• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.78.1-78.1
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• 2014

An ionization front (IF) surrounding an H II region is a sharp interface through which a cold neutral gas makes transition to a warm ionized phase by absorbing UV photons from central massive stars. We investigate the structure and instability of a plane-parallel D-type IF threaded by magnetic fields parallel to the front. We find that magnetic fields increase the maximum propagation speed of the IFs, while reducing the expansion factor, defined as the density ratio of neutral to ionized phases. IFs become unstable to distortional perturbations due to gas expansion across the fronts, exactly analogous to the Darrieus-Landau instability of ablation fronts in terrestrial flames. The growth rate of the IF instability is proportional linearly to the perturbation wavenumber as well as the upstream flow speed. The IF instability is stabilized by gas compressibility and becomes completely quenched when the front is D-critical. The instability is also stabilized by magnetic pressure when the perturbations propagate in the direction perpendicular to the fields. When the perturbations propagate in the direction parallel to the fields, on the other hand, it is magnetic tension that reduces the growth rate, completely suppressing the instability when ${\beta}$ < 1.5, with ${\beta}$ denoting the square of the ratio of the sound speed to the Alfven speed in the pre-IF region. When the front experiences an acceleration, the IF instability cooperates with the Rayleigh-Taylor instability to make the front more unstable. We discuss potential effects of IF instability on the evolution and dynamics of IFs in the interstellar medium.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.12 no.2
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• pp.44-52
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• 2003

This research suggests a cutting force model for the ball end milling processes. This model includes the effect of tool run out and tool deflection. In the proposed model, the flutes of ball end mills are considered as series of infinitesimal elements and each cutting edge is assumed to be straight for the analysis of the oblique cutting process, in which the small cutting edge element has been analyzed as an orthogonal cutting process n the plane including the cutting velocity and the chip-flow vector. Therefor, the cutting forces can be calculated through the model using the orthogonal cutting data obtained from the orthogonal cutting test. In order to enhance the performance of the model, the flutes of ball end mill are defined to keep geometric consistency at the peak of the ball part and the junction with the end mill part. The divided infinitesimal cutting edges are regulated to be even lengths. Some experiments show the validity of the developed model in the various cutting coalitions.

• Journal of Mechanical Science and Technology
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• v.20 no.2
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• pp.262-270
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• 2006

Direct numerical simulations (DNS) of turbulent channel flows up to $Re_{\tau}=1270$ are performed to investigate an elliptic feature and strain rate field on cross sections of coherent fine scale eddies (CFSEs) in wall turbulence. From DNS results, the CFSEs are educed and the strain rate field around the eddy is analyzed statistically. The principal strain rates (i.e. eigenvalues of the strain rate tensor) at the CFSE centers are scaled by the Kolmogorov length $\eta$ and velocity $U_k$. The most expected maximum (stretching) and minimum (compressing) eigenvalues at the CFSE centers are independent of the Reynolds number in each $y^+$ region (i. e. near-wall, logarithmic and wake regions). The elliptic feature of the CFSE is observed in the distribution of phase-averaged azimuthal velocity on a plane perpendicular to the rotating axis of the CFSE $(\omega_c)$. Except near the wall, phase-averaged maximum $(\gamma^{\ast}/\gamma_c^{\ast})$ and minimum $(\alpha^{\ast}/\alpha_c^{\ast})$ an eigenvalues show maxima on the major axis around the CFSE and minima on the minor axis near the CFSE center. This results in high energy dissipation rate around the CFSE.

• Journal of the Society of Naval Architects of Korea
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• v.57 no.2
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• pp.114-123
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• 2020

A high-order potential-based panel method based on Green's theorem, with piecewise-linear dipole strength on triangular panels, is formulated for the analysis of potential flow around a three-dimensional wing. Previous low-order panel methods adopt square panels with piecewise-constant dipole strength, which results in inherent errors. Square panels can not represent a high curvature lifting body, such as propellers, since the four vertices of the square panel do not locate at the same flat plane. Moreover the piecewise-constant dipole strength induces inevitable errors due to the steps in dipole strength between adjacent panels. In this paper a high-order panel method is formulated to improve accuracy by adopting a piecewise linear dipole strength on triangular panels. Firstly, the square panels are replaced by triangular panels in order to increase the geometric accuracy in representing the shape of the object with large curvature. Next, the step difference of the dipole strength between adjacent panels is removed by adopting piecewise-linear dipole strength on the triangular panels. The calculated results by the present method is compared with analytical ones for simple non-lifting geometries, such as ellipsoid. The results for an elliptic wing with zero thickness at finite angle of attack are compared with Jordan's results. The comparison shows reasonable agrements for the both lifting and non-lifting bodies.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.8 s.227
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• pp.887-894
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• 2004

A technique of mixing enhancement by using an elliptic jet screech reflector has been examined experimentally in an underexpanded sonic round jet where jet screech tone is generated. Since jet screech is known to enhance jet spreading, a reflector was designed to focus jet screech waves near the nozzle lip at an underexpanded jet. The reflector has an elliptic cross section of which one focus is located near the nozzle lip and the other in the jet screech source region in a plane including the jet axis. In the jet with the elliptic reflector, the mass flow rate showed a significant increase in the jet entrainment when compared to that for the small disk reflector. This was attributed to the increased screech amplitude near the nozzle lip as well as the mode change of the jet. The jet mixing was also increased by the amplified jet screech at two other underexpanded jets, but the jet oscillation mode did not change.

