• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.11
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• pp.1013-1019
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• 2011

Sound attenuation of periodically arranged cylindrical rods is studied numerically and experimentally. Cross section of the cylinder is circular and arrays are in a square lattice. Cylinders are made of steel, and consist of five groups with different diameters from 27.2 mm to 48 mm. Each group has 5 rows, while number of cylinders in a row varies from 17 to 31. The area filling fraction is about 60~61 %, which leads to the stop bandgap(2.9 kHz ~ 8.4 kHz). Sound attenuation is computed using two-dimensional BEM, and measurement is done by using a speaker and microphones in a semi-anechoic room. Comparison of the results by BEM and experiment shows that attenuation spectra are qualitatively in agreement, although experiment gives higher attenuations than BEM. After results by BEM are scaled up in accordance with cylinder diameter, it is observed that attenuation curves are in good agreement, which confirms that analysis by BEM is done correctly. It is also found that the measured bandgaps are shifted toward lower frequency by 0.5 kHz ~ 1.2 kHz, when compared to the predictions obtained from infinitely repeated two-dimensional cylinder arrays.

• Journal of Ocean Engineering and Technology
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• v.20 no.1 s.68
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• pp.48-54
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• 2006

In this paper, flowfield and acoustic-field around moving bodies are simulated by the Arbitrary Lagrangian Eulerian (ALE) formulation in the finite difference lattice Boltzmann method. Some effects are checked by comparing flaw about a square cylinder in ALE formulation and that in the fixed coordinates, and both agree very well. Matching procedure between the moving grid and fixed grid is also considered. The applied method in which the both grids are connected through buffer region is shown to be superior to moving overlapped grid. Dipole-like emissions of sound wave from harmonically vibrating bodies in two- and three-dimensional cases are simulated.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.5
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• pp.580-588
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• 2006

Two-dimensional turbulent flows past a square cylinder and cavity noise are simulated by the finite difference lattice Boltzmann method with subgrid turbulence model. The method, based on the standard Smagorinsky subgrid model and a single-time relaxation lattice Boltzmann method, incorporates the advantages of FDLBM for handling arbitrary boundaries. The results are compared with those by the experiments carried out by Noda & Nakayama and Lyn et al. Numerical results agree with the experimental ones. Besides, 2D computation of the cavity noise generated by flow over a cavity at a Mach number of 0.1 and a Reynolds number based on cavity depth of 5000 is calculated. The computation result is well presented a understanding of the physical phenomenon of tonal noise occurred primarily by well-jet shear layer and vortex shedding and an aeroacoustic feedback loop.

• Transactions on Electrical and Electronic Materials
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• v.13 no.1
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• pp.16-18
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• 2012

LED lighting systems that combine lighting capability, emotional and physiological characteristics are required for lighting source and multifunctional applications. In this work, Simulation studies using optical analysis software packages, Light Tools, are presented. This is done to estimate the uniformity ratio of illuminance and photosynthetic photon flux density (PPFD) of the periodic 2D lattice arrangements, such as square, diamond, two-way bias quadrangular, hexagonal, and Kagome lattices, under the same transmissivity, absorptance and reflectivity. It has been found out that the two-dimensional Kagome lattice arrangement exhibited high uniformity ratio of illuminance and PPFD compared to other lattices. Accordingly, these results can be used to guide a design and improve the lighting environment which in turn would maximize the uniform distributions of illuminance.

• Journal of the Optical Society of Korea
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• v.17 no.3
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• pp.262-268
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• 2013

We study the focusing properties of a two-dimensional square-lattice photonic crystal (PC) comprising silica and germanium partitioned cylinders in air background. The finite difference time domain (FDTD) method with periodic boundary condition is utilized to calculate the dispersion band diagram and the FDTD method incorporating the perfectly matched layer boundary condition is employed to simulate the image formation. In contrast to the common square PCs in which the negative refraction effect occurs in the first photonic band without negative phase propagation, in our suggested model system, the frequency with negative refraction exists in the second band and in near-infrared region. In this case, the wave propagates with a negative phase velocity and the evanescent waves can be supported. We also discuss the dependency of the image resolution and its location on surface termination, source location, and slab thickness. According to the simulation results, spatial resolution of the proposed PC lens is below the radiation wavelength.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.22 no.11
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• pp.727-734
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• 2010

