• Korea-Australia Rheology Journal
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• 제17권3호
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• pp.131-140
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• 2005

In this work we show the results of our most recent Direct Numerical Simulations (DNS) of turbulent viscoelastic channel flow using spectral spatial approximations and a stabilizing artificial diffusion in the viscoelastic constitutive model. The Finite-Elasticity Non-Linear Elastic Dumbbell model with the Peterlin approximation (FENE-P) is used to represent the effect of polymer molecules in solution, The corresponding rheological parameters are chosen so that to get closer to the conditions corresponding to maximum drag reduction: A high extensibility parameter (60) and a moderate solvent viscosity ratio (0.8) are used with two different friction Weissenberg numbers (50 and 100). We then first find that the corresponding achieved drag reduction, in the range of friction Reynolds numbers used in this work (180-590), is insensitive to the Reynolds number (in accordance to previous work). The obtained drag reduction is at the level of $49\%\;and\;63\%$, for the friction Weissenberg numbers 50 and 100, respectively. The largest value is substantially higher than any of our previous simulations, performed at more moderate levels of viscoelasticity (i.e. higher viscosity ratio and smaller extensibility parameter values). Therefore, the maximum extensional viscosity exhibited by the modeled system and the friction Weissenberg number can still be considered as the dominant factors determining the levels of drag reduction. These can reach high values, even for of dilute polymer solution (the system modeled by the FENE-P model), provided the flow viscoelasticity is high, corresponding to a high polymer molecular weight (which translates to a high extensibility parameter) and a high friction Weissenberg number. Based on that and the changes observed in the turbulent structure and in the most prevalent statistics, as presented in this work, we can still rationalize for an increasing extensional resistance-based drag reduction mechanism as the most prevalent mechanism for drag reduction, the same one evidenced in our previous work: As the polymer elasticity increases, so does the resistance offered to extensional deformation. That, in turn, changes the structure of the most energy-containing turbulent eddies (they become wider, more well correlated, and weaker in intensity) so that they become less efficient in transferring momentum, thus leading to drag reduction. Such a continuum, rheology-based, mechanism has first been proposed in the early 70s independently by Metzner and Lamley and is to be contrasted against any molecularly based explanations.

• Journal of the Korean Wood Science and Technology
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• 제22권4호
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• pp.19-25
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• 1994

이 연구는 저급합판이나 파아티클보오드의 표면단판 대용으로 얇은 하아드 보오드가 사용될 수 있는지의 여부를 측정하는데 그 목적이 있다. 이 하아드보오드의 제조를 위하여 사용되는 공시재료는 여러 형태의 폐지류를 여러 혼합조건으로, 접착제로서 합성수지와 함께 또는 합성수지를 첨가하지 않은 상태에서, 실험실적 장치로 하아드보오드를 제조하였다. 이들 폐지류는 상당의 리그닌을 함유한 폐골판지, 우유 및 씨리얼 포장지, 그리고 폐잡지를 포함하였다. 실험결과는 0.32cm 두께의 상업용 하아드보오드에 필적할 수 있는 0.21~0.16cm 두께의 하아드보오드를 이들 폐지류로부터 얻을 수 있었다. 이들 폐지류의 혼합효과는 영계수(MOE)와 Taber-마모성 실험을 비롯한 하아드보오드의 제 물리적 성질 즉 두께 팽윤율, 수분 흡수율 및 길이 팽창율에 현저히 나타남을 확인하였다. 이들 폐지류의 혼합과 사용한 합성수지는 하아드보오드의 비중, 영계수(MOE)와 제 물리적 성질에 민감하게 영향을 미쳤다. 이 하아드보오드의 이러한 제 물리적 성질은 저급합판이나 파아티클보오드의 표면단판 내용으로 사용할 수 있음을 보여 주었다.

• 대한치과보철학회지
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• 제41권3호
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• pp.288-299
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• 2003

