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장경식(Kyoungsik Chang) 한국전산유체공학회 2012 한국전산유체공학회지 Vol.17 No.2
The present study focuses on the case of developing flow with in a channel containing a long array of sinusoidal waves (2a/λ=0.1, λ=h, λ is the wavelength, 2a is the wave height, h is the mean channel depth) at the bottom wall. The Reynolds number defined with channel height, h and the mean velocity, U, is Re=6,700. The channel is sufficiently long such that transition is completed and the flow is fully developed over the downstream half of the channel. For the case of an incoming steady flow with no resolved turbulence, the instantaneous flow fields in the transition region are characterized by the formation of arrays of highly-organized large-scale hairpin vortices whose dimensions scale with that of the roughness elements. The paper explains the mechanism for the formation of these arrays of hairpin vortices and shows these eddies play the primary role in the formation of the large-scale streaks of high and low velocity over the wavy wall region. The presence of resolved turbulence in the incoming flow, reduces the streamwise distance needed for the streaks to develop over the wavy region, but does not affect qualitatively the transition process. In the fully-developed region, isolated and trains of large-scale hairpins play an important role in the dynamics of the streaks over the wavy wall.
Digital Design of DWPT Technique on FPGA for Power System Harmonics Estimation
Baraskar Savita,Tiwari Vinay K. 대한전기학회 2022 Journal of Electrical Engineering & Technology Vol.17 No.6
Real-time detection and quick amplitude estimation of power system harmonics are signifi cant issues among various power quality indices in the electrical power distribution system. In this paper digital design of the discrete wavelet packet transform (DWPT) technique is presented for accurate and fast amplitude estimation of power system harmonics in real-time applications. The DWPT technique provides a uniform frequency bandwidth of the power supply waveform at the output end. Consequently, the input signal's frequency spectrum is evenly divided into eight equal frequency bands that give accurate amplitude estimation of the harmonics by the proposed technique. Generally, the DWPT technique gives the output in the form of coeffi cients, so the proposed digital architecture incorporates arrangements for giving root mean square values directly. The DWPT based wavelet decomposition tree has been developed with the help of MAC-based wavelet fi lters that gives an advantage in saving the number of multipliers compared with the polyphase structure on the FPGA board. Also, the downsampling operation performed at each decomposition level reduces the wavelet coeffi cients by half which minimizes the computations involved in it. The digital design of the proposed DWPT technique has been implemented on the Xilinx Artix-7 FPGA AC-701 board with Xilinx System Generator (XSG)/Vivado design suite 18.3. Thereafter, standard experimental test signals have been generated on the MATLAB/Simulink software platform to validate the robustness and accuracy of the proposed digital hardware under various experimental test conditions, proving their applicability in real-time estimation of power system harmonics.
전기로 제강 더스트 배출 스크류 컨베이어의 냉각성능 수치해석
김병천(Byeongcheon Kim),장경식(Kyoungsik Chang),이상욱(Sang-Wook Lee) 한국전산유체공학회 2019 한국전산유체공학회지 Vol.24 No.2
A screw conveyor is used to discharge a large amount of slag and dust generated from an electric arc furnace in steel making process using scrap iron. Because it continuously operates in a high temperature environment, it is important to optimally design a cooling channel to minimize thermal corrosion damage. In this study, numerical analysis of conjugate heat transfer was carried out using Ansys Fluent, considering various geometric parameters of helically shaped flow guide fins in cooling channels, such as fin height and helical pitch. The higher the height of the helical fin and the smaller the pitch, the better the cooling performance. However, the pressure loss increases significantly. In particular, it can be seen that flow separation, which may degrade cooling performance, occurs in the inlet region of models with long pitch helical fin.