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저레이놀즈수 난류모델을 사용한 정익-동익 상호작용 해석
최창호,유정열,Choi, Chang Ho,Yoo, Jung Yul 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.10
A computational study on unsteady compressible flows has been performed by adopting a low Reynolds number $k-{\omega}$ turbulence model in conjunction with dual time stepping scheme. An explicit four-stage Runge-Kutta scheme for the Navier-Stokes equations and an approximate factorization scheme for the $k-{\omega}$ turbulence model equations are used. Computational results obtained for blade surface pressure distributions in the process of rotor-stator interaction in a turbine stage are in good agreement with extant experimental data. The effects of the wake from the stator on the boundary-layer transition over the rotor blade surface are discussed by showing that high intensity turbulence of the stator wake induces an early transition.
Implementation of Semi-infinite Boundary Condition for Dynamic Finite Element Analysis
최창호,정하익,Choi, Chang-Ho,Chung, Ha-Ik Korean Geotechnical Society 2006 한국지반공학회논문집 Vol.22 No.9
Dynamic numerical analysis of geotechnical problems requires a way to simulate the decrease of energy as the domain of interest gets larger. This phenomenon is usually referred to as radiation damping or geometric attenuation and it is distinguished from material damping in which elastic energy is actually dissipated by viscous, hysteretic, or other mechanism. The fact that the domain of analysis in numerical modeling must be chosen, however, causes a need for special attention at the boundary. This observation leads directly to the idea of determining the dynamic response of the interior region from a finite model consisting of the interior region subjected to a boundary condition which ensures that all energy arriving at the boundary is absorbed. This paper presents a simple methodology to simulate transmitting boundaries condition using viscoelastic infinite elements within the recently developed "OpenSees" finite element code. The methodology used here provides that the level of absorption for traveling waves is efficient enough for practical purposes, but unsatisfactory for the case of sharp incident angles. The effectiveness of the infinite elements for the absorption of incident waves at boundaries is evaluated via example analysis.
레이다용 낮은 위상잡음을 갖는 초고속 주파수 합성기에 관한 연구
최창호,이승주,Choi, Chang-Ho,Lee, Seung-Joo 한국정보기술전략혁신학회 2009 情報學硏究 Vol.12 No.4
In this paper, frequency synthesizer for radar system is designed and developed. Optimizing the phase noise and lock time, each module is designed as two-type PLL circuit, and then the performance of PLL frequency synthesizer is compared. The experiment result shows the lock time of 70 usec, the phase noise of less then 100 dBc, the bandwidth above 500MHz.
Implicit Numerical Integration of Two-surface Plasticity Model for Coarse-grained Soils
최창호,Choi, Chang-Ho Korean Geotechnical Society 2006 한국지반공학회논문집 Vol.22 No.9
The successful performance of any numerical geotechnical simulation depends on the accuracy and efficiency of the numerical implementation of constitutive model used to simulate the stress-strain (constitutive) response of the soil. The corner stone of the numerical implementation of constitutive models is the numerical integration of the incremental form of soil-plasticity constitutive equations over a discrete sequence of time steps. In this paper a well known two-surface soil plasticity model is implemented using a generalized implicit return mapping algorithm to arbitrary convex yield surfaces referred to as the Closest-Point-Projection method (CPPM). The two-surface model describes the nonlinear behavior of coarse-grained materials by incorporating a bounding surface concept together with isotropic and kinematic hardening as well as fabric formulation to account for the effect of fabric formation on the unloading response. In the course of investigating the performance of the CPPM integration method, it is proven that the algorithm is an accurate, robust, and efficient integration technique useful in finite element contexts. It is also shown that the algorithm produces a consistent tangent operator $\frac{d\sigma}{d\varepsilon}$ during the iterative process with quadratic convergence rate of the global iteration process.
최창호,노준구,김진한,Choi, Chang-Ho,Noh, Jun-Gu,Kim, Jin-Han 한국유체기계학회 2004 한국유체기계학회 논문집 Vol.20 No.2
This paper is concerned with the numerical analyses of an S-shaped duct for the inter-channel between compressor spools. For the compactness and lightweight of an engine, the length of the S-shaped duct is desired to be minimized. Shortening the S-shaped duct, however, flow separation is likely to occur. Numerical investigation using a three-dimensional Navier-Stokes flow solver was performed to determine the availability of the minimization of an S-shaped duct. Computations were performed introducing the experimental data as the inlet flow condition of the OGV in determining the minimum length of the S-shaped duct. Also, the leaning effect of the OGV which assists the flow to turn radially inward was studied adopting mixing-plane method to consider the rotor/OGV interaction.