Statement of problem: Many kinds of post and core systems are in the market, but there are no clear selection criteria for them. Purpose: The purpose of this study was to compare the flexural strength and modulus of elasticity of core materials, and measure the bending strength of post systems made of a variety of materials. Material and Methods: The flexural strength and elastic modulus of thirteen kinds core buildup materials were measured on beams of specimens of $2.0{\times}2.0{\times}24{\pm}0.1mm$. Ten specimens per group were fabricated and loaded on an lnstron testing machine at a crosshead speed of 0.25mm/min. A test span of 20 mm was used. The failure loads were recorded and flexural strength calculated with the measured dimensions. The elastic modulus was calculated from the slopes of the linear portions of the stress-stram graphs. Also nine kinds commercially available prefabricated posts made of various materials with similar nominal diameters, approximately 1.25mm, were loaded in a three-point bend test until plastic deformation or failure occurred. Ten posts per group were tested and the obtained data were anaylzed with analysis of variance and compared with the Tukey multiple comparison tests. Results: Clearfil Photo Core and Luxacore had flexural strengths approaching amalgam, but its modulus of elasticity was only about 15% of that of amalgam. The strengths of the glass ionomer and resin modified glass ionomer were very low. The heat pressed glass ceramic core had a high elastic modulus but a relatively low flexural strength approximating that of the lower strength composite resin core materials. The stainless steel, zirconia and carbon fiber post exhibited high bending strengths. The glass fiber posts displayed strengths that were approximately half of the higher strength posts. Conclusion: When moderate amounts of coronal tooth structure are to be replaced by a post and core on an anterior tooth, a prefabricated post and high strength, high elastic modulus core may be suitable. CLINICAL IMPLICATIONS In this study several newly introduced post and core systems demonstrated satisfactory physical properties. However when the higher stress situation exists with only a minimal ferrule extension remaining a cast post and core or zirconia post and pressed core are desirable.

• 기술혁신학회지
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• 제11권1호
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• pp.72-90
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• 2008

기술혁신이 경제성장에 미치는 영향은 어떠한가? 이는 전통적으로 지식스톡을 반영하는 성장회계법에 의해 분석되었다. R&D에 대한 수익률 추정은 특허와 같은 R&D 산출이 지식축적에서 기인하는 것으로 파악한다. Griliches(1973)는 이를 위해 회귀분석 방법을 사용하였다. 본 연구에서는 기존 성장회계법에서의 추정방법과 달리 R&D 효율성을 나타내는 파라미터가 시간이 지남에 따라 변동(time-varying)하는 것을 허용하는 상태공간 모형(state-space model)을 통해 한국경제의 R&D 효율성(fertility)을 추정하였다. R&D 스톡의 생산성에 대한 탄력성은 $0.120{\sim}0.135$ 정도로 추정되었다.

• 한국전산구조공학회논문집
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• 제24권1호
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• pp.55-60
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• 2011

본 논문에서는 NURBS의 기저함수를 이용하는 등기하 해석을 선형 탄성 문제에 적용하였다. 등기하 해석의 목적은 기하학적 모델링 (CAD)와 수치적 해석 (CAE)를 통합하는 것인데, 이는 계산 망으로써 NURBS에 의한 기하학적 모델링 결과를 직접 이용해서 이룰 수 있다. NURBS 곡면은 조정점과 노트 벡터들을 이용하여 정확한 기하학적 형상을 표현할 수 있으며, 또한 요소의 정밀화 과정이 상대적으로 용이하다는 장점이 있다. 본 연구를 통해 개발된 컴퓨터 코드의 정당성을 보이기 위해 비교적 단순한 형태의 두 가지 구조모델에 적용하였다 ; 1) 균일 내압을 받는 실린더, 2) 균일 인장력이 작용하는 중앙에 구멍이 있는 정사각형 판. 이 두 모델은 정해가 있는 경우로서 절점을 추가하는 h-정밀화와 기저함수의 차수를 증가하는 p-정밀화에 의한 등기하 해석법을 적용한 근사해의 수렴성을 분석하였다.

• Smart Structures and Systems
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• 제20권3호
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• pp.369-383
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• 2017

In this work, the effects of moisture and temperature on free vibration characteristics of functionally graded (FG) nanobeams resting on elastic foundation is studied by proposing a novel simple trigonometric shear deformation theory. The main advantage of this theory is that, in addition to including the shear deformation influence, the displacement field is modeled with only 2 unknowns as the case of the classical beam theory (CBT) and which is even less than the Timoshenko beam theory (TBT). Three types of environmental condition namely uniform, linear, and sinusoidal hygrothermal loading are studied. Material properties of FG beams are assumed to vary according to a power law distribution of the volume fraction of the constituents. Equations of motion are derived from Hamilton's principle. Numerical examples are presented to show the validity and accuracy of present shear deformation theories. The effects of hygro-thermal environments, power law index, nonlocality and elastic foundation on the free vibration responses of FG beams under hygro-thermal effect are investigated.

