• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.3
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• pp.211-217
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• 2014

This study analyzes the nonlinear interaction between two different vortex shedding frequencies from a cylinder with two diameters. In particular, two different vortex shedding frequencies are generated by preparing a cylinder having two diameters artificially. Flow velocity fluctuations behind the cylinder are measured three-dimensionally. Additionally, we fabricated a hole and placed a pressure transducer for measuring the pressure on the cylinder surface. The pressure signal from the pressure transducer is used as basic signal. A TSC(Trans Spectrum Coherence) is used for checking the strength of the nonlinear interaction between two different vortex shedding frequencies. As a result, the following are clarified: i) frequency distribution behind the cylinder, ii) three-dimensional flow state behind the cylinder through calculation of ensemble average, and iii) close relationship between the vertical vortex and change of low frequency by nonlinear interaction between two different vortex shedding frequencies from the cylinder with two diameters.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.9
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• pp.5778-5788
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• 2015

In this study, wet surface heat transfer and friction characteristics of compound enhanced fin-and-tube heat exchangers were experimentally investigated. Louver-finned heat exchangers were also tested for comparison purpose. The effect of fin pitch on j and f factor was negligible. Both j and f factor decreased as number of tube row increased. Compound enhanced fin samples yielded higher j and f factors than louver fin samples. For one row, j and f factors of compound enhanced fin samples were 11% and 43% higher than those of louver fin samples. For two row, those were 8% and 50%, and for three row, those were 17% and 53%. Heat transfer capacities at the same pressure drop of the compound enhanced fin samples were 2.0% for one row, 3.1% for two row and 8.4% for three row larger than those of louver fin samples, Data were compared with predictions of existing louver fin correlations.

• Journal of the Society of Naval Architects of Korea
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• v.39 no.3
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• pp.41-47
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• 2002

Turbulent wake behind a ship propeller has been investigated using the adaptive hybrid 2-frame PTV(Particle Tracking Velocimetry). 400 instantaneous velocity fields were measured according to 4 different blade phases and ensemble-averaged to investigate the spatial evolution of the vortical structure of near wake within one propeller diameter downstream. The phase averaged mean velocity fields show the potential wake and the viscous wake formed by the boundary layers developed on the blade surfaces. As the tip vortex evolves downstream, the slipstream is contracted and the turbulent intensity is decreased with viscous dissipation and turbulent diffusion.

• Korean Journal of Materials Research
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• v.8 no.5
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• pp.383-393
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• 1998

Cadmium telluride films were grown by close-spaced sublimation(CSS) technique. The effects of various deposition parameters such as ambient pressure, source- to-substrate spacings and temperatures on the growth rate and the microstructure were investigated. The growth mode of CdTe films showed a transition as the ambient pressure changed. This transition was interpreted in terms of the diffusion limited transport and the sublimation limited transport of Cd and $Te_2$ vapors. Experimental results indicated that the transition of growth mode was related with the mean free path of gas molecules. The growth rate and the microstructure of CdTe films were affected by the source type- bulk or powder. This change was due to the temperature difference at the source surface. XRD and SEM analysis showed that the growth rate was one of the main factors to determine CdTe microstructures.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.11
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• pp.1055-1063
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• 2012

Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a turbulent boundary layer. The coating used in this experiment was an open-cell, urethane-type foam with a porosity of approximately 50 ppi. The ultimate objective of the coating is to provide a mechanical filter to reduce the wall pressure fluctuations. The boundary layer on the flat plate was measured by using a hot wire probe, and the CPM method was used to determine the skin friction coefficient. The wall pressure autospectra and streamwise wavenumber-frequency spectra were compared to assess the attenuation of the wall pressure field by the coating. The coating is shown to attenuate the convective wall pressure energy. However, the relatively rough surface of the coating in this investigation resulted in a higher mean wall shear stress, thicker boundary layer, and higher low-frequency wall pressure spectral levels compared to a smooth wall.

