• 대한기계학회논문집
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• 제9권5호
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• pp.613-620
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• 1985

본 연구의 목적은 수평 Blasius유동의 열적 불안정성 문제에서 유동방향 좌표 에도 의존하는 교란양을 최초로 도입함으로써 보다 합리적인 안정성 해석을 수행하고 그 결과를 기존의 실험적 및 이론적인 자료들과 비교하며 이를 토대로 한 보다 실제적 인 열전달 상관관계를 얻고자 하는데 있다.

• 한국전산유체공학회지
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• 제19권1호
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• pp.57-63
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• 2014

This study performed numerical analysis for the characteristics of flow-induced forces and the flow instability depending on the cross-sectional shape of the cylinder in laminar flow. To implement the cylinder cross-section, we adopted an Immersed Boundary Method with marker particles. We analyzed flow characteristics based on the radius of corner curvature. Main parameters are corner radius and Reynolds number (Re). With Re = 40, 50, 150 we calculated the flow field, drag coefficient, RMS of lift coefficient, pressure coefficient and Strouhal number in conjunction with the corner radius variation. Also, we calculated critical Reynolds number ($Re_c$) depending on the corner radius variation.

• 한국음향학회지
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• 제39권4호
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• pp.371-377
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• 2020

종횡비(Length/Diameter, L/D)가 크고, 실린더-슬롯형 그레인을 적용한 고체로켓모터의 연소시험에서 연소불안정 현상이 발생하였다. 압력섭동을 스펙트럼 분석한 결과 중심축 길이방향 주파수가 지배적임이 확인되어, 음향노드와의 일치를 해소하기 위해 실린더 파트의 길이를 증가시켰다. 또한 고체로켓모터에서 발생하는 유동 구조에 의한 불안정성 발생 원인을 분석하기 위하여 음향모드해석과 유동해석을 수행하였다. 설계 변경 전후 그레인 형상을 이용하여 연소실 내 압력 진동 크기 및 주파수에 대해 정량적 비교를 통해 연소불안정이 저감됨을 확인하였다. 최종적으로 연소시험을 수행하여 해석결과와 같이 연소불안정 현상이 사라짐을 확인하였다.

• 한국추진공학회지
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• 제26권6호
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• pp.62-73
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• 2022

본 연구에서는 환형연소기 형태를 가지는 가스터빈 연소기에서 연소불안정성을 제어하는 방법을 제시하고자 하였으며, 노즐 배치와 이에 따른 당량비 변화를 통한 유동적인 대칭성파괴(Flow symmetry breaking) 효과를 비교하였다. 이를 위하여 FFT, Time signal 및 위상궤적의 모드 분석을 통하여 대칭성파괴 효과를 확인하였다. 또한, 모드분석으로 불안정한 영역과 안정된 영역을 확인하였고 이를 등고선 지도에 나타내었다. 본 연구를 통해 노즐의 당량비와 배치가 대칭이거나 노즐이 연속적으로 배치되면 불안정성이 발생하였으나, 노즐의 배치 및 당량비가 대칭성을 가지질 않는다면 당량비의 차이가 작더라도 연소불안정성이 극적으로 감소하는 것을 확인할 수 있었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3033-3038
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• 2007

In this research, Rayleigh instability of gas-liquid flow in annular pipe is studied in film boiling using viscous potential flow. Viscous potential flow is a kind of approximation of gas-liquid interface considering velocity field as potential including viscosity. A dispersion relation is obtained including the effect of heat and mass transfer and viscosity. New expression for dispersion relation in film boiling and critical wave number is obtained. Viscosity and heat and mass transfer have a stabilizing effect on instability and its effect appears in maximum growth rate and critical wave number. And the existence of marginal stability region is shown.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2008년도 춘계학술대회논문집
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• pp.242-249
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• 2008

