• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.7
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• pp.834-841
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• 2004

Experimental study on the three-dimensional topology of hairpin packet structures in turbulent boundary layers were carried out. Two different Reynolds number based on momentum thickness, Re$\sub$$\theta$/=514 and 934 were generated in a blowing type wind tunnel under the condition of zero pressure gradient. Simultaneous measurements of velocity fields at a wall-normal plane and wall-parallel plane by a plane PIV and a Stereo-PIV systems. The two Nd:Yag laser systems and three CCD cameras were synchronized to obtain instantaneous velocity fields at the same time. To avoid optical noise at the crossing line by the two laser light sheets, a new optical arrangement using polarization was applied. The obtained velocity fields show the existence of hairpin packet structure vividly and the idealized hairpin vortex signature is confirmed by experiment. Two counter-rotating vortex pair which reflects the cutting plane of hairpin legs are found both side of a strong streaky structure when the wall-normal plane cuts the hairpin head.

• The KSFM Journal of Fluid Machinery
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• v.12 no.2
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• pp.8-16
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• 2009

The objective of this study is to investigate the three-dimensional turbulence flow characteristics of a rotor passage of an one-stage axial flow gas turbine and to investigate the effects of a boundary layer fence installed on the hub endwall of the rotor passage. Secondary flows occurring within the rotor passage (e.g. horseshoe vortex, passage vortex, and cross flow) cause secondary loss and reduce turbine efficiency. To control these secondary flows, a boundary layer fence measuring half the height of the thickness of the inlet boundary layer was installed on the hub endwall of the rotor passage. This study was performed numerically. The results show that the wake and secondary flows generated by the stator reduced the rotor load to constrain the development of cross flow and secondary flow reinforced by the rotor passage. In addition, the secondary vortices occurring within the rotor passage were reduced by the rotation of the rotor. Although, the boundary layer fence induced additional vortices, giving rise to an additional loss of turbine, its presence was shown to reduce the total pressure loss when compared to effects of the case without fence regardless of the relative position of blades by enervating secondary vortices occurred within the rotor passage.

• Wind and Structures
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• v.36 no.3
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• pp.161-174
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• 2023

The analytical model of tornado vortices plays an essential role in tornado wind description and tornado-resistant design of civil structures. However, there is still a lack of guidance for the selection and application of tornado analytical models since they are different from each other. For single-cell tornado vortices, this study conducts a comparative study on the velocity characteristics of the analytical models based on numerically simulated tornado-like vortices (TLV). The single-cell stage TLV is first generated by Large-eddy simulations (LES). The spatial distribution of the three-dimensional mean velocity of the typical analytical tornado models is then investigated by comparison to the TLV with different swirl ratios. Finally, key parameters are given as functions of swirl ratio for the direct application of analytical tornado models to generate full-scale tornado wind field. Results show that the height of the maximum radial mean velocity is more appropriate to be defined as the boundary layer thickness of the TLV than the height of the maximum tangential mean velocity. The TLV velocity within the boundary layer can be well estimated by the analytical model. Simple fitted results show that the full-scale maximum radial and tangential mean velocity increase linearly with the swirl ratio, while the radius and height corresponding to the position of these two velocities decrease non-linearly with the swirl ratio.

• Journal of Environmental Science International
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• v.1 no.1
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• pp.35-43
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• 1992

Handling the emergency problems such as Chemobyl accident require real time prediction of pollutants dispersion. One-point real time sounding at pollutant source and simple model including turbulent-radiation process are very important to predict dispersion at real time. The stability categories obtained by one-dimensional numerical model (including PBL dynamics and radiative process) are good agreement with observational data (Golder, 1972). Therefore, the meteorological parameters (thermal, moisture and momentum fluxes; sensible and latent heat; Monin-Obukhov length and bulk Richardson number; vertical diffusion coefficient and TKE; mixing height) calculated by this model will be useful to understand the structure of stable boundary layer and to handling the emergency problems such as dangerous gasses accident. Especially, this simple model has strong merit for practical dispersion models which require turbulence process but does not takes long time to real predictions. According to the results of this model, the urban area has stronger vertical dispersion and weaker horizontal dispersion than rural area during daytime in summer season. The maximum stability class of urban area and rural area are "A" and "B" at 14 LST, respectively. After 20 LST, both urban and rural area have weak vertical dispersion, but they have strong horizontal dispersion. Generally, the urban area have larger radius of horizontal dispersion than rural area. Considering the resolution and time consuming problems of three dimensional grid model, one-dimensional model with one-point real sounding have strong merit for practical dispersion model.al dispersion model.

