• 한국소음진동공학회논문집
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• 제19권7호
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• pp.719-725
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• 2009

An efficient time-domain numerical method for the analysis of broadband noise generation and propagation due to turbulence-cascade interaction is developed. The core algorithm of the present method is based on the B-periodicity of the acoustic response function of the flat-airfoil cascade to the ingesting gust(B denotes the number of airfoils in the cascade). To confirm this periodicity, gust-cascade interaction problem are solved by using the time-domain method, which shows that the incident gust with the circumferential mode number having the same remainders when divided by the airfoil number excites the same acoustic response of the cascade. Using the proposed fast algorithm with this periodicity, we show that the total computation time for the model broadband problem using the total 525 incident gust modes can be reduced to about 1/4 of that taken in using the previous time-domain program.

• 한국전산유체공학회지
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• 제6권3호
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• pp.41-50
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• 2001

Effects of rotor-stator blade count ratio on the unsteady aerodynamic characteristics of a cascade was studied by using a Navier-Stokes code. Present Navier-Stokes code is a parallel code and works on a multi-cpu machine. It is based on the SIMPLE algorithm and uses QUICK scheme for convection terms and second order back difference for all temporal derivatives. Computations were carried out for two cases : case 1 is for 3 stator cascade passages subjected to two upstream wakes while case 2 is for 2 stator cascade passages subjected to three upstream wakes. Numerical solutions show that rotor-stator blade count ratio plays a significant role in the unsteady aerodynamic characteristics of the stator cascade. Case 2 shows smaller unsteady fluctuation than case 1, even if they show the same time averaged value. The smaller fluctuation of case 2 is believed due to strong interaction between unsteady vortices. The unsteady lift variation of case 2 is shown to have many high frequency fluctuations as more unsteady vortices travel around the cascade. The unsteady turbulent kinetic energy due to the upstream wake is also shown to decay faster through the cascade passage than in the free stream.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2009년도 춘계학술대회 논문집
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• pp.477-482
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• 2009

An efficient time-domain numerical method for the analysis of broadband noise generation and propagation due to turbulence-cascade interaction is developed. The core algorithm of the present method is based on the B-periodicity of the acoustic response function of the flat-airfoil cascade to the ingesting gust (B denotes the number of airfoils in the cascade). To confirm this periodicity, gust-cascade interaction problem are solved by using the time-domain method, which shows that the incident gust with the circumferential mode number having the same remainders when divided by the airfoil number excites the same acoustic response of the cascade. Using the proposed fast algorithm with this periodicity, we show that the total computation time for the model broadband problem using the total 525 incident gust modes can be reduced to about 1/4 of that taken in using the previous time-domain program.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.610-615
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• 2001

Effects of rotor-stator blade count ratio on the unsteady aerodynamic characteristics of a cascade was studied by using a Navier-Stokes code. Present Navier-Stokes code is a parallel code and works on a multi-cpu machine. It is based on the SIMPLE algorithm and uses QUICK scheme for convection terms and second order back difference for all temporal derivatives. Computations were carried out for two cases : case 1 is for 3 stator cascade passages subjected to two upstream wakes while case 2 is for 2 stator cascade passages subjected to three upstream wakes. Numerical solutions show that rotor-stator blade count ratio plays a significant role in the unsteady aerodynamic characteristics of the stator cascade. Case 2 shows smaller unsteady fluctuation than case 1, even if they show the same time averaged value. The smaller fluctuation of case 2 is believed due to strong interaction between unsteady vortices. The unsteady lift variation of case 2 is shown to have many high frequency fluctuations as more unsteady vortices travel around the cascade. The unsteady turbulent kinetic energy due to the upstream wake is also shown to decay faster through the cascade passage than in the free stream.

• 한국추진공학회지
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• 제9권4호
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• pp.66-72
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• 2005

본 연구에서는 초음속 충동형 터빈의 유동특성을 알아보기 위해 소형 초음속 풍동을 설계하였으며 Single pass Schlieren system을 이용하여 유동을 가시화하였다. 실험은 2차원 초음속 노즐과 익렬을 조합하여 블레이드 앞전 형상과 노즐-익렬 간극에 따라 실시하였다. 실험을 통해 충격파를 포함한 복잡한 유동 형태와 노즐-익렬, 충격파-경계층 상호작용 등을 관찰할 수 있었다.

