• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.5
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• pp.425-433
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• 2015

In this paper, we have numerically investigated two-dimensional dielectrophoretic (DEP) motions of a single particle suspended freely in a viscous fluid, interacting with a nearby nonconducting planar wall, under an externally applied uniform direct-current electric field. Particularly, we solve the Maxwell equation with a large sharp jump in the electric conductivity at the particle-fluid interface and then integrate the Maxwell stress tensor to compute the DEP force on the particle. Results show that, under an electric field parallel to the wall, one particle is always repelled to move far away from the wall and the motion depends strongly on the particle-wall spacing and the particle conductivity. The motion strength vanishes when the particle is as conductive as the fluid and increases as the conductivity deviates further from that of the fluid.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.5
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• pp.959-967
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• 1988

In this paper, 2-DOF vehicle suspension system with a semiactive on-off damper was studied for improving the ride comfort. It is known that a nonlinear hydraulic damper, which generates force proportional to the square of the relative velocity, can describe the actual fluid resisting type damper more properly than the traditional viscous damping model. On the other hand, hydraulic damper adoption in analysis makes the system nonlinear and causes difficulties to get the system response. In this work, time domain direct integration method was used to calculate system displacement and acceleration. first of all, the response of the suspension system experiencing a given road profile was optimized by Lagrangian multiplier method within the range of given damping coefficients. The appropriate on-loaf damping values were obtained by averaging the already calculated optimum damping coefficients from Lagrangian techniques. The criterion to control the on-off mechanism was determined by examining the suspension efficiency. It was found that the best out of practically applicable criteria is following the sign (positive and negative) of the multiplication of relative displacement and velocity. Judging from the theoretical calculations, it was proved that the semiactive on-off damper can increase suspension efficiency as much as 8-11% in object function.

• Advances in concrete construction
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• v.8 no.3
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• pp.173-185
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• 2019

This article presents a comparative study of the effect of steel layouts on the seismic behavior of transition steel-concrete composite connections, both experimental and analytical investigations of concrete filled steel tube-reinforced concrete (CFST-RC) and steel reinforecd concrete-reinforced concrete (SRC-RC) structures were conducted. The steel-concrete composite connections were subjected to combined constant axial load and lateral cyclic displacements. Tests were carried out on four full-scale connections extracted from a real project engineering with different levels of axial force. The effect of steel layouts on the mechanical behavior of the transition connections was evaluated by failure modes, hysteretic behavior, backbone curves, displacement ductility, energy dissipation capacity and stiffness degradation. Test results showed that different steel layouts led to significantly different failure modes. For CFST-RC transition specimens, the circular cracks of the concrete at the RC column base was followed by steel yielding at the bottom of the CFST column. While uncoordinated deformation could be observed between SRC and RC columns in SRC-RC transition specimens, the crushing and peeling damage of unconfined concrete at the SRC column base was more serious. The existences of I-shape steel and steel tube avoided the pinching phenomenon on the hysteresis curve, which was different from the hysteresis curve of the general reinforced concrete column. The hysteresis loops were spindle-shaped, indicating excellent seismic performance for these transition composite connections. The average values of equivalent viscous damping coefficients of the four specimens are 0.123, 0.186 and 0.304 corresponding to the yielding point, peak point and ultimate point, respectively. Those values demonstrate that the transition steel-concrete composite connections have great energy dissipating capacity. Based on the experimental research, a high-fidelity ABAQUS model was established to further study the influence of concrete strength, steel grade and longitudinal reinforcement ratio on the mechanical behavior of transition composite connections.

• Ocean Systems Engineering
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• v.5 no.3
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• pp.139-160
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• 2015

