• Water Engineering Research
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• v.3 no.3
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• pp.163-176
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• 2002

In this study, laboratory experiments have been performed to investigate characteristics of the velocity fields and turbulence for non-buoyant plane jet in the vicinity of the jet nozzle using PIV system. The experimental results show that, in the transition region, the lateral velocity profile is in good agreement with Gaussian distribution. However, the coefficient of Gaussian distribution, $\K_{u,}$, decreases with longitudinal distance in the transition region. The existing theoretical equation for the centerline velocity tends to overestimate the measured data in the transition region. A new equation for the centerline velocity derived by incorporating varying $k_{u}$ gives better agreement with the measured data than the previous equation. The results of the turbulence characteristics show peak values are concentrated on the shear layers. The Reynolds shear stress profile shows the positive peak in the upper layer and negative peak in the lower layer. The turbulent kinetic energy also provides double peaks at the shear layers. The peak of the Reynolds shear stress and the turbulent kinetic energy increases until x/B=8, and then it decreases afterwards.s.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.687-690
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• 2002

Multimode boundary-layer transition on a NACA0012 airfoil is experimentally investigated under periodically passing wakes and the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensity(Tu) at the leading edge of the airfoil is $0.5\;or\;3.5\;{\%}$. The Reynolds number ($Re_c$) based on chord length (C) of the alrfoil is $2.0{\times}10^5$, and Strouhal number ($St_c$) of the passing wake is about 0.7. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The wake-passing orientation changes pressure distribution on the airfoil in a different manner irrespective of the free-stream turbulence. Regardless of free-stream turbulence level, turbulent patches for the receding wakes propagate more rapidly than those for the approaching wake because adverse pressure gradient becomes larger. The patch under the high free-stream turbulence ($Tu=3.5{\%}$) grows more greatly in laminar-like regions compared with that under the low background turbulence ($Tu=0.5{\%}$) in laminar regions. The former, however, does not greatly change the original turbulence level in the very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually lose his identification, whereas the latter keep growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and for the receding wakes. The calmed region delays the breakdown further downstream and stabilizes more the boundary layer.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.5
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• pp.323-334
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• 2020

Since boundary layer transition has a significant impact on the aero-thermodynamic performance of hypersonic flight vehicles, capability of accurate prediction of transition location is essential for design and performance analysis. In this study, γ-Re_θt model is improved to predict transition of hypersonic boundary layers and validated. A coefficient in the production term of the intermittency transport equation that affects the transition onset location is constructed and applied as a function of Mach number, wall temperature, and freestream stagnation temperature based on the similarity numerical solution of compressible boundary layer. To take into account a Mach number dependency of transition onset momentum thickness Reynolds number and transition length, additional correlation equations are determined as function of Mach number and applied to Re_θc and F_length correlations of the baseline model. The suggested model is implemented to a commercial CFD code in consideration of practical use. Analysis of hypersonic flat plate and circular cone boundary layers is carried out by using the model for validation purpose. An improvement of prediction capability with respect to variation of Mach number and unit Reynolds number is identified from the comparison with experimental data.

• Transactions on Electrical and Electronic Materials
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• v.3 no.2
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• pp.32-37
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• 2002

This paper gives the basic mechanical properties of indium-tin-oxide (ITO) films on polymer substrates which are exposed to externally and thermally induced bending force. By using modified Storney formula including triple layer structure and bulge test measuring the conductive changes of patterned ITO islands as a function of bending curvature, the mechanical stability of ITO films on polymer substrates was intensively investigated. The numerical analyses and experimental results show thermally and externally induced mechanical stresses in the films are responsible for the difference of thermal expansion between the ITO film and the substrate, and leer substrate material and its thickness, respectively. Therefore, a gradually ramped heating process and an organic buffer layer were employed to improve the mechanical stability, and then, the effects of the buffer layer were also quantified in terms of conductivity-strain variations. As a result, it is uncovered that a buffer layer is also a critical factor determining the magnitude of mechanical stress and the layer with the Young's modulus lower than a specific value can contribute to relieving the mechanical stress of the films.

• Journal of Ship and Ocean Technology
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• v.8 no.4
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• pp.10-16
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• 2004

A response of a swept wing boundary layer to a single free-stream stationary axial vortex of a limited spanwise extent is considered as an example of typical problems that one can find in laminar-turbulent transition research and control. The response is dominated by streamwise velocity perturbations that grow quasi-exponentially downstream. It is shown that the formation of the boundary layer disturbance occurs for the most part close to the leading edge. The disturbance represents itself a wave packet consisted of the waves with characteristics specific for cross-flow instability. However, an admixture of growing disturbances whose origin can be attributed to transient effects and to a distributed receptivity mechanism is also identified.

