• Structural Engineering and Mechanics
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• v.66 no.3
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• pp.353-368
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• 2018

In this work, a high order hyperbolic shear deformation theory with four variables is presented to study the vibratory behavior of functionally graduated plates. The field of displacement of the theory used in this work is introduced indeterminate integral variables. In addition, the effect of porosity is studied. It is assumed that the material characteristics of the porous FGM plate, varies continuously in the direction of thickness as a function of the power law model in terms of volume fractions of constituents taken into account the homogeneous distribution of porosity. The equations of motion are obtained using the principle of virtual work. An analytical solution of the Navier type for free vibration analysis is obtained for a FGM plate for simply supported boundary conditions. A comparison of the results obtained with those of the literature is made to verify the accuracy and efficiency of the present theory. It can be concluded from his results that the current theory is not only accurate but also simple for the presentation of the response of free vibration and the effect of porosity on the latter.

• Advances in nano research
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• v.8 no.2
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• pp.135-148
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• 2020

The incorporation of carbon nanotubes in a polymer matrix makes it possible to obtain nanocomposite materials with exceptional properties. It's in this scientific background that this work was based. There are several theories that deal with the behavior of plates, in this research based on the Mindlin-Reissner theory that takes into account the transversal shear effect, for analysis of the critical buckling load of a reinforced polymer plate with parabolic distribution of carbon nanotubes. The equations of the model are derived and the critical loads of linear and parabolic distribution of carbon nanotubes are obtained. With different disposition of nanotubes of carbon in the polymer matrix, the effects of different parameters such as the volume fractions, the plate geometric ratios and the number of modes on the critical load buckling are analysed and discussed. The results show that the critical buckling load of parabolic distribution is larger than the linear distribution. This variation is attributed to the concentration of reinforcement (CNTs) at the top and bottom faces for the X-CNT type which make the plate more rigid against buckling.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.7
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• pp.911-917
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• 1999

The sonic nozzle is widely used as reference device for calibrating flowmeters In gas flow measurement and its use requires the Critical Flow Factor(CFF) based on the thermodynamic properties of the gas at the nozzle throat. ISO-9300 provides the calculating method of the factor. But since the CFF from this method show an error over ${\pm}0.5%$ In specific conditions and of ${\pm}0.1{\sim}{\pm}0.2%$ in common Natural Gas(NG) custody transfer condition. this method cannot be applied for gas flow measurement with sonic nozzle. Each research bodies or organizations of the world have joined in order to calculate the CFF more accurately. They have performed these works using their own method and compared the results with each other under the management of ISO. KOGAS have joined those works, because the high-pressure natural gas flow calibration facility of KOGAS will be constructed in late 1999, and then had necessities to calculate a CFF accurately. The calculation method of KOGAS was using the equation of state from AGA-8('94), high accuracy model of ideal gas properties and the solutions of thermodynamic equations. The evaluation results have had a very good consistency within ${\pm}0.05%$ in most NO custody transfer conditions compared to the speed of sound for methane and also shown that the CFF was within ${\pm}0.1%$ compared to the results of other works of the world.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.12
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• pp.1061-1069
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• 2010

In this study, a simplified axisymmetric model is proposed for the problem of compressible internal flow past a microgap. Using numerical and experimental methods, the phenomena of choked flows are observed; these flows are induced by the acceleration of subsonic flows past the narrow cross-section of an annular shape made by a microgap. The relation between mass flow rate and differential pressure is obtained, and by comparing the result with experimental results, the reliability of the numerical results is discussed. The generation of a supersonic jet flow and its diffraction are visualized by performing the numerical analysis of axisymmetric compressible Navier-Stokes equations. This investigation greatly extends the physical understanding of the axisymmetric compressible flow, which has a wide range of engineering applications, e.g., in the case of valves in automotive power systems.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.4 no.4
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• pp.201-207
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• 1992

The averaged non-linear term in the turbulence equations, suggested by Yeo (1987), is analyzed theoretically and experimentally. It was formulated by applying the filtering concepts to the convolution integral average definition with the Gaussian response function. This filtering approach seems to be superior to the conventional averaging methods in which all four terms at the doubly average vol must be defined separately, and it also gives a very useful tool in understanding the turbulence structures. By theoretically analyzing the newly derived description for the averaged non-linear terms, it is found that the vortex stretching can be explicitly accounted for. Furthermore, comparisons of the correlation coefficients based on the experimental data show that the vortex stretching acts most significantly on the turbulence residual stress. Thus, it strongly supports the claim that the vortex stretching is essential in the transfer of turbulence. In addition. a general form of turbulent energy models in LES is derived, by which it is recognized that the Smagorinsky, the vorticity and the SGS energy models are not distinctive.