The global performance of the 5MW OC4 semisubmersible floating wind turbine in random waves was numerically simulated by using the turbine-floater-mooring fully coupled and time-domain dynamic analysis program FAST-CHARM3D. There have been many papers regarding floating offshore wind turbines but the effects of second-order wave-body interactions on their global performance have rarely been studied. The second-order wave forces are actually small compared to the first-order wave forces, but its effect cannot be ignored when the natural frequencies of a floating system are outside the wave-frequency range. In the case of semi-submersible platform, second-order difference-frequency wave-diffraction forces and moments become important since surge/sway and pitch/roll natural frequencies are lower than those of typical incident waves. The computational effort related to the full second-order diffraction calculation is typically very heavy, so in many cases, the simplified approach called Newman's approximation or first-order-wave-force-only are used. However, it needs to be justified against more complete solutions with full QTF (quadratic transfer function), which is a main subject of the present study. The numerically simulated results for the 5MW OC4 semisubmersible floating wind turbine by FAST-CHARM3D are also extensively compared with the DeepCWind model test results by Technip/NREL/UMaine. The predicted motions and mooring tensions for two white-noise input-wave spectra agree well against the measure values. In this paper, the numerical static-offset and free-decay tests are also conducted to verify the system stiffness, damping, and natural frequencies against the experimental results. They also agree well to verify that the dynamic system modeling is correct to the details. The performance of the simplified approaches instead of using the full QTF are also tested.

• Coupled systems mechanics
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• v.2 no.3
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• pp.231-253
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• 2013

The impact of spar-nacelle-blade coupling on edgewise dynamic responses of spar-type floating wind turbines (S-FOWT) is investigated in this paper. Currently, this coupling is not considered explicitly by researchers. First of all, a coupled model of edgewise vibration of the S-FOWT considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar and mooring system, the hydrodynamic effects, the restoring moment and the buoyancy force is proposed. The aerodynamic loads are combined of a steady wind (including the wind shear) and turbulence. Each blade is modeled as a cantilever beam vibrating in its fundamental mode. The mooring cables are modeled using an extended quasi-static method. The hydrodynamic effects calculated by using Morison's equation and strip theory consist of added mass, fluid inertia and viscous drag forces. The random sea state is simulated by superimposing a number of linear regular waves. The model shows that the vibration of the blades, nacelle, tower, and spar are coupled in all degrees of freedom and in all inertial, dissipative and elastic components. An uncoupled model of the S-FOWT is then formulated in which the blades and the nacelle are not coupled with the spar vibration. A 5MW S-FOWT is analyzed by using the two proposed models. In the no-wave sea, the coupling is found to contribute to spar responses only. When the wave loading is considered, the coupling is significant for the responses of both the nacelle and the spar.

• Tribology and Lubricants
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• v.34 no.6
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• pp.247-253
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• 2018

Temperature rise due to viscous shear of the lubricating oil generates hydrodynamic pressure, even if the lubricating surfaces are parallel. This effect, known as the thermal wedge effect, varies significantly with film-temperature boundary conditions. The bearing conducts a part of the heat generated; hence, the oil temperature varies with the thermal conductivity of the bearing. In this study, we analyze the effect of thermal conductivity on the thermohydrodynamic (THD) lubrication of parallel slider bearings. We numerically analyze the continuity equation, Navier-Stokes equation, energy equation including the temperature-viscosity and temperature-density relations for lubricants, and the heat conduction equation for bearing by creating a 2D model of the micro-bearing using the commercial computational fluid dynamics (CFD) code FLUENT. We then compare the variation in temperature, viscosity, and pressure distributions with the thermal conductivity. The results demonstrate that the thermal conductivity has a significant influence on THD lubrication characteristics of parallel slider bearings. The lower the thermal conductivity, the greater the pressure generation due to the thermal wedge effect resulting in a higher load-carrying capacity and smaller frictional force. The present results can function as the basic data for optimum bearing design; however, the applicability requires further studies on various operating conditions.

• Tribology and Lubricants
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• v.38 no.6
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• pp.267-273
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• 2022

Surface texturing is the latest technology for processing grooves or dimples on the friction surface of a machine. When appropriately applied, it can reduce friction and significantly increase durability. Despite many studies over the past 20 years, most are isothermal (ISO) analyses in which the viscosity of the lubricant is constant. In practice, the viscosity changes significantly owing to the heat generated by the viscous shear of the lubricant and film-temperature boundary condition (FTBC). Although many thermohydrodynamic (THD) analyses have been performed on various sliding bearings, only few results for surface-textured bearings have been reported. This study investigates the effects of the FTBC and groove number on the THD lubrication characteristics of a surface-textured parallel thrust bearing with multiple rectangular grooves. The continuity, Navier-Stokes, and energy equations with temperature-viscosity-density relations are numerically analyzed using a commercial computational fluid dynamics code, FLUENT. The results show the pressure and temperature distributions, variations of load-carrying capacity (LCC), and friction force with four FTBCs. The FTBCs greatly influence the lubrication characteristics of surface-textured parallel thrust bearings. A groove number that maximizes the LCC exists, which depends on the FTBC. ISO analysis overestimates the LCC but underestimates friction reduction. Additional analysis of various temperature boundary conditions is required for practical applications.