The present study numerically investigates the motion of a solid body suspended in the square enclosure with natural convection. A two-dimensional circular cylinder levitated thermally has been simulated by using thermal lattice Boltzmann method(TLBM) with the direct-forcing immersed boundary method. To deal with the ascending, falling or levitation of a circular cylinder in natural convection, the immersed boundary method is expanded and coupled with the TLBM. The circular cylinder is located at the bottom of a square enclosure with no restriction on the motion and freely migrates due to the Boussinesq approximation which is employed for the coupling between the flow and temperature fields. For different density ratio between the cylinder and the fluid, the motion characteristics of the circular cylinder for various Grashof numbers have been carried out. The Prandtl number is fixed as 0.7.

• Proceedings of the Optical Society of Korea Conference
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• 2003.07a
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• pp.236-237
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• 2003

최근 공진기 형태가 없는 2차원 광결정(photonic crystal) 레이저가 그것의 2차원 되먹임과 거울 없는 발진 현상과 같은 특이한 특성으로 인해 많은 관심을 끌고 있다. 이러한 레이저는 photonic band edge에서 빛의 군속도의 감소에 근거하여 작동한다. 이 photonic crystal band edge 레이저는 2차원 형태의 Distributed-Feedback(DFB) 레이저 형태로 볼 수 있다. 지금까지 보고된 레이저는 0.1 정도의 매우 작은 굴절률 변화를 가지고 있고 크기가 (100$\mu$m)$^2$이상으로 상당히 크다. (중략)

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.7 s.262
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• pp.620-627
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• 2007

In this paper, we adapt a modified internal energy non-equilibrium first-order extrapolation thermal boundary condition to the thermal lattice Boltzmann model (TLBM). This model is the double populations approach to simulate hydrodynamic and thermal fields. The bounce-back boundary condition which is a traditional boundary condition of lattice Boltzmann method has only a first order in numerical accuracy at the boundary and numerical instability. A non-equilibrium first-order extrapolation boundary condition has been verified to be of better numerical stability than the bounce-back boundary condition and this boundary condition is proved to be of second-order accuracy for the flat boundaries. The two-dimensional natural convection flow in a square cavity with Pr=0.71 and various Rayleigh numbers are simulated. The results are found to be in good agreement with those of previous studies.

• Journal of computational fluids engineering
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• v.10 no.1
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• pp.8-15
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• 2005

Aerodynamic sounds generated by a uniform flow around a two-dimensional circular cylinder at Re=150 are simulated by applying the finite difference lattice Boltzmann method. Thethird-order-accurate up-wind scheme (UTOPIA) is used for the spatial derivatives, and the second-order-accurate Runge-Kutta scheme is applied for the time marching. We have succeed in capturing very small pressure fluctuations with the same frequency of the Karman vortex street compared with the pressure fluctuation around a circular cylinder. The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream due to the Doppler effect in the uniform flow. For the downstream, on the other hand, it is faster. It is also apparent that the amplitude of sound pressure is proportional to r /sup -1/2/,r being the distance from the center of the circular cylinder. To investigate the effect of the lattice dependence, furthermore, 2D computations of the tone noises radiated by a square cylinder and NACA0012 with a blunt trailing edge at high incidence and low Reynolds number are also investigate.

• Journal of Electrical Engineering and Technology
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• v.3 no.2
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• pp.276-279
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• 2008

We report on the enhancement of cathode-luminescence in an $In_xGa_{1-x}N/In_yGa_{1-y}$ green light emitting diode structure using two-dimensional photonic crystals. The square lattice arrays of photonic crystals with diameter/periodicity of 200/500 nm were fabricated by electron beam lithography. Inductively coupled plasma dry etching was used to etch and define photonic crystals. Three samples with different etch depths, i.e., 170, 95, and 65 nm, were constructed. Field emission scanning electron microscope analysis shows that air holes of photonic crystal structure with inverted-cone shapes were fabricated after dry etching. Cathode-luminescence measurement indicated that up to 30-fold enhancement of cathode-luminescence intensity has been achieved.