• Journal of Power System Engineering
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• v.17 no.5
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• pp.44-51
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• 2013

This paper proposes and analyzes recycling of optical phonons emitted by nonradiative decay, which is a major thermal management concern for high-power light emitting diodes (LED), by introducing an integrated, heterogeneous barrier cooling layer. The cooling is proportional to the number of phonons absorbed per electron overcoming the potential barrier, while the multi-phonon absorption rate is inversely proportional to this number. We address the theoretical treatment of photon-electron-phonon interaction/transport kinetics for optimal number of phonons (i.e., barrier height). We consider a GaN/InGaN LED with a metal/AlGaAs/GaAs/metal potential barrier and discuss the energy conversion rates. We find that significant amount of heat can be recycled by the barrier transition cooling layer.

• Current Applied Physics
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• v.18 no.11
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• pp.1393-1398
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• 2018

We have investigated Bernal-stacked tetralayer graphene as a function of interlayer distance and perpendicular electric field by using density functional theory calculations. The low-energy band structure was found to be very sensitive to the interlayer distance, undergoing a metal-insulator transition. It can be attributed to the nearest-layer coupling that is more sensitive to the interlayer distance than are the next-nearest-layer couplings. Under a perpendicular electric field above a critical field, six electric-field-induced Dirac cones with mass gaps predicted in tight-binding models were confirmed, however, our density functional theory calculations demonstrate a phase transition to a quantum valley Hall insulator, contrasting to the tight-binding model prediction of an ordinary insulator.

• Journal of Sensor Science and Technology
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• v.27 no.5
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• pp.306-310
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• 2018

In order to develop a highly sensitive infrared sensor, it is necessary to develop techniques for decreasing the rate of heat absorption and the transition of the absorption wavelength to a longer wavelength, both of which can be induced by decreasing the pixel size of the bolometer. Therefore, in this study, $1{\mu}m$ hole-arrays with a subwavelength smaller than the incident infrared wavelength were formed on the amorphous silicon-based microbolometer pixels in the absorber, which consisted of a TiN absorption layer, an a-Si resistance layer and a SiNx membrane support layer. We demonstrated that it is possible to reduce the thermal time constant by 16% relative to the hole-patternless bolometer, and that it is possible to shift the absorption peak to a shorter wavelength as well as increase absorption in the $4-8{\mu}m$ band to compensate for the infrared long-wavelength transition. These results demonstrate the potential for a new approach to improve the performance of high-resolution microbolometers.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1234-1243
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• 1990

Double coaxial are jets(annular and coaxial air jets) between which propane gas is fed was selected to study the structure of diffusion flames in turbulent shear flow. Schlieren and direct photographs are taken to visualize the flame structure. Mean and fluctuating temperatures and ion currents were measured to investigate the macroscopic and the instantaneous flame structure. The objective of this study is to understand the interaction between combustion and mixing process especially in the transition region of turbulent shear flow. The investigation reported in this paper focuses on the macroscopic and the instantaneous structures of three flames obtained. The increased mixing effect resulting from increase of Reynolds number of central air jet makes the flame bluish and short. When the velocity of surrounding air stream is higher than that of central air jet, the instantaneous flame structure is composed of coherent structure. It is considered that the flame structure of transitional region of mixing layer depends on the structure of mixing layer of non-reacting conditions.

• Applied Microscopy
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• v.45 no.3
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• pp.119-125
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• 2015

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention due to their unique and exotic properties attributed to their low dimensionality. In particular, semiconducting 2D TMDCs such as $MoS_2$, $WS_2$, $MoSe_2$, and $WSe_2$ have been demonstrated to be feasible for various advanced electronic and optical applications. In these regards, process to synthesize high quality 2D TMDCs layers with high reliability, wafer-scale uniformity, controllable layer number and excellent electronic properties is essential in order to use 2D TMDCs in practical applications. Vapor deposition techniques, such as physical vapor deposition, chemical vapor deposition and atomic layer deposition, could be promising processes to produce high quality 2D TMDCs due to high purity, thickness controllability and thickness uniformity. In this article, we briefly review recent research trend on vapor deposition techniques to synthesize 2D TMDCs.