• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.6 s.249
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• pp.560-567
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• 2006

Although various conjectures have been proposed to explain abnormal increase in thermal conductivity of nanofluids, the detailed mechanism could not be understood and explained yet. The main reason is primarily due to the lack of knowledge on the most fundamental factor governing the mechanisms such as Brownian motion, liquid layering, phonon transport, surface chemical effects and agglomeration. By applying surface complexation model for the measurement data of hydrodynamic size, zeta potential, and thermal conductivity, we have shown that sulfate charge state is mainly responsible for the increase in the present condition and may be the factor incorporating all the mechanisms as well. Moreover, we propose a new model including concepts of fractal and interfacial layer. The properties such as thickness and thermal conductivity of the layer are estimated from the surface charge states and the concept of electrical double layer. With this, we could demonstrate the pH dependences of the layer properties and eventually of the effective thermal conductivity of the nanofluid.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.141-146
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• 2000

Crack closure and growth behavior of physically short fatigue cracks under random loading are investigated by performing narrow- and wide-band random loading tests for various stress ratios. The importance of the crack closure phenomenon is examined by predicting the growth lives of short cracks using obtained crack opening behavior. Artificially prepared two-dimensional, short through-thickness cracks are used. The crack opening load of short cracks is much lower under random loading than under constant-amplitude loading corresponding to the largest load cycle in a random load history. This result indicates that the largest load cycle in a random load history has an effect to enhance crack opening of short cracks. Most of the life prediction ratios are within the factor of 2 scatter band except several data at very short crack sizes, indicating that crack growth predictions based on the measured crack opening data are excellent. From the results obtained in this study, it can be concluded that crack closure is the primary factor governing fatigue crack growth of short cracks under random loading as well as under constant-amplitude loading.

• Journal of Biosystems Engineering
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• v.38 no.2
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• pp.129-137
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• 2013

Purpose: Modeling has been used for elucidating the mechanism of complex biosystems. In spite of importance and uniqueness of adolescent calcium (Ca) metabolism characterized by a threshold Ca intake, its regulatory mechanism has not been covered and even not proposed. Hence, this study aims at model-based proposing potential mechanisms regulating adolescent Ca metabolism. Methods: Two different hypothetic mechanisms were proposed. The main mechanism is conceived based on Ca-protein binding which induces renal Ca filtration, while additional mechanism assumed that active renal Ca re-absorption regulated Ca metabolism in adolescents. Mathematical models were developed to represent the proposed mechanism and simulated them whether they could produce adolescent Ca profiles in serum and urine. Results: Simulation showed that both mechanisms resulted in the unique behavior of Ca metabolism in adolescents. Based on the simulation insulin-like growth factor-1 (IGF-1) is suggested as a potential regulator because it is related to both growth, a remarkable characteristic of adolescence, and Ca metabolism including absorption and bone accretion. Then, descriptive modeling is employed to conceptualize the hypothesized mechanisms governing adolescent Ca metabolism. Conclusions: This study demonstrated that modeling is a powerful tool for elucidating an unknown mechanism by simulating potential regulatory mechanisms in adolescent Ca metabolism. It is expected that various analytic applications would be plausible in the study of biosystems, particularly with combination of experimental and modeling approaches.

• The KSFM Journal of Fluid Machinery
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• v.7 no.6 s.27
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• pp.45-54
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• 2004

Governing equations and numerical solution methods are derived for the analysis of a combination-type-staggered-labyrinth seal used in high performance steam turbines. A bulk flow is assumed for each combination-type-staggered-labyrinth cavity. Axial flow through a throttling labyrinth strip is determined by Neumann's leakage equation and circumferential flow is assumed to be completely turbulent in the labyrinth cavity. Moody's wall-friction-factor formula is used for the calculation of wall shear stresses. For the reaction force developed by the seal, linearized zeroth-order and first-order perturbation equations are developed for small motion near the centered position. Integration of the resultant first-order pressure distribution along and around the seal defines the rotordynamic coefficients of the combination-type-staggered-labyrinth seal. Theoretical results of leakage and rotordynamic characteristics for the IP4-stage seal of USC (ultra super critical) steam turbine are shown with the effect of sump pressure, the number of throttling labyrinth strip, and rotor speed.

• Steel and Composite Structures
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• v.28 no.2
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• pp.195-207
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• 2018

First-order reliability method (FORM) is enhanced based on the search direction using relaxed conjugate reliability (RCR) approach for the embedded nanocomposite beam under buckling failure mode. The RCR method is formulated using discrete conjugate map with a limited scalar factor. A dynamical relaxed factor is proposed to control instability of proposed RCR, which is adjusted using sufficient descent condition. The characteristic of equivalent materials for nanocomposite beam are obtained by micro-electro-mechanical model. The probabilistic model of nanocomposite beam is simulated using the sinusoidal shear deformation theory (SSDT). The beam is subjected to external applied voltage in thickness direction and the surrounding elastic medium is modeled by Pasternak foundation. The governing equations are derived in terms of energy method and Hamilton's principal. Using exact solution, the implicit buckling limit state function of nanocomposite beam is proposed, which is involved various random variables including thickness of beam, length of beam, spring constant of foundation, shear constant of foundation, applied voltage, and volume fraction of ZnO nanoparticles in polymer. The robustness, accuracy and efficiency of proposed RCR method are evaluated for this engineering structural reliability problem. The results demonstrate that proposed RCR method is more accurate and robust than the excising reliability methods-based FORM. The volume fraction of ZnO nanoparticles and the applied voltage are the sensitive variables on the reliable levels of the nanocomposite beams.

