• Clinical and Experimental Pediatrics
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• v.50 no.8
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• pp.711-717
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• 2007

Sleep is a vital, highly organized process regulated by complex systems of neuronal networks and neurotransmitters. Normal sleep comprises non-rapid eye movement (NREM) and REM periods that alternate through the night. Sleep usually begins in NREM and progresses through deeper NREM stages (2, 3, and 4 stages), but newborns enter REM sleep (active sleep) first before NREM (quiet sleep). A period of NREM and REM sleep cycle is approximately 90 minutes, but newborn have a shorter sleep cycle (50 minutes). As children mature, sleep changes as an adult pattern: shorter sleep duration, longer sleep cycles and less daytime sleep. REM sleep is approximately 50% of total sleep in newborn and dramatically decreases over the first 2 years into adulthood (20% to 25%). An initial predominant of slow wave sleep (stage 3 and 4) that peaks in early childhood, drops off abruptly after adolescence by 40% from preteen years, and then declines over the life span. The hypothalamus is recognized as a key area of brain involved in regulation of sleep and wakefulness. The basic function of sleep largely remains elusive, but it is clear that sleep plays an important role in the regulation of CNS and body physiologic processes. Understanding of the architecture of sleep and basic mechanisms that regulate sleep and wake cycle are essential to evaluate normal or abnormal development of sleep pattern changes with age. Reduction or disruption of sleep can have a significant impact on daytime functioning and development, including learning, growth, behavior, and emotional regulation.

• Wind and Structures
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• v.26 no.6
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• pp.397-413
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• 2018

Excitation mechanism of interference effect between two tall buildings is investigated with wind tunnel experiments. Synchronized building surface pressure and flow field measurements by particle image velocimetry (PIV) are conducted to explore the relationship between the disturbed wind flow field and the consequent wind load modification for twin buildings in tandem. This reveals evident excitation mechanisms for the fluctuating across-wind loads on the buildings. For small distance (X/D < 3) between two buildings, the disturbed flow pattern of impaired vortex shedding is observed and the fluctuating across-wind load on the downstream building decreases. For larger distance ($X/D{\geq}3$), strong correlation between the across-wind load of the downstream building and the oscillation of the wake of the upstream building is found. By further analysis with conditional sampling and phase-averaged techniques, the coherent flow structures in the building gap are clearly observed and the wake oscillation of the upstream building is confirmed to be the reason of the magnified across-wind force on the downstream building. For efficient PIV measurement, the experiments use a square-section high-rise building model with geometry scale smaller than the usual value. Interference factors for all three components of wind loads on the building models being surrounded by another identical building with various configurations are measured and compared with those from previous studies made at large geometry scale. The results support that for interference effect between buildings with sharp corners, the length scale effect plays a minor role provided that the minimum Reynolds number requirement is met.

• Journal of the Society of Naval Architects of Korea
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• v.49 no.5
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• pp.440-446
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• 2012

A twisted cylinder has been newly designed by rotating the elliptic cross section along the spanwise direction in order to reduce the drag and vorticies in wake region. The flow around the twisted cylinder at a subcritical Reynolds number (Re) of 3000 is investigated to analyze the effect of twisted spiral pattern on the drag reduction and vortex suppression using large eddy simulation (LES). The instantaneous wake structures of the twisted cylinder are compared with those of a circular and a wavy cylinder at the same Re. The shear layer of the twisted cylinder covering the recirculation region is more elongated than that of the circular and the wavy cylinder. Successively, vortex shedding of the twisted cylinder is considerably suppressed, compared with those of the circular and the wavy cylinder. Consequently, the mean drag coefficient and the fluctuating lift of the twisted cylinder are less than those of the circular and the wavy cylinder.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.8
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• pp.729-736
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• 2009

In order to understand flow characteristics and flight mechanism of hummingbird's flapping flight, two-dimensional numerical analysis is carried out on the flapping motion of hummingbird, Selasphorus rufus. Hummingbird's flapping wing motion is realistically modeled from wind tunnel experimental data to perform numerical analysis. Numerical simulation shows that, as freestream velocity changes, wing trajectory is also adjusted and it substantially affects lift and thrust generation mechanism. According to this tendency, flight domain is separated as "low speed" and "high speed" regime, and each flight domain is studied for physical understanding. As a result, the lift generation during downstroke can be explained by the well-known effects, such as leading edge vortex effect, delayed stall, wake capture and so on. In addition, the lift generation during upstroke, the unique character of hummingbird, is also examined by detailed flow analysis. The thrust generation mechanism is investigated by examining the hummingbird's wing bone structure, vortex generation pattern and the resulting pressure gradient.

