• Journal of Mechanical Science and Technology
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• v.16 no.2
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• pp.235-245
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• 2002

The present article aims to perform numerical calculations for inter-spray impingement of two diesel sprays under a high injection pressure and to propose a new hybrid model for droplet collision on the basis of literature findings. The hybrid model is compared with the original O'Rourke's model, which has been widely used for spray calculations. The main difference between the hybrid model and the O'Rourke's model is mainly in determination of the collision threshold condition, in which the preferred directional effect of droplets and a critical collision radius are included. The Wave model involving the cavitation effect inside a nozzle is used for predictions of atomization processes. Numerical results are reported for different impingement angles of 60°and 90°in order to show the influence of the impinging angle on spray characteristics and also compared with experimental data. It is found that the hybrid model shows slightly better agreement with experimental data than the O'Rourke's model.

• Journal of Institute of Control, Robotics and Systems
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• v.5 no.6
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• pp.698-704
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• 1999

There are several types of defects of SMDs mounted on PCB, that is, missing components, misalignment, wrong parts and poor solder joints. This research study mainly focuses on measuring of deviation of SMD leads using the partial image of component, not using the full image. This processing based on the partial image has the advantage of the reduction in calculation time compared to the full image. Since position of lead is calculated with respect of the reduction in calculation time compared to the full image. Since position of lead is calculated with respect to pad, the accuracy of the system is not dependent on percise positioning stage. The grabbed image of gray scale is converted into binary format using a cutomatic threshold. After small fragments in the image is removed by a series of morphology operations such as opening and closing, the centroids of PCB pads and SMD leads is obtained together with labeling of blobs. Translational shift and rotationial angle of SMD are succedingly estimated using above information and chip data. The expression that can calculate the maximum deviation of leads with respect to PCB pads has been derived, and inferior mounting of SMD is judged by a given criterion. Some experiments have been executed to verify this measuring scheme.

• Journal of Information Display
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• v.10 no.4
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• pp.175-179
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• 2009

As the physical properties of nematic liquid crystals vary with respect to temperature, the performances of liquid crystal displays (LCDs) are highly dependent on temperature. Additionally, it is well known that the electro-optic characteristics of LCDs, such as transmittance and threshold voltage, also rely on the LCD switching modes. The temperature dependence of the electro-optic characteristics of the wide-viewing-angle LCD modes, such as in-plane switching (IPS), multidomain vertical alignment by patterned electrode (PVA), and fringe-field switching (FFS), have been studied, and the results showed that the FFS mode has lower temperature dependence compared to the IPS and PVA modes. Since the liquid crystal (LC) reorients in different ways in each mode, this result is associated with the temperature dependence of LC's bend and twist elastic constants, and also with the position of the main reorientation, either in the middle or on the surface of the LC layer.

• Transactions of the Korean Society of Automotive Engineers
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• v.1 no.3
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• pp.12-21
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• 1993

In recent years, high efficiency, high performance, and low pollutant emmision engines have been developed. Knock phenomenon has drawn interests because it became an hinderance to engine power and efficiency increase through higher compression ratio. Knock phenomenon is an abnormal combustion originated from autoignition of unburned gas in the end-gas region during the later stage of combustion process and accompanied a high pitched metallic noise. And this phenomenon is characterized by knock occurrence percentage, knock occurrence angle and knock intensity. A four cylinder spark ignition engine is used in our experiment, and its combustion chamber pressure is measured at various engine speeds, ignition timing. The data are analyzed by numerous methods in order to select the optimum methods and to achieve better understanding of knock characteristics. Methods using band-pass filter, third derivative and step method are shown to be the most suitable, while methods using frequency analysis are shown to be unsuitable. Because step method only uses signals above threshold value during knocking condition, pressure signal analyses with this method show good signal-to-noise ratio.

• Korean Journal of Optics and Photonics
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• v.3 no.4
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• pp.222-226
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• 1992

The lasing characteristics of a Nd:phosphate glass laser pumped by 514 nm of an $Ar^{+}$ laser has been investigated. The oscillator consists of a Nd:glass gain medium set at Brewster angle, and two concave mirrors, and a flat mirror with a reflectance of 98%. The $Ar^{+}$ laser pumping beam is focused longitudinally at the beam waist of laser mode for efficient pumping. The pumping beam is chopped at 100 Hz to reduce the heat loading to prevent the thermal damage of the gain medium by the latent heat from the absorbed pumping beam. The maximum laser output power of 70 mW at 1.5 W pumping and the threshold input power of 520 mW have been obtained.

