Jo Sep 23, 2022
Ro Won Myong, a lecturer at the Faculty of Applied Mathematics, has investigated necessary optimality conditions and a numerical method for solving variable-order fractional optimal control problems where the final time and state are simultaneously free and the dynamic system is described as a binomial variable-order fractional differential equation containing variable-order fractional derivatives in the sense of Caputo. These problems have been the focus of fractional order differential equations and their applications.
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Jo Sep 21, 2022
Digital watermarking is being widely applied in various fields such as copyright protection of paintings, music, movies and documents together with broadcast monitoring, transaction tracking, copy protection and ownership authentication nowadays when digital multimedia is rapidly spread through the Internet.
Recently, the most actively studied digital watermarking covers image watermarking, document watermarking and video watermarking. In particular, image watermarking holds a high proportion.
Image watermarking can be divided into several kinds according to representation, embedding domain and objectives of applications ― visible watermarking and invisible watermarking from the aspect of representation; spatial domain watermarking and transformed domain watermarking according to embedding domain; robust watermarking and fragile watermarking for objectives of applications.
There are several indicators of the performance of watermarking including imperceptibility, robustness, capacity, security and computational complexity. In particular, imperceptibility and robustness are the two major features that are trade off, so a number of studies for an effective solution to it have been conducted.
Among them are approaches to raise imperceptibility by embedding watermarks into regions with the lowest human visual characteristics, approaches to get the highest robustness under given imperceptibility levels and approaches to solve single object optimization problems of empirical weighted imperceptibility and robustness by advanced evolutionary algorithms. However, all these have several drawbacks since they cannot correspond properly to arbitrary images and every attack.
Son Chol Min, a researcher at the Faculty of Applied Mathematics, selected blocks with lower human visual characteristics and embedded watermarks into their DC-transformed domains so as to raise imperceptibility. On the basis of it, he has presented a design method of an optimal blind watermarking algorithm which guarantees the highest imperceptibility and robustness for arbitrary images and against any types of attacks by solving multi-objective optimization problems between imperceptibility and robustness based on MOEA/D.
The simulation results showed that the imperceptibility of images watermarked by the proposed method was very high and this method outperforms previous ones in terms of robustness against various attacks.
You can find detailed information about this in his paper “Design of Optimal Blind Watermarking Technique based on MOEA/D” presented to the SCI Journal “IET Image Processing”.
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Jo Sep 20, 2022
A shell and tube heat exchanger among a variety of heat exchangers is still widely used in many industrial applications as it has several advantages like a long working life, simple manufacture and wide operation range.
In multi-channel systems like the shell and tube heat exchanger, flow distributions significantly affect their performance and lifetime. Uniform distribution is commonly assumed in designing conventional heat exchangers, but flow maldistribution may be an inevitable occurrence in practice. In particular, flow distribution in the tube-side of the SSTHX with axial connections of bonnet nozzles is extremely non-uniform.
Therefore, numerical and experimental studies on flow distributions of the shell and tube heat exchangers have been widely carried out.
In particular, many attempts have been made for flow uniformity in multi-channel systems by means of different geometry of headers or manifolds. Most studies tended to be focused on shell-side flows due to their complexity in shell and tube heat exchangers. What is difficult in numerical studies on heat exchangers is that their geometries with lots of channels cannot be modeled as it is. One way to solve it is to model tube bundles as porous media. The most important problem is to determine porous medium parameters accurately. In most references, they were obtained by empirical correlations.
Various baffles have been proposed to improve velocity distributions and heat transfers of heat exchangers.
However, few researchers have addressed the problems concerning the tube-side flow of the shell and tube heat exchanger.
It should be noted that most studies have been focused on limited regime like laminar flow or turbulent flow.
Pak Sin Myong, a researcher at the Faculty of Heat Engineering, has clarified that introduction of new header baffles makes it possible to uniformalize dramatically flow distribution in the tube-side of SSTHX. He has also modeled tube bundles as porous media as they have lots of tubes, and he has obtained porous media parameters through CFD simulation for one isolated channel.
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Jo Sep 18, 2022
Recently, with the development of nanotechnology, it has become possible to change the size and the shape of nanoparticles, so a lot of investigations are being conducted into the optical properties of various shaped nanogold and semiconductor quantum dots. The properties are affected by their associate structure and a core-shell structure has been the main focus of most investigations. However, the optical property of a semiconductor quantum dot aqueous solution that contains nanogold has not been discussed.
While Discrete Dipole Approximation(DDA) and Finite Difference Time Domain(FDTD) methods are widely used in the optical property simulation of nanoparticles, Mie theory is employed in the simulation of the scattering property of various shaped nanoparticles.
Ri Myong Il, a researcher at the Faculty of Physical Engineering, has chemically prepared an aqueous solution of CdTe quantum dots with gold nanoparticles and measured its absorption and scattering spectra. On the basis of it, he has simulated the scattering property of this composite material by combining Mie theory with FDTD method.
