• 한국전산유체공학회지
• /
• 제22권1호
• /
• pp.81-87
• /
• 2017

The characteristics of flow past a square cylinder submerged under the free surface have been numerically studied. An immersed boundary method was adopted for implementation of the cylinder cross-section in a Cartesian grid system. Also, a level-set method was used to capture the interface of the two fluids. The case for Reynolds number 150 was examined. At the specific Reynolds number, by varying the gap ratio(0.25, 0.40, 0.55, 0.70, 1.00, 1.50, 2.50, 5.00) the effects of the free surface on the force coefficients and Strouhal number of vortex shedding were identified. The presence of the free surface very close to the cylinder significantly affects the shedding pattern, resulting in considerable deviation of the force coefficients and Strouhal number from those of the single-phase flow. In addition, the influence of Froude number was considered in this study. By increasing Froude number(0.2-0.4), flow topology change was identified at the specific gap ratios(0.40, 0.70, 1.50, 5.00).

• 대한기계학회논문집
• /
• 제18권10호
• /
• pp.2585-2596
• /
• 1994

Many particle filled materials like Poweder/Binder mixtures for poweder injection moldings, have complicated rheological behaviors such as an yield stress and slip phenomena. In the present study, numerical simulation programs via a finite element method and a finite difference method were developed for the quasi-three-dimensional flows and the two-dimensional flow models, respectively, with the slip phenomena taken into account in terms of a slip velocity. In order to qualitatively understand the slip effects, typical numerical results such as vector plots, pressure contours in the cross-channel plane, and isovelocity controus for the down-channel direction were discussed with respect to various slip coefficients. Slip velocities along the boudary surfaces were also investigated to find the effects of the slip coefficient and processing conditions on the overall flow behavior. Based on extensive numerical calculations varying the slip coefficients, pressure gradient, aspect ratio, and power law index, the screw characteristics of the extrusion process were studied in particular with comparisons between the slip model and non-slip model.

• Wind and Structures
• /
• 제31권4호
• /
• pp.335-350
• /
• 2020

In coastal residential communities, especially along the coastline, flooding is a frequent natural hazard that impacts the area. To reduce the adverse effects of flooding, it is recommended to elevate coastal buildings to a certain safe level. However, post storm damage assessment has revealed severe damages sustained by elevated buildings' components such as roofs, walls, and floors. By elevating a structure and creating air gap underneath the floor, the wind velocity increases and the aerodynamics change. This results in varying wind loading and pressure distribution that are different from their slab on grade counterparts. To fill the current knowledge gap, a large-scale aerodynamic wind testing was conducted at the Wall of Wind experimental facility to evaluate the wind pressure distribution over the surfaces of a low-rise gable roof single-story elevated house. The study considered three different stilt heights. This paper presents the observed changes in local and area averaged peak pressure coefficients for the building surfaces of the studied cases. The aerodynamics of the elevated structures are explained. Comparisons are done with ASCE 7-16 and AS/NZS 1170.2 wind loading standards. For the floor surface, the study suggests a wind pressure zoning and pressure coefficients for each stilt height.

• 자원환경지질
• /
• 제23권4호
• /
• pp.383-392
• /
• 1990

이 연구(硏究)에서는 경험적인 사용강도(便用强度) 방법(方法)과 파생수요(派生需要)의 경제적이론을 초월대수(超越代數) 수요(需要) 모델을 사용(使用)하여 결합하였다. 첫번째 단계로서 한 인구당 GDP와 사용강도(使用强度)의 실험적인 패턴인 비대칭(非對稱) 생명곡선(生命曲線)을 고려하였으며, 두번째 단계로서는 이러한 비대칭(非對稱) 생명곡선(生命曲線)이 수학적으로 단순한 초월대수(超越代數) 사용강도(使用强度) 모델로 바꿀수 있다는 것을 보여 주었으며, 마지막 단계로서 전통적인 수요이론(需要理論)이 가격(價格), 대체효과(代替效果), 기술(技術)의 변화(變化)를 포함한다는 것에 근거하여 초월(超越) 대수(代數) 수요(需要) 모델을 연장한 결과 전통적인 수요(需要)모델이나 사용강도(使用强度) 모델보다 더 정확하게 광물자원(鑛物資源) 수요(需要)를 예측(豫測)하였으며 사용강도(使用强度) 방법(方法)으로서도 자원(資源)의 수요(需要)를 예측(豫測)할 수 있음을 보여주었다.

• 한국통신학회논문지
• /
• 제29권9C호
• /
• pp.1330-1336
• /
• 2004

본 논문에서는 동영상 부호화 기의 성능을 개선하기 위한 최소 자송 기반 적응 움직임 벡터 예측 알고리즘을 제안 하고자 한다. 적응 움직임 벡터 예측 방식은 동영상 움직임 벡터의 국부 통계적인 특성의 돌연한 변화로 특정지어 진다는 것을 바탕으로 최소 자승(Least Squared) 기반의 선형 움직임의 계수들을 통계적 특성에 따라 최적화하는 방식이지만 상기 방식은 애우 높은 계산 량을 요구하는 단정을 지니고 있다. 본 논문에서는 공간적인 움직임 변화 방향성을 가지는 최소 자승 최적화를 기반으로 움직임 예측기의 계수를 적응적으로 조절하여 움직임 예측 오류뿐만 아니라 계산 량도 감소시키는 방식에 대해 기술한다. 실험을 통해 제안된 방식의 성능을 확인할 수 있었다.

