Jo Apr 19, 2024
Taylor-Couette flow, the flow in the gap between two independently rotating coaxial cylinders, is one of the most discussed problems in fluid mechanics, due to its conception simplicity and applications in processing technology.
In the case of a circular Couette flow with superimposed axial throughput, the flow patterns are expected to be more complicated due to strong interactions between axial and rotating flows.
Recently, there have been some reports that Taylor-Couette flow could be used for production of nano materials. Due to the very strong shear rate of particles in the Taylor-Couette flow, the shaking particles in the fluid tend to split with time and so this is one of the main reasons that Taylor-Couette systems are used to produce nano materials.
Ju Jong Min, a section head at the Faculty of Materials Science and Technology, has developed a machine for producing nano materials by deciding the geometrical dimension of a system and the rotating speed of inner cylinder to form TC flow in Taylor-Couette system without axial flow.
Observing Taylor-Couette vortex at Reynolds number of 500~2 000, he chose rotating speed, viscosity of fluid and geometrical structure of system like the radii of inner and outer cylinders and its ratio and the aspect ratio of system suitable for obtaining steady Taylor-Couette flow. He concluded from the experiment that the rotating speed of 800r/min, the viscosity of fluid of 0.1Pa•s and the radius ratio of 0.88 are appropriate for steady Taylor-Couette flow.
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Jo Apr 16, 2024
Specific algorithms for elaboration and digital filtering of pulse signals generated in X-ray detectors have been studied and optimized. A signal pulse at the output of a proper analog conditioning circuit is digitized and then the samples undergo a digital filtering procedure.
Jong Kyong Su, a researcher at the General Assay Office, has developed an algorithm for reliable detection of “X-ray informative” pulses partially buried in noise on digital boards which use programmable devices such as DSP or FPGA.
He focused on the application of least mean square (LMS) adaptive filter, which plays an important role in modern digital signal processing (DSP) such as telephone echo cancellation, noise cancellation, equalization of communications channels, biomedical signal enhancement, adaptive control systems, etc. The adaptive filter is a digital filter that can adjust its filter coefficients automatically to adapt input signals via an adaptive algorithm.
He compared two digital processing methods by traditional filters with fixed filter coefficients and a least-mean-squares (LMS) adaptive filter for applications in high resolution X-ray spectroscopy.
The requirements on peak reproducibility and SNR are better met by the LMS filter, thanks to the capability to adapt the parameters to the non-stationary environment of the detectors. The simulation showed that fast settlement of coefficients can be achieved with an algorithm of medium-level complexity, with elaboration time scaling linearly with the filter order.
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Jo Apr 15, 2024
Energy and environmental problems are two challenges today and production and use of recycled energy is the focus of world attention. More and more attention is being drawn to more reliable solar power generating systems, and more than 50GW solar power generating capacity is being constructed in the world every year.
At present, there are two widely-used types of solar power systems—central or distributed. The main problems in the construction of distributed solar power systems are to guarantee flexibility of distributed power sources, stability of voltage and frequency of the weak grids, optimized management and distribution of electric systems and improvement of electric power quality.
O Yong Nam, a section head at the Faculty of Physical Engineering, has designed a novel phase synchronization method for distributed power systems by combining good harmonic and DC offset cancellation characteristics of the discrete Fourier transform (DFT) and frequency adaptation characteristics of phase locked loop (PLL). The validation of the suggested method has been simulated with Microsoft Excel and MATLAB Simulink software.
The proposed method has shown to be effective for grid voltage signal synchronization in non-ideal grid conditions with frequency variation and harmonic distortion and for reference signal synchronization in multi-inverter standalone systems. Also, it is not sensitive to the DC offset in the measurement process.
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Jo Apr 14, 2024
Parallel robot mechanisms have been and are being used in a wide variety of applications such as motion simulators, high speed manipulation, machine tools and even nano-manipulators. Parallel robot mechanisms are closed-loop mechanisms presenting very good performances in terms of accuracy, rigidity and ability to manipulate large loads.
Researches on parallel robots can be divided into kinematics, dynamics and control, the most fundamental and important being the structural synthesis of parallel robot mechanisms in kinematics. Of many mechanisms that may be made by spatial combination of links and joints, finding a configuration of mechanisms with appropriate performances for applying to a necessary object as a parallel robot mechanism makes us understand the complexity and difficulty of the problems for the structural synthesis of parallel robot mechanisms. Here, quick calculation of correct degree of freedom is the basic requirement for the structural synthesis of parallel robot mechanisms.
Kim Yong Ho, a researcher at the Faculty of Mechanical Science and Technology, has proposed a new method of deriving information on mechanism’s degree of freedom by automatic generation of velocity constraint equations on computer without using any special analytical tools.
Under the condition that the endpoints of open kinematic chains－the legs of parallel robot mechanisms－are connected to one link, he automatically generated the symbolical velocity constraint equations to calculate the degree of freedom of parallel robot mechanisms by using the rank. The proposed method was applied to CPM mechanism, Bennett mechanism and Bricard mechanism. The results showed that the proposed method is correct, useful and prospective.
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Jo Apr 12, 2024
Nowadays taper threads and variable pitch threads are widely used in mechanical, medical, and other sectors. Taper threads are generally used for fixing objects to others such as wood. Variable pitch threads are mainly useful for fastening objects to soft materials with some elasticity due to their fastening characteristics changed with forward movement. For this reason, screws with the two characteristics combined are now designed and widely used. However, there are some disadvantages for machining these screws.
Kim Hak Song, a section head at the Faculty of Mechanical Science and Technology, has put forth the problems arising in the design and machining of variable pitch threads, and proposed a method for calculation of tool path and generation of NC program for cutting them on 5-axis CNC tool grinder and for virtual cutting simulation by virtual cutting simulation program VERICUT8.2.
The proposed method can be used in the design and manufacturing of implants and transmission system of taper threads with variable pitch.
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Jo Apr 10, 2024
Quantitative analysis of diffraction data usually refers to determination of amounts of different phases in multi-phase samples by using standard sample or simulation method. Quantitative analysis may also be understood as determination of particular characteristics of single phases including precise determination of crystal grain size and its structure and shape.
There are several methods of X-ray diffraction to quantify phases, but the Rietveld method has been perhaps the most useful tool in recent years as it accounts for the factors that affect the reproducibility of the intensity peaks (peak overlapping, presence of amorphous phases, and preferred orientation of crystallites).
Ro Thae Min, a researcher at the General Assay Office, has observed the quantitative value change when lattice constant is used, and not used, for fitting in the quantitative phase analysis of TiO2 by the Rietveld method.
The conclusions are as follows.
In qualitative analysis, the corresponding phase may not be correctly identified or unreconciled lattice constant in the ICSD database may be used. To avoid such mistakes, in the quantitative phase analysis, correct lattice parameters must be used for fitting. When lattice parameters are correct, Rietveld method is better than other X-ray diffraction quantitative phase methods such as internal standard method and direct method because it employs insturments, structure parameters and lattice constant of analogue phases altogether. Sometimes, however, lattice constant is not used for fitting. In this case, various lattice defects (crystallite size, rms strain, stacking and twin fault probabilities, etc.) are found in the whole diffraction pattern improvement process. Therefore, in the quantitative phase by Rietveld method, lattice constant should not be ignored to get correct quantitative value.
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