• Title, Summary, Keyword: critical velocity

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Mechanism analysis on fluidelastic instability of tube bundles in considering of cross-flow effects

  • Lai, Jiang;Sun, Lei;Gao, Lixia;Li, Pengzhou
    • Nuclear Engineering and Technology
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    • v.51 no.1
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    • pp.310-316
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    • 2019
  • Fluidelastic instability is a key issue in steam generator tube bundles subjected in cross-flow. With a low flow velocity, a large amplitude vibration of the tube observed by many researchers. However, the mechanism of this vibration is seldom analyzed. In this paper, the mechanism of cross-flow effects on fluidelastic instability of tube bundles was investigated. Analysis reveals that when the system reaches the critical state, there would be two forms, with two critical velocities, and thus two expressions for the critical velocities were obtained. Fluidelastic instability experiment and numerical analysis were conducted to obtain the critical velocity. And, if system damping is small, with increases of the flow velocity, the stability behavior of tube array changes. At a certain flow velocity, the stability of tube array reaches the first critical state, a dynamic bifurcation occurs. The tube array returns to a stable state with continues to increase the flow velocity. At another certain flow velocity, the stability of tube array reaches the second critical state, another dynamic bifurcation occurs. However, if system damping is big, there is only one critical state with increases the flow velocity. Compared the results of experiments to numerical analysis, it shows a good agreement.

The effect of nanoparticle in reduction of critical fluid velocity in pipes conveying fluid

  • Ghaitani, M.M.;Majidian, A.;Shokri, V.
    • Advances in concrete construction
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    • v.9 no.1
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    • pp.103-113
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    • 2020
  • This paper deal with the critical fluid velocity response of nanocomposite pipe conveying fluid based on numerical method. The pressure of fluid is obtained based on perturbation method. The motion equations are derived based on classical shell theory, energy method and Hamilton's principle. The shell is reinforced by nanoparticles and the distribution of them are functionally graded (FG). The mixture rule is applied for obtaining the equivalent material properties of the structure. Differential quadrature method (DQM) is utilized for solution of the motion equations in order to obtain the critical fluid velocity. The effects of different parameters such asCNT nanoparticles volume percent, boundary conditions, thickness to radius ratios, length to radius ratios and internal fluid are presented on the critical fluid velocity response structure. The results show that with increasing the CNT nanoparticles, the critical fluid velocity is increased. In addition, FGX distribution of nanoparticles is the best choice for reinforcement.

Experimental Study on Calculation of Critical Velocity in Accordance with Gradient of a Road Tunnel at Fire (도로터널 화재시 경사도에 따른 임계풍속산정에 관한 실험적 연구)

  • Kim, Jong-Yoon;Seo, Tae-Beom;Rie, Dong-Ho;Lim, Kyung-Bum;Yoo, Ji-Oh
    • Journal of the Korean Society of Safety
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    • v.21 no.5
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    • pp.1-5
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    • 2006
  • This study provides a basic data necessary to design a facility of smoke management after calculating the critical velocity of the gradient scale model tunnel and reviewing its adequacy to establish an optimum disaster prevention system for a road tunnel at fire. The experiment is carried out by using Froude scaling to a scale model which is about 1/29 as big as the real tunnel, and its critical velocity calculation is calculated to the 0-2% gradient of the tunnel. The result shows that the higher the gradient is, the stronger the critical velocity, but that it doesn't affect the critical velocity so much when the gradient is less 2%. In addition, this result is studied in comparison with the results done by other researchers to review the adequacy of the critical velocity.

Dynamic analysis of laminated nanocomposite pipes under the effect of turbulent in viscoelastic medium

  • Ghaitani, M.M.;Majidian, A.;Shokri, V.
    • Wind and Structures
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    • v.30 no.2
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    • pp.133-140
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    • 2020
  • In this paper, critical fluid velocity and frequency of laminated pipe conveying fluid are presented. Each layer of the pipe is reinforced by functionally graded carbon nanotubes (FG-CNTs). The internal fluid is assumed turbulent and the induced forces are calculated by momentum equations. The pipe is resting on viscoelastic foundation with spring, shear and damping constants. The motion equations are derived based on classical shell theory and energy method. Differential quadrature method (DQM) is used for solution and obtaining the critical fluid velocity. The effects of volume percent and distribution of CNT, boundary condition, lamina layer number, length to radius ration of pipe, viscoelastic medium and fluid velocity are shown on the critical fluid velocity. Results show that with increasing the lamina layer number, the critical fluid velocity increases.

An Experimental Analysis for the Stability Investigation of Slope on Saemangeum Lake Dykes (새만금 방수제 축조사면의 안정성 검토를 위한 실험적 분석)

  • Jang, Dong-Gi;Kim, Ki-Nyun;Kim, Dong-Hwan;Seo, Kwan-Seok;Son, Moon-Joon
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.687-697
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    • 2010
  • This study was designed to carry out studies on critical seepage velocity and critical hydraulic gradient using a piping test targeting SM and ML which are widely distributed ahead of and behind the depth of E.L(-)10m in Saemangeum area in order to examine stability of embankment built on the ground vulnerable to piping. The effects of relative densities on critical hydraulic gradient and critical velocity were also compared and analyzed using empirical formula and theoretical formula, and relative densities were set up as respectively 9%, 25%, 50%, and 75% for this experiment. As a result, for critical hydraulic gradient, most of specimens detected piping at lower values than the empirical formula of Terzaghi(1922). It is, therefore, considered that the empirical formula devised by Kalin(1977) or Hayashi(1978) is more reasonable to be conservative. It was also found that critical velocity decreased as relative density increased, and critical velocity predicted was mostly lower than the theoretical formula.

