• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.2B
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• pp.249-259
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• 2008

The present paper studies the effect of the slope gradient of a fully permeable submerged breakwater using a newly developed numerical model that is able to consider the flow through a porous midium with inertial, laminar and turbulent resistance terms, i.e. simulate directly WAve-Structure (submerged breakwater)-Sand seabed interaction and can determine the eddy viscosity with LES turbulence model in 2-Dimensional wave field (LES-WASS-2D). The developed model was validated through the comparison with an existing experimental data, and further used for various numerical experiments in oder to investigate the complicated hydrodynamics on the varying slope gradient of permeable submerged breakwater. We found an acceptable phenomenon, as we expect intuitively, that reflection and transmission coefficients decrease simultaneously as slope gradient decrease. In addition, the breaking point, the circulation flow and mean vorticity around a submerged breakwater are throughly discussed.

• Journal of Astronomy and Space Sciences
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• v.37 no.2
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• pp.77-84
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• 2020

We performed high-resolution three-dimensional global magnetohydrodynamic (MHD) simulations to study the interaction between the Earth's magnetosphere and a prolonged steady southward interplanetary magnetic field (IMF) (Bz = -2nT) and slow solar wind. The simulation results show that dayside magnetic reconnection continuously occurs at the subsolar region where the magnetosheath magnetic field is antiparallel to the geomagnetic field. The plasmoid developed on closed plasma sheet field lines. We found that the vortex was generated at the magnetic equator such as (X, Y) = (7.6, 8.9) R_E due to the viscous-like interaction, which was strengthened by dayside reconnection. The magnetic field and plasma properties clearly showed quasiperiodic variations with a period of 8-10 min across the vortex. Additionally, double twin parallel vorticity in the polar region was clearly seen. The peak value of the cross-polar cap potential fluctuated between 17 and 20 kV during the tail reconnection.

• Wind and Structures
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• v.23 no.1
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• pp.45-57
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• 2016

Open-cross-section composite beam (OCB) tends to suffer vortex-induced vibration (VIV) due to its bluff aerodynamic shape. A cable-stayed bridge equipped with typical OCB is taken as an example in this paper to conduct sectional model wind tunnel test. Vortex-induced vibration is observed and maximum vibration amplitudes are obtained. CFD approach is employed to calculate the flow field around original cross sections in service stage and construction stage, as well as sections added with three different countermeasures: splitters, slabs and wind fairings. Results show that flow separate on the upstream edge and cause vortex shedding on original section. Splitters can only smooth the flow field on the upper surface, while slabs cannot smooth flow field on the upper or lower surface too much. Thus, splitters or slabs cannot serve as valid aerodynamic means. Wind tunnel test results show that VIV can only be mitigated when wind fairings are mounted, by which the flow field above and below the bridge deck are accelerated simultaneously.

• Wind and Structures
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• v.4 no.4
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• pp.333-352
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• 2001

A two dimensional discrete vortex method (DIVEX) has been developed to predict unsteady and incompressible flow fields around closed bodies. The basis of the method is the discretisation of the vorticity field, rather than the velocity field, into a series of vortex particles that are free to move in the flow field that the particles collectively induce. This paper gives a brief description of the numerical implementation of DIVEX and presents the results of calculations on a recent suspension bridge deck section. The predictions for the static section demonstrate that the method captures the character of the flow field at different angles of incidence. In addition, flutter derivatives are obtained from simulations of the flow field around the section undergoing vertical and torsional oscillatory motion. The subsequent predictions of the critical flutter velocity compare well with those from both experiment and other computations. A brief study of the effect of flow control vanes on the aeroelastic stability of the bridge is also presented and the results from DIVEX are shown to be in accordance with previous analytical and experimental studies. In conclusion, the results indicate that DIVEX is a very useful design tool in the field of wind engineering.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.11
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• pp.3003-3013
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• 1995

The characteristics of the flow field in the tangentially fired furnace are presented. Experiments are conducted in the simplified cold type isothermal flow model. In the measurement of flow field, a hot wire anemometer is used. The hot wire was calibrated by lookup table method. The mean velocity field and turbulence characteristics are showed with changing the nozzle angle. In the center of the model, the low speed, unstable flow region is formed. The size and position of these regions are varied with changing the nozzle angle. It can be used as fundamental data in the design of the large furnace. From the experimental results, various turbulent characteristics of swirling flow field is obtained. And the entrainment mechanism of the jet flow field is described from the distribution of the skewness and the flatness. It can be used the raw data of approximate calculation and turbulent modelling.

