• Journal of Communications and Networks
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• v.16 no.4
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• pp.397-406
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• 2014

Radio frequency (RF) jamming is a denial of service attack targeted at wireless networks. In resource-hungry scenarios with constant traffic demand, jamming can create connectivity problems and seriously affect communication. Therefore, the vulnerabilities of wireless networks must be studied. In this study, we investigate a particular type of RF jamming that exploits the semantics of physical (PHY) and medium access control (MAC) layer protocols. This can be extended to any wireless communication network whose protocol characteristics and operating frequencies are known to the attacker. We propose two efficient jamming techniques: A low-data-rate random jamming and a shot-noise based protocol-aware RF jamming. Both techniques use shot-noise pulses to disrupt ongoing transmission ensuring they are energy efficient, and they significantly reduce the detection probability of the jammer. Further, we derived the tight upper bound on the duration and the number of shot-noise pulses for Wi-Fi, GSM, and WiMax networks. The proposed model takes consider the channel access mechanism employed at the MAC layer, data transmission rate, PHY/MAC layer modulation and channel coding schemes. Moreover, we analyze the effect of different packet sizes on the proposed jamming methodologies. The proposed jamming attack models have been experimentally evaluated for 802.11b networks on an actual testbed environment by transmitting data packets of varying sizes. The achieved results clearly demonstrate a considerable increase in the overall jamming efficiency of the proposed protocol-aware jammer in terms of packet delivery ratio, energy expenditure and detection probabilities over contemporary jamming methods provided in the literature.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.5
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• pp.510-519
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• 2013

In this study, the spray characteristics of a pressure swirl atomizer classified into a liquid sheet-type swirl nozzle for Urea-SCR system were investigated experimentally with the variation of injection pressure. The length to diameter ratio ($l_o/d$) of the nozzle was 3.1, and the swirler was set inside the nozzle tip to give injecting fluid angular momentum. The injection duration of the nozzle was controlled by PWM (pulse width modulation) modes. The development processes of the spray were imaged by a 2-D PIV system, and the change of spray angle was measured. The atomization characteristics, including axial velocity and SMD, were measured using a 2-D PDA system with the injection pressures at room temperature and ambient pressure conditions. As the experimental results, the injection pressure had a significant impact on the spray structure showing a different shape around the spray leading edge, and the smaller SMD was observed with increasing injection pressures, which was similar to that of the previous work.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.11
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• pp.1163-1168
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• 2011

Compared with the spark-ignition gasoline engine, the compression-ignition diesel engine has reduced fuel consumption due to its higher thermal efficiency. In addition, this reduction in the fuel consumption also leads to a reduction in $CO_2$ emission. Diesel engines do not require spark-ignition systems, which makes them less technically complex. Thus, diesel engines are very suitable target engines for using biofuels with high cetane numbers. In this study, the spray characteristics of biofuels such as vegetable jatropha oil and soybean oil were analyzed and compared with those of diesel oil. The injection pressures and blend ratios of jatropha oil and diesel oil (BD3, BD5, and BD20) were used as the main parameters. The injection pressures were set to 500, 1000, 1500, and 1600 bar. The injection duration was set to $500{\mu}s$. Consequently, it was found that there is no significant difference in the characteristics of the spray behavior (spray angle) in response to changes in the blend ratio of the biodiesel or changes in the injection pressure. However, at higher injection pressures, the spray angle decreased slightly.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.7
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• pp.629-636
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• 2009

Mechanical stimulation is known to play a vital role on the differentiation of mesenchymal stem cells (MSCs) to pre-osteoblasts. In this research, we developed a tensile cell stimulator, composed of a DC motor-driven actuator and LVDT sensor for measuring linear displacement, to study the effects of tensile stress on osteogenic differentiation of MSCs. First, we demonstrated the reliability of this device by showing the uniform strain field in the silicon substrate. Secondly, we investigated the effects of tensile stretching on osteogenic differentiation. We imposed a pre-set cyclic strain at a fixed frequency on cell monolayer cultured on a flexible silicon substrate while varying its amplitude and duration. 60 min of resting period was allowed between 30 min of cyclic stretching and this cycle is repeated up to 7 days. Under the combined stimulation with osteogenic media and mechanical stretching, the osteogenic markers such as alkaline phosphatase (ALP), osterix, and osteopontin began to get expressed as early as 4 days of stimulation, which is much shorter than what is typically required for osteogenic media induced differentiation. Moreover, different markers were induced at different magnitudes of the applied strains. Lastly, for the case of ALP, we observed the antagonistic effects of osteogenic media when combined with mechanical stretching.

