• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.680-685
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• 2008

The ejector is a simple device which can transport a low-pressure secondary flow by using a high-pressure primary flow. In general, it consists of a primary driving nozzle, a mixing section, and a diffuser. The ejector system entrains the secondary flow through a shear action generated by the primary jet. Until now, a large number of researches have been made to design and evaluate the ejector systems, where it is assumed that the ejector system has an infinite secondary chamber which can supply mass infinitely. However, in almost all of the practical applications, the ejector system has a finite secondary chamber implying steady flow can be possible only after the flow inside ejector has reached an equilibrium state after the starting process. To the authors' best knowledge, there are no reports on the starting characteristics of the ejector systems and none of the works to date discloses the detailed flow process until the secondary chamber flow reaches an equilibrium state. The objective of the present study is to investigate the starting process of an ejector-diffuser system. The present study is also planned to identify the operating range of ejector-diffuser systems where the steady flow assumption can be applied without uncertainty. The results obtained show that the one and only condition in which an infinite mass entrainment is possible is the generation of a recirculation zone near the primary nozzle exit. The flow in the secondary chamber attains a state of dynamic equilibrium at this point.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제14권3호
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• pp.1313-1336
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• 2020

Cognitive radio (CR) is regarded as an effective approach to avoid the inefficient use of spectrum. However, CRNs have more special security problems compared with the traditional wireless communication systems due to its open and dynamic characteristics. Primary user emulation attack (PUEA) is a common method which can hinder secondary users (SUs) from accessing the spectrum by transmitting signals who has the similar characteristics of the primary users' (PUs) signals, and then the SUs' quality of service (QoS) cannot be guaranteed. To handle this issue, we first design a multiple-phase energy detection scheme based on the cooperation of multiple SUs to detect the PUEA more precisely. Second, a joint SUs scheduling and power allocation scheme is proposed to maximize the weighted effective capacity of multiple SUs with a constraint of the average interference to the PU. The simulation results show that the proposed method can effectively improve the effective capacity of the secondary users compared with the traditional overlay scheme which cannot be aware of the existence of PUEA. Also the good delay QoS guarantee for the secondary users is provided.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제12권4호
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• pp.1489-1503
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• 2018

Spectrum reuse has attracted much concern of researchers and scientists, however, the dynamic spectrum access is challenging, since an individual secondary user usually just has limited sensing abilities. One key insight is that spectrum usage forecasting among secondary users, this inspiration enables users to obtain more informed spectrum opportunities. Therefore, spectrum usage forecasting is vital to cognitive radio networks (CRNs). With this insight, a spectrum usage forecasting model for the occurrence of primary users prediction is derived in this paper. The proposed model is based on auto regressive enhanced primary user emergence reasoning (AR-PUER), which combines linear prediction and primary user emergence reasoning. Historical samples are selected to train the spectrum usage forecasting model in order to capture the current distinction pattern of primary users. The proposed scheme does not require the knowledge of signal or of noise power. To verify the performance of proposed spectrum usage forecasting model, we apply it to the data during the past two months, and then compare it with some other sensing techniques. The simulation results demonstrate that the spectrum usage forecasting model is effective and generates the most accurate prediction of primary users occasion in several cases.

• 한국인터넷방송통신학회논문지
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• 제13권5호
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• pp.55-63
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• 2013

