• 한국음향학회지
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• 제23권5호
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• pp.362-369
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• 2004

수층과 사질 퇴적층이 이루는 경계면이 평탄할지라도 고주파수 (30∼120 ㎑) 음파의 반사손실은 퇴적물의 입도와 입사파의 파장에 따라 거친 경계면에서의 반사 효과를 갖게 된다. 경계면 거칠기 영향은 음향학적 거칠기 (acoustical roughness, g/sub R/)로 표현하며 사질 퇴적물의 경우 g/sub R/∼ O(1)이고 주파수에 따른 종속성을 가진다. 따라서 입도분포에 따른 반사손실의 편차 (deviation)와 주파수에 따른 종속성을 포함하는 개선된 해저면 반사손실 모델 (HYBRL 모델, Hanyang University Bottom Reflection Loss model)을 제안한다. 그리고 주파수 종속성과 해저면 물성이 갖는 편차를 포함하는 반사손실 모델을 검증하기 위해 수조실험과 해상실험을 실시하였다. 수조실험 및 해상실험에서 측정된 해저면 반사손실 결과 사질 퇴적물에서 모델의 특성을 잘 반영한다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.574-579
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• 2000

In this study. an off-design performance prediction program for centrifugal pumps is developed. To estimate the losses in an impeller flow passage, two-zone model and two-element in series(TEIS) model are used. At impeller exit. the mixing process occurs with an increase in entropy. In two-zone model. there are both primary zone and secondary zone for an isentropic core flow and an average of all non-isentropic streamtubes respectively. The level of the core flow diffusion in an impeller was calculated by using TEIS model. While internal losses in an impeller an automatically estimated by using the above models, some empirical correlations far estimating external losses. far example, disk friction loss, recirculation loss and leakage loss are used. In order to analyze the vaneless diffuser flow. the momentum equations for the radial and tangential directions are used and solved together with continuity and energy equations.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2011년도 제42회 하계학술대회
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• pp.822-823
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• 2011

The authors analyzed harmonic current based loss of a high temperature superconducting (HTS) DC model cable. The loss in HTS DC cable is generated due to the variation of magnetic field caused by harmonic current in a HVDC transmission system. The authors designed and fabricated two meters of HTS DC model cable for verification of real loss characteristic. In this paper, the loss characteristics caused by harmonic current in the HTS DC model cable are analyzed using commercial finite element method software package. The loss of the HTS DC cable is much less than the loss of the HTS AC cable but the loss should be considered to decide a proper capacity of cooling system.

• 대한전자공학회논문지TC
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• 제39권12호
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• pp.547-552
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• 2002

본 논문에서는 국내지형환경에서 차세대 이동통신인 IMT-2000을 위한 경로손실을 측정 및 고찰하였다. 이를 위해 국내지형환경을 분류하였으며, 주파수 1.9201GHz에서 비가시(non line of sight) 경로로 수신전력을 측정하였다. 또한 이를 토대로 안테나 복사패턴을 고려하여 경로손실을 계산하였다. 국내지형환경에서 경로손실을 고찰하기 위해 각 지형환경에 대해 계산된 경로손실 데이터를 비교기준모델인 COST-231 HATA Urban Model 기울기로 fitting하고 비교하였다. 결과를 살펴볼 때 국내지형환경의 경우, 비교기준 모델인 COST-231 HATA Urban Model에 비해 도심환경의 경우 약 5dB, 한산한 도심환경의 경우 약 8dB, 밀집한 부심환경의 경우 약 12dB, 부심환경의 경우 약 13dB, 한산한 부심환경의 경우 약 19dB, 도로환경의 경우 약 29dB가 적게 나타남을 확인할 수 있었다.

• 한국초전도ㆍ저온공학회논문지
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• 제15권1호
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• pp.29-34
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• 2013

AMR (Active Magnetic Regenerative) refrigerators require the large variation of the magnetic field and a HTS magnet can be used. The amount of AC loss is very important considering the overall efficiency of the AMR refrigerator. However, it is very hard to estimate the precise loss of the HTS magnet because the magnetic field distribution around the conductor itself depends on the coil configuration and the neighboring HTS wires interact each other through the distorted magnetic field by the screening current Therefore, the AC loss of HTS magnet should be calculated using the whole configuration of the HTS magnet with superconducting characteristic. This paper describes the AC loss of the HTS magnet by an appropriate FEM approach, which uses the non-linear characteristic of HTS conductor. The analysis model is based on the 2-D FEM model, called as 'magnetic field formulation and edge-element model', for whole coil configuration in cylindrical coordinates. The effects of transport current and stacked conductors on the AC loss are investigated considering the field-dependent critical current. The PDE model of 'Comsol multiphysics' is used for the FEM analysis with properly implemented equations for axisymmetric model.