• Korean Journal of Optics and Photonics
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• v.6 no.4
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• pp.282-287
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• 1995

We have studied the characteristics of a cw Ti:sapphire laser pumped by an Ar-ion laser. Ti:sapphire is one of the most attractive materials for the generation of ultra-short pulses because of its very broad gain bandwidth. We used a 4.1 mm thick, 0.15 wt. % $Ti^{3+}$ -doped, Brewster-angled Ti:sapphire crystal and made a folded cavity to create a strong focusing mode. The folding angle of the cavity was adjusted to $15.4^{\circ}$ to compensate for the astigmatism from the Brewster-angled Ti:sapphire crystal. 5 WAr-ion laser was used as a pumping source. We observed that the Ti:sapphire cw output power was sensitively changed with respect to the condition of the folded cavity. The threshold pumping power was 2 Wand the slope efficiency was 16% when an output coupler of 10% transmission was used. The maximum output power was more than 450 mW at 5 W pumping. mping.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.29A no.9
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• pp.36-45
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• 1992

The Modified Multi-Quantum Well(MMQWAl) structures have been grown by Mental-Organic chemical Vapor Deposition(MOCVD) method and stripe type MMQW laser diodes have been investigated. In the case of GaAs/AlGaAs superlattice and quantum well growth by MOCVD, the periodicity, interface abruptess, Al compositional uniformity and layer thickness have been confirmed though the shallow angle lapping technique, double crystal x-ray diffractometry (DCXD) and photoluminescence (PL) measurement. stripe-type MMQW laser diodes have been fabricated using the process technology of photolithography, chemical etching, ohmic contact, back side removing and cleaving. As the result of the electrical and opticalmeasurement of these laser diodes, we have achieved the series resistance of $1[\Omega}~2{\Omega}$ by current-voltage measurements, the threshold current of 200-300mA by currnt-light measurements and the lasing wavelength of 8000-8400$\AA$ by lasing spectrum measurements.

• Journal of the Korean GEO-environmental Society
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• v.17 no.5
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• pp.5-16
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• 2016

A rainfall induced slope failure is a common natural hazard in mountainous areas worldwide. Sudden and rapid failures which have a high possibility of occurrence in a steep slope are always the most dangerous due to their suddenness and high velocities. Based on a series of experiments this study aimed to determine a critical angle which could be considered as an approximate threshold for a sudden failure. The experiments were performed using 0.42 mm mean grain size sand in a 200 cm long, 60 cm wide and 50 cm deep rectangular flume. A numerical model was created by integrating a 2D seepage flow model and a 2D slope stability analysis model to predict the failure surface and the time of occurrence. The results showed that, the failure mode for the entire material will be sudden for slopes greater than $67^{\circ}$; in contrast the failure mode becomes retrogressive. There is no clear link between the degree of saturation and the mode of failure. The simulation results in considering matric suction showed good matching with the results obtained from experiment. A subsequent discarding of the matric suction effect in calculating safety factors will result in a deeper predicted failure surface and an incorrect predicted time of occurrence.

• Korean Journal of Remote Sensing
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• v.9 no.1
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• pp.1-19
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• 1993

A simple bispectral threshold technique which reflects the temporal and spatial characteristics of the analysis area has been developed to classify the cloud type and estimate the cloud cover from GMS/S-VISSR(Stretched Visible and Infrared Spin Scan Radiometer) imagery. In this research, we divided the analysis area into land and sea to consider their different optical properties and used the same time observation data to exclude the solar zenith angle effects included in the raw data. Statistical clear sky radiance(CSRs) was constructed using maximum brightness temperature and minimum albedo from the S-VISSR imagery data during consecutive two weeks. The CSR used in the cloud anaysis was updated on the daily basis by using CSRs, the standard deviation of CSRs and present raw data to reflect the daily variation of temperature. Thresholds were applied to classify the cloud type and estimate the cloud cover from GMS/S-VISST imagery. We used a different thresholds according to the earth surface type and the thresholds were enough to resolve the spatial variation of brightness temperature and the noise in raw data. To classify the ambiguous pixels, we used the time series of 2-D histogram and local standard deviation, and the results showed a little improvements. Visual comparisons among the present research results, KMA's manual analysis and observed sea level charts showed a good agreement in quality.

• Steel and Composite Structures
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• v.52 no.5
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• pp.515-524
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• 2024

The principal goal of the present study is to examine the aeroelastic analysis of cylindrical laminated shells with curvilinear fibers. To attain this objective, the equations of motion are firstly extracted according to the first-order shear deformation theory (FSDT). The linear piston theory is then implemented to estimate aerodynamic loads for various airflow angles over the cylindrical shell area, providing the aeroelastic equations. The well-known isogeometric analysis based on the NURBS basis functions is subsequently developed to discretize the aeroelastic equations of the considered problem. Finally, by writing the resultant equations in the standard form of an eigenvalue problem, the panel flutter analysis of a cylindrical variable stiffness composite laminated (VSCL) shell will be carried out. The comparison and validation of achieved results with the results of references mentioned in the literature are made to demonstrate the accurateness of the present formulation. Also, the influence of various parameters, including the airflow angle, fiber path orientation, radius of curvature, and converting symmetric lay-up to unsymmetrical lay-up on the flutter threshold is studied.