• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.1
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• pp.41-51
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• 2004

A direct numerical simulation is performed to analyze the effects of a localized time-periodic blowing on a turbulent boundary layer flow at R $e_{+}$=300. Main emphasis is placed on the blowing frequency effect on near-wall turbulent flow structures at downstream. Wall-normal velocity on a spanwise slot is varied periodically at different frequencies (0.004$\leq$ $f^{＋}$$\leq$0.080). The amplitude of periodic blowing is $A^{＋}$=0.5 in wall nit, which corresponds to the value of $v_{rms}$ at $y^{＋}$=15 without blowing. The frequency responses are scrutinized by examining the phase or time-averaged turbulent statistics. The optimal frequency ( $f^{＋}$=0.03) is observed, where maximum increase in Reynolds shear stress, streamwise vorticity fluctuations and energy redistribution occurs. The phase-averaged stretching and tilting term are investigated to analyze the increase of streamwise vorticity fluctuations which are closely related to turbulent coherent structures. It is found that the difference between PB and SB at a high blowing frequencies is negligible.e.e.

• Structural Engineering and Mechanics
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• v.72 no.5
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• pp.541-555
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• 2019

Wind induced dynamic responses on hyperbolic cooling tower (HCT) shells are complicated functions of structure and wind properties, such as the fundamental frequency f_min, damping ratio ζ, wind velocity V, correlationship in meridian direction and so on, but comprehensions on the sensitivities of the dynamic responses to these four factors are still limited and disagree from each other. Following the dynamic calculation in time domain, features of dynamic effects were elaborated, focusing on the background and resonant components σ_B and σ_R, and their contributions to the total rms value σ_T. The σ_R is always less than σ_B when only the maximum σ_T along latitude is concerned and the contribution of σ_R to σ_T varies with responses and locations, but the σ_R couldn't be neglected for structural design. Then, parameters of the above four factors were artificially adjusted respectively and their influences on the gust responses were illustrated. The relationships of σ_R and the former three factors were expressed by fitted equations which shows certain differences from the existing equations. Moreover, a new strategy for wind tunnel tests aiming at surface pressures and the following dynamic calculations, which demands less experiment equipment, was proposed according to the influence from meridian correlationship.

• Korean Journal of Applied Biomechanics
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• v.26 no.1
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• pp.115-126
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• 2016

Objective: The purpose of this study was to investigate plantar foot pressure and static balance according to the type of insole in the elderly. Methods: Thirteen elderly (mean age: $67.08{\pm}2.25years$, mean height: $159.63{\pm}9.64cm$, mean body weight: $61.48{\pm}9.06kg$) who had no previous injury experience in the lower limbs and a normal gait pattern participated in this study. Three models of insoles of the normal, 3D, and triangle types were selected for the test. The Pedar-X system and Pedar-X insoles, 3.3 km/h of walking speed, and a compilation of 20 steps walking stages were used to analyze foot-pressure distribution. Static balance test was conducted using Gaitview AFA-50, and balance (opening eyes, closing eyes) was inspected for 20 s. One-way ANOVA was conducted to test the significance of the results with the three insoles. p-value of less than .05 was considered statistically significant. Results: The mean foot pressure under the forefoot regions was the lowest with the 3D insole during treadmill walking (p<.05). The mean value under the midfoot was the highest with the 3D insole (left: p<.05, right: p<.01). The mean value under the rearfoot was the lowest with the 3D insole (p<.001). The maximum foot pressure value under the foot regions was the lowest on both sides of the forefoot with the 3D insole. A statistically significant difference was seen only in the left foot (p<.01). The maximum value under the midfoot was the highest with the 3D insole (p<.001). No statistically significant difference was detected on the values under the rearfoot. In the case of vertical ground reaction force (GRF), statistically significant difference was seen only in the left side rearfoot (p<.01). However, static balance values (ENV, REC, RMS, Total Length, Sway velocity, and Length/ENV) did not show significant differences by the type of insole. Conclusion: These results show that functional insoles can decrease plantar pressure and GRF under the forefoot and rearfoot. Moreover, functional insoles can dislodge the overload of the rearfoot and forefoot to the midfoot. However, functional insoles do not affect the static balance in the elderly.

