• Journal of Astronomy and Space Sciences
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• v.8 no.1
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• pp.11-23
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• 1991

The dynamical evolution of globular clusters is studied using the orbit-averaged multicomponent Fokker-Planck equation. The original code developed by Cohn(1980) is modi-fied to include the effect of stellar evolutions. Plommer's model is chosen as the initial density distribution with the initial mass function index $\alpha$=0.25, 0.65, 1.35, 2.35, and 3.35. The mass loss rate adopted in this work follows that of Fusi-Pecci and Renzini(1976). The stellar mass loss acts as the energy source, and thus affects the dynamical evolution of globular clusters by slowing down the evolution rate and extending the core collapse time Tcc. And the dynamical length scale $$R_c, $$R_h is also extended. This represents the expansion of cluster due to the stellar mass loss.

• Wind and Structures
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• v.5 no.2_3_4
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• pp.379-392
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• 2002

Numerical flow computations around an aeroelastic 3D square cylinder immersed in the turbulent boundary layer are shown. Present computational code can be characterized by three numerical aspects which are 1) the method of artificial compressibility is adopted for the incompressible flow computations, 2) the domain decomposition technique is used to get better grid point distributions, and 3) to achieve the conservation law both in time and space when the flow is computed a with moving and transformed grid, the time derivatives of metrics are evaluated using the time-and-space volume. To provide time-dependant inflow boundary conditions satisfying prescribed time-averaged velocity profiles, a convenient way for generating inflow turbulence is proposed. The square cylinder is modeled as a 4-lumped-mass system and it vibrates with two-degree of freedom of heaving motion. Those blocks which surround the cylinder are deformed according to the cylinder's motion. Vigorous oscillations occur as the vortex shedding frequency approaches cylinder's natural frequencies.

• Journal of Astronomy and Space Sciences
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• v.33 no.2
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• pp.69-73
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• 2016

We use the non-stationary three dimensional two-layer outer gap model to explain gamma-ray emissions from a pulsar magnetosphere. We found out that for some pulsars like the Geminga pulsar, it was hard to explain emissions above a level of around 1 GeV. We then developed the model into a non-stationary model. In this model we assigned a power-law distribution to one or more of the spectral parameters proposed in the previous model and calculated the weighted phase-averaged spectrum. Though this model is suitable for some pulsars, it still cannot explain the high energy emission of the Geminga pulsar. An Inverse-Compton Scattering component between the primary particles and the radio photons in the outer magnetosphere was introduced into the model, and this component produced a sufficient number of GeV photons in the spectrum of the Geminga pulsar.

• Journal of Astronomy and Space Sciences
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• v.11 no.1
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• pp.81-92
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• 1994

The magnetic field measurements from the KitSat-1 and KitSat-2 were tested by comparing with the IGRF model. The magnetic data have been collected by a three-axis fluxgate magnetometer on each satellite at an latitude of 1,325 km and 820 km, respectively. To avoid highly variable magnetic disturbances at the polar region, the field map has been within the limits of 50 degrees in latitude. Each data is averaged over the square of $5{\times}5$ degrees in both latitude and longitude. In these results, the relatively quiet periods were selected and the sampling rate was 30 seconds. It is shown that the results from these measurements are consistent with the IGRF map over the global surface map.

• International Journal of Aeronautical and Space Sciences
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• v.15 no.3
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• pp.232-240
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• 2014

Transonic flow past a NASA SC(2)-0710 airfoil with deployments of a spoiler up to $6^{\circ}$ was studied numerically. We consider angles of attack from $-0.6^{\circ}$ to $0.6^{\circ}$ and free-stream Mach numbers from 0.81 to 0.86. Solutions of the unsteady Reynolds-averaged Navier-Stokes equations were obtained with a finite-volume solver using several turbulence models. Both stationary and time-dependent deployments of the spoiler were examined. The study revealed the existence of narrow bands of the Mach number, angle of attack, and spoiler deflection angle, in which the flow was extremely sensitive to small perturbations. Simulations of 3D flow past a swept wing confirmed the flow sensitivity to small perturbations of boundary conditions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.4
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• pp.507-513
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• 2001

