• 인터넷정보학회논문지
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• 제9권4호
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• pp.21-27
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• 2008

TCP의 수신자는 송수신자간 신뢰성이 보장되는 전송을 위하여 ACK를 송신자에 전달하며, 이 ACK 데이터가 정상적으로 전달되지 못할 때 TCP의 성능이 저하된다. 본 논문은 백그라운드 트래픽으로 인해 역방향 네트워크가 혼잡한 경우 TCP 송신자에서 ACK의 수신이 원활하지 못하기 때문에 전송 성능이 급격히 떨어지는 문제를 해결하는 기법을 제안한다. 본 기법은 라우터나 게이트웨이 등 별도의 하드웨어나 수신자의 도움 없이 송신자에서 역방향 혼잡을 감지하는 기법으로, 송신자는 수신자로부터 전달받는 ACK들의 수신시간 차이와 그 ACK에 실려있는 타임스탬프(Timestamp)값들의 시간 차이를 각각 비교한다. 그 차이 값을 성능저하의 판단 근거로 하여 역방향 혼잡을 판단할 뿐만 아니라 역방향 혼잡 상황이 발생했을 때의 대응 방안을 제안한다. NS-2 기반 시뮬레이션을 수행한 결과, 역방향 네트워크가 혼잡하고 무선 링크의 에러율이 1%인 유무선 혼합 환경에서는 제안 기법이 Reno에 비해 20%, New Reno에 비해 150%, Westwood에 비해 450%의 성능 향상을 보인다.

• 한국광학회지
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• 제6권4호
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• pp.331-336
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• 1995

미세 반도체가 첨가된 유리에 암색화에 따른 정상상태 포화흡수 변화를 조사하였고 축퇴 4광파 혼합 실험으로 영구 회절격자를 형성시켜 회절 효율을 연구하였다. 사용된 레이저는 Q-switch된 Nd:YAG레이저의 제2조화파로 펼스폭은 15ns이었다. 시료가 암색화된에 따라 포화 흡수세기 $I_s$는 증가하였고, 축퇴 4광파 혼합 실험 장치로부터 전방 펌프빔과 조사빔에 의한 큰 간격의 영구 회절격자를 형성하여 후방 펌프빔의 세기에 따라 회절 되어 나오는 빛의 세기를 측정하였다. 영구 회절격자는 암색화 현상에 의한 것으로 회절효율은 회절 되는 후방 펌프빔의 세기가 크지 않을 때 그 세기에 비례하는데, 이는 암색화에 의해 형성된 회절격자 사이의 흡수 차이에 의한 것임을 알 수 있었다.

• 대한소아치과학회지
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• 제25권1호
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• pp.197-208
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• 1998

This study was carried out to compare the amount of the maxillary bone remodeling and tooth displacement in each three maxillary superimposition methods, Ricketts, Best-fit, Structural method. Forty cases of the lateral cephalometric radiographs from 27 boys and 13 girls who had been treated to correct anterior cross-bite were selected for the study. The initial radiographs were taken at about 8-year-old and the second radiographs were taken in about 3.3 years later. Followings were the results: 1. With the Structural method, backward movement was shown in PNS, while forward movement was observed in ANS and point A. With the Ricketts method, however, all structures were shown significant backward movement comparing with Structural method(P<0.05). With the Best-fit method, the amount of horizontal movement was similar to that of the Structural method(P>0.05). 2. The palate seemed to be moved downward with Structural method, but there was no measured downward remodeling on nasal floor with Ricketts and Best-fit method(P<0.05). 3. Comparing with Structural method, Ricketts and Best-fit method significantly underestimated the eruption of the teeth by 20% to 30% (P<0.05). 4. The Structural method showed the anteroinferior rotation (43%) and posteroinferior rotation(57%) of the palatal plane, while the Best-fit method showed mostly anterosuperior rotation(87%), but no change was found in the Ricketts method. 5. With the Structural method, there was a statistically significant correlation between the amount of the rotation of the palatal plane and that of N-S line(r=0.86). 6. The measured angles of the long axis of the incisors and molars showed no significant difference in each 3 methods(P>0.05).