• Bulletin of the Society of Naval Architects of Korea
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• v.12 no.2
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• pp.59-66
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• 1975

Recently, as the dimensions of energy carriers increase, especially in draft, a reliable prediction of the ship motions at finite depths of water becomes necessary. The purpose of this paper is to probe the effect of finite water depth on the hydrodynamic forces and ship motions, particularly heave and pitch, in longitudinal regular head waves, by comparing the experimental value of Freakes and Keay with the author's theoretical value obtained by applying the modified strip theory to the Mariner class ship. It is confirmed that generally the hydrodynamic coefficients in the equations of motion increase with decreasing water depth, and the wave exciting forces and moments decrease with decreasing water depth. Amplitudes of heave and pitch in longitudinal regular head waves decrease as the water depth in the range where the length of the incident wave is comparatively long. The effects of Froude Number on the hydrodynamic coefficients increase with decreasing water depth and is more noticeable in the case of heave than pitch. In heave, generally the discrepancy between the experimental value and the theoretical value is relatively small in the case of $F_n=O$, but it is very large in the case of $F_n=0.2$. It is considered that the trend stems from the ignorance of the three dimensional effect and the other effects due to shallowness of water on the hydrodynamic coefficients in the theoretical calculation. An extension of methods for calculating the two dimensional hydrodynamic forces to included the effect of forward speed should be recommended. It is required that more experimental works in finite water depths will be carried out for correlation studies between the theoretical calculation, according tp modified strip theory, and model experiments.

• Journal of the Korea Safety Management & Science
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• v.14 no.1
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• pp.23-31
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• 2012

Recent research on stand-alone focused on the improvement and development of functions for solving problems such as the limited operating time of stand-alone installed at dwelling and their low reliability caused by false alarms, but it is more urgent to study on the operating range of stand-alone sensor in consideration of the impacts of combustion products on residents because the primary goal of fire safety is minimizing casualties. This study purposed to propose the optimized operating range of stand-alone sensor in consideration of the impacts of combustion products on residents. For this purpose, we made a mathematical approach to the change of temperature over the lapse of time in compartment fires similar to house fires, and established the standards of the body's response against heat and smoke based on literature review. In addition, we surveyed domestic and foreign technological standards for stand-alone sensor, and converted them to standards for residents of the body's response against heat and smoke using mathematical model equations and analyzed them comparatively.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2002.04a
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• pp.123-130
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• 2002

A general formulation for shape design sensitivity analysis over three dimensional beam structure is developed based on a variational formulation of the beam in linear elasticity. Sensitivity formula is derived based on variational equations in cartesian coordinates using the material derivative concept and adjoint variable method for the displacement and Von-Mises stress functionals. Shape variation is considered for the beam shape in general 3-dimensional direction as well as for the orientation angle of the beam cross section. In the sensitivity expression, the end points evaluation at each beam segment is added to the integral formula, which are summed over the entire structure. The sensitivity formula can be evaluated with generality and ease even by employing piecewise linear design velocity field despite the bending model is fourth order differential equation. For the numerical implementation, commercial software ANSYS is used as analysis tool for the primal and adjoint analysis. Once the design variable set is defined using ANSYS language, shape and orientation variation vector at each node is generated by making finite difference to the shape with respect to each design parameter, and is used for the computation of sensitivity formula. Several numerical examples are taken to show the advantage of the method, in which the accuracy of the sensitivity is evaluated. The results are found excellent even by employing a simple linear function for the design velocity evaluation. Shape optimization is carried out for the geometric design of an archgrid and tilted bridge, which is to minimize maximum stress over the structure while maintaining constant weight. In conclusion, the proposed formulation is a useful and easy tool in finding optimum shape in a variety of the spatial frame structures.

• Journal of the Korean Society of Propulsion Engineers
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• v.5 no.1
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• pp.18-25
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• 2001

The results of systematic numerical experiments of secondary gas injection thrust vector control are presented. The effects of secondary injection system such as injection location and nozzle divergent cone angle onto the overall performance parameters such as thrust ratio, specific impulse ratio and axial thrust augmentation, are investigated. Complex nozzle exhaust flows induced by the secondary jet penetration is numerically analyzed by solving unsteady three-dimensional Reynolds-averaged Navier-Stokes equations with Baldwin-Lomax turbulence model for closure. Numerical simulations compared with the experiments of secondary air injection into the rocket nozzle of $9.6^{\cire}$ divergent half angle showed good agreement. The results obtained in terms of overall performance parameters showed that locating the secondary injection orifice further downstream of primary nozzle ensures the prevention of occurrence of reflected shock wave, therefore is suitable for efficient and stable thrust vectoring over a wide range of use.

• Steel and Composite Structures
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• v.20 no.2
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• pp.227-249
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• 2016

The objective of this work is to present a zeroth-order shear deformation theory for free vibration analysis of functionally graded (FG) nanoscale plates resting on elastic foundation. The model takes into consideration the influences of small scale and the parabolic variation of the transverse shear strains across the thickness of the nanoscale plate and thus, it avoids the employ use of shear correction factors. Also, in this present theory, the effect of transverse shear deformation is included in the axial displacements by using the shear forces instead of rotational displacements as in available high order plate theories. The material properties are supposed to be graded only in the thickness direction and the effective properties for the FG nanoscale plate are calculated by considering Mori-Tanaka homogenization scheme. The equations of motion are obtained using the nonlocal differential constitutive expressions of Eringen in conjunction with the zeroth-order shear deformation theory via Hamilton's principle. Numerical results for vibration of FG nanoscale plates resting on elastic foundations are presented and compared with the existing solutions. The influences of small scale, shear deformation, gradient index, Winkler modulus parameter and Pasternak shear modulus parameter on the vibration responses of the FG nanoscale plates are investigated.