• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.695-698
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• 2004

STSAT RWA (Reaction Wheel Assembly) micro-vibration is measured using KISTLER dynamic plate that can provide the time signals of three orthogonal forces and torques simultaneously up to 400Hz. In the post-processing, measured data are evaluated with respect to the wheel spin rate in both time and frequency domains, and the static/dynamic unbalances are evaluated from the extracted first harmonic component. Also the friction torque profile at each wheel speed is estimated from the measured data. Several higher order harmonic components are observed, that comes from its rotor shape as well as the wheel bearing characteristics. One of the most peculiar characteristics of this wheel is that the dynamic properties of two radial unbalance components are much different from each other as the RWA mounting configuration on a spacecraft is different from conventional RWA mounting configuration. Rocking mode is not appeared below 400Hz for all operating speed because the wheel size is very small. The post-processed results will be used for jitter analysis of STSAT due to RWA micro-vibration.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.39.1-39.1
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• 2019

We trace the cosmological origin of satellites around isolated dwarf galaxies using a very high resolution (12 pc/h) cosmological hydrodynamic zoom simulation. To realistically describe the formation and evolution of small-mass stellar satellites, our model includes a full baryonic physics treatment. We find that the mini-halos form objects resembling dwarf galaxies. The majority of their star forming gas is accreted after reionization, thus the survival of a mini-halo's gas to reionization is not an important factor. Instead, the key factor seems to be the ability for a mini-halo to cool its recently accreted gas, which is more efficient in more massive halos. Although the host galaxy is only a dwarf galaxy itself, we find that ram pressure is an efficient means by which accreted mini-halos lose their gas content, both by interacting with hot halo gas but also in direct collisions with the gas disk of the host. The satellites are also disrupted by the tidal forces near the center of the host galaxy. Compared to the disrupted satellites, surviving satellites are relatively more massive, but tend to infall later into the host galaxy, thus reducing the time they are subjected to destructive environmental mechanisms and dynamical friction.

• Steel and Composite Structures
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• v.45 no.4
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• pp.501-511
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• 2022

In this study, a two-dimensional model of the contact problem has been examined using the finite element method (FEM) based software ANSYS and based on the multilayer perceptron (MLP), an artificial neural network (ANN). For this purpose, a functionally graded (FG) half-infinite layer (HIL) with a crack pressed by means of two rigid blocks has been solved using FEM. Mass forces and friction are neglected in the solution. Since the problem is analyzed for the plane state, the thickness along the z-axis direction is taken as a unit. To check the accuracy of the contact problem model the results are compared with a study in the literature. In addition, ANSYS and MLP results are compared using Root Mean Square Error (RMSE) and coefficient of determination (R2), and good agreement is found. Numerical solutions are made by considering different values of external load, the width of blocks, crack depth, and material properties. The stresses on the contact surfaces between the blocks and the FG HIL are examined for these values, and the results are presented. Consequently, it is concluded that the considered non-dimensional quantities have a noteworthy influence on the contact stress distributions, and also, FEM and ANN can be efficient alternative methods to time-consuming analytical solutions if used correctly.

• Geomechanics and Engineering
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• v.35 no.2
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• pp.209-219
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• 2023

This paper presents numerical investigations into the particle crushing effect on the shear properties of gravel under direct shear condition. A novel particle crushing model was developed based on the octahedral shear stress criterion and fragment replacement method. A series of direct shear tests were carried out on unbreakable particles and breakable particles with different strengths. The evolutions of the particle crushing, shear strength, volumetric strain behavior, and contact force fabric during shearing were analyzed. It was observed that the number of crushed particles increased with the increase of the shear displacement and axial pressure and decreased with the particle strength increasing. Moreover, the shear strength and volume dilatancy were obviously decreased with particle crushing. The shear displacement of particles starting to crush was close to that corresponding to the peak shear stress got. Besides, the shear-hardening behavior was obviously affected by the number of crushed particles. A microanalysis showed that due to particle crushing, the contact forces and anisotropy decreased. The mechanism of the particle crushing effect on the shear strength was further clarified in terms of the particle friction and interlock.