• Proceedings of the KOR-BRONCHOESO Conference
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• 1993.05a
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• pp.79-79
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• 1993

The retrospective analysis of 285 cases with diagnoses of foreign bodies in the food and air passages that were managed at the Department of Otolaryngology, KangNam St.Mary's Hospital from Nov.1980 to Oct.1992 are reported. The results are as follows : 1) Among 285 cases, 254 cases(89.2%) were lodged in the esophagus and 31 cases (10.8%) in the air passage. The ratio between the foreign bodies in the food and those in the air passage was about 8.2 : 1 2) Among all cases, 171 cases were males(59.8%) and 114 cases were females(40.2%). 3) The group under 5 years old having foreign bodies was over half of all cases(56.5%), and the group over 51 years old was 15.5% of all cases. 4) The incidence of foreign bodies in order of frequency was coin(35.6%), fish bone (21.8%) and chicken bone(7.9%) in the food passage. Beans and peanuts were the most common foreign bodies in the air passage. 5) In the location of foreign bodies of the food passage, 208 cases(82.2%) were found in the first esophageal narrowing. In the air passage, the frequent sites of lodgement were the bronchus, trachea and larynx in order. 6) The 67.0% of all cases was removed within 24 hours. The duration of lodgement showed differences in relation with varieties of the foreign bodies.

• Proceedings of the KOR-BRONCHOESO Conference
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• 1977.06a
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• pp.8.5-9
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• 1977

Foreign bodies in food and air passage, especially the latter in infants and children requires emergency treatment. The frequent symptoms are coughing, wheezing, and dyspnea, So may be confused as asthma, pneumonia and acute Laryngitis. The species and incidence of the foregin bodies are variable according to the age, method of life, environment and economic condition. The authors experienced 35 cases of foreign body in air passage during the period of last 3yrs which were treated by ventilating Bronchoscopy, and thoractomy in this hospital. One of them was lead to death. The authors analysed above cases and report with it's literature consideration. Results: 1. The prevalent foreign bodies were peanut, metal, food particles in order of frequency. 2. In frequent symptoms were dyspnea, coughing and wheezing in the order. 3. In the age incidence, 48% of the foreign bodies were 1∼5yrs. 4. In sex distribution, male to female ratio was 2.1: 1 5. In duration of lodgment, 65% of the foreign body were removed within 24hrs. 6. Most prevalent site of the foreign body was Rt. main bronchus Rt. to Lt. ratis was 2 : 1 was 7. Treatment was done; 25 cases were by ventilating bronchoscopy.

• Journal of the Korean Association of Oral and Maxillofacial Surgeons
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• v.28 no.5
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• pp.348-352
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• 2002

Ameloblastoma of the maxilla is an unusual epithelial tumor of odontogenic origin. According to many authors and reports, ameloblastoma account for approximately 1% of all tumors of the jaws, but when pseudotumors and cysts are excluded, the ratio rises to 11%. Of these tumors,80% originate in the mandible, while 20% originate in the maxilla. Although it is considered benign histopathologically, it can behave in a slowly growing infiltrative fashion, with multiple recurrences and eventual intracranial, or even distant, spread. We clinically analyzed common site in maxilla, radiographic findings, recurrence rate, duration between treatment and recurrence, the presence and site of distant metastasis in 15 patients who were diagnosed as ameloblastoma of the maxilla and took treatments from 1985 to 1999 in Dept. of Oral and Maxillofacial Surgery, Dental Hospital, Seoul National University. In this paper, treatment outcomes and our clinical experiences of maxillary ameloblastoma are reported with review of literatures.

