relative dielectric constant may be obtained by averaging over each cell. This averaging is expected to play an important role in regularizing the high-frequency effect due to noise.