The aqueous solution of CdTe quantum dots (3nm in diameter) with gold nanoparticles (30nm in diameter) was prepared by the chemical crystal growth method. The scattering property of the aqueous solution of CdTe quantum dots with gold nanoparticles was simulated by the Finite Difference Time Domain (FDTD) method and it was verified by an absorption spectrophotometer.The results showed that compared with a pure aqueous solution of CdTe quantum dots, it has stronger absorption and scattering spectra, that is, 1.5 times higher in scattering intensity. Therefore, it is concluded that aqueous solutions of CdTe quantum dots with gold nanoparticles can be widlely used in the fields of medicine and optics.
More information can be found in his paper “Scattering Property of Aqueous Solution of CdTe Quantum dots with Gold Nanoparticles” presented to the SCI Journal “Optics Communications”.
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Jo Sep 16, 2022
Most practical engineering problems are currently solved by the well-developed standard finite element method (FEM). However, the FEM has its inherent shortcomings as it is a numerical method that relies on meshes. While conventional mesh-based techniques such as FEM can accurately approximate governing equations, labor and computational costs associated with creating the conforming mesh undermine the efficiency of such methods. The Boundary Element Method (BEM) is another technique that can alleviate the difficulties associated with the implementation of the standard FEM, but it still relies on meshes.
Thus, an idea of getting rid of meshes in the process of numerical treatments has naturally evolved, and the concepts of meshfree or boundary meshfree methods have been shaped up.
Boundary meshfree methods can be roughly sorted into two: MFS (Method of Fundamental Solutions)-based type and the BIE (Boundary Integral Equation)-based type. The former is based on the concept of the MFS. The traditional MFS uses only a fundamental solution, which is a response due to a concentrated point source, in the construction of the solution of a problem without using any integrals. It is a natural boundary meshfree method.
Studies show that the MFS has been developed by improving the locations or types of sources. In the traditional MFS, a fictitious boundary is required to have the source points on it to avoid the singularity of fundamental solutions. The determination of the distance between the real boundary and the fictitious boundary is based on experience and therefore it is troublesome. In recent years, a number of efforts have been made aiming to remove this barrier in the MFS so that the source points can directly be placed on the real boundary.
And the MFS has been improved by replacing concentrated point sources of the traditional MFS with area-distributed sources covering the source points for 2D problems (volume-distributed sources for 3D problems).
Kim Un Ok, a lecturer at the Faculty of Applied Mathematics, has successfully improved the boundary meshfree method with area-distributed sources placed on the real boundary called boundary distributed source (BDS) method, by moving distributed sources outside the boundary.
To show the effectiveness of the new improved method, she has plotted the relative errors in the computed results using the BDS method and her new improved method respectively.
She has demonstrated the accuracy of the new method by solving 2D potential problems in a square domain covering 0≤x and y≤1 with Dirichlet BC,Ф(x, y)=x2-y2. She has moved sources 1.5 times d, average distance between the original boundary distributed sources, outside the boundary in the direction of outward normal vectors.
When BDS method is used, the solution is inaccurate near the boundary regions. However, the proposed method works well in improving the accuracy of the numerical solution. The results say that moving distributed sources outside the boundary can improve the BDS method.
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Jo Aug 29, 2022
Stability of earth and rockfill dams (ERD) usually includes sliding stabilities of steady seepage (at levels of normal water or flood water), earthquake and transient seepage (drawdown or heading-up).
In transient seepage such as rapid drawdown (RDD) unlike steady seepage, the phreatic surface, shear strength, boundary condition and effective stress of the slope soil, etc. dynamically change and they have coupled seepage-stress and nonlinear characteristics. These cause a number of problems with evaluating stability for transient seepage.
It can be a great expense to examine one by one the stability of earth and rockfill dams for rapid drawdown in civil engineering practice.
ERDs frequently met in civil engineering practice can be classified into homogeneous, sloping upstream core and central core according to whether they have an impervious core or not and where the impervious core exists.
Ri Yong Nam, a researcher at the Faculty of Earth Science and Technology, has proved that fully coupled seepage-stress analysis makes it possible to select the safest type for rapid drawdown among all common types of earth and rockfill dams and to approximately estimate the minimum safety factor for RDD from the steady safety factors.
The slope stability for RDD is very important in embankments such as a reservoir dam of relatively small storage capacity than its height, embankments under the effects of the high tide and the ebb tide, dams of agricultural reservoir, etc. In these cases, a central core dam (CCD) to improve the stability of ERD for RDD is recommended.
On the contrary, slope stability for RDD is not so important in embankments such as the reservoir dams of relatively large storage capacity compared to its height, embankments of gentle water level variation, etc. In these cases, an ERD which has larger steady stability for the same slope gradients is economically better and thus, a sloping upstream core dam (SUCD) is recommended.
The details about this is carried in his paper “Study on Stability Reduction Characteristics of Earth and Rockfill Dams under Rapid Drawdown Using Fully Coupled Seepage-Stress Analysis” presented to the SCI Journal “Advances in Civil Engineering”.
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