• Economic and Environmental Geology
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• v.29 no.3
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• pp.357-367
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• 1996

A total of 113 samples (basalts, tuffs, and siltstones from coal-bearing sediments) was collected from 14 sites of the Tertiary Changgi basin in southeastern Korea, and studied palaeomagnetically. Site-mean declination of the ChRM from 5 sites was found to be deflected clockwise about $30^{\circ}$. Other 5 sites showed no vertical-axis deflection of ChRM direction. In consideration of previous palaeomagnetic data from other Tertiary basins in the vicinity, it is interpreted that the deflection of ChRM directions has been caused by NNW-SSE simple shear associated with the opening of the East Sea, and the time of rotation should be about 16 Ma. Other 2 sites showed counterclockwise deflection of site-mean ChRM. These sites might be located among lager tectonic blocks which were rotating clockwise. AMS (anisotropy of magnetic susceptibility) study revealed $NE{\rightarrow}SW$ directed magnetic lineation at two tuffaceous sites. This might indicate flow direction of tuffs during the time of deposition. Most of the other sites showed load-foliation lying subparallel to the bedding plane. This must have been caused by gravitational loading acted vertically to the strata.

• Coupled systems mechanics
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• v.8 no.3
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• pp.199-218
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• 2019

This paper studies the forced vibration of the hydro-elastic system consisting of the anisotropic (orthotropic) plate, compressible viscous fluid and rigid wall within the scope of the exact equations and relations of elastodynamics for anisotropic bodies for describing of the plate motion, and with utilizing the linearized exact Navier-Stokes equations for describing of the fluid flow. For solution of the corresponding boundary value problem it is employed time-harmonic presentation of the sought values with respect to time and the Fourier transform with respect to the space coordinate on the coordinate axis directed along the plate length. Numerical results on the pressure acting on the interface plane between the plate and fluid are presented and discussed. The main aim in this discussion is focused on the study of the influence of the plate material anisotropy on the frequency response of the mentioned pressure. In particular, it is established that under fixed values of the shear modulus of the plate material a decrease in the values of the modulus of elasticity of the plate material in the direction of plate length causes to increase of the absolute values of the interface pressure. The numerical results are presented not only for the viscous fluid case but also for the inviscid fluid case.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.793-798
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• 2004

The vibration of a curved pipe conveying fluid is studied when the pipe is clamped at both ends. To consider the geometric nonlinearity, this study adopts the Lagrange strain theory for large deformation and the extensible dynamics based on the Euler-Bernoulli beam theory for slenderness assumption. By using the extended Hamilton principle, the non-linear partial differential equations are derived for the in-plane motions of the pipe. The linear and non-linear terms in the governing equations are compared with those in the previous study, and some significant differences are discussed. To investigate the vibration characteristics of the system, the discretized equations of motion are derived from the Galerkin method. The natural frequencies varying with the flow velocity are computed from the two cases, which one is the linear problem and the other is the linearized problem in the neighborhood of the equilibrium position. From these results, we should consider the geometric nonlinearity to analyze the dynamics of a curved pipe conveying fluid more precisely.

• Korean Journal of Geomatics
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• v.2 no.1
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• pp.83-89
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• 2002

The cadastral maps as many as about 750,000 map sheets to cover 34,751,000 parcels of land of Korea, are being digitalized. The problem of shrinkage-expansion of the paper cadastral maps has to be resolved for the new digital maps, where the nodes and vertices of the parcel boundaries are represented by coordinates. The photo coordinate refinement techniques, two dimensional projective transformation and local area transformation as in the reseau grid method, were introduced for this distortion correction. Using the fact that original maps drawn on the plane tables in field from 1910 to 1918 have grid lines and have been preserved well, a strategic flow to apply the refinement techniques to the digital maps with the original maps as controls was developed. To accommodate the presence or absence of the original maps and grid lines, and different scales and sizes of the maps, the strategy was implemented by a computer program package. Various distortions and corrections were simulated and errors were evaluated. The RMS errors in the corrected digital maps were allowable, thus, the method developed in this study was to be applicable for the digital cadastral maps.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.4
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• pp.555-562
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• 1998

Under the influence of non-uniform magnetic field, melt flow in steady state and oxygen concentration in unsteady state are numerically investigated. The strength of the applied characteristic magnetic fields are B=0.1T, 0.2T, and 0.3T, respectively. The buoyancy effects due to the crucible wall heating and the thermocapillary effects due to the surface tention at the free surface are suppressed differentially by the non-uniform magnetic fields. As the intensity of characteristic magnetic fields is increasing, the recirculation region in the meridional plane is moving toward the growing crystal, and is diminishing. The oxygen concentration on the growing surface of crystals is decreasing and the uniformity of the oxygen concentration is increasing as the intensity of the magnetic fields is increasing.