• Journal of the Korean Society of Propulsion Engineers
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• v.13 no.2
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• pp.1-8
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• 2009

Present studies of these experiments was conducted to using water, over a range of cross-flow velocities from 42 to 136 m/s, with injection frequencies from 35.7 to 166.2 Hz. In cross-flow field, main parameters of liquid jet for secondary breakup were cross-flow drag rather than pressure pulse frequency. As oscillation of the periodic pressure, liquid jet was moved up and down. Also, a bulk of liquid jet puff was detected at upper field of liquid surface. Because of pressure pulsation frequency, an inclination of SMD for the structured layer was evanescent. Cross-sectional characteristics of SMD at downstream area were non-structured distributions. The tendency of volume flux value for various frequency of pressure pulse was same distribution. And volume flux was decreased when the frequency of pressure pulse increasing.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.1
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• pp.41-52
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• 2021

During atmospheric ascent of a launch vehicle, airborne acoustic loads act on the vehicle and its effect becomes pronounced at transonic speed. In the present study, acoustic loads acting on a hammerhead launch vehicle at a transonic speed have been analyzed using ��-ω SST based IDDES and the results including mean Cp, Cprms, and PSD are compared to available wind-tunnel test data. Mesh dependency of IDDES results has been investigated and it has been concluded that with an appropriate turbulence scale-resolving computational mesh, the characteristic flow features around a transonic hammerhead launch vehicle such as separated shear flow at fairing shoulder and its reattachment on rear body as well as large pressure fluctuation in the region of separated flow behind the boat-tail can be predicted with reasonable accuracy for engineering purposes.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.315-315
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• 2010

결정질 실리콘을 포함하는 태양전지의 광전효율은 표면에 입사되는 태양광의 반사를 제외하면 흡수된 광자에 의해 생성되는 전자-정공쌍의 상대적인 비율인 내부양자효율에 의존하게 된다. 실제 생성된 전자-정공쌍은 기판재료의 결정상태와 전기광학적 물성 등에 의해 일부가 재결합되어 2차적인 광자의 생성이나 열로서 작용하고 최종적으로 전자와 정공이 완전히 분리되고 전극에 포집되어 실질적인 유효전류로 작용한다. 16% 이상의 고효율 결정질 실리콘 태양전지양산이 요구되고 있는 현실에서 광전효율 개선 위해 가장 우선적으로 고려되어야 할 변수는 입력 태양광스펙트럼에 대한 결정질 실리콘 표면반사율을 최소화하여 광흡수를 극대화하는 것이라 할 수 있다. 이의 해결을 위하여 대기와 실리콘표면 사이의 굴절률차이가 크면 클수록 태양광스펙트럼에 대한 결정질 실리콘의 광반사는 증가하기 때문에 상대적으로 낮은 굴절률의 $SiO_x$나 $SiN_x$와 같은 반사방지막을 광입력 실리콘표면에 증착하여 광반사율 저감공정을 적용하고 있다. 이와 더불어 결정질 실리콘표면을 화학적으로 혹은 플라즈마이온으로 50-100nm 직경의 바늘형 피라미드형상으로 texturing 함으로 광자들의 다중반사 등에 기인하는 광흡수율의 증가를 기대할 수 있기 때문에 태양전지효율 개선에 긍정적인 영향을 미치는 것으로 이해된다. 본 실험에서도 고효율 다결정 실리콘 태양전지 양산공정에 적용 가능한 ICP-RIE기반 결정질 실리콘표면에 대한 texturing 공정기술을 연구하였다. Double Langmuir 플라즈마 진단시스템(DLP2000)을 적용하여 사용한 $SF_6$와 $O_2$ 개스유량과 챔버압력, 플라즈마 파워에 따른 이온밀도, 전자온도, 포화이온전류밀도, 플라즈마포텐셜의 공간분포를 모니터링하였고 texturing이 완료된 시료에 대하여 A1.5G 표준태양광스펙트럼의 300-1100nm 파장대역에서 반사율을 측정하여 그 변화를 관찰하였다. 본 연구에서 얻어진 결과를 간략히 정리하면 Si texturing에 가장 적합한 플라즈마파워는 100W, $SF_6/O_2$ 혼합비는 18:22, 챔버압력은 30mtorr 등이고 이에 상응하는 플라즈마의 이온밀도는 $2{\sim}3{\times}10^8\;ions/cm^3$, 전자온도는 14~15eV, 포화전류밀도는 $0.014{\sim}0.015mA/cm^2$, 플라즈마포텐셜은 38~39V 범위 등이었다. 현재까지 얻어진 최소 평균반사율은 14.2% 였으며 최적의 texturing패턴 플라즈마공정 조건은 이온에 의한 Si표면원자들의 스퍼터링과 화학반응에 의한 증착이 교차하는 플라즈마 에너지 및 밀도 상태인 것으로 해석된다.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.340.1-340.1
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• 2014