• Earthquakes and Structures
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• v.27 no.2
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• pp.127-142
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• 2024

The amount of wave propagation through a rubber concrete construction is the subject of the current investigation. Rubber tire waste was used to make two different types of cement mixtures. One type contains sand substitute in amounts ranging from 15% to 60% of the total volume, while the other has gravel with diameters of 3/8 and 8/15 and 15% sand in the same mixture. A wide variety of concrete forms and compositions were created, and their viscous and solid state characteristics were assessed, along with their short-, medium-, and long-term strengths. Diffusion, density, mechanical strength resistance to compressive force, and ultrasound wave propagation were also assessed. The water-to-cement ratio and plasticizer were used in this investigation. In the second part of the study, an analytical model is presented that simulates the experimental model in predicting the speed of waves and the frequencies accompanying them for this type of mixture. Higher order shear deformation beam theory for wave propagation in the rubberized concrete beam is developed, considering the bidirectional distribution, which is primarily expressed by the density, the Poisson coefficient, and Young's modulus. Hamilton's concept is used to determine the governing equations of the wave propagation in the rubberized concrete beam structure. When the analytical and experimental results for rubber concrete beams were compared, the outcomes were very comparable. The addition of rubber gravel and sandy rubber to the mixture both resulted in a discernible drop in velocities and frequencies, according to the data.

• Journal of Korean Ophthalmic Optics Society
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• v.6 no.2
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• pp.85-97
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• 2001

A mathematical model was proposed to analyze the damped motion of contact lens which is initially displaced from the equilibrium position. The model incorporates the differential equations and their numerical solution program, based on the formulations of restoring force arising from the capillary action in the tear-film layer between the lens and cornea. The model predicts the capillary action induced surface tension, time dependence of displacement of lens when it is released from the equilibrium position. It seems that the motion of lens is similar to the typical over-damped oscillation caused by the large viscous friction in the liquid layer between the cornea and lens. The effect of variables such as base curves, lens diameters and thickness of tear film layer were illustrated by the computer simulation of the derived program. The time required for the lens to return to the original position increases as the liquid layer thickness increases and it decreases as the diameter of lens increases. With the certain value of base curve the time interval is found to be minimum. The free vibrations of lenses were also simulated varying the parameters such as base curve, diameter, layer thickness. The resonant frequencies are inversely proportional to the liquid layer thickness and it increases as the lens diameter increases. The resonant frequency of lens has a maximum when the diameter is of certain value. If the external impulse or force of the same frequency as the natural frequency of contact lens acted on the cornea in vivo it may cause an excessive movement and thus it might cause the distortion 10 the lens or be pulled off the eye.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.7
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• pp.675-682
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• 2013

To verify the effect of inner- and outer-stage gas jets, a shear coaxial injector was designed to analyze the axial velocity profile and breakup phenomenon with an increase in the measurement distance. When the measurement position was increased to Z/d=100, the axial flow showed a fully developed shape due to the momentum transfer, aerodynamic drag effect, and viscous mixing. An inner gas injection, which induces a higher momentum flux ratio near the nozzle, produces the greater shear force on atomization than an outer gas injection. Inner- and Outer-stage gas injection do not affect the mixing between the inner and outer gas flow below Z/d=5. The experiment results showed that the main effect of liquid jet breakup was governed by the gas jet of an inner stage. As the nozzle exit of the outer-stage was located far from the liquid column, shear force and turbulence breaking up of the liquid jets do not fully affect the liquid column. In the case of an inner-stage gas injection momentum flux ratio within 0.84, with the increase in the outer gas momentum flux ratio, the SMD decreases. However, at an inner-stage gas jet momentum flux ratio over 1.38, the SMD shows the similar distribution.