• Wind and Structures
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• v.21 no.3
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• pp.289-309
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• 2015

Vertical profiles of mean and fluctuating wind velocities over water waves were studied, by performing Large-Eddy Simulations (LES) on a fully developed turbulent boundary layer over simplified water waves. The water waves were simplified to two-dimensional, periodic and non-evolving. Different wave steepness defined by $a/{\lambda}$ (a : wave amplitude; ${\lambda}$ : wavelength) and wave age defined by $c/U_b$ (c: phase velocity of the wave; $U_b$ : bulk velocity of the air) were considered, in order to elaborate the characteristics of mean and fluctuating wind profiles. Results shows that, compared to a static wave, a moving wave plays a lesser aerodynamic role as roughness as it moves downstream slower or a little faster than air, and plays more aerodynamic roles when it moves downstream much faster than air or moves in the opposite direction to air. The changes of gradient height, power law index, roughness length and friction velocity with wave age and wave amplitude are presented, which shed light on the wind characteristics over real sea surfaces for wind engineering applications.

• Transactions of the Korean Society of Automotive Engineers
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• v.9 no.1
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• pp.1-7
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• 2001

Oxygen sensor taken from the aftertreatment systems of 4 vehicles which had been aged in domestic field examined for aging effects on emissions using a idel engine bench. also the artificial sensor signal generated by function generator was supplied to ECU, instead of oxygen sensor to simulate aging effects of oxygen sensor. This study shows that reduction of amplitude, deformation and shift of mean value of aged sensors seriously affect on the engine out emissions and the performance of TWC.

• Journal of the Korean Society for Precision Engineering
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• v.15 no.10
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• pp.113-119
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• 1998

Ship structures and aircraft structures are consisted of thin sheet alloy, so it is very important to understand the characteristics of fatigue crack propagation of that material and to establish the data base. The data for fatigue crack propagation behavior scatter very much even under identical experimental conditions with constant loading. The behavior of fatigue crack propagation under regular and irregular cyclic loadings is known to be highly affected by complicated factors such as plastic zone developed at the vicinity of crack tip and reduction of cross sectional area. In this paper, the controlled stress amplitude and overload fatigue crack propagation tests have been conducted to investigate the effect of varying factors such as plastic zone size near the crack tip and area reduction factor (AF) on the fatigue crack propagation behavior A better simulation of fatigue crack propagation behavior is found to be obtainable by using Wheeler and Willenborg models with AF effect.

• Journal of Power Electronics
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• v.5 no.4
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• pp.312-318
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• 2005

This paper presents a compensator for reduction of the reactive lateral current in multiple autonomously controlled uninterruptible power supplies (UPS) connected in parallel. This compensator acts directly on the control equation for voltage amplitude and it provides an improved current distribution especially in the case of parallel connection of UPSs with different output power ratings. Observations show that the original control equation for output voltage amplitude is efficient for voltage regulation but it causes great variation of voltage levels. A compensator with the same structure is added to counterbalance the variation caused by the original control equation. Simulations show promising results with the employment of the proposed compensator. Our simulations are confirmed by experimental results using three UPSs with different output ratings and voltage limiters ($1\%$) connected in parallel under various conditions.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.195-198
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• 2006

The possibility of skin friction reduction in laminar channel flow is investigated when the flow is subjected to stationary distributed surface blowing and suction. Blowing and suction provided at the channel walls is steady in time but varies as a sine function along the streamwise direction. The skin friction changes depending on the wavelength and amplitude of the actuation. Especially, the skin friction is reduced below that of fully developed laminar flow as the wavelength decreases and amplitude increases. The optimal wavelength of producing minimum skin friction is $\pi/2{\delta}$, where $\delta$ is the channel half-height It is observed that the distributed blowing and suction induces strong negative Reynolds shear stress in the near-wall region at the end of the suction part.

• Korean Journal of Biological Psychiatry
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• v.16 no.3
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• pp.181-189
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• 2009

Objectives : N100 amplitude slope(the intensity dependence of the cortical auditory evoked potentials) is widely considered as an indirect indicator of central serotonergic neurotransmission. However, there are only a few studies about N100 amplitude slopes of major psychiatric disorders. In this study, we examined N100 amplitude slope differences among major depressive disorder(MDD), bipolar disorder(BD), schizophrenia (SCZ) and normal controls(NC). Methods : We measured the N100 amplitude slopes of 35 patients with MDD, 33 patients with BD, 27 patients with SCZ and 35 NC subjects. Amplitude differences from N1 to P2 at the five different sound intensities(55, 65, 75, 85 and 95dB) were examined at Cz electrode. The N100 amplitude slope was calculated as the linear regression of five N1/P2 peak-to-peak amplitudes across stimulus intensities. Results : BD patients showed significantly reduced N100 amplitude slope compared with NC(0.54${\pm}$0.70 vs. 0.96${\pm}$0.72, p=0.035). N100 amplitude slope of SCZ patients was significantly reduced compared with NC(0.50${\pm}$0.47 vs. 0.96${\pm}$0.72, p=0.027). N100 amplitude slope of BD patients was significantly lower than that of MDD patients(0.54${\pm}$0.70 vs. 0.94${\pm}$0.60, p=0.046). SCZ patients also showed significant reduction of N100 amplitude slope compared with MDD patients(0.50${\pm}$0.47 vs. 0.94${\pm}$0.60, p=0.036). There was no significant difference of N100 amplitude slope between MDD patients and NC(0.94${\pm}$0.60 vs. 0.96${\pm}$0.72, p=1.000). Conclusion : Interestingly, the N100 amplitude slopes of BD and SCZ were reduced compared to NC and MDD patients. Our results suggest the predictive use of N100 amplitude slope in making differential diagnoses of major psychiatric disorders. Clinical implications of N100 amplitude slope in major psychiatric disorders were discussed.