• 한국소음진동공학회논문집
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• 제16권3호
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• pp.263-269
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• 2006

The objective of the present work is to develop a time-domain numerical method of broadband noise in a cascade of airfoils. This paper focuses on dipolar broadband noise sources, resulting from the interaction of turbulent inflows with the flat-plate airfoil cascade. The turbulence response of a two-dimensional cascade is studied by solving both of the linearised and the full nonlinear Euler equations employing accurate higher order spatial differencing, time stepping techniques and non-reflecting inflow/outflow boundary condition. The time-domain result using the linearised Euler equations shows good agreement with the analytical solution using the modified LINSUB code. Through the comparison of the nonlinear time-domain result using the full nonlinear Euler equations with the linear, it is found that the acoustic mode amplitude of the nonlinear response is less than that of the linear response due to the energy cascade from low frequency components to the high frequency ones. Considering the merits of the time-domain methods over the typical time-linearised frequency-domain analysis, the current method is expected to be promising tools for analyzing the effects of the airfoil shapes, non-uniform background flow, linear-nonliear regimes on the broadband noise due to turbulence-cascade interaction.

• 조명전기설비학회논문지
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• 제23권5호
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• pp.101-109
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• 2009

본 논문에서는 변압비가 다른 2개의 삼상 변압기를 이용한 새로운 방식의 cascade 3상 IHCML(Isolated H-bridge Cascade Multi-Level) 인버터 및 제어 방식을 제안한다. 또한 cascade 3상 IHCML 인버터를 제어하기 위해 최근접 벡터를 이용한 벡터제어 기법을 사용하였다. 이로 인해 많은 전력을 담당하는 대용량 H-bridge의 스위칭 주파수는 출력 기본주파수와 거의 동일하여 제어 스위치 손실이 작으며, 작은 전력을 담당하는 저용량 H-bridge의 스위칭 주파수는 높게 제어 하므로 출력전압의 품질이 향상되었다.

• 한국지열·수열에너지학회논문집
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• 제11권1호
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• pp.15-20
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• 2015

In this study, the optimization and performance characteristics of a cascade heat pump system was analyzed with the variation of low stage refrigerant charge amount. The cascade heat pump was designed and constructed with R134a and R410A as the refrigerant for high stage and low stage cycle, respectively. Experiments were conducted by varying the low stage charge amount and the performance characteristics of the cascade heat pump were studied. The refrigerant charge amount of the low stage cycle was varied between the ranges of -15% and +10% of the optimum charge amount. The performance variation experienced in the cascade heat pump due to the variation of refrigerant charge amount shows greater effect in the undercharge regions than the overcharge regions. COP reduction in the undercharge region is larger than the decrease in the overcharge region. Some cycle variation such as power consumption and cycle pressure according to low stage refrigerant charge amount showed different trends comparing with those according to high stage refrrgerant charge amount. Therefore, the optimum charge amount of the cascade heat pump should be determined based on the experimental data obtained by the variation of high and low stage refrigerant charge amount.

• 조명전기설비학회논문지
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• 제27권5호
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• pp.95-103
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• 2013

Protection of the electrical and electronic equipment against surges in low voltage AC power distribution systems is based on wide applications of surge protective devices(SPDs). Cascade application of SPDs located at the service entrance of a building and near sensitive equipment is intended to ensure the optimal voltage protection level and energy sharing among cascade SPDs. In this paper, when surges impinge at the service entrance of the building of interest, the protection characteristics of two-stage cascade SPD systems were investigated. The influence of the distance between the upstream and downstream SPDs on the voltage protection level and energy sharing of the two-stage cascade SPD systems were analyzed experimentally. It was found that the energy sharing of two-stage cascade SPD systems strongly depends on the distance between the two SPDs and the component of SPD. As the distance between the two SPDs increases, the energy absorbed by the upstream SPD increases while the energy absorbed by the downstream SPD decreases. Consequently, it is desirable to select the upstream and downstream SPDs having the proper energy capability with due consideration of the distance between the two SPDs.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1999년도 춘계 학술대회논문집
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• pp.155-161
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• 1999

The three-dimensional turbulent cascade flows with and without endwall fences are numerically investigated by solving the incompressible Navier-Stokes equations with a high-Reynolds number $k-{\varepsilon}$ turbulence closure model. A projection method based algorithm is used in the finite-volume formulation, with the second order upwind-differencing scheme for the convective terms. First, assessments on accuracy of the present method are made by comparing the static pressure distributions at the mid-span of the cascade with measured data, and also by confirming the experimental observations on the choice of an optimal fence height for the secondary flow control. In understanding the three-dimensional nature of the secondary flow in turbine cascade, the limiting streamline patterns and the static pressure contours at the suction surface of the blade as well as on the cascade endwall are employed to visualize the effectiveness of the endwall fence for the secondary flow control. Analysis on the streamwise vorticity contour maps along the cascade with the three-dimensional representation of their iso-surfaces reveals the strucuture of the complicated vortical flow in the turbine cascade with endwall fence, and also leads to an understanding on formation of the counter-rotating streamwise vortex over the endwall fence, in explaining the mechanisms of controlling the secondary flow and also for the proper selection of an optimal fence height.