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Dynamic characteristics of flexibly supported infinite beam subjected to an axial force and a moving load (이동하중과 축하중이 작용하는 유연한 기초위에 지지된 무한보의 동특성)

  • 홍동균;김광식
    • Journal of the korean Society of Automotive Engineers
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    • v.4 no.3
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    • pp.56-68
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    • 1982
  • This paper presents analytic solutions of defection and their resonance diagrams for a uniform beam of infinite length subjected to an constant axial force and moving transverse load simultaneously. Steady solutions are obtained by a time-independent coordinate moving with the load. The supporting foundation includes damping effects. The influences of the axial force, the damping coefficient and the load velocity on the beam response are studied. The limiting cases of no damping and critical damping are also investigate. The profiles of the deflection of the beam are shown graphically for several values of the load speed, the axial force and damping parameters. Form the results, following conclusions have been reached. 1. The critical velocity .THETA.cr decreases as the axial compressive force increases, but increases as the axial tensile force increase. 2. At the critical velocity .THETA.cr the deflection have a tendency to decrease as the axial tensile force increases and to increase gradually as the axial compressive force increases. 3. In case if relatively small dampings, the deflection increases suddenly as the velocity of the moving load approaches the critical velocity, and it reachs its maximum at the critical velocity, and it decreases and become greatly affected by the axial force as the velocity increases further. 4. in case of relatively large dampings, as the velocity increases the deflection decreases gradually and it is affected little by the axial load.

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An Experimental Study of Critical Velocity in Sloping Tunnel Fires (경사 터널내 화재시 임계속도에 관한 실험적 연구)

  • 이성룡;김충익;유홍선;김혁순;전명배
    • Fire Science and Engineering
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    • v.18 no.1
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    • pp.49-53
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    • 2004
  • In this study, reduced-scale experiments were conducted to analyze an effect of tunnel slope on critical velocity. The 1/20 scale experiments were carried out under the Froude scaling using ethanol pool fire. Square pools ranging from 2.47 to 12.30㎾ were used experiments. Critical velocity varied with one-fourth power of the heat release rate. As the slope of the tunnel increases the critical velocity comes to be fast due to the increase of the chimney effect.

A comparative study on the relationship between estimates of critical velocity and number of jet fans for smoke control - A 'Fire-JF' contour map in road tunnels (임계속도와 제연팬 용량의 상관관계 연구 - 도로터널의 제연팬 특성도 연구)

  • Kim, Hyo-Gyu;Kim, Eun-Soo;Kim, Nam-Young;Lee, Chang-Woo
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.6 no.4
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    • pp.269-278
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    • 2004
  • Recently, critical velocity has become a topic to attract most interests from the researchers in the field of tunnel safety. As the minimum velocity to prevent smoke backlayering during a fire, many equations have been proposed so far, and the following three equations are being considered as a standard method in Korea to calculate the capacity of smoke extraction fans. Equation by Kennedy based on Froude number, Tetzner' s equation with additional variable, ${\beta}$ to modify the Kennedy's equation, and the equation with the concept of super critical velocity by Wu are studied in this paper for the comparative purpose. A contour map describing the relationship between the critical velocity and the capacity of smoke extraction fans is proposed as a tool to calculate the number of jet fans for smoke control during a fire in the local tunnels.

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Oxidation Effect on the Critical Velocity of Pure Al Feedstock Deposition in the Kinetic Spraying Process (저온분사 공정에서 알루미늄 분말의 산화가 임계 적층 속도에 미치는 영향)

  • Kang, Ki-Cheol;Yoon, Sang-Hoon;Ji, Youl-Gwun;Lee, Chang-Hee
    • Journal of Welding and Joining
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    • v.25 no.4
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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.

The Influence of K-ratio and Seepage Velocity on Piping Occurrence (Piping현상 발생에 미치는 투수계수비와 침투유속의 영향에 대한 연구)

  • Huh, Kyung-Han;Chang, Ock-Sung
    • Journal of the Korean Society of Hazard Mitigation
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    • v.8 no.2
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    • pp.129-138
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    • 2008
  • In case of judging the stability of dike or dam structures which need hydraulic interception, the first thing to do is to examine whether a piping phenomenon occurred or not. Generally, dike or dam structures are constructed while layer compacting is executed, so permeability is likely to be anisotropic- different from each other in hydraulic conductivity in the horizontal direction [$k_x$] and hydraulic conductivity in the vertical direction[$k_y$]. This study looked into exit hydraulic gradient and Seepage velocity by conducting an Seepage analysis subsequent to various hydraulic conductivity ratios[k-ratio = ky / kx] and examined the influence on piping by comparing & examining critical Seepage Velocity based on critical hydraulic gradient in theoretical equation and critical Seepage Velocity in empirical equation. As the research result, it was found that hydraulic conductivity ratio operates as a very important factor in case the stability against piping occurrence is considered with the concept of critical hydraulic gradient, but relatively the hydraulic conductivity ratio is very low in its importance in relation to the concept of critical Seepage Velocity.