• Water Engineering Research
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• 제1권4호
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• pp.259-266
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• 2000

Scour hole is formed due to the high shear stress of the jet flow at the outlet of a hydraulic structure and vortex erosion occurs in the scour hole. It is important to determine the amount of vortex erosion occurs in the scour hole. It is important to determine the amount of vortex erosion for the design of bed protection. If the vortex erosion continues and reaches to the hydraulic structure, it causes the deformation of the structure itself. To obtain the amount of the vortex erosion, it is necessary to determine the shear velocity of the line vortex in the scour hole was derived by the theory of energy conservation and found to be related to the upstream overflow velocity. The amount of vortex erosion from the scour hole was obtained using entrainment equation for given value of shear velocity. For a design purpose, if the flow velocity at the end of an apron and the properties of bed material are given, the amount of vortex erosion was obtained.

• 한국공간구조학회논문집
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• 제17권2호
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• pp.79-87
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• 2017

The retractable roof structure is used in various fields and it is classified for steel retractable system and soft retractable system. For the domestic industry, it is in the initial phase now and the demand of the retractable roof structure is expected to increase in the future. Therefore, this paper is classified for steel retractable system and soft retractable system from the retractable roof structures in overseas to survey and analyze the cases of wind velocity on retractable roof structure in Japan that uses the same wind velocity criteria like Korea regarding the open-close time and average open-close time for retractable roof area.

• 대한기계학회논문집B
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• 제28권9호
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• pp.1032-1041
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• 2004

In this study we decompose the 3-dimensional velocity field of isotropic turbulent flow into the coherent and the incoherent structure using the discrete wavelet. It is shown that the coherent structure, 3% wavelet modes, has 98% energy and 88% enstrophy and its statistical characteristics are almost same as the original turbulence structure. And it is confirmed that the role of the coherent structure is that it produces the turbulent kinetic energy at the inertia range then transfers energy to the dissipation range. The incoherent structure, with residual wavelet modes, is uncorrelated and has the Gaussian probability density function but it dissipates the kinetic energy in dissipation range. On the procedure, we propose a new but easy way to get the threshold by applying the energy partition percentage concept about coherent structure. The vorticity field extracted from the wavelet-decomposed velocity field has the same structure as the result of the precedent studies which decomposed vorticity field directly using wavelet. Therefore it has been shown that velocity and vorticity field are on the interactive condition.

• International Journal of Aeronautical and Space Sciences
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• 제2권1호
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• pp.12-19
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• 2001

Study of turbulent mixing layers has been a popular subject from the point of view of both practical application and phenomenological importance in engineering field. Turbulent mixing layers can be applied in many fields where rapid transition to turbulence is desirable in order to prevent boundary layer separation or to enhance mixing. The ability to control mixing, structure and growth of the shear flow would obviously have a considerable impact on many engineering applications. In addition to practical applications, free shear flows are one of the simplest flows to understand the fundamental mechanism in the transition process to turbulence. After the discovery of large-scale vortical structure in free shear flows many researchers have investigated the physical mechanism of generation and dissipation processes of the vortical structure. This study investigated the role of the large-scale vortical structures in the turbulent mixing layer using LES(Large-Eddy Simulation). The result shows that the pairing interaction of the vortical structure plays an important role in the growth rate of a mixing layer. It is found that the turbulence quantities depend strongly on the velocity ratio. It is also found that the vorticity in the high-velocity-side can extract energy from the mean flow, while the vorticity in the low-velocity-side lose energy by the viscous dissipation. Finally the results suggest the guideline to obtain the desired flow by control of the velocity ratio.

• Ocean Systems Engineering
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• 제5권3호
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• pp.181-198
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• 2015

An OSV (Offshore Support Vessel) is being used to install a structure which is laid on its deck or an adjacent transport barge by lifting off the structure with its own crane, lifting in the air, crossing splash zone, deeply submerging, and lastly landing it. There are some major considerations during these operations. Especially, when lifting off the structure, if operating conditions such as ocean environmental loads and lifting off velocity are not suitable, the collision can be occurred due to the relative motion between the structure and the OSV or the transport barge. To solve this problem, this study performs the physics-based simulation of the lifting off step while the OSV installs the structure. The simulation includes the calculation of dynamic responses of the OSV and the structure, including the collision detection between the transport barge and the structure. To check the applicability of the physics-based simulation, it is applied to a problem of the lifting off step by varying the ocean environmental loads and the lifting off velocity. As a result, it is confirmed that the operability of the lifting off step are affected by the conditions.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1995년도 하계학술대회 논문집 B
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• pp.742-744
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• 1995