• Tribology and Lubricants
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• v.39 no.6
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• pp.273-277
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• 2023

This paper presents a molecular dynamics simulation-based numerical investigation of the influence of surface energy on water lubrication. Models composed of a crystalline substrate, half cylindrical tip, and cluster of water molecules are prepared for a tribological-characteristic evaluation. To determine the effect of surface energy on lubrication, the surface energy between the substrate and water molecules as well as that between the tip and water molecules are controlled by changing the interatomic potential parameters. Simulations are conducted to investigate the indentation and sliding processes. Three different normal forces are applied to the system by controlling the indentation depth to examine the influence of normal force on the lubrication of the system. The simulation results reveal that the solid surface's surface energy and normal force significantly affect the behavior of the water molecules and lubrication characteristics. The lubrication characteristics of the water molecules deteriorate with the increasing magnitude of the normal force. At a low surface energy, the water molecules are readily squeezed out of the interface under a load, thus increasing the frictional force. Contrarily, a moderate surface energy prevents expulsion of the water molecules due to squeezing, resulting in a low frictional force. At a high surface energy, although squeezing of the water molecules is restricted, similar to the case of moderate surface energy, dragging occurs at the soil surface-water molecule interface, and the frictional force increases.

• Steel and Composite Structures
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• v.50 no.1
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• pp.77-87
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• 2024

In this study, the two-dimensional crack problem was investigated by using the finite element method (FEM)-based ANSYS package program and the artificial neural network (ANN)-based multilayer perceptron (MLP) method. For this purpose, a half-infinite functionally graded (FG) layer with a crack pressed through two rigid blocks was analyzed using FEM and ANN. Mass forces and friction were neglected in the solution. To control the validity of the crack problem model exercised, the acquired results were compared with a study in the literature. In addition, FEM and ANN results were checked using Root Mean Square Error (RMSE) and coefficient of determination (R²), and a well agreement was found. Numerical solutions were made considering different geometric parameters and material properties. The stress intensity factor (SIF) was examined for these values, and the results were presented. Consequently, it is concluded that the considered non-dimensional quantities have a noteworthy influence on the SIF. Also FEM and ANN can be logical alternative methods to time-consuming analytical solutions if used correctly.

• Korean Journal of Materials Research
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• v.17 no.1
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• pp.31-36
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• 2007

Nanoimprint lithography (NIL) is a new lithographic method that offers a sub-10nm feature size, high throughput, and low cost. One of the most serious problems of NIL is the stiction between mold and resist. The antistiction layer coating is very effective to prevent this stiction and ensure the successful NIL results. In this paper, an antistiction layer was deposited by VSAM (vapor self assembly monolayer) method on silicon samples with FOTS (perfluoroctyltrichlorosilane) as a precursor for making an antistiction layer. A specially designed LPCVD (low pressure chemical vapor deposition) was used for this experiment. All experiments were achieved after removing the humidity. First, the evaporation test of FOTS was performed for checking the evaporation temperature at low pressure. FOTS was evaporated at 5 Tow and $110^{\circ}C$. In order to evaluate the temperature effect on antistiction layer, chamber temperature was changed from 50 to $170^{\circ}C$ with 0.1ml of FOTS for 1 minute. Good hydrophobicity of all samples was shown at about $110^{\circ}$ of contact angle and under $20^{\circ}$ of hysteresis. The surface energies of all samples calculated by Lewis acid/base theory was shown to be about 15mN/m. The deposited thicknesses of all samples measured by ellipsometry were almost 1nm that was similar value of the calculated molecular length. The surface roughness of all samples was not changed after deposition but the friction force showed relatively high values and deviations deposited at under $110^{\circ}$. Also the white circles were founded in LFM images under $110^{\circ}$. High friction forces were guessed based on this irregular deposition. The optimized VSAM process for FOTS was achieved at $170^{\circ}C$, 5 Torr for 1 hour. The hot embossing process with 4 inch Si mold was successfully achieved after VSAM deposition.

