• Coupled systems mechanics
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• 제2권2호
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• pp.159-174
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• 2013

The present work aims at investigating the nonlinear dynamics, bifurcations, and stability of an axially accelerating beam with an intermediate spring-support. The problem of a parametrically excited system is addressed for the gyroscopic system. A geometric nonlinearity due to mid-plane stretching is considered and Hamilton's principle is employed to derive the nonlinear equation of motion. The equation is then reduced into a set of nonlinear ordinary differential equations with coupled terms via Galerkin's method. For the system in the sub-critical speed regime, the pseudo-arclength continuation technique is employed to plot the frequency-response curves. The results are presented for the system with and without a three-to-one internal resonance between the first two transverse modes. Also, the global dynamics of the system is investigated using direct time integration of the discretized equations. The mean axial speed and the amplitude of speed variations are varied as the bifurcation parameters and the bifurcation diagrams of Poincare maps are constructed.

• 한국진공학회지
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• 제21권3호
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• pp.130-135
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• 2012

드리프트-확산 근사식을 이용한 1차원 유체 방정식으로부터 선형적 안정성 이론을 전개하여 Tosend 방전에서 전자 확산이 불안정성에 미치는 영향을 관찰하였다. 본 연구에서 관찰된 바에 따르면 Townsend 불안정성은 전자 확산과 공간 전하에 의해 형성된 전기장의 효과가 결합되어 발생하며, 공간전하에 의한 효과가 작은 영역, 즉 방전 전류가 낮은 영역에서는 전자 확산 효과가 커질수록 불안정이 더 빨리 진행된다는 것이 발견되었다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2003년도 The Fifth Asian Computational Fluid Dynamics Conference
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• pp.77-79
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• 2003

The stability of flows induced by a surface acoustic wave (SAW) propagating along the deformable walls in a confined parallel-plane microchannel or slab in the laminar flow regime is investigated. The governing equation which was derived by considering the nonlinear coupling between the deformable or waving interface and viscous fluids is linearized and then the problem is solved by a verified code based on the spectral method together with the associated interface and boundary conditions. The value of the critical Reynolds number was found to be near 1439 which is much smaller than the rigid-wall case: 5772 for conventional pressure-driven flows.

• Wind and Structures
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• 제4권2호
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• pp.85-100
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• 2001

The mechanism of wind-induced ovalling vibrations of cylindrical shells is numerically investigated by using a vortex method. The subject of this paper is limited to a two-dimensional structure in the subcritical regime. The aerodynamic stability of the ovalling vibrations in the second to fourth circumferential modes is discussed, based on the results of a forced-vibration test. In the analysis, two modal configurations are considered; one is symmetric and the other is anti-symmetric with respect to a diameter parallel to the flow direction. The unsteady pressures acting on a vibrating cylinder are simulated and the work done by them for one cycle of a harmonic motion is computed. The effects of a splitter plate on the flow around the cylinder as well as on the aerodynamic stability of the ovalling vibrations are also discussed. The consideration on the mechanism of ovalling vibrations is verified by the results of a free-vibration test.

• 대한기계학회논문집B
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• 제20권12호
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• pp.3949-3959
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• 1996

A new interfacial instability between two vertical fluid layers of different densities is studied. The two layers are flowing between two parallel vertical plates vertically upward or downward, forming counter- or concurrent flows. In order to extend the study to highly-nonlinear regime in future studies, a nonlinear interface evolution equation is derived, and the stability analysis is performed based on the evolution equation. Among the parameters studies are the ratios of the fluid densities and layer thicknesses and the net flow rate.