• Journal of Korea Water Resources Association
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• v.42 no.8
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• pp.619-629
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• 2009

The objective of this study is to estimate bed-resistance in gravel-bed rivers using the equivalent roughness height($k_s$). We calculated the friction factor(f) with the measured data from 8 domestic gravel-bed rivers and investigated the size distributions of the bed materials. The averaged $k_s$ in each cross-section, which is determined under the hypothesis that the vertical velocity distribution follows the logarithmic law, is compared with the reach $k_s$ which is calculated with the cumulative grain diameter distribution curve of bed materials. Moreover, the applicability of existing formulae, such as Strickler type equations, is examined by comparing with Manning's n value converted from the $k_s$. According to the results, the reach $k_s$ proves to be a good indicator of representative characteristic of bed materials in a reach, and the Manning's n based on the reach $k_s$ is appropriate for practical estimation of the bed-resistance, for RMS errors between calculated and measured Manning's n is less than 0.003. The correlation between the $k_s$ and specified bed-material size($D_i$) is very low, so it is difficult to select a proper one among the existing empirical equations.

• Clean Technology
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• v.25 no.2
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• pp.147-152
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• 2019

Electrostatic precipitator that shows a good performance for the removal of particulate matter is important for controlling emissions from industrial facilities and power plants. The efficiency of the electrostatic precipitator on the removal of particulate matter is highly affected by the flow pattern inside the electrostatic precipitator. A number of studies have been conducted to obtain uniform flow distribution inside electrostatic precipitators. An electrostatic precipitator (ESP) with a length of 3.5 m and a height of 0.875 m was designed and installed in this study. The ESP included an inlet duct, diffuser, body, and contractor. Three perforated plates were installed in the diffuser of the ESP. Five pitot tubes were installed vertically and used to measure flow distribution in the cross section of the ESP body. Root mean square deviation value (RMS%) was used to examine the flow distribution inside the ESP when the perforated plates were installed in the diffuser. Flow distribution was also investigated in relation to the porosity of the perforated plate. The results showed that the perforated plates improved greatly the flow distribution inside the electrostatic precipitator. In addition, the most uniform flow distribution was found with 40%, 50%, and 50% porous perforated plates located from the inlet of the diffuser.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.113.1-113.1
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• 2012

The characteristics of Doppler shifts in a quiet region of the Sun are investigated by comparing between the $H{\alpha}$ line and the Caii infrared line at 854.2 nm. A small area of $16^{\prime\prime}{\times}40^{\prime\prime}$ was observed for about half an hour with the Fast Imaging Solar Spectrograph (FISS) of the 1.6 meter New Solar Telescope (NST) at Big Bear Solar Observatory. The observed area contains a network region and an internetwork region, and identified in the network region are $H{\alpha}$ fibrils, Caii fibrils and bright points. We infer the Doppler velocity from each line profile at a point with the lambdameter method as a function of half wavelength separation ${\Delta}{\lambda}$. It is confirmed that the bisector of the spatially-averaged Caii line profile has an inverse C-shape of with a significant peak redshift of +1.8 km/s. In contrast, the bisector of the spatially-averaged $H{\alpha}$ line profile has a different shape; it is almost vertically straight or, if not, has a C-shape with a small peak blueshift of -0.5 km/s. In both the lines, the bisectors of bright network points are much different from those of other features in that they are significantly redshifted not only at the line centers, but also at the wings. We also find that the spatio-temporal fluctuation of Doppler shift inferred from the Caii line is correlated with those of the $H{\alpha}$ line. The strongest correlation occurs in the internework region, and when the inner wings rather than the line centers are used to determine Doppler shift. In this region, the RMS value of Doppler shift fluctuation is the largest at the line center, and monotonically decreases with ${\Delta}{\lambda}$. We discuss the physical implications of our results on the formation of the $H{\alpha}$ line and Caii 854.2 nm line in the quiet region chromosphere.