본 실험에서는 Ag두께 변화에 따른 투과율과 Energy bandgap의 변화를 알아보기 위해 RF Sputter장비와 Evaporator장비를 사용하여 IGZO, ZnO, AZO OMO 구조로 Low-e 코팅된 Glass를 제작하였다. $3cm{\times}3cm$의 Corning1737 유리기판에 RF Sputtering 방식으로 Oxide layer를 증착 하였고 Evaporator장비로는 Metal layer인 Ag막을 증착하였다. Oxide layer 증착 시 RF Sputter장비의 조건은 $3.0{\times}10^{-6}Torr$이하로 하였으며, 증착압력은 $6.0{\times}10^{-3}Torr$, 증착온도는 실온으로 고정하였다. Metal layer 증착 시 Evaporator장비의 조건은 $5.0{\times}10^{-6}Torr$이하, 전압은 0.3 V, Rotate 2 rpm으로 고정하였다. 실험 변수로는 Ag 두께를 5,7,9,11,13 nm로 변화를 주어 실험을 진행하였다. 투과도 측정 장비를 사용하여 각 샘플을 측정한 결과 IGZO의 경우 가시광영역의 평균 투과율이 80% 이상이며 Ag두께가 5nm일 때부터 자외선 영역의 빛을 차단하여 low-e 특성을 나타내었다. 이는 산화물인 IGZO가 결정질인 AZO, ZnO 보다 낮은 표면거칠기를 가지기 때문이다. Ag 두께에 따른 각 물질의 Optical energy bandgap 분석결과 Ag 두께가 증가할수록 IGZO는 4.65~4.5 eV, AZO는 4.6~4.4 eV, ZnO는 4.55~4.45 eV로 Energy bandgap은 감소하였다. AFM장비를 이용하여 각 샘플의 표면 Roughness 측정 결과 Ag 두께가 증가할수록 표면거칠기도 증가하는 경향을 나타내었다.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.137-137
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• 2011

펄스 직류 $BCl_3$ 플라즈마의 전기적 특성과 GaAs의 건식식각을 연구하였다. 공정변수는 펄스 직류 전압 (350~550 V), 펄스 직류 주파수 (100~250 kHz), 리버스 시간 (0.4~1.2 ${\mu}s$)이었다. 전기적 특성은 오실로스코프를 이용하여 분석하였다. 펄스 직류 전원의 경우 평균 전압이 일정하더라도 주파수가 커지거나 리버스 시간이 커지면 peak-to-peak 전압이 증가한다는 사실을 이해하였다. GaAs 식각 실험 후 샘플의 식각률, 식각 선택비, 표면 형상을 비교, 분석하였다. GaAs의 식각 결과는 식각 속도, 식각 선택비, 표면 형상, 잔류 물질 분석을 실시하엿다. 본 실험에서는 1대의 기계적 펌프만을 상ㅇ하여 진공 압력을 유지하였다. GaAs의 식각 속도는 10 sccm $BCl_3$를 사용한 경우 최대 0.4 ${\mu}m$까지 얻을 수 있었다. 감광제에 대한 최대 식각 선택비는 약 2.5 : 1이었다. BCl₃ 플라즈마의 경우 75 mTorr의 저진공 조건에서는 500 V, 250 kHz, 0.7 ${\mu}s$의 실험에서 가장 좋은 식각 특성을 얻을 수 있엇다. X-레이 광전 분석기 데이터에 의하면, 식각된 GaAs의 표면을 깨끗하였으며, 염소와 관련된 잔류 물질은 거의 발견되지 않았다.