• International Journal of Aeronautical and Space Sciences
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• v.1 no.1
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• pp.36-47
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• 2000

Three-dimensional compressible turbulent flow fields within the passage of a diffusing S-duct have been simulated by solving the Navier-Stokes equations with SIMPLE scheme. The average inlet Mach number is 0.6 and the Reynolds number based on the inlet diameter is $1.76{\times}10^6$ The extended $k-{\varepsilon}$ turbulence model is applied to modeling the Reynolds stresses. Computed results of the flow in a circular diffusing S-duct provide an understanding of the flow structure within a typical engine inlet system. These are compared with experimental wall static-pressure, total-pressure fields, and secondary velocity profiles. Additionally, boundary layer thickness, skin friction values, and streamlines in the symmetric plane are presented. The computed results depict the interaction between the low energy flow by the flow separation and the high energy flow by the reversed duct curvature. The computed results obtained using the extended $k-{\varepsilon}$ turbulence model.

• Journal of Ocean Engineering and Technology
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• v.8 no.2
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• pp.31-37
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• 1994

3-Dimensional panel method is now developed to the level that one can calculate the lift of a three dimensional body with the same accuracy of wind tunnel test and some current codes can consider the boundary layer effects due to the viscosity and unsteady motion in the calculation of lift. This paper is also aimed to develop these kinds of computing programs, and as a beginning, the authors restricted the problems to the steady potential flow cases. The calculation of 3-Dimensional body, wing and tandem wing carried out, using source panel and vortex ring panel. Finally, the interactions between 3-Dimension symmetric body and a wing are also calculated.

• Journal of Korean Society for Atmospheric Environment
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• v.14 no.3
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• pp.219-228
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• 1998

In the present investigation, a numerical model developed for the prediction of the wind flow over complex terrain is validated by comparing with the field experiments. For the solution of the Reynolds - Averaged Clavier- stokes equations which are the governing equations of the microscale atmospheric flow, the model is constructed based on the finite-volume formulation and the SIMPLEC pressure-correction algorithm for the hydrodynamic computation. The boundary- fitted coordinate system is employed for the detailed depiction of topography. The boundary conditions and the modified turbulence constants suitable for an atmospheric boundary- layer are applied together with the k- s turbulence model. The full- scale experiments of Cooper's Ridge, Kettles Hill and Askervein Hill are chosen as the validation cases . Comparisons of the mean flow field between the field measurements and the predicted results show good agreement. In the simulation of the wind flow over Askervein Hill , the numerical model predicts the three dimensional flow separation in the downslope of the hill including the blockage effect due to neighboring hills . Such a flow behavior has not been simulated by the theoretical predictions. Therefore, the present model may offer the most accurate prediction of flow behavior in the leeside of the hill among the existing theoretical and numerical predictions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.10
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• pp.917-923
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• 2010

Direct numerical simulation (DNS) of a spatially developing turbulent boundary layer (TBL) with regularly arrayed cubical roughness elements was performed to investigate the effects of three-dimensional (3D) surface elements. The staggered cubes downstream were periodically arranged in the streamwise and spanwise directions with pitches of $p_x$/k=8 and $p_z$/k=2, where $p_x$ and $p_z$ are the streamwise and spanwise spacings of the cubes; the roughness height (k) was k=$1.5{\theta}_{in}$, where ${\theta}_{in}$ is the momentum thickness at the inlet. Spatially developing characteristics over the 3D cubical roughness were compared with the data obtained from the DNS over the two-dimensional (2D) rod roughened wall and smooth wall. Introduction of the cubical roughness on the TBL affected the turbulent Reynolds stresses not only in the roughness sublayer but also in the outer layer; and these effects are consistent with those observed over the 2D rough wall.

• Journal of the korean society of oceanography
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• v.38 no.1
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• pp.11-16
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• 2003

The three dimensional structure of thermohaline circulation in a D-plane is investigated using a conceptual two layer model and a scaling argument. In this simple model, the water mass formation region is excluded. The upper layer represents the oceans above the main thermocline. The lower layer represents the deep ocean below the thermocline and is much thicker than the upper layer. In each layer, geostrophy and the linear vorticity balance are assumed. The cross interfacial velocity that compensates for the deep water mass formation balances downward heat diffusion from the top. From the above relations, we can determine the thickness of the upper layer, which is the same as thermocline depth. The results we get is basically the same as that we get for an f-plane ocean or the classical thermocline theory. Mass budget using the velocity scales from the scaling argument shows that western boundary and interior transports are much larger than the net meridional transport. Therefore in the thermohaline circulation, horizontal circulation is much stronger than the vertical circulation occuring on a meridional plane.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.585-590
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• 2000

The hydrodynamic instability of the three-dimensional boundary layer on a rotating disk introduces a periodic modulation of the mean flow in the form of stationary cross flow vortices. Detailed numerical values of the growth rates, neutral curves and other characteristics of the two instabilities have been calculated over a wide range of parameters. Presented are the neutral stability results concerning the two instability modes by solving new linear stability equations reformulated not only by considering whole convective terms but by correcting some errors in the previous stability equations. The present stability results are agree with the previously known ones within reasonable limit. The flow is found to be always stable for a disturbance whose dimensionless wave number at Re=1200 is greater than 0.75. Also, the spatial amplification contours have been calculated for the moving disturbance wave, whose azimuth angle is between ${\varepsilon}=15^{\circ}$ and $12.5^{\circ}$.