• Journal of the Korean Society of Visualization
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• v.18 no.2
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• pp.51-58
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• 2020

Research on shock structures of supersonic jet through visualization experiments in low-pressure environment have not been actively conducted. Therefore, in this study, shock waves and supersonic jets were analyzed and compared by numerical analysis and Schlieren technique at low-pressure. Schlieren technique is commonly used to visualize the shock waves generated by density gradient as interferometric methods. Pressure ratio of entrance and ambient was set around 4 to observe moderate under-expanded jet. For validation of experimental and numerical results, the shock structure and frequency were compared. In the case of ST and C nozzle, the results were shown that the difference of shock cell distance was within 10%. The Mach number gradually decreased due to energy reduction, and the error rate was within 7%. D nozzle was not fitted to be observing the shock structure. Because the interface between rarefaction fan and supersonic jet was ambiguous and oscillating phemenoma occurred at end of jet, the supersonic jet in low ambient pressure was observed and analyzed.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.32 no.6C
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• pp.285-293
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• 2012

The surface tension, which is generated in the unsaturated soils, increases the stiffness of the soils. The objective of this study is to estimate the effect of the surface tension, which varies according to the temperature, on the shear wave velocity. Nine specimens, which have the different degree of saturation (0%, 2.5%, 5%, 10%, 20%, 40%, 60%, 80%, 100%), are prepared by using sand-silt mixtures. Experiments are carried out in a nylon cell designed for the measurement of shear waves. A pair of bender elements, which are used for the generation and detection of shear waves, is installed as a cross-hole type. The shear waves are continuously monitored and measured as the temperature of specimens decreases from $15^{\circ}C$ to $1^{\circ}C$. The results show that shear wave velocities of the fully saturated and fully dried specimens change a little bit as the temperatures of specimens decrease. However, the shear wave velocities of the specimens with the degree of saturations of 2.5%, 5%, 10%, 20%, 40%, 60% and 80% continuously increase as temperature decreases from $15^{\circ}C$ to $1^{\circ}C$. Furthermore, a fully saturated specimen is dried at the temperature of $70^{\circ}C$ in order to observe the shear waves according to degree of saturation. The shear wave velocities measured at the temperature of $70^{\circ}C$ are generally lower than those measured at temperature of $15^{\circ}C$. This study demonstrates that the dependence of shear wave velocities on the temperature according to the degree of saturation should be taken into account in both laboratory and field tests.

• Journal of the Korean Vacuum Society
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• v.22 no.4
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• pp.218-224
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• 2013

We measure solar currents transformed from quantum efficiency as a function of incident angles of solar lights. According to conventional models for solar cells, solar currents can be induced when electrons are separated into electrons and holes in the presence of incident solar lights. On the contrary, solar currents can be possible at the time when pinned charge density waves go beyond the pinning potential barrier under the influence of incident solar beams suggested by some authors. In this experiment, measured solar currents and our theory are in good correspondence to confirm the angle dependence of solar lights.

• Journal of Internet Computing and Services
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• v.18 no.4
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• pp.89-97
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• 2017

The purpose of the present study was to estimate the effect of adolescents' depression on cell phone dependency and to identify mediating effects of parenting style and peer isolation. The study used data from the $6^{th}$ waves of 2015 KCYPS(Korean Children and Youth Panel Survey). Structural Equation Model(SEM) was conducted to examine the mediating effects of parenting style and peer isolation. The results showed that adolescents' depression had positive effects on cell phone dependency. In addition, the relationship between adolescents' depression and cell phone dependency was partially mediated by parenting style, not mediated by peer relationships. Based on these findings, further suggestions are provided to reduce adolescents' cell phone dependency by decreasing parent's over-involvement and improve adolescents' self control opportunities.

• Journal of computational fluids engineering
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• v.12 no.1
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• pp.53-59
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• 2007

An axisymmetric supersonic screeching jet is numerically simulated to examine the length scales of screech frequency as well as screech tone generation mechanism. The axisymmetric Reynolds-averaged Navier-Stokes equations in conjuction with a modified Spalart-Allmaras turbulence model are employed. It is demonstrated that the axisymmetric jet screech tones can be simulated correctly and the numerical results are in good agreement with the experimental data. Instability waves, shock-cell structures and the phenomena of shock motion are investigated in detail to identify the screech tone generation mechanism. Shock spacings and standing wave length are analyzed to determine the dominent length scale crucial to the screech frequency formulation.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.389-392
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• 2001

To investigate the generation mechanism of the shock-associated noise, an underexpanded supersonic jet from an axisymmetic nozzle is simulated under the conditions of the Nozzle exit Mach number of 2 and the exit pressure ratio of Pe/Pe =1.5. The present simulation is performed based on the high-order accuracy and high-resolution ENO (Essentially Non-Oscillatory) scheme to capture the time-dependent flow structure representing the sound source. It was found that the shock-associated noise is generated by the weak interaction between the downstream propagating large turbulence structures of the jet flow and the quasi-periodic shock cell structure during the one is passing through the other. The directivity of propagating waves to the upstream is clearly shown in the visualization of pressure field. It is shown that the present calculation of the centerline pressure distribution is in fare agreement with the experimental data at the location of first shock cell.

