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Nonlinear response of viscoelastic fluids under LAOS (large amplitude oscillatory shear) flow
현규,안경현,이승종 한국화학공학회 2007 화학공학의이론과응용 Vol.10 No.1
The rheological properties are usually investigated by two classes of flow. One is the linear viscoelastic experiment, the other is a non-linear viscoelastic experiment which can be designed and performed in various ways. The large amplitude oscillatory shear (LAOS) can provide plentiful additional information. Furthermore, it allows both strain amplitude and time scale to be controlled independently, and it is easy to generate because it does not involve any sudden jump in speed or position. In this study, we will investigate various viscoelastic fluid (polymer solution, suspension, polymer blend, and polymer nanocomposite) under LAOS. As the strain amplitude increases, however, the stress curve becomes distorted and some important information may be lost during data processing. Thus, we need to investigate the stress data more precisely and systematically. We have obtained the stress data using high performance ADC (analog digital converting) card, and investigated the nonlinear response using Fourier transformation (FT) rheology.
Nonlinear response of polypropylene (PP)/Clay nanocomposites underdynamic oscillatory shear flow
현규,Tag Lim,안경현 한국유변학회 2012 Korea-Australia rheology journal Vol.24 No.2
Dynamic oscillatory shear flow tests, i.e. small, medium, and large amplitude oscillatory shear (SAOS,MAOS, and LAOS), are useful to study polymer composite systems. In this study, MAOS and LAOS tests were used to investigate the dynamic behavior of electrically activated polypropylene (PP)/Clay nanocomposites. The morphology of PP/Clay nanocomposites could be controlled by the applied time, type (AC and DC), and strength of the electric field. Various electrically activated PP/Clay nanocomposites were compared in terms of I3/1, which was determined from FT-Rheology within the MAOS and LAOS region. Nonlinear-Linear viscoelastic Ratio (NLR), which developed by Lim et al. (2011), was calculated to measure the dispersion quality of the PP/Clay nanocomposites.
김윤환,박균법,노정호,현규리,송용규 한국항공우주학회 2011 한국항공우주학회 학술발표회 논문집 Vol.2011 No.11
고속에서 고기동을 하는 미사일의 공력계수들은 복잡한 테이블로 표현되고 비선형성이 강하기 때문에 이를 정확히 추정하여 이용하는 것은 비행 제어 시스템을 설계하는데 있어 중요한 문제이다. 이에 본 연구에서는 STT미사일을 모델링하여 일정 마하수에서 테이블과 curve fitting식을 이용하여 시뮬레이션하고 그 결과를 이용하여 공력계수를 추정하였다. 수식을 통한 시뮬레이션 결과에 대한 추정결과는 미세한 오차를 가지는 계수가 일부 존재하지만 나머지 계수에 대해서는 참값을 추정하였고 테이블을 통한 시뮬레이션 결과로 추정한 경우에는 기동이 나타난 total angle of attack의 범위에 대해서 입력된 테이블 값에 대한 만족스러운 추정결과를 보여주었다. Aerodynamic coefficients of high velocity maneuverable missiles are expressed in complex tables and because it has strong nonlinearity it is important issue to estimate correctly and use it to design flight control system. This study uses STT missile as its model and uses tables and curve fitting equations to simulate of certain Mach number and uses its results to estimate coefficient. Estimated results found from the simulation through equations bring fine error to some coefficient but estimated results found from other coefficients which true values are estimated and the tables are used for simulation shows maneuvers through total angle of attack range satisfying estimated results.
Characterization of polyethylene/silica nanocomposites using different rheological analyses
김민근,현규 한국유변학회 2021 Korea-Australia rheology journal Vol.33 No.1
Polyethylene (PE)/silica polymer nanocomposites (PNCs) were investigated mainly using rheological measurements. Various PEs [linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and low-density polyethylene (LDPE)] were selected as polymer matrices, and two comparable particulate silicas were utilized as favorable (hydrophobic, R202) or unfavorable (hydrophilic, A90) fillers. Small amplitude oscillatory shear (SAOS), large amplitude oscillatory shear (LAOS), elongational rheometry, tensile tests, and TEM were adopted to characterize PE/silica PNCs. LDPE/silica nanocomposites showed the best filler dispersion in TEM images but were the poorest in terms of rheological property enhancement. Linear viscoelasticity as determined by SAOS testing was discordant with their morphology. However, nonlinear viscoelasticity as determined by FT-rheology in LAOS testing adequately determined dispersion states. Elongational viscosity revealed LDPE/silica PNCs show strain-hardening behavior, which resulted in intense mixing conditions and best compatibilization on LDPE/silica PNCs. To quantify the strain-hardening effect, strain hardening coefficients (SHCs) were calculated for LDPE/silica PNCs and found to decrease with increasing silica concentration. In addition, tensile testing showed mechanical properties deteriorated with silica content.
Nonlinear material functions under medium amplitude oscillatory shear (MAOS) flow
송형용,현규 한국유변학회 2019 Korea-Australia rheology journal Vol.31 No.4
Dynamic oscillatory shear flow has been widely used to investigate viscoelastic material functions. In particular, small amplitude oscillatory shear (SAOS) tests have become the canonical method for characterizing the linear viscoelastic properties of complex fluids based on strong theoretical background and plenty of experimental results. Recently, there has been increasing interest in the use of large amplitude oscillatory shear (LAOS) tests for the characterization of complex fluids. However, it is difficult to define material functions in LAOS regime due to an infinite number of higher harmonic contributions. For this reason, many recent studies have focused on intrinsic nonlinearities obtained in medium amplitude oscillatory shear (MAOS) regime, which is a subdivision of the full LAOS regime. In this study, we reviewed recent experimental and theoretical results of nonlinear material functions in the MAOS regime, which contain four MAOS moduli (two first-harmonic moduli and two third-harmonic moduli) from Fourier and power series of shear stress, and a nonlinear material function Q0 and its elastic and viscous parts from Fourier-transform rheology (FT rheology). Furthermore, to identify linear-to-nonlinear transitions in stress response of model polystyrene (PS) solutions, we presented Pipkin diagrams in frequency ranges from the rubbery plateau region to the terminal region.