• Journal of Welding and Joining
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• 제25권4호
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• pp.35-41
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• 2007

In kinetic spraying process, the critical velocity is an important criterion which determines the deposition of a feedstock particle onto the substrate. In other studies, it was experimentally and numerically proven that the critical velocity is determined by the physical and mechanical properties and the state of materials such as initial temperature, size and the extent of oxidation. Compared to un-oxidized feedstock, oxidized feedstock required a greater kinetic energy of in-flight particle to break away oxide film during impact. The oxide film formed on the surface of particle and substrate is of a relatively higher brittleness and hardness than those of general metals. Because of its physical characteristics, the oxide significantly affected the deposition behavior and critical velocity. In this study, in order to investigate the effects of oxidation on the deposition behavior and critical velocity of feedstock, oxygen contents of Al feedstock were artificially controlled, individual particle impact tests were carried out and the velocities of in-flight Al feedstock was measured for a wide range of process gas conditions. As a result, as the oxygen contents of Al feedstock increased, the critical velocity increased.

• Wind and Structures
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• 제5권2_3_4호
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• pp.257-266
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• 2002

In the recent years flow around bridges are investigated using computer modeling. Selvam (1998), Selvam and Bosch (1999), Frandsen and McRobie (1999) used finite element procedures. Larsen and Walther (1997) used discrete vorticity procedure. The aeroelastic instability is a major criterion to be checked for long span bridges. If the wind speed experienced by a bridge is greater than the critical wind speed for flutter, then the bridge fails due to aeroelastic instability. Larsen and Walther (1997) computed the critical velocity for flutter using discrete vortex method similar to wind tunnel procedures. In this work, the critical velocity for flutter will be calculated directly (free oscillation procedure) similar to the approaches reported by Selvam et al. (1998). It is expected that the computational time required to compute the critical velocity using this approach may be much shorter than the traditional approach. The computed critical flutter velocity of 69 m/s is in reasonable comparison with wind tunnel measurement. The no flutter and flutter conditions are illustrated using the bridge response in time.

• 한국지반공학회논문집
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• 제29권1호
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• pp.5-12
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• 2013

철도노반에서 임계속도효과는 시간영역에서의 유사-공진현상으로서 차량의 주행속도와 노반표면파의 군속도대역이 중첩되면서 에너지가 증폭되는 현상을 의미한다. 과거에는 열차의 주행속도가 낮고 지반의 군속도가 높았기 때문에 문제가 되지 않았으나, 열차속도가 고속화되면서 임계속도효과가 궤도틀림에 영향을 미치는 것으로 보고되고 있다. 현재까지는 임계속도에 대하여 주로 이론적인 분석만 제시되었는데 실질적인 임계속도효과를 효율적으로 평가하기 위해서는 궤도 및 노반의 지지강성을 현장조건과 유사하게 고려하는 것이 필요하다. 그래서 본 논문에서는 유한요소해석을 이용하여 궤도 및 노반의 지지조건을 고려한 임계속도해석을 수행하였다. 궤도조건은 자갈궤도와 콘크리트궤도로 구분하였으며 노반의 지지강성은 10~300MPa범위에서의 임계속도영향을 평가하였다. 해석결과 노반의 지지강성에 따른 변형증폭을 확인하였으며, 궤도지지조건에 대한 임계속도영향도 매우 큰 것으로 나타났다.

• 한국터널지하공간학회 논문집
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• 제6권1호
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• pp.51-59
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• 2004

본 연구에서는 3차원 전산유체역학 기법을 이용하여 열차터널 내 10MW급 화재발생 시, 대피환경에 대한 1차원 임계속도의 유효성을 평가하였다. 또한 터널의 입구속도가 1m/s, 2m/s (임계속도) 그리고 3m/s일 때의 터널 내 기류분포, 온도분포, 가시거리분포 및 오염물질분포가 대피환경에 미치는 영향을 각각 검토하였다. 그 결과, 세 가지 경우모두, 승객의 안전한 대피환경을 충분하게 제공하지 못할 것으로 예상되어 승객들은 유동방향 하류로 대피하여야 한다. 그러나 3m/s 입구속도의 경우는 1m/s, 2m/s의 경우 보다 승객의 대피환경에 있어서 좀 더 나은 결과를 보인다. 따라서 터널의 방재시스템의 설계 시, 안전한 대피환경을 확보하기 위해서는 임계속도보다 큰 입구속도의 시용이 요구된다.

• Ecology and Resilient Infrastructure
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• 제10권4호
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• pp.258-265
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• 2023

대부분의 어류가 생존을 위해 회유 또는 이동하는 특성이 있음에도 불구하고 국내에 설치된 어도는 평상시에 고유속과 저수심의 흐름이 발생할 수 있어 어류의 이동에 장애가 되고 있는 실정이다. 또한 어도에 관한 국내 설계기준상 국내 하천에 서식하는 다양한 어종별 유영특성을 고려하지 못하는 한계가 있다. 따라서 어도의 기능을 확보하기 위한 설계기준의 마련이 필요하며, 이를 위해 국내 회유성 어종의 유영특성과 어도의 수리특성에 대한 연구가 필요하다. 이에 본 연구에서는 붕어를 대상으로 증진유속방법과 고정유속방법을 수행하여 가능한 객관적인 어류의 유영특성을 분석하고 한계유영유속을 제시하였다. 증진유속방법 실험을 수행한 결과 붕어는 0.7 m/s ~ 0.8 m/s로 하는 것이 바람직한 것으로 확인되었다. 고정유속방법 실험을 수행한 결과 붕어의 경우는 0.8 m/s 정도로 확인되었다. 따라서 두 실험 방법을 종합적으로 분석한 결과 어도설계를 위한 한계유영유속을 결정하면 붕어의 경우는 약 0.8 m/s로 판단된다. 향후 다양한 회유성 어종에 대해 유영특성의 실험적 연구를 수행하여 어도설계를 위한 대상어종별 유영특성을 고려한 어도설계기준 마련이 필요하다.