• Wind and Structures
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• v.28 no.5
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• pp.285-298
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• 2019

For super long-span bridges, the aerodynamic forces induced by the flow passing the box girder should be considered carefully. And the Reynolds number sensitively of aerodynamic characteristics is one of considerable issue. In the study, a numerical study on the Reynolds number sensitivity of aerodynamic characteristic (flow pattern, pressure distribution and aerodynamic forces) of a twin-box girder were carried out using large eddy simulation (LES) with the dynamic Smagorinsky-Lilly subgrid model. The results show that the aerodynamic characteristics have strong correlation with the Reynolds number. At the leading edge, the flow experiences attachment, departure, and reattachment stages accompanying by the laminar transition into turbulence, causing pressure plateaus to form on the surface, and the pressure plateaus gradually shrinks. Around the gap, attributing that the flow experiences stages of laminar cavity flow, the wake with alternate shedding vortices, and turbulent cavity flow in sequence with an increase in the Reynolds number, the pressures around the gap vary greatly with the Reynold number. At the trailing edge, the pressure gradually recovers as the flow transits to turbulence (the flow undergoes wake instability, shear layer transition-reattachment station), In addition, at relative high Reynolds numbers, the drag force almost does not change, however, the lift force coefficient gradually decreases with an increase in Reynolds number.

• Journal of Information Processing Systems
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• v.9 no.1
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• pp.41-52
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• 2013

One of the obstacles preventing the Zigbee protocol from being widely used is the excessive power consumption of Zigbee devices in low bandwidth and low power requirement applications. This paper proposes a protocol that resolves the power efficiency problem. The proposed protocol reduces the power consumption of Zigbee devices in beacon-enabled networks without increasing the time taken by Zigbee peripherals to communicate with their coordinator. The proposed protocol utilizes a beacon control mechanism called a "sleep pattern," which is updated based on the previous event statistics. It determines exactly when Zigbee peripherals wake up or sleep. A simulation of the proposed protocol using realistic parameters and an experiment using commercial products yielded similar results, demonstrating that the protocol may be a solution to reduce the power consumption of Zigbee devices.

• Journal of computational fluids engineering
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• v.10 no.1
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• pp.1-7
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• 2005

Three-dimensional numerical analysis of the flow around rectangular cylinders with various side ratios, D/H, from 0.2 to 2.0, was carried out for Reynolds number of 10³ by using a multi-directional finite difference method on a regular-arranged multi-grid. The predicted results are in good agreement with the experimental data. It is found that fluid dynamic characteristics of rectangular cylinders alternate between the high-pressure mode and the low-pressure mode of the base pressure for D/H=0.2-0.6. We show that this phenomenon is induced by the change of the flow pattern around rectangular cylinders.

• Journal of Distribution Research
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• v.1 no.2
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• pp.221-251
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• 1996

The retail industry of Korea has, owing to the increase of purchasing power of her people and the changing pattern of consumption appetite, marked a considerable development. Nevertheless the retail industry of Korea is still in primitive stage with its smallness of size, density of stores and inefficiency of management compared to those in the developed countries. It is a high time for Korean distribution market to boost its competitive power in the wake of the opening of distribution industry since January of 1996. Accordingly, it should be the goal of these markets to maximize its positive aspects while trying their best to minimize the negative aspect in this transition time for globalization.

• Journal of Mechanical Science and Technology
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• v.18 no.10
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• pp.1869-1879
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• 2004

The present study has numerically investigated two-dimensional flow over three circular cylinders in an equidistant side-by-side arrangement at a low Reynolds number. For the study, numerical simulations are performed, using the immersed boundary method, in the range of g* < 5 at Re= 100, where g* is the spacing between two adjacent cylinder surfaces divided by the cylinder diameter. Results show that the flow characteristics significantly depend on the gap spacing and a total of five kinds of wake patterns are observed over the range: modulation-synchronized (g* (equation omitted) 2), inphase-synchronized (g* (equation omitted) 1.5) , flip-flopping (0.3 < g* (equation omitted) 1.2) , deflected (g* (equation omitted) 0.3), and single bluff-body patterns (g* < 0.3). Moreover, the parallel and symmetric modes are also observed depending on g* in the regime of the flip-flopping pattern. The corresponding flow fields and statistics are presented to verify the observations.

• Sleep Medicine and Psychophysiology
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• v.14 no.1
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• pp.5-12
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• 2007

Sleep changes substantially with age. There is a phase advance in the circadian sleep cycle and increased waking after sleep onset. The elderly people wake more frequently during the night and experience fragmented sleep and excessive daytime sleepiness. The prevalence of sleep disorders increases with age, and the composition of sleep disorders in the elderly differs from that in the young. The most frequently encountered sleep disorders are psychophysiologic insomnia, sleep disturbance due to dementia, sleeprelated respiratory disorder, restless legs syndrome and periodic limb movement disorder, and REM sleep behavior disorder. To treat the elderly sleep problem appropriately, it is important to know how sleep pattern changes as we age and to understand the cause of sleep-related symptoms. This article will review the sleep physiology and common sleep disorders in the elderly.