• Progress in Superconductivity
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• 제8권2호
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• pp.193-201
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• 2007

Since the margins for the minimum quench energy and for the operating current in the superconducting magnet determine the operating regime of the magnet, a thermal stability analysis for the KSTAR superconducting magnet system is performed using 1-D Gandalf code. The result shows that the minimum quench energy is about 500 mJ/cc and the operating current margin is about 70 %. These values are larger than those of the KSTAR design criteria and the KSTAR superconducting magnet system can be operated stably under various experimental environments.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2003년도 학술대회지
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• pp.215-223
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• 2003

Viscosity index improver (VII) is one of the major additives to the modern multigrade lubricants for the viscosity stability against temperature rise. However, it causes shear thinning effects which make the film thickness lessened very delicately at high shear rate $(over\;10^5\;s^{-1})$ of general EHL contact regime. In order to exactly verify the VII's performance of viscosity stability at such high shear rate, it is necessary to make the measurement of EHL film thickness down to $\~100nm$ with fine resolution for the preliminary study of viscosity control. In this work, EHL film thickness of VII added lubricant is measured with the resolution of $\~5nm$, which will give very informative design tool for the synthesis of lubricants regarding the matter of load carrying capacity at high shear rate condition.

• 한국수자원학회논문집
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• 제54권12호
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• pp.1223-1232
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• 2021

본 연구에서는 연중 일유량의 불균등 정도를 파악하기 위해 "유량 지니계수"라는 새로운 개념의 분석방법을 제시한다. 유량 지니계수는 오름차순한 연중 일유량 발생일자의 누계백분율과 일최빈유량의 오름차순 누계백분율과의 면적관계를 이용하여 산정한다. 유량 지니계수는 0~1 사이의 값으로 표현되며, 불균등의 정도는 5단계로 구분할 수 있다. 유량 지니계수를 이용하면 상류 지점에 대한 하류 지점의 유량 안정도를 산정할 수 있다. 또한, 유황분석 기준유량별로 불균등에 미치는 영향의 정도를 수치적으로 파악할 수 있다. 4대강 본류 상·하류 8개 지점의 장기간 일유량 자료를 이용하여 유량 지니계수의 적용성을 검토하였다. 유량지니계수는 상류 댐에 의한 하류에서의 유량조절 효과를 분석하는 데에도 활용될 수 있을 것이다.

• Korean Chemical Engineering Research
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• 제59권4호
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• pp.626-631
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• 2021

전기투석장치, 전기화학 전지, 미세유체역학 분석 장치 등에서 사용하는 이온 교환 막 근처의 대표적인 비선형 전기동역학 현상은 전기와류 불안정성이다. 전기투석 장치에서 전기와류 불안정성은 물질 전달 속도 증폭을 통해 물질 전달에 대한 이점을 제공한다. 그러나 전기화학 전지나 미세유체역학 장치에서 발생하는 불안정성은 원치 않는 물질 전달 기작을 유발시킨다. 본 연구에서는 전기와류 불안정성의 제어하기 위해, 인가 전압과 공간 제약 효과의 전기와류 불안정성에 대한 영향을 연구하였다. 그 결과, 인가 전압과 공간 제약의 정도에 따라 불안정성의 동역학이 안정 영역 - 고정 영역 - 혼돈 영역 순으로 전이됨을 밝혀내었다. 더불어, 동역학 전이에 대한 안정성 곡선을 수치적으로 결정하였다. 결론적으로, 공간 제약 효과는 전기동역학적 혼돈을 제어할 수 있는 효과적인 기작으로 활용 가능할 것이다.

• 한국가시화정보학회지
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• 제8권3호
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• pp.35-40
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• 2010

We have investigated different radial temperature gradient effect on the stability of Taylor-Couette flow. The radius ratio and aspect ratio of the model was 0.825 and 48, respectively. Two heating exchangers were used for generating different temperature gradient along the radial direction. The change of flow regime in the Taylor-Couette flow was studied by increasing the Reynolds number. The results showed that: as Gr is increased in helical vortex flow regime, the vortices with the same direction of convection flow increased in size, and the vortex moving velocity also increased. It is also shown that the presence of temperature gradient obviously increased the flow instability when the Richardson number is larger than 0.0045.