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Effects of gap size for parallel 45 degree angled rib turbulators
Park, Jongmyung,Park, Samgyu,Ligrani, Phillip M Emerald Group Publishing Limited 2016 International journal of numerical methods for hea Vol.26 No.6
<P>Purpose - Turbulent air flows within a channel with 45 degrees angled rib turbulators on the top and bottom walls are numerically predicted using the numerical code. For the predictions, a v(2)-f turbulence model (velocity variance scale and elliptic relaxation factor model) is utilized. The paper aims to discuss these issues. Design/methodology/approach - Three different rib arrangements with or without gap are investigated to present information on the effects of gap size on flow structure and heat transfer characteristics. Three-dimensional turbulent transport, and detailed flow structural characteristics are considered to provide new insight into the mechanisms which result in surface heat transfer augmentations. Findings - Compared to the baseline rib arrangement, the numerically predicted results show that the parallel ribs with gap (where the width of the gap is two times of rib height) shows the highest local Nusselt number ratios. This is a result of locally increased vorticity distributions, as well as augmented local magnitudes of mixing, secondary flows, and turbulent transport. Local transport changes are less pronounced when the gap width of gap is 0.5 times of rib height. As a result, associated local and spatially averaged Nusselt number ratios are also lower for this arrangement. Practical implications - Results will give improved heat transfer augmentation technologies. Originality/value - The present investigation provides new information and insight into flow structural characteristics in a channel with rib turbulators, both with and without gaps, especially the mechanisms which result in surface heat transfer augmentations, which are not available in any other existing numerical or experimental investigation.</P>
CFD modeling of ventilation ducts for improvement of air quality in closed mines
Jongmyung Park,Samgyu Park,이동길 한국자원공학회 2016 Geosystem engineering Vol.19 No.4
Decommissioned, dead-end mining tunnels or natural caves are employed to investigate new usage in Korea. Ventilation ducts are designed to allow control of flow velocities, mean age of air, and temperatures of blown air. Turbulent air flows within a dead-ended tunnel are theoretically predicted using the numerical code with a κ − ε turbulence model, and 3.5 M hexahedral grids. Three-dimensional turbulent transport, detailed flow structural characteristics, dimensionless temperature, and local mean age of air are considered to provide new insight into the mechanisms needed to design the ventilation duct for tunnels. Time-averaged turbulent flow characteristics, velocity shape path lines, fluid age distributions, and temperature distributions are presented for 5% inlet turbulence intensity and 12 m/s inlet velocity. From CASE1 to CASE6, six different geometries of ventilation duct are used to optimize the design for better air quality. Overall, the numerically predicted results show the air is fresher in the long duct and average temperatures in the tunnel with long ventilation ducts are slightly lower than those in a tunnel with short ducts.
EPCglobal Gen 2 Q 알고리즘에서 C 모델에 따른 태그 인식 성능 평가
박종명(Jongmyung Park),조성호(Sung Ho Cho) 한국통신학회 2009 韓國通信學會論文誌 Vol.34 No.12b
본 논문은 EPCglobal C1 Gen 2 표준에서 제안한 Q 알고리즘에서 사용되는 C값에 대한 다양한 모델을 제시하고 시뮬레이션을 통하여 그 성능을 비교, 평가한다. EPCglobal C1 Gen 2 표준에서는 다중 태그 인식을 위해서 slot-count (Q) Selection Algorithm을 제안하고 있지만, Q 알고리즘에서 태그의 충돌과 무응답의 상태에 따라서 Q값을 변화시키는 값인 C 값에 대한 정확한 정의가 내려져있지 않다. Q 알고리즘에서는 태그 충돌의 경우 C를 Q에 더하고 무응답인 경우에는 C를 감산하여 변화되는 Q값으로 태그들의 새로운 slot-count를 결정하기 때문에 다중 태그 인식 환경에 있어서 이 C값은 태그 인식 속도에 커다란 영향을 준다. 하지만 기존 연구들에서는 C값에 따른 태그 인식 속도 성능 평가나 비교 없이, Q 알고리즘을 변형하거나 새로운 방법을 제안하여 태그 인식 속도를 늘리기 위한 연구들이 존재한다. 본 연구에서는 EPCglobal C1 Gen 2 표준을 만족하는 C값의 다양한 모델을 제시하고 각각에 대해 다중 태그 인식 환경에 있어서 그 성능을 비교하고 평가한다. 본 연구의 결과물은 향후 EPCglobal C1 Gen 2 C 모델에 대한 연구나 태그 인식 성능 연구를 위한 하나의 지표로 쓰일 수 있다. This paper first proposes diverse C determining models in the Q algorithm which is proposed in the EPCglobal C1 Gen 2 standard and then compares and analyzes its performance. EPCglobal C1 Gen 2 standard proposes the slot-count (Q) selection algorithm for multiple tag identification environment, but there is no such definition for the C value which modifies the Q value depending on collision or no reply. During the tag anti-collision process, the Q algorithm adds C to the Q when there is a collision and reduces the Q by C when there is no reply. The modified Q value updates new slot-counts for tags which determines the tag identification speed, so the C value is an important factor. However, many researches only intend to increase the tag identification speed by proposing a new method or modifying the Q algorithm without any research about the C value. This paper suggests diverse C models which satisfies the EPCglobal C1 Gen 2 and analyzes their performance in the multi tag identification environment. The result of this paper can be used as an index for future researches on EPCglobal C1 Gen 2 C models and multiple tag identification performance.
Hybrid Selector With Excellent Selectivity and Fast Switching Speed for X-Point Memory Array
Park, Jaehyuk,Yoo, Jongmyung,Song, Jeonghwan,Sung, Changhyuck,Hwang, Hyunsang IEEE 2018 IEEE electron device letters Vol.39 No.8
<P>In this letter, we demonstrate a new concept of threshold selector by combining Ag-based selector with NbO<SUB><I>x</I></SUB> insulator metal transition (IMT) selector. To overcome intrinsic limits of slow turn-off speed ( <TEX>$> 1\mu \text{s}$</TEX>) of the Ag-based selector, we adopted NbO<SUB><I>x</I></SUB> IMT selector with fast turn-off speed. The hybrid device exhibits excellent selector characteristics, such as extremely low off-current (<1 pA) and fast turn-off speed ( <TEX>$< 1~\mu \text{s}$</TEX>). It shows excellent read-out margin (>75% at >2<SUP>10</SUP> word lines) in a fast sequential operation of cross point memory array.</P>