• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.41.1-41.1
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• 2019

We consider dynamical dark energy models based on a minimally coupled scalar field with three different potentials: the inverse power-law, SUGRA and double exponential potentials. For each model, we derived perturbation initial conditions in the early epoch and performed the Markov Chain Monte Carlo (MCMC) analysis to explore the parameter space that is favored by the current cosmological observations like Planck CMB anisotropy, type Ia supernovae, and baryon acoustic oscillation data. The analysis has been done by using the modified CAMB/COSMOMC code in which the dynamical evolution of the scalar field perturbations are fully considered. The MCMC constraints on the cosmological as well as potential parameters are derived. In the talk we will present a progress report.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.79.1-79.1
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• 2019

We present an algorithm for the fast computation of the general N-point spatial correlation functions of any discrete point set embedded within an Euclidean space of ${\mathbb{R}}n$. Utilizing the concepts of kd-trees and graph databases, we describe how to count all possible N-tuples in binned configurations within a given length scale, e.g. all pairs of points or all triplets of points with side lengths < rmax. Through benchmarking we show the computational advantage of our new graph-based algorithm over more traditional methods. We show that all 3-point configurations up to and beyond the Baryon Acoustic Oscillation scale (~200 Mpc in physical units) can be performed on current Sloan Digital Sky Survey (SDSS) data in reasonable time. Finally we present the first measurements of the 4-point correlation function of ~0.5 million SDSS galaxies over the redshift range 0.43< z <0.7. We present the publicly available code GRAMSCI (GRAph Made Statistics for Cosmological Information; bitbucket.org/csabiu/gramsci), under a GNU General Public License.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.47.3-47.3
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• 2021

In this talk I will review recent progress that the SDSS-IV / eBOSS collaboration has made in constraining cosmology from the clustering of galaxies, quasars and the Lyman-alpha forest. The SDSS-IV / eBOSS collaboration has measured the baryon acoustic oscillation (BAO) and redshift space distortion (RSD) features in the correlation function in redshift bins from z~0.15 to z~2.33. These features constitute measurements of angular diameter distances, Hubble distances, and growth rate measurements. A number of consistency tests have been performed between the BAO and RSD datasets and additional cosmological datasets such as the Planck cosmic microwave background constraints, the Pantheon Type Ia supernova compilation, and the weak lensing results from the Dark Energy Survey. Taken together, these joint constraints all point to a broad consistency with the standard model of cosmology LCDM + GR, though they remain in tension with local measurements of the Hubble parameter.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.5
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• pp.342-350
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• 2008

The effects of combustion instability on flow structure and flame dynamics with the inlet configurations in a model gas turbine combustor were investigated using large eddy simulation (LES). A G-equation flamelet model was employed to simulate the unsteady flame behaviors. As a result of mean flow field, the change of divergent half angle($\alpha$) at combustor inlet results in variations in the size and shape of the central toroidal recirculation (CTRZ) as well as the flame length by changing corner recirculation zone (CRZ). The case of ${\alpha}=45^{\circ}$ show smaller size and upstream location of CTRZ than those of $90^{\circ}$ and $30^{\circ}$ by the development of higher swirl velocity. The flame length in the case of ${\alpha}=45^{\circ}$ is shorter than other cases, while the case of ${\alpha}=30^{\circ}$ yields the longest flame length due to the decrease of effective reactive area with the absence of CRZ. Through the analysis of pressure fluctuation, it was identified that the case of ${\alpha}=45^{\circ}$ shows the largest damping effect of pressure oscillation in all configurations and brings in the noise reduction of 2.97dB, compared to that of ${\alpha}=30^{\circ}$ having the largest pressure oscillation. These reasons were discussed in detail through the analysis of unsteady phenomena related to recirculation zone and flame surface. Finally the effects of flame-acoustic interaction were evaluated using local Rayleigh parameter.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.24 no.3
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• pp.213-218
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• 2014

CFD flow simulation of vehicles with open sunroof and passenger window help the automotive OEM(original equipment manufacturer) to identify the low frequency noise levels in the cabin. The lock-in and lock-off phenomena observed in the experimental studies of sunroof buffeting is well predicted by CFD speed sweep calculations over the operating speed range of the vehicle. The trend of the shear layer oscillation frequency with vehicle speed is also well predicted. The peak SPL from the CFD calculation has a good compromise with the experimental value after incorporating the real world effects into the CFD model by means of artificial compressibility and damping correction. The entire process right from modeling to flow analysis as well as acoustic analysis has been performed within the single environment i.e., STAR-CCM+.

