• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.10
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• pp.1340-1346
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• 1999

The turbulent shear flow around a surface-mounted vertical fence was investigated using the two-frame PTV system. The Reynolds number based on the fence height(H) was 2950. From this study, it is revealed that at least 400 instantaneous velocity field data are required for ensemble average to get reliable turbulence statistics, but only 100 field data are sufficient for the time-averaged mean velocity information. Various turbulence statistics such as turbulent intensities, turbulence kinetic energy and Reynolds shear stress were calculated from 700 instantaneous velocity vector fields. The fence flow has an unsteady recirculation region behind the fence, followed by a slow relaxation to the flat-plate boundary layer flow. The time-averaged reattachment length estimated from the streamline distribution is about 11.2H. There exists a region of negative Reynolds shear stress near the fence top due to the highly convex (stabilizing) streamline-curvature of the upstream flow. The large eddy structure in the separated shear layer seems to have significant influence on the development of the separated shear layer and the reattachment process.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.3
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• pp.365-370
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• 2004

Simultaneous measurement with PLIF(Planar Laser-Induced Fluorescence) and Stereo-PIV(Stereo Particle Image Velocimetry) was performed to investigate the structural characteristics of flow field in Rushton Turbine Mixer. Instantaneous 3D velocity fields are measured by two 2K${\times}$2K CCD cameras focused on an object plane with the angular displacement methods while the concentration fields are obtained through the measurement of the fluorescence intensity of Rhodamine B tracer excited by the second pulse of Nd:Yag laser light. Image distortion due to the camera view-angle is compensated by a mapping function. Finally, the spatial structures of turbulent flow around Rushton turbine were identified by the calculation of synchronized data of the velocity field and concentration field.

• 전기의세계
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• v.23 no.2
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• pp.55-61
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• 1974

Nature of magnetic creep phenomena in low coercive force films(Ni 80%-Fe 20%) in form of narrow channels imbedded in high coercive force films is studied in this work. Aluminium is evaporated on the hot glass substrate and eched free in the shape of narrow channels by photoetoetching method. then, Permalloy(Ni 80%, Fe 20%) is deposited on these Aluminium substrate under the uniform field of 30(Oe) to introduce anisotropy. Permalloy film on Al has a high coercive force and one on the substrate devoid of Al has how coercive force. Magnetic revers domain which is introduced at the end of channel grows under the a.c field in hard axis direction, in spite of very weak d.c field in easy axis direction. This creeping is investigated as a function of external fields and channel widths. Permalloy film thickness is 500.angs.-900.angs. and channel widths are 40, 51, 65, 81, 115.mu. respectively. Creeping increases as external field increases while it decreases with channel width decrease. Creep velocity in channels depends on the a.c field along hard axis, d.c field along easy axis and channel widths and its range is 1-10cm/sec in this experiment. From study of dependence of creep velocity on channel width, it can be concluded that creep velocity is expressed in form of v=v$_{0}$ exp .alpha.(H-H$_{0}$) where .alpha. is a function of a.c field along hard axis and H is driving d.c field along easy axis, H$_{0}$ is not a coercive force of film as usuall expected but the d.c threshold field along easy axis which is a function of channel width. This characteristic is also confirmed by the study of dependence of creep velocity upon easy axis field strength. Value of .alpha. obtained is 1.3-2.3cm/sec We depending upon film charactor, hard axis field strength and frequency.uency.

• Journal of the Korean Society of Hazard Mitigation
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• v.5 no.1 s.16
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• pp.63-68
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• 2005

LSIV (Large Scale Image Velocimetry), a technique of image analysis on velocity measurement, was applied to a hydraulic model experiment of river confluence. The surface velocities measured by using LSIV showed similar results with the mean velocities by using a traditional velocimeter, While a general velocimeter can measure only local point velocity, LSIV can measure whole velocity field with one shot. When it is applied to river confluence or around a bridge pier where local flow is dominant, LSIV may be a powerful tool to measure velocity field.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.69.2-69.2
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• 2012

NGC 6861 is the brightest S0 galaxy in the Telescopium group. It has unusually high central stellar velocity dispersion (~400 km/s) and clear rotation (~250 km/s). Considering the well-known M-sigma relation, this large central dispersion implies that the central supermassive black hole (SMBH) has mass comparable to the most massive black holes in the Universe. However, the mass implied by the bulge luminosity-SMBH mass relation is an order of magnitude lower than that predicted by the M-sigma relation. In order to determine the origin of this inconsistency, we obtain integral field spectroscopy using the Wide Field Spectrograph (WiFeS) on the ANU 2.3m telescope. The data are used to map the velocity and velocity dispersion fields which show that our measurements are consistent with those from the other literature. The large field of view the WiFeS observations have allows us to map the kinematics of a much greater portion of NGC 6861 and reveals that the eastern part of the galaxy has higher velocity and dispersion than the rest of halo. We discuss the origin of the unusual fast rotation and the discrepancy of two SMBH mass estimations from three plausible perspectives: 1) the interaction between subgroups of NGC 6861 and its counterpart, NGC 6868; 2) the inhibited growth of the stellar bulge by the AGN activity which leads to an underestimate the SMBH mass when using the bulge luminosity-SMBH mass relation; and 3) gas rich minor mergers that could be crucial for increasing both rotation velocity and velocity dispersion during the evolution of NGC 6861.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.1
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• pp.21-28
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• 2000