In position control system using variable structure control, the velocity of control object is controlled to approach the desired specified velocity patterns, and eventually the position of control object is correctly at reference position. Here, this intention can be success by means of variable structure control. In this paper, the PI velocity feedback control is also used sliding mode controller. The design of position controller under specified velocity profiles in variable structure control's constraints is studied.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2003년도 춘계 학술발표회논문집
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• pp.45-49
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• 2003

The velocity structure beneath the CHNB broadband station is determined by receiver function analysis using by from teleseismic P waveforms. The detailed broadband receiver functions are obtained by stacking method for source-equalized vertical, radial and tangential components of teleseismic P waveforms. A time domain inversion uses the stacked radial receiver function to determine vertical P wave velocity structure beneath the station. The crustal velocity structures beneath the stations are estimated using the receiver function inversion method in the case at the crustal model parameterized by many thin, flat-tying, homogeneous layers. The result of crust at model inversion shows the crustal velocity structure beneath the CHNB station varies smoothly with increasing depth, and there are six discontinuity around 2.5km, 6.25km, 12.5km, 22.5km and 27.5km depth, with Moho discontinuity at about 32.5km depth.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2003년도 추계 학술발표회논문집
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• pp.3-7
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• 2003

The velocity structure beneath the CHNB broadband station is determined by receiver function analysis using by from teleseismic P waveforms. The detailed broadband receiver functions are obtained by stacking method for source-equalized vertical, radial and tangential components of teleseismic P waveforms. A time domain inversion uses the stacked radial receiver function to determine vertical P wave velocity structure beneath the station. The crustal velocity structures beneath the stations are estimated using the receiver function inversion method in the case at the crustal model parameterized by many thin, flat-lying, homogeneous layers. Events divide into 4 groups. four azimuths corresponding to events in group a(southwest), b(south), c(southeast), d(northeast). The result of crust at model inversion shows the crustal velocity structure beneath the CHNB station varies smoothly with increasing depth. The conard discontinuity lies around 18 km and moho discontinuity lies range from 30 to 34 km.

• Journal of Mechanical Science and Technology
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• 제14권12호
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• pp.1386-1395
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• 2000

The mear field structure of round turbulent jets with initially asymmetric velocity distributions is investigated experimentally. Experiments are carried out using a constant temperature hot-wire anemometry system to measure streamwise velocity in the jets. The measurements are undertaken across the jet at various streamwise stations in a range starting from the jet exit plane and up to a downstream location of twelve diameters. The experimental results include the distributions of mean and instantaneous velocities, vorticity field, turbulence intensity, and the Reynolds shear stresses. The asymmetry of the jet exit plane was obtained by using circular cross-section pipes with a bend upstream of the exit. There pipes used here include a straight pipe, and 90 and 160 degree-bend pipes. Therefore, at the upstream of the upstream of the pipe exit, secondary flow through the bend mean streamwise velocity distribution could be controlled by changing the curvature of pipes. The jets into the atmosphere have two levels of initial velocity skewness in addition to an axisymmetric jet from a straight pipe. In case of the curved pipe, a six diameter-long straight pipe section follows the bend upstream of the exit. The Reynolds number based on the exit bulk velocity is 13,400. The results indicate that the near field structure is considerably modified by the skewness of an initial mean velocity distribution. As the skewness increases, the decay rate of mean velocity at the centerline also increases.

• 한국의류학회지
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• 제22권5호
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• pp.565-574
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• 1998

The purpose of the study was to determine the effect of air velocity on the thermal resistance of wool ensembles. Three suits for men with different weaving structure and density were made with the same design and size for the study. In addition, Y-shirt, underwear, and socks were prepared for constructing the ensembles. Thermal insulation of air layer and 3 ensembles were measured by using thermal manikin in environmental chamber controlled at 2$0^{\circ}C$ and 65% RH with various air velocity. The results were as follows: 1. Thermal resistance of air layer was 0.079 m2.$^{\circ}C$/W with no air velocity(less than 0.2m/sec). 2. Thermal resistance of air layer decreased with increasing the air velocity rapidly. When the air velocity was 0.25 and 2.89 m/sec, the decreasing rate was 15% and 61%, respectively compared with no air velocity. 3. While there was little difference among the effective thermal insulation of 3 ensembles having different weaving structure and density with no air velocity, there was sharp difference among them when the air velocity increased. That is, the decreasing rate of effective thermal insulation of the ensemble which has higher air permeability was higher. 4. The decreasing rates of the effective thermal resistances of plain, twill and satin ensemble were 61, 54, and 49%, respectively when the air velocity was 2.89 m/sec which was a maximum air velocity in this study.