• Proceedings of the KIEE Conference
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• 1991.07a
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• pp.611-614
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• 1991

In variable speed driving system of three phase induction motor controlled by an inverter, because of the switching of semiconductor devices in inverter, an appreciable amount of harmonic components of voltage waveform can cause the motor to generate losses, torque ripple, acoustic noise and oscillation of semiconductor devices. In this paper a new PAM type PWM inverter using IGBT is described. The output waveforms in the proposed PAM type PWM inverter are investigated both theoretiically and experimentally. The line-voltage waveform is composed of fundamental component and the sidebands of carrier frequency. The lower order harmonics are not included in the output wave form. As each inverter arm does not operate during two-thirds period, the heats, generated in the devices are reduced. That is, the size of the inverter system can he minimized because of the reduction in the heat dissipating equipment.

• 한국전산유체공학회:학술대회논문집
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• 2001.05a
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• pp.189-194
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• 2001

In this paper, non-reacting and reacting flowfields were computed using a preconditioned Navier-Stokes solver. The preconditioning technique of Merkle et al. and TVD scheme or Chakravarthy and Osher was employed and the results obtained using developed code have a good agreement with the previous results and experimental data. The preconditioned Wavier-Stokes equation set with low Reynolds number $\kappa-\epsilon$ equation and species continuity equations, are discretized with strongly implicit manner and time integrated with LU-SSOR scheme. For the purpose of treating unsteady problem the duel-time stepping scheme was employed. For the validation of the code in incompressible flow regime, steady driven square cavity flow was considered and calculation result shows reasonably good agreement with the result of incompressible code. Shock wave/boundary layer interaction problem was considered to show the shock capturing performance of preconditioned-TVD scheme. To validate unsteady flow, acoustic oscillation problem was calculated, and supersonic premix flame of $H_2$-air reaction problem which is calculated with turbulence model, 9-species/18-reaction step reaction model, shows reasonable agreement with the previous results. As a result, the preconditioning method has an advantage to calculate incompressible and compressible flow through one code and preconditioned solver easily developed from standard compressible code with minor efforts. But additional computational time and computer memory is required due to preconditioning matrix.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.27 no.2
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• pp.84-94
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• 2013

In case when the existing TRIAC phase controlled dimmer is drove for the LED lighting equipments, there are many problems such as the LED flicker in low phase-angles, the acoustic noise and elements damage by increase of the peak voltage in the input filter capacitor, mulfunction by insufficiency of the TRIAC holding current, and the abnormal oscillation by LC resonant. In this paper, we proposes the single-stage quasi-resonant PSR(Primary Side Regulation) PWM converter, and the design, the simulation and experiment are performed. As a result, it could confirm that the proposed PWM converter is the lighting equipments for LED drive which can alternate the existing 60W class incandescent bulbs and it has the high drive performance of the efficiency 80% and over, the power factor 0.95 and over under the normal voltage 220V. Finally, total harmonic distortion(THD) is gratified with a standard[1] of the lighting equipments and the durability is evaluated as the high reliablilty of 150,000 hours and over.

• Journal of the Korean Society of Propulsion Engineers
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• v.7 no.1
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• pp.40-48
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• 2003

Large eddy simulation is used to investigate the compressible flow over a open cavity, The sub-grid scale stresses are modeled using the dynamic model. The compressible Navier-Stokes equations are solved with the sixth order accurate compact finite difference scheme in the space and the 4th order Runge-Kutta scheme in the time. The results show a typical flow pattern of the shear layer mode of oscillation over the cavity. The votical disturbances, the roll-up of vorticity, and impingement and scattering of vorticity at the downstream cavity edge can be seen in the shear layer. Predicted acoustic resonant frequency is in good agreement with that of the empirical formula. The mean flow streamlines are nearly horizontal along the mouth of the cavity. The pressure has its minimum value in the vortex core inside the cavity.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.16 no.5
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• pp.90-95
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• 2007

Single side final polishing is a very important role to stabilize a wafer finally before the device process on the wafer is executed. In this study, the machining variables, such as pressure, machining time, and the velocity of pad table were adopted. These parameters have the major influence on the characteristics of wafer polishing. We investigated the surface roughness changing these variables to find the optimal polishing condition. Pad, slurry, slurry quantity, and oscillation distance were set to the fixed variables. In order to reduce defects and find a stable machining condition, a hall sensor was used on the polishing process. AE sensor was attached to the polishing machine to verify optimal condition. Applying data analysis of the sensor signal, experiments were performed. We can get better surface roughness from loading the quasi static force and improving wafer-holding method.