• 한국전자파학회논문지
• /
• 제8권4호
• /
• pp.403-414
• /
• 1997

각종 원인에 기인하는 궤도별 우주폐기물의 분포는 우주환경에 있어서 위성통신 방송 및 통신신호의 전파전파 에 방해요소로 작용하게 된다. 본 논문에서는 수치해석적 접근에 의해 우주폐기물의 공간분포밀도와 사용 주파 수의 변화에 따른 EM파의 정규화 산란계수를 계산하고 우주폐기물에 의한 감쇠도 및 위상변화를 구하였다. 그 결과 우주폐기물에 의한 위성통신 방송선호 전파에 었어서 감쇠도는 베셀함수에 의해 표현된 전계 요소의 진폭 함수의 실수부에 의해 가장 큰 영향을 받게 되고 위상 변화량은 허수부에 의해 주로 결정됨을 분석하였다. 본 논 문의 결과는 우주폐기물의 기하급수적 증가에 따른 우주환경의 새로운 인식과 우주폐기물을 고려한 위성통신망 의 링크버짓 설계의 활용에 적합하다.

• Nuclear Engineering and Technology
• /
• 제23권4호
• /
• pp.410-419
• /
• 1991

국내 남동지역에서 수집된 4가지 벤토나이트 시료를 대상으로 X-선 회절과 화학조성을 분석한 결과 Ca-벤토나이트인 것으로 나타났다. 4가지 시료의 비표면적, 양이온교환능 및 팽윤도를 비교하여, 분배계수 측정을 위한 적절한 재료로서 동해 A 시료를 선택하였다. Cs, Co 및 Am의 흡착평형은 약 10일 정도에서 이루어졌으며, Sr의 경우는 이보다 훨씬 발리 이루어졌다. 분배계수 측정 결과로부터 국산 벤토나이트가 높은 흡착능을 가지고 있음을 알았으며, 농도변화에 대한 분배계수 감은 약 $10^{-7}$ mo1/$\ell$의 농도범위에서 최고를 나타내었다.

• Journal of Korean Society for Atmospheric Environment
• /
• 제18권E3호
• /
• pp.165-174
• /
• 2002

A dual -column biofilter system with two different composts was used to investigate the macro-kinetics of dim-ethyl disulfide (DMDS) degradation. The biofilter columns were filled with compost mixtures up to one meter, The gas How rate and DMDS concentration to the biofilters were varied to study their effect on the removal characteris-tics of DMDS. It was found that the biodegradation of DMDS was governed by zero-order reaction -limited macro-kinetics for inlet DMDS concentrations between 10 and 55 ppmv. The overall average zero-order kinetic coeffi-cient for DMDS removal by compost was 0.50 ($\pm$0.1) ppm/sec for both compost mixtures studied. Variations in individual kinetic coefficients were observed due to varying environmental conditions, such as pH and temperature. The kinetic coefficients determined are specific to the system discussed in this work. During high acidity conditions in the filter beds, methyl mercaptan (MM) was observed in the gas samples collected. Appearance of MM was pro-bably due to decreased microbial activity in the lower portions of the biofilter. Considering the neutral pH range required and the presence of methyl mercaptan, it is likely that the microorganisms present in the biofilters used in this research are similar to the T. thioparus (strain E6) species.

• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2010년도 제35회 추계학술대회논문집
• /
• pp.9-12
• /
• 2010

본 논문은 연료 과농 연소 환경 하의 이중 와류 동축형 분사기의 유량 통과 특성 파악을 위해 수행한 실험결과를 수록하였다. 액체산소와 케로신(Jet A-1)을 사용하여 연소시험을 수행하고 유량 통과 특성을 유량계수로 표현하였다. 유량계수 산출을 위해 유량, 압력, 온도를 계측하였다. 연료 분사기의 경우, 산화제 측 분사기 형상, 연소압, 혼합비에 관계없이 일정한 유량 계수 값을 보였다. 이에 반해 산화제 분사기는 연소압과 혼합비 변화에 영향을 받는 것으로 나타났다. 화염 형성 변화가 유량계수 변화에 특히 산화제 측에 영향을 주고 있음을 밝혔다.

• Wind and Structures
• /
• 제15권5호
• /
• pp.385-407
• /
• 2012

Bridge hanger vibrations have been reported under icy conditions. In this paper, the results from a series of static and dynamic wind tunnel tests on a circular cylinder representing a bridge hanger with simulated thin ice accretions are presented. The experiments focus on ice accretions produced for wind perpendicular to the cylinder at velocities below 30 m/s and for temperatures between $-5^{\circ}C$ and $-1^{\circ}C$. Aerodynamic drag, lift and moment coefficients are obtained from the static tests, whilst mean and fluctuating responses are obtained from the dynamic tests. The influence of varying surface roughness is also examined. The static force coefficients are used to predict parameter regions where aerodynamic instability of the iced bridge hanger might be expected to occur, through use of an adapted theoretical 3-DOF quasi-steady galloping instability model, which accounts for sectional axial rotation. A comparison between the 3-DOF model and the instabilities found through two degree-of-freedom (2-DOF) dynamic tests is presented. It is shown that, although there is good agreement between the instabilities found through use of the quasi-steady theory and the dynamic tests, discrepancies exist-indicating the possible inability of quasi-steady theory to fully predict these vibrational instabilities.