인지무선네트워크에서 기 사용자(primary user)의 위치를 정확하게 파악하는 것은 2차 사용자(secondary user)들의 스펙트럼 이용 효율 향상 및 적절한 전력제어를 통한 기 사용자에 대한 간섭을 야기하는 것을 회피하는데 사용될 수 있기 때문에 인지무선네트워크에서 매우 중요한 연구 주제이다. 노드의 위치를 추정하는 다양한 기법들 중, 수신신호세기(RSS) 기반의 위치 추정 기법은 추가적인 하드웨어 자원 없이 쉽게 구현할 수 있어 일반적으로 가장 많이 사용되는 기법으로 인지무선네트워크에서도 사용될 수 있다. 하지만, 수신신호세기 기반의 위치 추정에서 노드간 거리 측정은 수신된 신호 세기를 기반으로 이루어져 무선 채널 환경의 페이딩, 쉐도잉 그리고 장애물 등으로 인해서 거리 추정의 오차가 생긴다. 따라서 본 논문에서는 수신신호세기 기반으로 기 사용자의 위치 측정의 정확도를 향상시키기 위하여 전송 손실 지수(path loss exponent) 추정기반의 기 사용자 위치 검출 기법을 제안한다. 시뮬레이션 결과를 통해 제안된 방식이 기존 방식보다 기 사용자 위치 측정의 오차를 줄어 기 사용자에 대한 간섭률을 더 줄일 수 있음을 보였다.

• Imaging Science in Dentistry
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• 제52권3호
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• pp.327-332
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• 2022

Mucocele of the paranasal sinuses is a benign, slow-growing, expansile lesion. Maxillary sinus mucoceles are usually associated with painless bulging of the cheek; however, orbital expansion is rarely observed. Maxillary sinus mucoceles can be classified as primary or secondary according to their etiology. An impediment to sinus ostium ventilation is thought to be the cause of primary mucocele, while sequestering of residual mucosa after surgery in the wound and long-term retention of tissue fluid have been suggested to lead to the formation of secondary mucocele. This report presents 2 cases of primary and secondary mucoceles, with a focus on radiographic features. As primary and superiorly positioned secondary maxillary sinus mucoceles are uncommon and their close proximity to the orbit predisposes the patient to significant morbidity, the authors expect that this report will contribute to a better understanding and diagnosis of maxillary sinus mucocele involving the orbit.

• Journal of Communications and Networks
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• 제18권3호
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• pp.340-350
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• 2016

The performance of large-scale cognitive radio (CR) networks with secondary users sustained by opportunistically harvesting radio-frequency (RF) energy from nearby primary transmissions is investigated. Using an advanced RF energy harvester, a secondary user is assumed to be able to collect ambient primary RF energy as long as it lies inside the harvesting zone of an active primary transmitter (PT). A variable power (VP) transmission mode is proposed, and an energy-based opportunistic spectrum access (OSA) strategy is considered, under which a secondary transmitter (ST) is allowed to transmit only if its harvested energy is larger than a predefined transmission threshold and it is outside the guard zones of all active PTs. The transmission probability of the STs is derived. The outage probabilities and the throughputs of the primary and the secondary networks, respectively, are characterized. Compared with prior work, the throughput can be increased by as much as 29%. The energy-based OSA strategy can be generally applied to a non-CR setup, where distributed power beacons (PBs) are deployed to power coexisting wireless signal transmitters (WSTs) in a wireless powered sensor network.

• 공학교육연구
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• 제21권2호
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• pp.17-27
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• 2018

This study aims to investigate the effect of design thinking-based STEAM program for primary and secondary students. The design thinking-based STEAM program dealt with advanced scientific technology issues and it consisted of 8 topics. A questionnaire was developed to explore the change of Science concept awareness, Interest, Self-efficacy, Career consciousness of students through the program. The questionnaire consisted of following 4 areas, such as 'Science concept awareness', 'Interest', 'Career consciousness' and 'Self efficacy'. Pre-post test was conducted to 75 primary and secondary students. As a result of analyzing post-test compared to pre-test, 'Science concept awareness', 'Career consciousness' and 'Self efficacy' of students were improved. And the 'Interest' wasn't changed significantly. Through this research, it can be suggested that the design thinking-based STEAM program can be useful at school education to improve the Science concept awareness, Career consciousness and Self-efficacy of primary and secondary students. It is also proposed that further research of the various areas of interest and other affective variables will be needed.