• Physical Therapy Rehabilitation Science
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• 제8권1호
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• pp.40-44
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• 2019

Objective: The purpose of this study was to examine the changes in postural stability according to ankle fixation in healthy university students. Design: Cross-sectional study. Methods: Thirty healthy subjects (15 males and 15 females, 20.13 years, 167.49 cm, 65.87 kg) were recruited on a voluntary basis. The BT4 system (HUR Laps Oy, Tampere, Finland) was used to measure the static (standing posture with eyes open and eyes closed) and dynamic (external perturbation and limits of stability (LOS) in the forward, backward, left, and right side) balance abilities. External perturbation was measured by the subject's postural sway velocity and area for 20 seconds after being impacted by a gym ball. Static and dynamic stabilities were measured with ankle joint fixation and non-fixation conditions. Ankle fixation was provided using Mueller tape on both ankle joints. Results: For static stability under the standing posture, there was no significant difference between standing with ankle joint fixation and non-fixation conditions. However, dynamic stability (external perturbation and LOS in the forward, backward, left, and right side) was significantly higher in the standing with the non-fixation condition compared to the standing with ankle joint fixation condition (p<0.05). Conclusions: Our results reveal that ankle joint fixation can influence dynamic stability during standing. Thus, we believe that this result provides basic information for making improvements in postural control and may be useful in balance training for fall prevention.

• 한국전문물리치료학회지
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• 제28권2호
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• pp.123-131
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• 2021

Background: Only 2% of falls in older adults result in serious injuries (i.e., hip fracture). Therefore, it is important to differentiate injurious versus non-injurious falls, which is critical to develop effective interventions for injury prevention. Objects: The purpose of this study was to a. extract the best features of surface electromyography (sEMG) for classification of injurious falls, and b. find a best model provided by data mining techniques using the extracted features. Methods: Twenty young adults self-initiated falls and landed sideways. Falling trials were consisted of three initial fall directions (forward, sideways, or backward) and three knee positions at the time of hip impact (the impacting-side knee contacted the other knee ("knee together") or the mat ("knee on mat"), or neither the other knee nor the mat was contacted by the impacting-side knee ("free knee"). Falls involved "backward initial fall direction" or "free knee" were defined as "injurious falls" as suggested from previous studies. Nine features were extracted from sEMG signals of four hip muscles during a fall, including integral of absolute value (IAV), Wilson amplitude (WAMP), zero crossing (ZC), number of turns (NT), mean of amplitude (MA), root mean square (RMS), average amplitude change (AAC), difference absolute standard deviation value (DASDV). The decision tree and support vector machine (SVM) were used to classify the injurious falls. Results: For the initial fall direction, accuracy of the best model (SVM with a DASDV) was 48%. For the knee position, accuracy of the best model (SVM with an AAC) was 49%. Furthermore, there was no model that has sensitivity and specificity of 80% or greater. Conclusion: Our results suggest that the classification model built upon the sEMG features of the four hip muscles are not effective to classify injurious falls. Future studies should consider other data mining techniques with different muscles.

• 대기
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• 제32권2호
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• pp.119-133
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• 2022

From 2015 to 2021, high-concentration fine dust episodes with a daily average PM_2.5 concentration of 50 ㎍ m^-3 or higher were selected and classified into 3 types [long range transport (LRT), mixed (MIX) and Local emission and stagnant (LES)] using synoptic chart and backward trajectory analysis. And relationships between the fine particle data (PM_2.5 and PM₁₀ concentration and PM_2.5/PM₁₀ ratio) and meteorological data (PBLH, Ta, WS, U-wind, and Rainfall) were analyzed using hourly observation for the classification episodes on the Korean Peninsula and the Seoul metropolitan area (SMA). In LRT, relatively large particles such as dust are usually included, and in LES, fine particle is abundant. In the Korean peninsula, the rainfall was relatively increased centered on the middle and western coasts in MIX and LES. In the SMA, wind speed was rather strong in LRT and weak in LES. In LRT, rainfall was centered in Seoul, and in MIX and LES, rainfall appeared around Seoul. However, when the dust cases were excluded, the difference between the LRT and other types of air quality was decreased, but the meteorological variables (Ta, RH, Pa, PBLH, etc.) were further strengthened. In the case of the Korean Peninsula, it is difficult to find a clear relationship because regional influences (topographical elevation, cities and coasts, etc.) are complexly included in a rather wide area. In the SMA, it is analyzed that the effects of urbanization such as the urban heat island centered on Seoul coincide with the sea and land winds, resulting in a combination of high concentrations and meteorological phenomena.

