• Journal of the Korean Ceramic Society
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• v.29 no.12
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• pp.926-934
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• 1992

Uniform diamond films in a few $\textrm{mm}^2$ size and locally isolated diamond single crystals in size of 60 $\mu\textrm{m}$ were synthesized on Si-wafer and Al2O3 substrate by the method of acetylene flame. The effects of substrate temperature and flow ratio of oxygen to acetylene on the morphology of deposited diamond were investigated. According to the observations of growth behavior of diamond on Si substrate with respect to substrate surface pretreatment and flow ratio, it was shown that well faceted diamonds could grow uniformly when flow ratio was above 0.9 and substrates were densely scratched. With increasing substrates temperature, the crystal morphology changes from octahedron bounded by only {111} plane below 850$^{\circ}C$ to cubo-octahedron with almost equal development of {111} and {100} plane in the temperature range of 850∼950$^{\circ}C$. Between 950∼1050$^{\circ}C$, the {111} faces become rough and concave. Above 1050$^{\circ}C$, new crystallites begin to grow on concave {111} surface and overall morphology looks like cubo-octahedron with degenerated {111} faces. These changes of morphology can be understood in terms of the different growth mode of each crystallographic plane with respect to the substrate temperature and supersaturation. And the observed phenomena on {111} planes can be related to the face instability and twin generation.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.05a
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• pp.1185-1190
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• 2007

In this paper the vibration system is consisted of a rotating cantilever pipe conveying fluid. The equation of motion is derived by using the Lagrange's equation. Also, the equation of motion is derived applying a modeling method that employs hybrid deformation variables. Generally, the system of pipe conveying fluid becomes unstable by flutter. So, we studied about the influences of the rotating angular velocity, mass ratio and the velocity of fluid flow on the stability of a cantilever pipe by the numerical method. The influences of mass ratio, the velocity of fluid, the angular velocity of a cantilever pipe and the coupling of these factors on the stability of a cantilever pipe are analytically clarified. The critical fluid velocity$(u_{cr})$ is proportional to the angular velocity of the cantilever pipe. In this paper Flutter(instability) always occur in the second mode of the system.

• Advances in biomechanics and applications
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• v.1 no.3
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• pp.211-225
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• 2014

The usual assumption that the increase of fractures in aging bone is due entirely to lower bone density is taken back with respect to the possibility that aging bone fractures result from a loss of stability, or buckling, in the structure of the bone lattice. Buckling is an instability mode that becomes likely in end-loaded structures when they become too slender and lose lateral support. The relative importance of bone density and architecture in etiology bone fractures are poorly understood and the need for improved mechanistic understanding of bone failure is at the core of important clinical problems such as osteoporosis, as well as basic biological issues such as bone formation and adaptation. These observations motivated the present work in which simplified adaptive-beam buckling model is formulated within the context of the adaptive elasticity (Cowin and Hegedus 1976, Hegedus and Cowin 1976). Our results indicate that bone loss activation process leads systematically to the apparition of new elastic instabilities that can conduct to bone-buckling mechanism of fracture.

• The Journal of the Acoustical Society of Korea
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• v.25 no.1E
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• pp.32-35
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• 2006

The screech tone of underexpanded jet is numerically calculated without any specific modeling for the screech tone itself. Fourth-order optimized compact scheme and fourth-order Runge-Kutta method are used to solve the 2D axisymmetric Euler equation. Adaptive nonlinear artificial dissipation model and generalized characteristic boundary condition are also used. The screech tone, generated by a closed loop between instability waves and quasi-periodic shock cells at the near field, is reasonably analyzed with present numerical methods for the underexpanded jet having Mach number 1.13. First of all, the centerline mean pressure distribution is calculated and compared with experimental and other numerical results. The instantaneous density contour plot shows Mach waves due to mixing layer convecting supersonically, which propagate downstream. The pressure signal and its Fourier transform at upstream and downstream shows the directivity pattern of screech tone very clearly. Most of all, we can simulate the axisymmetric mode change of screech tone very precisely with present method. It can be concluded that the basic phenomenon of screech tone including the frequency can be calculated by using high-order and high-resolution schemes without any specific numerical modeling for screech tone feedback loop.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.05a
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• pp.49-52
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• 2009

The response times of monopropellant thrusters at a pulse mode were investigated experimentally as design parameters and feed pressure conditions. Five different model thrusters as injection direction/uniformity, aspect ratio of reactor, volumes of manifold and chamber were designed. As a results, two parameters, aspect ratio and manifold volume, were directly related to response characteristics. Additionally, chugging instability at reaction chamber was observed when pressure drop across the catalyst bed was increased due to high aspect ratio or when low pressure was built at reaction chamber.