• Korean Chemical Engineering Research
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• 제58권3호
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• pp.430-437
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• 2020

본 연구에서는 다단제어(Cascade Control)를 위한 안티 와인드업(Anti-windup) 기술을 개발하였다. 다단제어는 외란을 보다 효과적으로 제어하기 위해 내부에 되먹임(Feedback) 제어루프를 추가한 제어구조이다. 단일루프제어(Single-loop Control)와는 다르게 두 개의 제어루프로 이루어져 내부의 종속제어루프(Secondary Control)가 외부의 주제어루프(Primary Control)에 영향을 미치는 특징을 가지고 있다. 산업현장에서 다단제어에 적용하는 기존의 안티 와인드업 기술은 주로 주제어기(Primary Controller)와 종속제어기(Secondary Controller)에 각각 로컬 역연산법(Back Calculation)을 적용하는 것이다. 하지만 이 방법은 종속제어기의 제어출력이 포화되었을 때 발생하는 주제어기의 적분누적(Integral-windup) 현상을 효과적으로 제거하지 못한다. 이를 해결하기 위해 기존 로컬 안티 와인드업 기술을 확대 적용한 두 가지의 다단제어용 안티 와인드업 기술을 제안한다. 첫 번째는 다단 조건부 적분법(Cascade Conditional Integration)이고 두 번째는 다단 역연산법(Cascade Back Calculation)으로 단일루프제어에 적용되어 왔던 로컬 조건부 적분법과 역연산법을 다단제어로 확대 적용하여 다단제어 시스템에서 PID제어기의 적분누적 현상을 방지하고자하였다. 개발한 다단 조건부 적분법과 다단 역연산법은 간단하고 직관적이면서도 종속제어기의 제어출력의 포화로 인해 발생한 적분누적현상을 효과적으로 방지할 수 있고 공정이나 제어기 형태에 상관없이 우수한 제어성능을 유지할 수 있었다. 향후 실제 공정 적용을 통해 신뢰성을 확보한다면 산업현장의 다단제어기 성능을 개선하는데 크게 기여를 할 수 있을 것이다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제9권9호
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• pp.3335-3356
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• 2015

In this paper, we analyze the connectivity of cognitive radio ad-hoc networks in a log-normal shadow fading environment. Considering secondary user and primary user's locations and primary user's active state are randomly distributed according to a homogeneous Poisson process and taking into account the spectrum sensing efficiency of secondary user, we derive mathematical models to investigate the connectivity of cognitive radio ad-hoc networks in three aspects and compare with the connectivity of ad-hoc networks. First, from the viewpoint of a secondary user, we study the communication probability of that secondary user. Second, we examine the possibility that two secondary users can establish a direct communication link between them. Finally, we extend to the case of finding the probability that two arbitrary secondary users can communicate via multi-hop path. We verify the correctness of our analytical approach by comparing with simulations. The numerical results show that in cognitive radio ad-hoc networks, high fading variance helps to remarkably improve connectivity behavior in the same condition of secondary user's density and primary user's average active rate. Furthermore, the impact of shadowing on wireless connection probability dominates that of primary user's average active rate. Finally, the spectrum sensing efficiency of secondary user significantly impacts the connectivity features. The analysis in this paper provides an efficient way for system designers to characterize and optimize the connectivity of cognitive radio ad-hoc networks in practical wireless environment.

• 한국인터넷방송통신학회논문지
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• 제15권1호
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• pp.105-110
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• 2015

인지 통신 네트워크에서 1차 송신단과 수신단 사이의 신호 대 잡음 비가 높을 경우 1차 네트워크의 수신 품질이 우수함으로 네트워크 간 지속적인 협력을 요구하지 않는다. 그러므로 협력을 필요로 하는 2차 시스템(보조)에게 스펙트럼을 제공하지 않는다. 본 논문에서는 간섭이 존재하지 않을 때 스펙트럼 공유의 빈도를 연구하였다. 또한 인지네트워크의 1차 시스템은 2차 시스템(보조)으로 인해 신호 대 잡음 비가 낮아지게 되며 이로 인한 스펙트럼 공유의 빈도의 증가에 초점을 맞추었다. 모의실험 및 이론적 결과를 바탕으로 기존의 기법에 비해 제안된 기법 스펙트럼 효율향상을 증명한다.