Time averaged pressure distributions in a high-speed centrifugal compressor channel diffuser at design and off-design flow rates are investigated. Pressure distributions from the impeller exit to the channel diffuser exit are measured for various flow rates from choke to near surge condition, and the effects of operating condition are discussed. The strong non-uniformity in the pressure distribution is obtained over the vaneless space and semi-vaneless space caused by the impeller-diffuser interaction. As the flow rate increases, flow separation near the throat, due to large incidence angle at the vane leading edge, increases aerodynamic blockage and reduces the aerodynamic flow area downstream. Thus the minimum pressure location occurs downstream of the geometric throat, and it is named as the aerodynamic throat. And at choke condition, normal shock occurs downstream of this aerodynamic throat. The variation in the location of the aerodynamic throat is discussed.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.548-554
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• 2000

The aim of this paper is to understand the time averaged pressure distributions in a high-speed centrifugal compressor channel diffuser at design and off-design flow rates. Pressure distributions from the impeller exit to the channel diffuser exit are measured and discussed far various flow rates from choke to near surge condition, and the effect of operating condition is discussed. The strong non-uniformity in the pressure distribution is obtained over the vaneless space and semi-vaneless space caused by the impeller-diffuser interaction. As the flow rate increases, flow separation near the throat, due to large incidence angle at the vane leading edge, increases aerodynamic blockage and reduces the aerodynamic flow area downstream. Thus the minimum pressure location occurs downstream of the geometric throat, and it is named as the aerodynamic throat. And at choke condition, normal shock occurs downstream of this aerodynamic throat. The variation in the location of the aerodynamic throat is discussed.

• International Journal of Aeronautical and Space Sciences
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• v.11 no.1
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• pp.41-48
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• 2010

In the present study, the turbulent flow fields around a circular cylinder at $Re=3.6{\times}10^6$ were investigated based on an unstructured mesh technique, and the comparisons between URANS(S-A, SST) and hybrid RANS/LES(DES, SAS) methods for the simulation of high Reynolds number flow have been conducted. For this purpose, unsteady characteristics of vortex shedding and time-averaged quantities were compared. A quasi-steady solution-adaptive mesh refinement was also made for the URANS and hybrid RANS/LES approaches. The results showed that the simple changes in the turbulent length scale or source term of turbulent models made the flow fields less dissipative and more realistic in hybrid RANS/LES methods than the URANS approaches.

• International Journal of Aeronautical and Space Sciences
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• v.14 no.3
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• pp.210-214
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• 2013

The sensitivity of transonic flow past a Whitcomb airfoil to deflections of an aileron is studied at free-stream Mach numbers from 0.81 to 0.86 and vanishing or negative angles of attack. Solutions of the Reynolds-averaged Navier-Stokes equations are obtained with a finite-volume solver using the $k-{\omega}$ SST turbulence model. The numerical study demonstrates the existence of narrow bands of the Mach number and aileron deflection angles that admit abrupt changes of the lift coefficient at small perturbations. In addition, computations reveal free-stream conditions in which the lift coefficient is independent of aileron deflections of up to 5 degrees. The anomalous behavior of the lift is explained by interplay of local supersonic regions on the airfoil. Both stationary and impulse changes of the aileron position are considered.

• 연구논문집
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• s.28
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• pp.39-47
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• 1998

Industrial electrostatic precipitation is a very complex process, which involves multiple-way interaction between the electric field, the fluid flow, and the particulate motion. This paper describes a strongly coupled calculation procedure for the rigorous computation of particle dynamics during electrostatic precipitation. The turbulent gas flow and the particle motion under electrostatic forces are calculated by using the commercial computational fluid dynamics (CFD) package FLUENT linked to a finite-volume solver for the electric field and ion charge. Particle charge is determined from both local electrical conditions and the cell residence time which the particle has experienced through its path. Particle charge density and the particle velocity are averaged in a control volume to use Lagrangian information of the particle motion in calculating the gas and electric fields. The turbulent particulate transport and the effects of particulate space charge on the electrical current flow are investigated. The calculated results for poly-dispersed particles are compared with those for mono-dispersed particles, and significant differences are demonstrated.