• International Journal of Concrete Structures and Materials
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• v.10 no.4
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• pp.539-553
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• 2016

Polymer concrete (PC) has been favoured over Portland cement concrete when low permeability, high adhesion, and/or high durability against aggressive environments are required. In this research, a new class of PC incorporating Multi-Walled Carbon Nanotubes (MWCNTs) is introduced. Four PC mixes with different MWCNTs contents were examined. MWCNTs were carefully dispersed in epoxy resin and then mixed with the hardener and aggregate to produce PC. The impact strength of the new PC was investigated by performing low-velocity impact tests. Other mechanical properties of the new PC including compressive, flexural, and shear strengths were also characterized. Moreover, microstructural characterization using scanning electron microscope and Fourier transform infrared spectroscopy of PC incorporating MWCNTs was performed. Impact test results showed that energy absorption of PC with 1.0 wt% MWCNTs by weight of epoxy resin was significantly improved by 36 % compared with conventional PC. Microstructural analysis demonstrated evidence that MWCNTs significantly altered the chemical structure of epoxy matrix. The changes in the microstructure lead to improvements in the impact resistance of PC, which would benefit the design of various PC structural elements.

• International Journal of Fluid Machinery and Systems
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• v.4 no.3
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• pp.341-348
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• 2011

The study of bacterial flagellar swimming motion remains an interesting and challenging research subject in the fields of hydrodynamics and bio-locomotion. This swimming motion is characterized by very low Reynolds numbers, which is unique and time reversible. In particular, the effect of rotation of helical flagella of bacterium on swimming motion requires detailed multi-disciplinary analysis. Clear understanding of such swimming motion will not only be beneficial for biologists but also to engineers interested in developing nanorobots mimicking bacterial swimming. In this paper, computational fluid dynamics (CFD) simulation of a three dimensional single flagellated bacteria has been developed and the fluid flow around the flagellum is investigated. CFD-based modeling studies were conducted to find the variables that affect the forward thrust experienced by the swimming bacterium. It is found that the propulsive force increases with increase in rotational velocity of flagellum and viscosity of surrounding fluid. It is also deduced from the study that the forward force depends on the geometry of helical flagella (directly proportional to square of the helical radius and inversely proportional to pitch).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.1
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• pp.32-38
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• 2003

Staged combustion, such as air- and fuel-staging, is a relatively well-known technique fur reduction of NOx emission and used in combination with other techniques nowadays. However, the design variables are still selected depending upon operating conditions. There are many variables tested to investigate the NOx emission characteristics fur changing of fuel or air velocity, swirl intensity, and staging ratio of air and fuel in multi-staged burner. In air-staging case, the fuel-rich condition of the primary combustion zone is very helpful to reduce NOx emission and its range is known to be restricted by the increase of carbon monoxide. However, in many cases carbon monoxide level is not too high to be restricted operating condition. So we tried to expand the equivalence ratio range to the richer condition in the primary combustion zone and certificate the function of each burner component and its contribution to the overall NOx production.

• Proceedings of the Korea Water Resources Association Conference
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• 2009.05a
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• pp.1437-1441
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• 2009

기존의 ADCP는 1m 이하의 저수심 지점에서 측정이 불가능하였으며, 유속계 측정장비는 저유속 지점에서 측정이 불가능한 경우가 많아 유량측정에 어려움을 겪는 경우가 많다. 최근 RDInstruments에서 제작한 StreamPro는 1m 이하의 수심, 0.1m/s 이하의 유속에서 측정이 가능하기 때문에 이를 이용하면 저수심, 저유속 지점에서 유량측정이 가능하다. ADCP를 이용한 측정의 경우 보다 정확한 측정성과를 얻기 위하여 유속의 50% 속도로 이동하며 측정하여야 하며, 4회 측정유량값의 상대오차가 5% 이내일 때 그 평균값을 측정치로 한다. 그러나 0.1m/s 이하의 유속이 나타나는 지점은 0.05m/s 이하의 속도로 이동하며 측정을 실시하여야 하며 이 경우 측정시간이 너무 길어지는 문제가 있다. 따라서 본 연구에서는 StreamPro를 이용한 유량측정의 적용성 평가와 함께 적절한 신뢰도를 얻을 수 있는 실용적인 측정회수 분석을 실시하였다. StreamPro를 이용한 측정성과의 적용성을 평가하기 위하여 StreamPro와 동시에 Price 유속계 측정을 실시하여 이를 비교하였다. 실용적인 적정 측정회수 분석은 측정유량의 상대오차를 Student-t분포에 적용하여 불확실도값을 기준으로 분석하였다. StreamPro와 Price 유속계 측정성과를 비교한 결과, 평균 상대오차가 약 3.5%로 적절한 값을 나타냈으며, 저수심, 저유속 지점에서의 실용적인 측정회수는 약 11회로 나타났다.