• Proceedings of the Korea Electromagnetic Engineering Society Conference
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• 2003.11a
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• pp.8-12
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• 2003

To protect digital communications and broadcasting that are expected to be increasingly used at frequencies above 1GHz, there is a need to determine permissible disturbance limits for frequency bands above 1GHz according to indicates that better correlates with interference level in digital systems. The quality of digital communications is expressed in terms of bit error rate(BER) or throughput. In this regard, it has been reported that degradation of BER or throughput can be estimated from the amplitude probability distribution (APD) of disturbance amplitude. Therefore, if the APD can simply be measured with a high accuracy, APD will be an optimal index for the assessment of interference in digital communications. In this paper, we show that a measurement method using statistic model of APD has compatibility about performance reduction estimation of WLAN. And through approach of this inspection, we propose numeral models about interference effects of peak value and time rate of spurious.

• Geomechanics and Engineering
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• v.13 no.2
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• pp.269-284
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• 2017

A new technique is proposed to obtain more effective screening efficiency against the ground vibration using intermittent geofoam (IF) in-filled trench. The numerical analysis is performed by employing two-dimensional finite element method under dynamic condition. Vertically oscillated strip foundation is considered as the vibration source. In presence of the ground vibration, the vertical displacements at different locations (pick-up points) along the ground surface are captured to determine the amplitude reduction factor (ARF), which helps to assess the efficiency of the vibration screening technique. The efficiency of IF over continuous geofoam (CF) in-filled vibration barriers is assessed by varying the geofoam density, the location of trench and the frequency of excitation. The results from this study indicate that a significant reduction in ARF can be achieved by using intermittent geofoam as compared to continuous geofoam. Further, it is noticed that the efficiency of IF increases with an increase in the frequency of the vibrating source. These encouraging results put forward the potential of utilising intermittent geofoam as a vibration screening material.

• Tribology and Lubricants
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• v.38 no.5
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• pp.205-212
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• 2022

This paper presents experimental measurements of the structural characteristics of a two-pad beam-type gas foil journal bearing and its rotordynamic performance for a high-speed motor-driven turbocompressor. The test bearing had two top foils and two beam foils, each with an arc length of ~180°. Each beam foil was etched to obtain 40 beams with six geometries of different lengths and widths. The insertion of beam foils into the bearing housing produces equivalent beam heights. The structural tests of the bearing with a non-rotating journal revealed a smaller bearing clearance and larger structural stiffness for the load-on-pad configuration than for the load-between-pads configuration. Rotordynamic performance measurements during driving tests up to 100 krpm demonstrated synchronous vibrations and subsynchronous vibrations with large amplitudes. The test was repeated after inserting the shim between the top foil and beam foil to reduce the bearing radial clearance. The reduced bearing clearance resulted in a reduction in the peak amplitude of the synchronous vibrations and an increase in the speed at which the peak amplitude occurred. In addition, the onset speed and amplitude of the subsynchronous vibrations were dramatically increased and diminished, respectively. The rotor coast-down tests at 100 krpm show that the reduction in the bearing clearance extends the time to rotor stop, thus implying an improvement in hydrodynamic pressure generation and a reduction in bearing frictional torque.

• Wind and Structures
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• v.22 no.2
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• pp.185-209
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• 2016

Across-wind aerodynamic damping ratios are identified from the wind-induced acceleration responses of 15 aeroelastic models of rectangular super-high-rise buildings in various simulated wind conditions by using the random decrement technique. The influences of amplitude-dependent structural damping ratio and natural frequency on the estimation of the aerodynamic damping ratio are discussed and the identifying method for aerodynamic damping is improved at first. Based on these works, effects of turbulence intensity $I_u$, aspect ratio H/B, and side ratio B/D on the across-wind aerodynamic damping ratio are investigated. The results indicate that turbulence intensity and side ratio are the most important factors that affect across-wind aerodynamic damping ratio, whereas aspect ratio indirectly affects the aerodynamic damping ratio by changing the response amplitude. Furthermore, empirical aerodynamic damping functions are proposed to estimate aerodynamic damping ratios at low and high reduced speeds for rectangular super-high-rise buildings with an aspect ratio in the range of 5 to 10, a side ratio of 1/3 to 3, and turbulence intensity varying from 1.7% to 25%.