In this paper, flow-induced flutter instability of cantilever carbon nanotubes conveying fluid and modelled as a thin-walled beam is investigated. Non-classical effects of transverse shear and rotary inertia are incorporated in this study. The governing equations and the associated boundary conditions are derived through Hamilton's principle. Numerical analysis is performed by using extend Galerkin method which enables us to obtain more exact solutions compared with conventional Galerkin method. Cantilevered carbon nanotubes are damped with decaying amplitude for flow velocity below a certain critical value, however, beyond this critical flow velocity, flutter instability may occur. Variations of critical flow velocity with both radius ratio and length of carbon nanotubes are investigated and pertinent conclusion is outlined.

• 한국소음진동공학회논문집
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• 제18권6호
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• pp.654-662
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• 2008

In this paper, flow-induced flutter instability of cantilever carbon nanotubes conveying fluid and modelled as a thin-walled beam is investigated. Non-classical effects of transverse shear and rotary inertia are incorporated in this study. The governing equations and the associated boundary conditions are derived through Hamilton's principle. Numerical analysis is performed by using extend Galerkin method which enables us to obtain more exact solutions compared with conventional Galerkin method. Cantilevered carbon nanotubes are damped with decaying amplitude for flow velocity below a certain critical value, however, beyond this critical flow velocity, flutter instability may occur. Variations of critical flow velocity with both radius ratio and length of carbon nanotubes are investigated and pertinent conclusion is outlined.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제16권4호
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• pp.225-234
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• 2012

We present two high-order potential flow models for the evolution of the interface in the Rayleigh-Taylor instability in two dimensions. One is the source-flow model and the other is the Layzer-type model which is based on an analytic potential. The late-time asymptotic solution of the source-flow model for arbitrary density jump is obtained. The asymptotic bubble curvature is found to be independent to the density jump of the fluids. We also give the time-evolution solutions of the two models by integrating equations numerically. We show that the two high-order models give more accurate solutions for the bubble evolution than their low-order models, but the solution of the source-flow model agrees much better with the numerical solution than the Layzer model.

• 대한전기학회논문지:전력기술부문A
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• 제48권3호
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• pp.190-196
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• 1999

Continuation power flow has been developed to remove the ill-condition problem caused by singularity of power flow Jacobian at and near at steady-state voltage instability point in conventional power flow. Continuation power flow consists of predictor and corrector. In prddictor, the direction vector at the resent solution is caluculated and the initial guess of next solution is determined at the distance of step length. The selection of step length is a very important part, since computational speed and convergence performance are both greatly affected by the choice of the step length. This paper presents the practical step length selection algorithm using the reactive power generation sensitivith. In numulation, the proposed algorithm is compared with step length selection algorithm using TVI(tangent vector index).

• 한국연소학회지
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• 제7권1호
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• pp.23-30
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• 2002

Instability of oblique detonation waves (ODW) at off-attaching condition was investigated through a series of numerical simulations. Two-dimensional wedge of finite length was considered in $H_2/O_2/N_2$ mixtures at superdetonative condition. Numerical simulation was carried out with a compressible fluid dynamics code and a detailed hydrogen-oxygen combustion mechanism. Present result reveals that there is a chemical kinetic limit of the ODW detachment, in addition to the theoretical limit predicted by Rankine-Hugoniot theory with equilibrium chemistry. Result also presents that ODW still attaches at a wedge as an oblique shock-induced flame showing periodically unstable motion, if the Rankine-Hugoniot limit of detachment is satisfied but the chemical kinetic limit is not. Mechanism of the periodic instability is considered as interactions of shock and reaction waves coupled with chemical kinetic effects. From the investigation of characteristic chemical time, condition of the periodic instability is identified as follows; at the detaching condition of the Rankine-Hugoniot theory, (1) flow residence time is smaller than the chemical characteristic time, behind the detached shock wave with heat addition, (2) flow residence time should be greater than the chemical characteristic time, behind an oblique shock wave without heat addition.