• Structural Engineering and Mechanics
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• v.21 no.5
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• pp.539-551
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• 2005

Finite elements based on isoparametric formulation are known to suffer spurious stiffness properties and corresponding stress oscillations, even when care is taken to ensure that completeness and continuity requirements are enforced. This occurs frequently when the physics of the problem requires multiple strain components to be defined. This kind of error, commonly known as locking, can be circumvented by using reduced integration techniques to evaluate the element stiffness matrices instead of the full integration that is mathematically prescribed. However, the reduced integration technique itself can have a further drawback - rank deficiency, which physically implies that spurious energy modes (e.g., hourglass modes) are introduced because of reduced integration. Such instability in an existing stiffness matrix is generally detected by means of an eigenvalue test. In this paper we show that a knowledge of the dimension of the solution space spanned by the column vectors of the strain-displacement matrix can be used to identify the instabilities arising in an element due to reduced/selective integration techniques a priori, without having to complete the element stiffness matrix formulation and then test for zero eigenvalues.

• Nuclear Engineering and Technology
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• v.52 no.7
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• pp.1417-1428
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• 2020

The design of a nuclear reactor core requires basic thermal-hydraulic information concerning the heat transfer regime at which onset of nucleate boiling (ONB) will occur, the pressure drop and flow rate through the reactor core, the temperature and power distributions in the reactor core, the departure from nucleate boiling (DNB), the condition for onset of flow instability (OFI), in addition to, the critical velocity beyond which the fuel elements will collapse. These values depend on coolant velocity, fuel element geometry, inlet temperature, flow direction and water column above the top of the reactor core. Enough safety margins to ONB, DNB and OFI must-emphasized. A heat transfer package is used for calculating convection heat transfer coefficient in single phase turbulent, transition and laminar regimes. The main objective of this paper is to study the possibility of power upgrading of WWR-S research reactor from 2 to 10 MW_th. This study presents a one-dimensional mathematical model (axial direction) for steady-state thermal-hydraulic design and analysis of the upgraded WWR-S reactor in which two types of plate fuel elements are employed. FOR-CONV computer program is developed for the needs of the power upgrading of WWR-S reactor up to 10 MW_th.

• Journal of Ocean Engineering and Technology
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• v.24 no.1
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• pp.10-19
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• 2010

ANSYS multi-physics software was applied to solve the coupled dynamic problem related to a full-scale TLP foundation for floating wind turbines. In this coupled dynamics simulation, the forced oscillation imposed on the tethers' top resulting from the sway of the wind turbine platform and the self-excited vortex-induced vibration (VIV) along the tether span have been taken into account. The stability of this tensioned tether system has been validated in the form of separate static and dynamic analyses. The dynamic characteristics of the tensioned tether linked to the floating wind turbine were analyzed by the resultant modal form and its corresponding vortex shedding pattern. The calculated result shows that even a slight forced oscillation imposed on the tethers' top leads to the VIV amplification and enhances the risk of instability in the case of low pretension. It is also found that the "synchronization" would be aggravated when the top tension decreases and the "2P" vortex shedding mode takes place. The increased top tension imposed on the tethers contributes to the stability of the tensioned legs by diminishing the oscillation amplitude markedly.

• International Journal of Fluid Machinery and Systems
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• v.1 no.1
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• pp.10-23
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• 2008

The development of pre-set wavelength G$\ddot{o}$rtler vortices are studied in the boundary-layer flows on concave surfaces of 1.0 and 2.0 m radius of curvature. The wavelengths of the vortices were pre-set by thin wires of 0.2 mm diameter placed 10 mm upstream and perpendicular to the concave surface leading edge. Velocity contours were obtained from velocity measurements using a single hot-wire anemometer probe. The most amplified or dominant wavelength is found to be 15 mm for free-stream velocity of 2.1 m/s and 3.0 m/s on the concave surface of R = 1 m and 2 m, respectively. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different regions can be identified based on the growth rate of the vortices. The occurrence of a secondary instability mode is also shown in the form of mushroom-like structures as a consequence of the non-linear growth of the G$\ddot{o}$rtler vortices. By pre-setting the vortex wavelength to be much larger and much smaller than the most amplified one, the splitting and merging of G$\ddot{o}$rtler vortices can be respectively observed.

• Journal of Korean Association for Spatial Structures
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• v.16 no.4
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• pp.45-52
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• 2016

The single layer latticed domes have attracted many designers and researchers's attention all of the world, because these structures as spatial structure are of great advantage in not only mechanical rationality but also function, fabrication, construction and economic aspect. But single layer latticed domes are apt to occur the unstable phenomena that are called "buckling" because of the lack of strength of members, instability of structural shape, etc. In the case of latticed dome, there are several types of buckling mode such as overall buckling, local buckling, and member buckling according to the shape of dome, section type of member, the size of member, junction's condition of member and so on. There are many methods to increase the buckling strength of the single layer latticed dome, that is, with the change of geometrical shape of dome, the reinforcement of buckled member, etc. Therefore, the purpose of this study is to verify the reinforcement effect of buckled member when designers reinforce the buckled member to increase the buckling strength of single layer latticed dome with 3-way grid.