• The Korean Journal of Health Service Management
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• v.13 no.2
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• pp.81-92
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• 2019

Objectives: The purpose of this study was to discuss the overall and oral health conditions that affect obesity in middle-aged women in Jeju and to help them operate the effective obesity program in the future. Methods: Participants of this study were 171 middle-aged women living in Seogwipo City, Jeju Special Self - Governing Province. Data were collected through the convenience sampling method. Body mass index and waist circumference of each participant were measured, and a self-report questionnaire was applied. Results: The overall health status of obesity comprised depression, stress, diabetes, and menopause. The oral health conditions affecting obesity were bad breath and chewing. Hypertension was the main factor affecting body mass index and depression was the main factor influencing waist circumference. Obesity was more influenced by overall health status factors than by oral health status factors. Conclusions: The results of this study are expected to be useful as basic data for efficient health care services in the prevention of obesity.

• Journal of Environmental Science International
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• v.26 no.9
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• pp.999-1011
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• 2017

In this study, the Chiu-2D velocity-flow rate distribution based on theoretical background of the entropy probability method was applied to actual ADCP measurement data of Gangjung Stream in Jeju from July 2011 to June 2015 to predict the parameter that take part in velocity distribution of the stream. In addition, surface velocity measured by SIV (Surface Image Velocimeter) was applied to the predicted parameter to calculate discharge. Calculated discharge was compared with observed discharge of ADCP observed during the same time to analyze propriety and applicability of depth of water velocity average conversion factor. To check applicability of the predicted stream parameter, surface velocity and discharge were calculated using SIV and compared with velocity and flow based on ADCP. Discharge calculated by applying velocity factor of SIV to the Chiu-2D velocity-flow rate distribution and discharge based on depth of water velocity average conversion factor of 0.85 were $0.7171m^3/sec$ and $0.5758m^3/sec$, respectively. Their error rates compared to average ADCP discharge of $0.6664m^3/sec$ were respectively 7.63% and 13.64%. Discharge based on the Chiu-2D velocity-flow distribution showed lower error rate compared to discharge based on depth of water velocity average conversion factor of 0.85.

• Structural Engineering and Mechanics
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• v.67 no.5
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• pp.527-544
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• 2018

In this paper, the effect of homogenization models on stress analysis is presented for functionally graded plates (FGMs). The derivation of the effective elastic proprieties of the FGMs, which are a combination of both ceramic and metallic phase materials, is of most of importance. The majority of studies in the last decade, the Voigt homogenization model explored to derive the effective elastic proprieties of FGMs at macroscopic-scale in order to study their mechanical responses. In this work, various homogenization models were used to derive the effective elastic proprieties of FGMs. The effect of these models on the stress analysis have also been presented and discussed through a comparative study. So as to show this effect, a refined plate theory is formulated and evaluated, the number of unknowns and governing equations were reduced by dividing the transverse displacement into both bending and shear parts. Based on sinusoidal variation of displacement field trough the thickness, the shear stresses on top and bottom surfaces of plate were vanished and the shear correction factor was avoided. Governing equations of equilibrium were derived from the principle of virtual displacements. Analytical solutions of the stress analysis were obtained for simply supported FGM plates. The obtained results of the displacements and stresses were compared with those predicted by other plate theories available in the literature. This study demonstrates the sensitivity of the obtained results to different homogenization models and that the results generated may vary considerably from one theory to another. Finally, this study offers benchmark results for the multi-scale analysis of functionally graded plates.

• Structural Engineering and Mechanics
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• v.31 no.4
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• pp.453-475
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• 2009

The literature regarding the free vibration analysis of Bernoulli-Euler and Timoshenko beams on elastic soil is plenty, but the free vibration analysis of Reddy-Bickford beams on elastic soil with/without axial force effect using the Differential Transform Method (DTM) has not been investigated by any of the studies in open literature so far. In this study, the free vibration analysis of axially loaded Reddy-Bickford beam on elastic soil is carried out by using DTM. The model has six degrees of freedom at the two ends, one transverse displacement and two rotations, and the end forces are a shear force and two end moments in this study. The governing differential equations of motion of the rectangular beam in free vibration are derived using Hamilton's principle and considering rotatory inertia. Parameters for the relative stiffness, stiffness ratio and nondimensionalized multiplication factor for the axial compressive force are incorporated into the equations of motion in order to investigate their effects on the natural frequencies. At first, the terms are found directly from the analytical solutions of the differential equations that describe the deformations of the cross-section according to the high-order theory. After the analytical solution, an efficient and easy mathematical technique called DTM is used to solve the governing differential equations of the motion. The calculated natural frequencies of one end fixed and the other end simply supported Reddy-Bickford beam on elastic soil using DTM are tabulated in several tables and figures and are compared with the results of the analytical solution where a very good agreement is observed and the mode shapes are presented in graphs.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.41 no.1
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• pp.39-48
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• 2021

In this paper, an analytical model is proposed for assessing the natural frequency of barrettes subjected to vertical loading. The differential equation governing the free vibration of rectangular friction piles embedded in inhomogeneous soil is derived. The governing equation is numerically integrated by Runge-Kutta technique and the eigenvalue of natural frequency is computed by Regula-Falsi method. The numerical solutions for the natural frequency of barrettes compare well with those obtained from finite element analysis. Illustrated examples show that the natural frequencies increase with an increase of the cross-sectional aspect ratio, the friction resistance ratio and the soil stiffness ratio, and decrease with an increase of the friction aspect ratio, the slenderness ratio and the load factor, respectively.