• 공업화학
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• 제7권2호
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• pp.341-349
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• 1996

폴리우레탄 수지의 염색성을 향상시키기 위하여 염착좌석을 갖는 저분자량의 디올류를 쇄연장제로 활용하였다. 쇄연장제와 폴리올의 종류를 변화시키고, 또한 하드세그멘트 (HS)/소프트세그멘트 (SS) 비율을 변화시키면서 폴리우레탄 수지를 합성하였다. HS/SS가 1.4이고, dimethylolpropionic acld(DMPA), N-butyldiethanolamine(BDEA)를 염착좌석용 쇄연장제(DCE)로 활용한 경우 반응의 불균일성으로 인하여 기계적 물성이 좋지 못하였으며, 특히 에스테르계 폴리올인 poly(butylene/ethylene adipate) glycol(PBEAG)로 합성한 경우 내가수분해성이 현저히 저하되었다. 그러나 DCE로 N-methyldiethanol amine(MDEA)를 사용하고 HS/SS를 1.3으로 조절한 경우 기계적 물성과 염색성이 향상되었으며, MDEA를 선형 쇄연장제(CE)인 1,4-butanediol(1,4-BD)과 에테르형 폴리올인 poly[oxyteramethylene] glycol(PTMG)과 반응시킨 경우 기계적 물성과 내가수분해성이 현저하게 향상되었다. 특히 분자설계적 측면에서 DCE를 HS와 SS내의 배분과 1,6-hexanediol(1,6-HD) 및 neopentylglycol(NPG)과의 공쇄연장으로 초기탄성률, 인장강도, 신장률을 제어 할 수 있음을 알 수 있다.

• Journal of the Korean Wood Science and Technology
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• 제18권2호
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• pp.67-78
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• 1990

This study was performed to determine the characteristics of physical and mechanical properties of domestic wood based materials; plywood, particleboard, medium density fiberboard. Main items of tested properties were panel size, moisture content, water absorption, linear expansion and thickness swelling, glue bond shear strength, bending properties(stress at proportional limit, modulus of rupture. modulus of elasticity), tensile strength, screw holding strength, and internal bond as neccessary. the results were discussed mainly with Korean Standards. The obtained conclusions are as follows; 1. Length and width of 3mm thin plywood(3-ply) and 12mm thick plywood(7-ply) were satished with KS-standard, but thicknesses of these panels were not- passed tolerance limit except one of eight makers. 2. Length and width of particleboard and medium density fiberboard were greater than the tolerance limit value of KS standard, but the thicknesses of these panels were passed this value. 3. Moisture contents of 12mm thick and 3mm thin plywood were satisfied with KS-standard except one mill made 3mm thin plywood. 4. Moisture absortion of plywood was not passed tolerance limit of KS-standard but particleboard was satisfied with this standard value. 5. Dry and wet shear strengths in glue bond of 3mm thin plywood were not reached to KS-standard, but those of 12mm thick plywood were sufficiently satisfied with KS standrad. 6. Modulus of ruptures, parallel to grain and perpendicular to grain of plywood, and particleboard and medium density fiberboard were satisfied with KS-standard. 7. Tensile strengths, parallel to grain and perpendicular to grain of plywood were satisfied with allowance stress of US product standard PS 1-74. 8. Screw holding strength of particleboard was not reached to KS standard, but internal bond was satisfied with KS standard.

• Smart Structures and Systems
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• 제1권1호
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• pp.83-101
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• 2005

This paper summarizes the application of a rational methodology for the structural assessment of older reinforced concrete Tunisian bridges. This methodology is based on ambient vibration measurement of the bridge, identification of the structure's modal signature and finite element model updating. The selected case study is the Boujnah bridge of the Tunis-Msaken Highway. This bridge is made of a continuous four-span simply supported reinforced concrete slab without girders resting on elastomeric bearings at each support. Ambient vibration tests were conducted on the bridge using a data acquisition system with nine force-balance accelerometers placed at selected locations of the bridge. The Enhanced Frequency Domain Decomposition technique was applied to extract the dynamic characteristics of the bridge. The finite element model was updated in order to obtain a reasonable correlation between experimental and numerical modal properties. For the model updating part of the study, the parameters selected for the updating process include the concrete modulus of elasticity, the elastic bearing stiffness and the foundation spring stiffnesses. The primary objective of the paper is to demonstrate the use of the Enhanced Frequency Domain Decomposition technique combined with model updating to provide data that could be used to assess the structural condition of the selected bridge. The application of the proposed methodology led to a relatively faithful linear elastic model of the bridge in its present condition.

• Steel and Composite Structures
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• 제34권5호
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• pp.643-655
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• 2020

In the present work, the buckling behavior of a single-layered graphene sheet (SLGS) embedded in visco-Pasternak's medium is studied using nonlocal four-unknown integral model. This model has a displacement field with integral terms which includes the effect of transverse shear deformation without using shear correction factors. The visco-Pasternak's medium is introduced by considering the damping effect to the classical foundation model which modeled by the linear Winkler's coefficient and Pasternak's (shear) foundation coefficient. The SLGS under consideration is subjected to compressive in- plane edge loads per unit length. The influences of many parameters such as nonlocal parameter, geometric ratio, the visco-Pasternak's coefficients, damping parameter, and mode numbers on the buckling response of the SLGSs are studied and discussed.