• Advances in aircraft and spacecraft science
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• v.4 no.4
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• pp.353-369
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• 2017

Flutter is a dangerous phenomenon encountered in flexible structures subjected to aerodynamic forces. This includes aircraft, buildings and bridges. Flutter occurs as a result of interactions between aerodynamic, stiffness, and inertia forces on a structure. In an aircraft, as the speed of the flow increases, there may be a point at which the structural damping is insufficient to damp out the motion which is increasing due to aerodynamic energy being added to the structure. This vibration can cause structural failure, and therefore considering flutter characteristics is an essential part of designing an aircraft. Scientists and engineers studied flutter and developed theories and mathematical tools to analyze the phenomenon. Strip theory aerodynamics, beam structural models, unsteady lifting surface methods (e.g., Doublet-Lattice) and finite element models expanded analysis capabilities. Periodic Structures have been in the focus of research for their useful characteristics and ability to attenuate vibration in frequency bands called "stop-bands". A periodic structure consists of cells which differ in material or geometry. As vibration waves travel along the structure and face the cell boundaries, some waves pass and some are reflected back, which may cause destructive interference with the succeeding waves. This may reduce the vibration level of the structure, and hence improve its dynamic performance. In this paper, for the first time, we analyze the flutter characteristics of a wing with a periodic change in its sandwich construction. The new technique preserves the external geometry of the wing structure and depends on changing the material of the sandwich core. The periodic analysis and the vibration response characteristics of the model are investigated using a finite element model for the wing. Previous studies investigating the dynamic bending response of a periodic sandwich beam in the absence of flow have shown promising results.

• Geophysics and Geophysical Exploration
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• v.22 no.2
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• pp.56-61
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• 2019

In this paper, we studied efficient modeling strategies when we simulate the 3D time-domain acoustic wave propagation using a cell-based finite difference method which can handle the variations of both P-wave velocity and density. The standard finite difference method assigns physical properties such as velocities of elastic waves and density to grid points; on the other hand, the cell-based finite difference method assigns physical properties to cells between grid points. The cell-based finite difference method uses average physical properties of adjacent cells to calculate the finite difference equation centered at a grid point. This feature increases the computational cost of the cell-based finite difference method compared to the standard finite different method. In this study, we used additional memory to mitigate the computational overburden and thus reduced the calculation time by more than 30 %. Furthermore, we were able to enhance the performance of the modeling on several media with limited density variations by using the cell-based and standard finite difference methods together.

• Journal of the Korean Society of Combustion
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• v.11 no.4
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• pp.9-13
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• 2006

Recent advances in energetic materials modeling and high-resolution hydrocode simulation enable enhanced computational analysis of bio-medical treatments that utilize high-pressure shock waves. Of particular interest is in designing devices that use such technology in medical treatments. For example, the generated micro shock waves with peak pressure on orders of 10 GPa can be used for treatments such as kidney stone removal, transdermal micro-particle delivery, and cancer cell removal. In this work, we present a new computational methodology for applying the high explosive dynamics to bio-medical treatments by making use of high pressure shock physics and multi-material wave interactions. The preliminary calculations conducted by the in-house code, GIBBS2D, captures various features that are observed from the actual experiments under the similar test conditions. We expect to gain novel insights in applying explosive shock wave physics to the bio-medical science involving drug injection. Our forthcoming papers will illustrate the quantitative comparison of the modeled results against the experimental data.

• 한국연소학회:학술대회논문집
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• 2006.04a
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• pp.213-217
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• 2006

Recent advances in energetic materials modeling and high-resolution hydrocode simulation enable enhanced computational analysis of bio-medical treatments that utilize high-pressure shock waves. Of particular interest is in designing devices that use such technology in medical treatments. For example, the generated micro shock waves with peak pressure on orders of 10 GPa can be used for treatments such as kidney stone removal, trans-dermal micro-particle delivery. and cancer cell removal. In this work, we present a new computational methodology for applying the high explosive dynamics to bio-medical treatments by making use of high pressure shock physics and multi-material wave interactions. The preliminary calculations conducted by the in-house code, GIBBS2D, captures various features that are observed from the actual experiments under the similar test conditions. We expect to gain novel insights in applying explosive shock wave physics to the bio-medical science involving drug injection. Our forthcoming papers will illustrate the quantitative comparison of the modeled results against the experimental data.