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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.
천이효과를 고려한 BWB UCAV 형상의 공력 특성 전산해석
조영희(Young-Hee Jo),장경식(Kyoungsik Chang),신동진(Dong-Jin Sheen),박수형(Soo Hyung Park) 한국항공우주학회 2014 韓國航空宇宙學會誌 Vol.42 No.7
비세장형, 둥근 앞전을 가지고 스팬이 1.0m로 축소된 BWB형 UCAV에 대해 완전난류, 천이 모델을 사용하여 전산해석을 수행하였다. 자유류는 받음각 ?4도부터 26도까지 50㎧이며, 평균 시위 기준 레이놀즈수는 1.25×10?이다. 멀티블록 6면체 격자와 함께 완전난류 모델과 천이 모델의 결과를 비교하여 천이효과가 공력 특성에 미치는 영향을 살펴보았다. 풍동 실험과 비교한 결과 양/항력 계수는 해석범위 내에서 잘 일치하였으며, 피칭모멘트는 높은 받음각에서 작게 예측됨과 동시에 난류모델에 따라 결과가 크게 달라졌다. 압력분포와 skin friction line, 축 방향 속도장을 이용하여 와류구조의 거동과 천이현상이 미치는 영향을 살펴본 결과, 천이효과를 고려하는 것이 UCAV의 정확한 와류 구조와 공력특성 예측에 필요한 것으로 확인하였다. A computational simulation for a nonslender BWB UCAV configuration with rounded leading edge and span of 1.0m was performed to analyze its aerodynamic characteristics. The freestream is 50㎧ over ?4 to 26 degree A.o.A.s. Reynolds number based on the mean chord length is 1.25×10?. 3D multi block hexahedral grids are used which allow good grid quality and ease to capture boundary layer. γ-Re<SUB>θ</SUB> model as well as κ-ω SST model is employed to assess the effect of transition for flow behavior. Drag and lift of the UCAV were well predicted while C<SUB>M</SUB> is under predicted at high angle of attacks and influenced by the turbulence models strongly. After assessing pressure distribution, skin friction lines and velocity field around the UCAV configuration, it was found that transition effect should be considered to enhance the prediction of aerodynamic behavior by a vortical flowfield.