Although commercial PIV systems have been widely used for the non-intrusive velocity field measurement of fluid flows, they are still under development and have considerable room for improvement. In this study, a single-frame double-exposure PIV system using a high-resolution CCD camera was developed. A pulsed Nd:Yag laser and high-resolution CCD camera were synchronized by a home-made control circuit. In order to resolve the directional ambiguity problem encountered in the single-frame PIV technique, the second particle image was genuinely shifted in the CCD sensor array during the time interval dt. The velocity vector field was determined by calculating the displacement vector at each interrogation window using cross-correlation with 50% overlapping. In order to check the effect of spatial resolution of CCD camera on the accuracy of PIV velocity field measurement, the developed PIV system with three different resolution modes of the CCD camera (512 ${\times}$ 512, lK ${\times}$ IK, 2K ${\times}$ 2K) was applied to a turbulent flow which simulate the Zn plating process of a steel strip. The experimental model consists of a snout and a moving belt. Aluminum flakes about $1{\mu}m$ diameter were used as scattering particles for the liquid flow in the zinc pot and the gas flow above the zinc surface was seeded with atomized olive oil with an average diameter of 1-$3{\mu}m$. Velocity field measurements were carried out at the strip speed $V_s$=1.0 m/s. The 2K ${\times}$ 2K high-resolution PIV technique was significantly superior compared to the smaller pixel resolution PIV system. For the cases of 512 ${\times}$ 512 and 1K ${\times}$ 1K pixel resolution PIV system, it was difficult to get accurate flow structure of viscous flow near the wall and small vortex structure in the region of large velocity gradient.

• 한국해양학회지
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• v.30 no.6
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• pp.543-549
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• 1995

The inverse method has been used to estimate a surface velocity field from sequential AVHRR/SST data. In the model, equation system was composed of heat equation and horizontal divergence minimization and the velocity field contained in the advective term of the heat equation, which was linearized in grid system, was estimated. A constraint was the minimization of horizontal divergence with weighting factor and introduced to compensate the null space(Menke, 1984) of the velocity solutions for the heat equation. The experiments were carried out to set up the range of weighting factor and the matrix equation was solved by SVD(Singular Value Decomposion). In the experiment, the scales of horizontal temperature gradient and divergence of synthetic velocity field were approximated to those of real field. The neglected diffusive effect and the horizontal variation of heat flux in the heat equation were regarded as random temperature errors. According to the result of experiments, the minimum of relative error was more desirable than the minimum of misfit as the criteria of setting up the weighting factor and the error of estimated velocity field became small when the weighting factor was order of $10^{-1}$

• Journal of the Korean Society of Visualization
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• v.8 no.1
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• pp.13-18
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• 2010

An experimental investigation of the flow around a vertical fence was carried out using a PIV velocity field measurement technique. The vertical fence was embedded in a turbulent boundary layer. The instantaneous velocity fields measured at cross-sectional planes reveal complex longitudinal vortices that vary in size and strength, developing from the upstream location. In the instantaneous vorticity and velocity field data, the shear flow separated from the fence top is highly turbulent and shows unsteady flow characteristics. The topography of the ensemble averaged velocity fields, especially the separation bubble formed behind the fence, shows that the spatial distributions of streamwise velocity (U) and vertical (V) are symmetric, the spanwise velocity (W) is skew-symmetric with respect to the central xy-plane(z=0).

• Journal of the Korean Society of Visualization
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• v.16 no.3
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• pp.59-64
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• 2018

Numerical simulations were carried out to analyze the flow characteristics of the wind tunnel. Flow field characteristics with velocity uniformity at the test sections are largely affected by inlet conditions of air flow rate and temperature. Axial average velocity of the flow field inside the test area was almost linearly decreased by 0.026% each 1m. The uniformity distributions of axial velocity showed the highest reduction rate of about 24% between nozzle outlets 1 ~ 2m. In addition, average velocity and the uniformity are increased with air temperature in the wind tunnel due to density variation. The results of this paper are expected to be useful for the basic design of wind tunnel and to be used for efficient design.

• International Journal of Fluid Machinery and Systems
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• v.8 no.4
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• pp.283-293
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• 2015

The effect of the inlet swirling flow in a hydraulic turbine draft tube is a very complex phenomenon, which has been extensively investigated both theoretically and experimentally. In fact, the finding of the optimal flow distribution at the draft tube inlet in order to get the best performance has remained a challenge. Thus, attempting to answer this question, it was assumed that through an automatic optimization process a Genetic Algorithm would be able to manage a parameterized inlet velocity profile in order to achieve the best flow field for a particular draft tube. As a result of the optimization process, it was possible to obtain different draft-tube flow structures generated by the automatic manipulation of parameterized inlet velocity profiles. Thus, this work develops a qualitative and quantitative analysis of these new draft tube flow field structures provoked by the redesigned inlet velocity profiles. The comparisons among the different flow fields obtained clearly illustrate the importance of the flow uniformity at the end of the conduit. Another important aspect has been the elimination of the re-circulating flow area which used to promote an adverse pressure gradient in the cone, deteriorating the pressure recovery effect. Thanks to the evolutionary optimization strategy, it has been possible to demonstrate that the optimized inlet velocity profile can suppress or mitigate, at least numerically, the undesirable draft tube flow characteristics. Finally, since there is only a single swirl number for which the objective function has been minimized, the energy loss factor might be slightly affected by the flow rate if the same relation of the axial-tangential velocity components is maintained, which makes it possible to scale the inlet velocity field to different operating points.