• Journal of Power Electronics
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• 제12권6호
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• pp.960-973
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• 2012

The winding loss analysis of a flyback transformer is difficult and ambiguous because the primary side current and the secondary side current differs both in shape and phase, especially for DCM (Discontinuous Conduction Mode) operation. Meanwhile, the fringing field caused by the air gaps further makes the traditional 1-D loss analysis model not directly applicable. The paper gives a thorough investigation into the phase shift of winding currents, which indicates that the phase shift of the high order harmonics is still close to $180^{\circ}$ out-of-phase. Based on the analysis, a simplified 2-D winding loss analytical model for flyback transformers considering the effects of low order harmonics is proposed. By neglecting the y components of the fringing field, the proposed model has an acceptable accuracy and a simple form that is similar to the conventional 1-D model. The power loss calculated with the proposed analysis model is verified by FEA (Finite Element Analysis) simulations and experimental results.

• 한국공간구조학회논문집
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• 제20권4호
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• pp.159-167
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• 2020

The resilience performance evaluation method of a structure can evaluate the ability to recover after an earthquake disaster, and this study deals with the consideration and introduction of the resilience performance evaluation method. The resilience evaluation method can be expressed as a quantified number by constructing a loss estimation model and a recovery evaluation model. The recovery evaluation model should consider downtime in addition to the repair time, and the loss estimation model should consider not only direct loss to structures and non-structures, but also indirect loss due to functional loss of the building. In addition, to build a loss estimation model, the structure should be simplified to perform an efficient analysis. Therefore, in this study, the equivalent terminal induction system proposed cantilever-type and rahmen-type SDOF, and it is evaluated somewhat conservatively compared to the example structure, and it is judged that there is a need to improve the hysteresis characteristics by applying the stiffness reduction factor of the SDOF model.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제11권10호
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• pp.4698-4716
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• 2017

Wireless signal transmission is influenced by environmental effects. These effects have also been challenging for Vehicular Ad hoc Network (VANET) in real-time communication. More specifically, in an urban environment, with high mobility among vehicles, a vehicle's status from the transmitter can instantly trigger from line of sight to non-line of sight, which may cause loss of real-time communication. In order to overcome this, a deterministic signal propagation model is required, which has less complexity and more feasibility of implementation. Hence, we propose a realistic path loss model which adopts ray tracing technique for VANET in a grid urban environment with less computational complexity. To evaluate the model, it is applied to a vehicular simulation scenario. The results obtained are compared with different path loss models in the same scenario based on path loss value and application layer performance analysis. The proposed path loss model provides higher loss value in dB compared to other models. Nevertheless, the performance of vehicle-vehicle communication, which is evaluated by the packet delivery ratio with different vehicle transmitter density verifies improvement in real-time vehicle-vehicle communication. In conclusion, we present a realistic path loss model that improves vehicle-vehicle wireless real-time communication in the grid urban environment.

• 한국통신학회논문지
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• 제38A권9호
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• pp.765-772
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• 2013

본 논문에서는 인천공항 Concourse 지역에서 항공정보통신 무선 채널의 경로 손실 모델에 관해 연구하였다. 항공정보통신 주파수 밴드인 VHF/UHF 채널 중에 두 개의 주파수에 대해 전파 측정을 수행하였다. 현재 운영 중인 송신 사이트에서 변조 신호를 제거한 캐리어 파워를 송신하였으며, 전파 측정을 위해 수신기를 장착한 이동 차량을 이용하여 Concourse 지역에서 전파 측정을 수행하였다. 송신 파워, 주파수, 안테나 위치 등은 현재 운용 조건과 같다. 경로 손실 계수 및 실험적인 경로 손실 식은 기본적인 경로 손실 모델 및 하타 모델 등을 이용하여 추출하였다. Concourse 지역에서 추출된 LOS/NLOS 경로 손실 계수는 128.2MHz 및 269.1MHz에서 각각 3.1/3.13 및 3.01/3.38이었고 예측 에러의 편차는 각각 2.77/3.17 및 4.01/3.66이었다. 추출된 경로 손실 계수를 이용하여 Concourse 지역에서 전파 경로 손실식과 실험적인 경로 손실 계수를 도출하였으며 또한 다른 전파 패스 모델과 비교하였다. 이러한 결과는 항공정보통신 사이트 최적 위치 선정 및 항공정보통신 서비스 평가에 도움이 될 것이다.

• Journal of Communications and Networks
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• 제6권3호
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• pp.235-244
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• 2004

There have been a lot of approaches to evaluate and predict transmission control protocol (TCP) performance in a numerical way. Especially, under the recent advance in wireless transmission technology, the issue of TCP performance over wireless links has come to surface. It is because TCP responds to all packet losses by invoking congestion control and avoidance algorithms, resulting in degraded end-to-end performance in wireless and lossy systems. By several previous works, although it has been already proved that overall TCP performance is largely dependent on its loss recovery performance, there have been few works to try to analyze TCP loss recovery performance with thoroughness. In this paper, therefore, we focus on analyzing TCP's loss recovery performance and have developed a simple model that facilitates to capture the TCP sender's behaviors during loss recovery period. Based on the developed model, we can derive the conditions that packet losses may be recovered without retransmission timeout (RTO). Especially, we have found that TCP Reno can retransmit three packet losses by fast retransmits in a specific situation. In addition, we have proved that successive three packet losses and more than four packet losses in a window always invoke RTO easily, which is not considered or approximated in the previous works. Through probabilistic works with the conditions derived, the loss recovery performance of TCP Reno can be quantified in terms of the number of packet losses in a window.