• 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.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.8
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• pp.1532-1537
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• 2009

Nowadays the diversity and large-capacity of electric appliances are strong effect on electrical fires augment in an alarming way. But, as the inactive response characteristics of the existing RCD (Residual Current protective Device) used on low voltage power distribution lines, so control of overload and electric short circuit faults, major causes of electrical fires, are not enough. Therefore this paper is confirmed the unreliability of the existing RCD by electrical faults simulation and is proposed a auxiliary trip device of RCD by using a high precision current sensor (namely, reed switch) for the prevention of electrical disasters in low voltage power distribution lines caused by overload or electric short circuit faults. The sensitive reed switch in the proposed ATD (auxiliary trip device) exactly detects the increased magnetic flux with the overload or the short current caused by a number of electrical faults, and then rapidly cuts off the existing RCD. The proposed auxiliary trip device of RCD is confirmed the excellent characteristics in response velocity and accuracy in comparison with the conventional circuit breaker through various operation performance analysis. The proposed ATD can also prevent electrical disaster, like as electrical fires, which resulted from the malfunction and inactive response characteristics of the existing RCD.

• The Proceedings of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.6 no.5
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• pp.27-37
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• 1992

We studied the dielectric and dynamic mechanical losses according to the quenching condition in low density polyethylene being used to power cables. According to severe quenching condition, characteristics of the temperature in internal friction los peak have decreased the magnitude of loss peak as amorphous region lengthen. From now on, the frequency dependent characteristics of dielectric loss have investigated at room temperature, and the dielectric loss peak due to interface polarization, between crystal and amorphous region, occurs about 30[Hz], and that, the peak due to orientation polarization in correspondence to the loss peak in internal friction has observed at about 3 [MHz]. As quenching velocity increased, the effect on quenching condition about the dielectric loss has decreased the magnitude of the loss peak. Thus, estimation has been carried out on the activation energies nd the degree of crystallinity by means of X-ray diffraction are obtained as follows: room quenching : 26.4 [kal/mole] and 54.73 [%], ice quenching : 25.6 [kcal/mole] and 48.47 [%], liquid nitrogen quenching specimens : 22.56 [kcal/mole] and 40.95 [%].

• Journal of the Korean Society of Combustion
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• v.18 no.4
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• pp.53-58
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• 2013

Lots of Coal power plants (about 30) using bituminous coals are being run in Korea. The use of high volatile low grade sub-bituminous coal is increasingly extended because of imbalance between the worldwide coal supply and demand. Mill-fire has been an important issue since the use of such sub-bituminous coal. In existing coal plants of Korea, shutdown of coal and air supplies could be only a way, and an alternative has not been found in suppressing the mill fire. The inside fowfield in the mills has a highly fuel-rich, low temperature, and high velocity and non-reactive such that it could be a nonreactive system essentially. Nevertheless, occasional fire-occurrence could be attributed to the existence of an ignition source. However it has not been so far investigated in detail. The current work has a focus on suppressing the mile fire via some parametric experimental study such as effects of temperature, residence time, ignition source, and inert gas mixing. The results show that an small amount of $CO_2$- or $N_2$-mixing with air is very effective in suppressing fire formation even at high temperatures or flying sparks. The results suggest that exhaust gas recirculation into the mill should be an alternative to suppress mill fire.

• Transactions of the Korean Society of Automotive Engineers
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• v.17 no.4
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• pp.118-125
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• 2009

Bumper is designed to protect the automotive frame without damage at low velocity. Expanded polypropylene (EPP) foam is used in the bumper as an energy absorbing material. In order to exactly predict the energy absorbing performance of the foam material under impact loading condition, it is important to use high strain rate material properties. In this study, a new apparatus for dynamic compression tests was developed to investigate the high strain rate behavior of EPP foams. Three kinds of EPP foams which have different expansion ratios were tested to investigate the quasi-static and dynamic compression behavior. Quasi-static compressions were performed at low strain rates of 0.001/s, 0.1/s and 1/s. The dynamic compressions were carried out at high strain rates of 50/s and 100/s with the developed apparatus. It was observed that the EPP foam has significant strain rate effect as compared to quasi-static behavior.

• International Journal of Aeronautical and Space Sciences
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• v.18 no.1
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• pp.48-55
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• 2017

Aeroacoustic computation of a fully-developed turbulent pipe flow at $Re_{\tau}=175$ and M = 0.1 is conducted by LES/LPCE hybrid method. The generation and propagation of acoustic waves are computed by solving the linearized perturbed compressible equations (LPCE), with acoustic source DP(x,t)/Dt attained by the incompressible large eddy simulation (LES). The computed acoustic power spectral density is closely compared with the wall shear-stress dipole source of a turbulent channel flow at $Re_{\tau}=175$. A constant decaying rate of the acoustic power spectrum, $f^{-8/5}$ is found to be related to the turbulent bursts of the correlated longitudinal structures such as hairpin vortex and their merged structures (or hairpin packets). The power spectra of the streamwise velocity fluctuations across the turbulent boundary layer indicate that the most intensive noise at ${\omega}^+$ < 0.1 is produced in the buffer layer with fluctuations of the longitudinal structures ($k_zR$ < 1.5).