• Steel and Composite Structures
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• 제30권3호
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• pp.281-292
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• 2019

In this paper, analysis of critical fluid velocity and heat transfer in the nanocomposite pipes conveying nanofluid is presented. The pipe is reinforced by carbon nanotubes (CNTs) and the fluid is mixed by $AL_2O_3$ nanoparticles. The material properties of the nanocomposite pipe and nanofluid are considered temperature-dependent and the structure is subjected to magnetic field. The forces of fluid viscosity and turbulent pressure are obtained using momentum equations of fluid. Based on energy balance, the convection of inner and outer fluids, conduction of pipe and heat generation are considered. For mathematical modeling of the nanocomposite pipes, the first order shear deformation theory (FSDT) and energy method are used. Utilizing the Lagrange method, the coupled pipe-nanofluid motion equations are derived. Applying a semi-analytical method, the motion equations are solved for obtaining the critical fluid velocity and critical Reynolds and Nusselt numbers. The effects of CNTs volume percent, $AL_2O_3$ nanoparticles volume percent, length to radius ratio of the pipe and shell surface roughness were shown on the critical fluid velocity, critical Reynolds and Nusselt numbers. The results are validated with other published work which shows the accuracy of obtained results of this work. Numerical results indicate that for heat generation of $Q=10MW/m^3$, adding 6% $AL_2O_3$ nanoparticles to the fluid increases 20% the critical fluid velocity and 15% the Nusselt number which can be useful for heat exchangers.

• 한국기계가공학회지
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• 제20권8호
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• pp.25-32
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• 2021

Washboarding is a crucial problem occurring on unpaved roads. This phenomenon involves the formation of ripples on the surface of the unpaved road and causes a critical problem to vehicles and riders. The phenomenon is affected by several parameters, but we focused on the velocity and the size of the rotating body. In the precedent research, we observed that a critical velocity existed for the occurrence of the phenomenon, and the phenomenon's grade was related to the velocity. Therefore, this study, using a discrete element method, aimed to analyze the relation between the velocity and the size of the rotating body for the occurrence of the phenomenon and perform a fast Fourier transform (FFT) analysis to determine the correlation between the phenomenon and the period. The study observed that the critical velocity could vary from the velocity and the size of the rotating body, and there was a certain range of frequency for the occurrence of the Washboarding phenomenon.

• Structural Engineering and Mechanics
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• 제66권1호
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• pp.45-59
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• 2018

The bi-material elastic system consisting of the pre-stressed hollow cylinder and pre-stresses surrounding infinite elastic medium is considered and it is assumed that the mentioned initial stresses in this system are caused with the compressing or stretching uniformly distributed normal forces acting at infinity in the direction which is parallel to the cylinder's axis. Moreover, it is assumed that on the internal surface of the cylinder the ring load which moves with constant velocity acts and within these frameworks it is required to determine the influence of the aforementioned initial stresses on the critical velocity of the moving load. The corresponding investigations are carried out within the framework of the so-called three-dimensional linearized theory of elastic waves in initially stresses bodies and the axisymmetric stress-strain state case is considered. The "moving coordinate system" method is used and the Fourier transform is employed for solution to the formulated mathematical problem and Fourier transformation of the sought values are determined analytically. However, the originals of those are determined numerically with the use of the Sommerfeld contour method. The critical velocity is determined from the criterion, according to which, the magnitudes of the absolute values of the stresses and displacements caused with the moving load approaches an infinity. Numerical results on the influence of the initial stresses on the critical velocity and interface normal and shear stresses are presented and discussed. In particular, it is established that the initial stretching (compressing) of the constituents of the system under consideration causes a decrease (an increase) in the values of the critical velocity.

• 한국소음진동공학회논문집
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• 제17권10호
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• pp.976-982
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• 2007

In this paper the vibration system is consisted of a rotating cantilever pipe conveying fluid and tip mass. The equation of motion is derived by using the Lagrange's equation. The system of pipe conveying fluid becomes unstable by flutter. Therefore, the influence of a rotating angular velocity, mass ratio, the velocity of fluid flow and tip mass on the stability of a cantilever pipe by the numerical method are studied. The critical flow velocity for flutter is proportional to the angular velocity and tip mass of the cantilever pipe. Also, the critical flow velocity and stability maps of the pipe system are obtained by changing the mass ratios.

• Journal of Mechanical Science and Technology
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• 제14권8호
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• pp.816-822
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• 2000

In the core of the nuclear power plant of PWR, several cases of fuel failure by unknown causes have been experienced for various fuel types. From the common features of the failure pattern, failure lead time, flow conditions, and flow induced vibration characteristics in nuclear fuel bundles, it is deduced that the fretting wear failure of the fuel rod at the spacer grid position is due to the fluidelastic vibration. In the past, fluidelastic vibration was simulated by quasi -static semi-analytical model, so called the static model, which could not account for the interaction between the rods within a bundle. To overcome this defect and to provide for more flexibilities applicable to the fuel bundle, Tanaka's unsteady model was modified to accomodate the geometrical differences and governing parameter changes during the operations such as the number of rods, pitch to diameter ratio (P/D), spring force, damping coefficient, etc. The critical velocity was calculated by solving the governing equations with the MATLAB code. A comparison between the estimated critical velocity and the test result shows a good agreement. Finally, the level of decrease of the critical velocity due to the reduction in the spring force and reduced damping coefficient due to the radiation exposure is also estimated.