• Journal of the Korean Association of Oral and Maxillofacial Surgeons
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• v.32 no.4
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• pp.384-390
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• 2006

Pleomorphic adenoma is the most common salivary benign tumor, constituting over 60% of parotid gland tumor, 25% of sublingual gland tumor and 50% of minor salivary gland tumor. It is somewhat more frequent in the fourth to sixth decades. The recurrent rate which enforces only a simple enucleation is very high $(20{\sim}45%)$. Histologically, it contains the epithelial cell, the myo-epithelial cell and mesenchymal ingredient, which is various aspect. We analyzed clinicopathologically and immunohistochemically the patients(34 cases) who are diagnosed with pleomorphic adenoma in Dept. of Oral & Maxillofacial Surgery, College of Dentisty, Dankook university since 1998. The results are as follow: 1. The incidence of the tumor was most frequent in age 30 to 50. The ratio of male to female was 1:1.43. 2. The most chief complain was a painless mass(94.1%) and the duration time was more than decade in 18 cases(52.9%). 3. Palate(soft & hard palate) was the most occurred site(64.7%). In major salivary glands, the parotid gland was the most frequent site(17.6%). 4. The tumor size was 2 to 3cm on the average. Most of tumors were with capsule(91.2%). 5. Surgical excision was a main treatment method(20 cases, 58.8%) and 14 cases were excised with a glandectomy, 1 case was treated with a partial maxillectomy. Only 1 case of all cases was recurred. 6. Histopathologically, 9 cases(26.5%) were cellular type, 11 cases(32.4%) were intermediate(classic) type and 14 cases(41.1%) were myxoid type. 7. Immunohistochemically, the specimen of all tumors reacted positively to cytokeratin and vimentin marker.

• Journal of the Korean Association of Oral and Maxillofacial Surgeons
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• v.41 no.6
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• pp.293-298
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• 2015

Objectives: Infection involving the orbit, zygomatic space, lateral pharyngeal space, or hemifacial and oral floor phlegmon is referred to as cervicofacial infection (CFI). When diagnosis and/or adequate treatment are delayed, these infections can be life-threatening. Most cases are the result of odontogenic infections. We highlight our experiences in the management of this life-threatening condition. Materials and Methods: This was a retrospective study of patients who presented with CFI from December 2005 to June 2012 at the Oral and Maxillofacial Surgery Clinic or the Accident and Emergency Unit of Ahmadu Bello University Teaching Hospital (Zaria, Nigeria). The medical records of all patients who presented with either localized or diffuse infection of the maxillofacial soft tissue spaces were retrospectively collected. Data collected was analyzed using SPSS version 13.0 and are expressed as descriptive and inferential statistics. Results: Of the 77 patients, 49 patients (63.6%) were males, a male to female ratio of 1:7.5. The ages ranged from two years to 75 years with a mean of $35.0{\pm}19.3$ years, although most patients were older than 40 years. The duration of symptoms prior to presentation ranged from 6 to 60 days, with a mean of $11.0{\pm}9.4$ days. More than 90% of the patients presented to the clinic within the first 10 days. The most commonly involved anatomical space was the submandibular space (n=29, 37.7%), followed by hemifacial space (n=22, 28.6%) and buccal space (n=7, 9.1%). Ludwig angina accounted for about 7.8% of the cases. Conclusion: CFI most commonly involves the submandibular space, typically affects individuals with a low level of education, and is influenced by traditional medical practices. Despite improved health care delivery, CFI remains a significant problem in developing countries.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.23 no.1
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• pp.17-26
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• 2010

This study presents a decision making process for installation of wind barrier which is used to reduce the wind acting on running vehicle on expressway. At the first stage of this study, the lateral deviations of running vehicles under side winds were computed from the commercial softwares, CarSim and TruckSim, and then the critical wind speeds for car accident were evaluated from predefined risk index. To determine whether it is needed to install wind barrier or not, cost and benefit from wind barrier are calculated during lifetime. In obtaining car accidental risk, probabilistic distribution of wind speed, daily traffic volume, mixture ratio in the volume, and duration time for wind speed range are considered. It is recommended to install wind barrier if benefit from the barrier installation exceed construction cost. In the numerical examples, case studies were shown for risk and benefit calculation and main risky regions on Korean highway were all evaluated to identify the number of installation sites.