• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.8
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• pp.684-691
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• 2015

A sudden increase in combustion pressure is observed in the ducted rocket combustion test equipped with pipe shaped and converging nozzle exhaust tubes. This study aims to understand the physical mechanism of abrupt change in combustion pressure using thermal choking in the exhaust tube. Results confirmed that the thermal choking of the flow inside the exhaust tube was responsible for the sudden increase in combustion pressure. Also, high pressure exponent of solid propellants is critical sensitive to the occurrence of thermal choking exhaust pipe. Additionally, numerical simulation showed that the sudden increase in combustion pressure was less possible in diverging pipe because thermal choking is more reluctant to occur.

• Journal of Elementary Mathematics Education in Korea
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• v.19 no.4
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• pp.609-624
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• 2015

It is well-known that every learner has his/her own learning style and the learning style has a great effect on his/her learning characteristics, processes and achievements. This study aims to induce some implications for designing the gifted mathematics programs which go with gifted students' learning styles. To do this, we applied the gifted mathematics program of nine lessons to 52 third grade gifted students and examined the characteristics of the programs according Kolb's learning style. To investigate the effect of the program, the pretest and posttest for applying the programs were taken and the results of two tests were analysed by t-test. As a result, the students of diverging style increased and the ones of converging style decreased, while there is little change in the accommodating style and the assimilating style. It was also shown that the average of the concrete experience(CE) of the subjects has increased significantly. This study also contains some pedagogical implications based on discussing about the results of analysis.

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.4
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• pp.1-8
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• 2014

Overexpanded flow of an axisymmetric thruster nozzle is numerically simulated to investigate effects of nozzle pressure ratio (NPR) on the shock structure and thrust performance. The Reynolds-averaged Navier-Stokes equations with k-${\omega}$ SST turbulence model are solved utilizing FLUENT solver. As the NPR is raised, thrust performance monotonically increases with the shock structure and flow-separation point being pushed toward the nozzle exit. It is also discussed that the flow structure at nozzle-exit plane which is immediately affected by a position of nozzle-interior shocks and expansion waves, has strong influence upon the thrust performance of thruster nozzle.

• Journal of the Korean Society of Propulsion Engineers
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• v.6 no.2
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• pp.29-36
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• 2002

Experiments have been performed to investigate the effect of secondary venturi tip angle on flow and spray characteristics in gas turbine combustor with a swirl cup assembly. Three variations of secondary venturi tip angle are made: converging, straight and diverging angles. It is found that the variation of venturi tip angle results in the significant changes of flow and spray characteristics in gas turbine combustors, such as the size and location of recirculation zones. drop size and mass distribution affecting combustion efficiency and NOx emissions. In diverge case, central toroidal recirculation zone(CTRZ) exists near the exit, which is known to be beneficial for flame stability. But in converge case, the finest SMD distribution and uniform mass distribution are found and CTRZ is longer than other cases. Consequently, high combustion efficiency and low pollutant emission are expected in converge case.

• Korean Journal of Metals and Materials
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• v.49 no.3
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• pp.221-230
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• 2011

The cold spray process is an emerging technology that utilizes high velocity metallic particles for surface coating. Metallic powder particles are injected into a converging-diverging de Laval nozzle and accelerated to a high velocity by a supersonic gas flow. The cold spray process normally uses a circular nozzle that has a rather narrow spraying range. To overcome this fault, a slit nozzle was considered in this study. The slit nozzle is anticipated to reduce the coating process time because it has a wider coating width than the circular nozzle. However, the slit nozzle can reduce the coating efficiency because it does not allow as much gas and particle velocity as the circular nozzle. To improve the coating efficiency of a slit nozzle, the shape of the slit nozzle was modified. And the results of gas flow and particle behaviour according to the nozzlers shape were compared by the a numerical analysis. As a results, as Expansion Ratio(ER) of 7.5 was found to be the most optimal condition for enhancing the spraying efficiency when the ER was changed by the variation of nozzle neck and exit size.

• Journal of The Korean Association For Science Education
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• v.35 no.4
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• pp.573-584
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• 2015

Inquiry has been consistently emphasized in science education as a crucial element for learning. Although many researchers came to agree on the importance of scientific inquiry, authentic inquiry activities are hard to be actualized in an educational context. Therefore it is required to critically examine what teachers have difficulty in teaching inquiry. In this article, we looked into inquiry-based science activities in a small group setting where pre-service science teachers faced dilemmas between cognition and practice of inquiry. A case study was conducted on eight undergraduate students who are majoring in science education. The participants attended a weekly science program for middle school students in low SES as teaching assistants and mentors, and took full care of his/her mentees during open-inquiry activities. The results were drawn by analyzing participants' personal and group interviews, participant observations, self-reports, and others. The pre-service teachers viewed the knowledge and procedure of science as an essential factor in inquiry activities along with student's spontaneous attitude. However, in the process of performing inquiry, they faced several dilemmas between ideal cognition and real activities. The aspects of dilemmas could be summarized in three pairs of opposing concepts: 'diverging inquiry or converging science', 'interest-centered inquiry or learning-centered inquiry', and 'student as the subject or student with the insufficient expertise.' We discussed ways of resolving dilemmas and alternative perspectives on scientific inquiry.

• Korean Journal of Legislative Studies
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• v.22 no.2
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• pp.5-52
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• 2016

Several criticisms notwithstanding, the major political parties in South Korea seem to have developed relatively coherent 'collective identities' with regard to the unification question between South and North Korea. A comparative analysis in this paper reveals a certain pattern of convergence and divergence in unification policies among major parties. First, diachronically, the two major political parties in the history of South Korea, which I call for simplicity "conservative parties" and "democratic parties" respectively, have converged into allelosubjective attitudes towards North Korea since President Park Chung-hee's proclamation of peaceful unification plan in 1970. The governments of conservative parties since then promoted allelosubjective relations between South and North Korea, which the governments of democratic parties succeeded and developed into a partial integration policy. Though the succeeding governments of conservative parties of Lee Myung-bak and Park Geun-hye seem to have stepped back from the allelosubjective attitudes, seemingly they have not yet withdrawn to the monosubjective stance before 1970. Next, synchronically, an analysis of the platforms of major parties and their campaign promises in the 20th general election in 2016 reveals converging and diverging points in their unification policies. All the major parties show relatively allelosubjective attitudes towards North Korea, with significant differences. "Saenuri Party," the current conservative party, maintains quite bit of monosubjective attitudes towards North Korea and requires unilateral changes of North Korea in the process of unification. "Justice Party," the minor progressive party, is the most allelosubjective in that it presupposes the co-existence and mutual survival of the two Koreas in unification. In between lie "The Minjoo Party of Korea" and "The People's Party", the two parties separated in the democratic party bloc.