• Journal of Korean Ophthalmic Optics Society
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• v.13 no.1
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• pp.5-13
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• 2008

Purpose: This review article was written to investigate what kind of forces are acting on the contact lens fitted on the cornea and its subsequent motion. Methods: A capillary action-induced force develops in the tear layer between the lens and cornea, which leads to the restoring force due to difference in layer thickness according to lens rotation. The characteristics of the lens movement can be determined by the various factors such as friction between eyelid and lens, acceleration force based on blinking and the restoring force incorporated with the viscous damping force. A mathematical model which consists of the differential equations and their numerical solution was proposed to analyze the damped motion of lenses. The model predicts the time dependence of lenses during and after the blink varying the BC, blink period and eyelid pressure. Results: It was found that both the blink period and lid pressure increases the movement increases because of the enhanced lid friction. As the BC increases the viscous damping reduces due to the lacrimal layer's increase which resulted in the enhanced lens motion. After blink the lens illustrates the damped oscillation because of the restoring force by the increased lacrimal layer thickness and reduced viscous resistance. The time for the lens to return to the equilibrium shortens as the BC increase because of the resistance reduction. Conclusions: The movement of the contact lens is governed by the characteristics of the lacrimal layer between the lens and cornea as well as the lid blink.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.24 no.6
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• pp.390-399
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• 2012

This research observed the cross section current of 7 survey lines in Seokmo Channel of Gyeonggi bay with a lot of freshwater inflow and S-shaped for 13 hours during flood season and neap tide. We indicated the distribution of the current velocity by comprehending the speed and direction of the current velocity of each line during maximum flood, ebb tide and observed the distribution of salinity. Moreover, in order to understand what lateral momentum causes the lateral flow in each survey line, we practiced the momentum analysis through the observation data. As a result, the lateral baroclinic pressure gradient force and vertical friction of the Seokmo channel during neap tide were the strongest, and this is why the flow by the distribution of salinity and stratification most often occurs. In north of the Seokmo channel, where have wide intertidal and a lot of freshwater inflow, the secondary circulation is caused by balance of lateral baroclinic pressure gradient force and other forces, and the vertical friction was strong in the lines with small depth. On the other hand, in the southern part of the Seokmo channel where the water is deep and the waterway is curved, the advective acceleration and centrifugal force become stronger by the geographical causes during ebb and the influence of fresh water. Therefore, the lateral flow in the Seokmo channel was caused by the distribution of the momentum that differs by location, depth, curve, etc.

• The korean journal of orthodontics
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• v.35 no.2 s.109
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• pp.116-126
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• 2005

The object of this study was to evaluate how friction that occurs during the sliding movement of an orthodontic archwire through orthodontic brackets is differently affected by variant designs and ingredients of brackets and archwires and bracket-archwire angles. In order to simulate the situations which could occur during orthodontic treatment with fixed appliances, 4 types of brackets (Gemini, a stainless steel twin bracket, Mini Uni-Twiu. a stainless steel bracket with a single bracket design and narrow mesio-distal width; Clarity, a metal-reinforced ceramic bracket; Transcend, a ceramic bracket) and 3 types of orthodontic archwires $(0.016',\; 0.016{\times}0.022'\;stainless\;steel,\;0.016'\;Nitinol)$ were used and the bracket-archwire angles were controlled as $0^{\circ},\;3^{\circ}\;6^{\circ},\;and\;9^{\circ}$ Gemini significantly show and the lowest static and kinetic frictions (P<0.001) Clarity showed the highest static and kinetic frictions with a bracket-archwire angle of $0^{\circ}$. and Transcend at $6^{\circ}\;and\;9^{\circ}$ (P<0.001). An $0.016{\times}0.022'$ stainless steel rectangular archwire significantly showed the highest static and kinetic frictions (P<0.01). The lowest static and kinetic frictions were observed when the bracket-archwire angles were $0^{\circ}\;and\;3^{\circ}$ with 0.010' stainless steel round archwires (P<0.01), and $6^{\circ}\;and\;9^{\circ}$ with 0.016 Nitinol (P<0.001). The static and kinetic frictions were increased as the bracket-archwire angles were increased (P<0.001)