• 대한토목학회논문집
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• 제14권1호
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• pp.131-141
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• 1994

특성곡선을 고려한 세가지 연산자 분리방법을 오염원의 종확산 문제에 적용하여, 그 결과를 Eulerian 기법들의 계산결과와 비교하였다. 연산자 분리방법의 이송방정식에 대한 수치 기법들로는 generalized upwind, two-point fourth-order 및 sixth-order Holly-Preissmann 기법들을 각각 적용하였으며, 확산 방정식에 대한 수치기법으로는 Crank-Nicholson 기법을 적용하였다. Holly-Preissmann 기법을 사용하는 연산자 분리방법들이 Eulerian 기법들에 비하여 매우 정확한 계산결과를 나타내었다. Eulerian 기법들의 경우에는 이송항의 근사방법으로서 중앙차분을 취하는 기법들은 수치진동을, 후방차분을 취하는 기법들은 수치분산을 각각 보였으며, 이러한 현상들은 종확산계수의 값이 작을수록 더욱 뚜렷하게 나타났다.

• 한국전산유체공학회지
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• 제6권4호
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• pp.26-34
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• 2001

The flow past a circular cylinder forced to vibrate transversely is numerically simulated by solving the two-dimensional Navier-Stokes equations modified by the vibration velocity of a circular cylinder at a Reynolds number of 164. The higher-order finite difference scheme is employed for the spatial discretization along with the second order Adams-Bashforth and the first order backward-Euler time integration. The calculated cylinder vibration frequency is between 0.60 and 1.30 times of the natural vortex-shedding frequency. The calculated oscillation amplitude extends to 25% of the cylinder diameter and in the case of the lock-in region it is 60%. It is made clear that the cylinder oscillation has influence on the wake pattern, the time histories of the drag and lift forces, power spectral density and phase diagrams, etc. It is found that these results include both the periodic (lock-in) and the quasi-periodic (non-lock-in) state. The vortex shedding frequency equals the driving frequency in the lock-in region but is independent in the non-lock-in region. The mean drag and the maximum lift coefficient increase with the increase of the forcing amplitude in the lock-in state. The lock-in boundaries are also established from the present direct numerical simulation.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2001년도 춘계 학술대회논문집
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• pp.181-188
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• 2001

The flow past a circular cylinder forced to vibrate transversely is numerically simulated by solving the two-dimensional Wavier-Stokes equations modified by the vibration velocity of a circular cylinder at a Reynolds number of 164. The higher-order finite difference scheme is employed for the spatial discretization along with the second order Adams-Bashforth and the first order backward-Euler time integration. The calculated cylinder vibration frequency is between 0.60 and 1.30 times of the natural vortex-shedding frequency. The calculated oscillation amplitude extends to $25\%$ of the cylinder diameter and in the case of the lock-in region it is $60\%$. It is made clear that the cylinder oscillation has influence on the wake pattern, the time histories of the drag and lift forces, power spectral density and phase diagrams, etc. It is found that these results include both the periodic (lock-in) and the quasi-periodic (non-lock-in) state. The vortex shedding frequency equals the driving frequency in the lock-in region but is independent in the non-lock-in region. The mean drag and the maximum lift coefficient increase with the increase of the forcing amplitude in the lock-in state. The lock-in boundaries are also established from the present direct numerical simulation.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2001년도 추계 학술대회논문집
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• pp.84-90
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• 2001

Secondary instability of flow past a circular cylinder is examined using direct numerical simulation at Reynolds number 220 and 250. The higher-order finite difference scheme is employed for the spatial distributions along with the second order Adams-Bashforth and the first order backward-Euler time integration. In x-y plane, the convection term is applied by the 5th order upwind scheme, and the pressure and viscosity terms are applied by the 4th order central difference. In spanwise, Navier-Stokes equation is distributed using Spectral Method. The critical Reynolds number for this instability is found to be about Re=190. The secondary instability leads re three-dimensionality with a spanwise wavelength about 4 cylinder diameters at onset (A-mode). Results of three-dimensional effect in wake of a circular cylinder are represented with spanwise and streamwise vorticity contours as Reynolds numbers.