• Journal of computational fluids engineering
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• v.15 no.2
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• pp.14-20
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• 2010

Shock-focusing concave reflectors can have parabolic, circular or elliptic cross-sections. They produce effectively a very high pressure at the focusing point. In the past, many optical images have been obtained on shock focusing via experiments. Measurement of field variables is, however, difficult in the experiment. Using the wave propagation algorithm and the Cartesian embedded boundary method, we have successfully obtained numerical Schlieren images that appear very much like the experimental results. In addition, we obtained the detailed field variables such as pressure, velocity, density and vorticity in the unsteady domain. The present numerical results have made it possible to understand the shock focusing phenomenon in more detail than before.

• Journal of the Korea Computer Graphics Society
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• v.14 no.4
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• pp.1-5
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• 2008

In this paper, we propose an efficient technique for improving the grid based fluid simulation by sub-grid visuals. The detailed turbulency generated efficiently by Vortex Particle Method are blended with the flow fields coming from the traditional incompressible Navier-Stokes solver. The algorithm enables large- and small- scale detail to be edited separately.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.7
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• pp.769-778
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• 2005

The velocity and pressure fields of a ship's Weis-Fogh type propulsion mechanism are studied in this paper using an advanced vortex method. The wing (NACA0010 airfoil) and channel are approximated by source and vortex panels. and free vortices are introduced away from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from an integral, based on the instantaneous velocity and vorticity distributions in the flow field. Two-dimensional unsteady viscous flow calculations of this propulsion mechanism are shown. and the calculated results agree qualitatively with the measured thrust and drag due to un-modeled large fluctuations in the measured data.

• Proceedings of the KSRS Conference
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• 2005.10a
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• pp.52-53
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• 2005

Time series of gridded surface wind and wind-stress vectors over the world ocean have been constructed by satellite scatterometer data. The products are derived from the ERS-l,2 covering 9 years during 1992-2000 and the Sea Winds on board QuikSCAT (Qscat) which has been operating up to the present since June 1999, so they allows us to analyze variabilities with various time scales. In this study, we focus on interannual variability of the wind stress in the mid- and high-latitude region of North Pacific. These are compared with those by numerical weather prediction(NWP) ones (NCEP Reanalysis). We also examine variability in the wind-stress curl field that is an important factor for ocean dynamics and focus its time and spatial characters in the northwestern Pacific around Japan. It is found that the vorticity field in the lower atmosphere tends to increase gradually with time, suggesting the enhancement of the North Pacific subtropical gyre.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2002.04a
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• pp.64-64
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• 2002

Two dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using digital particle image velocimetry (PIV). Mean velocities, turbulence intensities, vorticity field and volume dilatation field are obtained from 300 instantaneous PIV images using 0.33 $\mu\textrm{m}$ $TiO_2$ particle. The seeding particle of larger size, 1.4 $\mu\textrm{m}$ $TiO_2$, is also used for the experimental measurements of velocity lag downstream of shock waves according to particle sizes. The results have been compared and analyzed with schlieren photographs and computational fluid dynamics (CFD) results for the velocity distribution, the locations of shock waves and over-expanded shock structure. It was shown that the locations of normal shock and shock waves can be resolved by the axial or radial velocities, and the velocity lag is more significantly increased due to particle inertia as a particle size increases. And it was also found that over-expanded shock structures call be predicted by volume dilatation fields, and streamwise turbulence intensities are influenced significantly by normal shock waves.