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Numerical simulation of a short flexible pipe subject to forced motion and vortex-induced vibration

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Acta Oceanologica Sinica 2009年第6期28卷 70-83页

作者： chen Zhengshou kim Wu-joan CHOI Yoon-rak Engineering College Ocean University of China Qingdao 266100 China Department of Naval Architecture ＆ Marine Engineering Mokpo National University Mokpo534-729 Korea School of Naval Architecture ＆ Ocean Engineering University of Ulsan Ulsan 680-749 Korea

A series of numerical sinmlations about a small scale （aspect ratio： 63.2） flexible pipe undergoing forced harmonious oscillation and vortex-induced vibration （VIV） have been taken into account. The wake hydrodynamics and pipe deformation were accomplished by ANSYS MFX solution strat- egy designed for fluid-structure interaction （FSI） problem with well-performed LES model. The configuration of structured mesh, multi-domain design, different mesh stiffness admeasured by User Fortran ensured that the numerical task was competent to deal with large deformation related to this case. The introduction of instantaneous amplitude definition and modeless component decom- position method （chen and kim, 2008） was helpful to reveal much more information from modal analysis. Most results from numerical simulation are generally consistent with those from model test （Choi and Hong, 2000） via the comparison between them. As supplementary to model test, visualization of the vortex wake was also provided. It has been proved that the forced oscillation doesn＇t only excite a complicated dumbbell-like wake pattern around the outer thimble, but also results in inner flow inside the PVC pipe. The velocity of the inner flow increases with the frequency of forced oscillation.

关键词： force oscillation vortex-induced vibration (VIV) fluid-structure interaction (FSI) ANAYS MFX instantaneous amplitude

Effect of Upward Internal Flow on Dynamics of Riser Model Subject to Shear Current

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China Ocean Engineering 2012年第1期26卷 95-108页

作者：陈正寿 kim Wu-joan 熊丛博 School of Naval Architecture and Civil Engineering Zhejiang Ocean University Department of Ocean Engineering Mokpo National University The First Institute of Oceanography State Oceanic Administration

Numerical study about vortex-induced vibration （V/V） related to a flexible riser model in consideration of internal flow progressing inside has been performed. The main objective of this work is to investigate the coupled fluid-structure interaction （FSI） taking place between tensioned riser model, external shear current and upward-progressing internal flow （from ocean bottom to surface）. A CAE technology behind the current research which combines structural software with the CFD technology has been proposed. According to the result from dynamic analysis, it has been found that the existence of upward-progressing internal flow does play an important role in determining the vibration mode （/dominant frequency）, vibration intensity and the magnitude of instantaneous vibration amplitude, when the velocity ratio of internal flow against external current is relatively high. As a rule, the larger the velocity of intemal flow is, the more it contributes to the dynamic vibration response of the flexible riser model. In addition, multi-modal vibration phenomenon has been widely observed, for asymmetric curvature along the riser span emerges in the case of external shear current being imposed.

关键词： internal flow upward-progressing fluid-structure interaction velocity ratio

Esophagogastric junction distensibility assessed using the functional lumen imaging probe

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World Journal of Gastroenterology 2017年第7期23卷 1289-1297页

作者： joan W chen Joel H Rubenstein Department of Gastro-enterology and Hepatology University of Michigan Health SystemsAnn ArborMI 48109United States

To assess reference values in the literature for esophageal distensibility and cross-sectional area in healthy and diseased subjects measured by the functional lumen imaging probe (FLIP).METHODSSystematic search and review of articles in Medline and Embase pertaining to the use of FLIP in the esophagus was conducted in accordance with the PRISMA guidelines. Cross-sectional area and distensibility at the esophagogastric junction (EGJ) were abstracted for normal subjects, achalasia, and gastroesophageal reflux disease (GERD) patients, stratified by balloon length and volume of *** achalasia studies (n = 154), 3 GERD (n = 52), and 5 studies including healthy controls (n = 98) were included in the systematic review. Normative data varied widely amongst studies of healthy volunteers. In contrast, studies in achalasia patients uniformly demonstrated low point estimates in distensibility ≤ 1.6 mm2/mmHg prior to treatment that increased to ≥ 3.4 mm2/mmHg following treatment at 40mL bag volume. In GERD patients, distensibility fell to the range of untreated achalasia (≤ 2.85 mm2/mmHg) following *** may be a useful tool in assessment of treatment efficacy in achalasia. The drastic drop in EGJ distensibility after fundoplication suggests that FLIP measurements need to be interpreted in the context of esophageal body motility and highlights the importance of pre-operative screening for dysmotility. Future studies using standardized FLIP protocol and balloon size are needed.

关键词： Impedance planimetry Gastroesophageal reflux disease Esophageal distensibility Achalasia

A Maximum-Entropy Method for Estimating the Spectrum

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China Ocean Engineering 2007年第4期21卷 587-596页

作者：陈正寿王莉萍于定勇 Wu-joan kim Engineering College Ocean University of China Mathematics Department Ocean University of China Department of Naval Architecture & Marine Engineering Mokpo National University

Based on the maximunl-entropy （ME） principle, a new power spectral estimator for random waves is derived in the form of S（ω）=a/8H^2^-（2π）^（d＋2）exp[-b（2π/ω）^n],1）y solving a variational problem subject to some quite general constraints. This robust method is comprehensive enough to describe the wave spectra even in extreme wave conditions and is superior to periodogranl method that is not suit＇able to process comparatively short or intensively unsteady signals for its tremendous boundary effect and some inherent defects of FKF. Fortunately, the newly derived method for spectral estimation works fairly well, even though the sample data sets are very short and unsteady, and the reliability and efficiency of this spectral estimator have been preliminarily proved.

关键词： maximum-entropy method (MEM) maximum-entropy ,spectral estimator PDF periodogram method

Measuring nanoscale friction at graphene step edges

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Friction 2020年第4期8卷 802-811页

作者： Zhe chen Seong H.kim Department of Chemical Engineering and Materials Research Institute Pennsylvania State UniversityUniversity ParkPA 16802USA

Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer thick graphene step edges was studied using atomic force microscopy (AFM) with a Si tip in dry nitrogen atmosphere. It is found that, when the tip slides over a'buried' graphene step edge, there is a resistive force during the step-up motion and an assistive force during the step-down motion due to the topographic height change. The magnitude of these two forces is small and the same in both step-up and step-down motions. As for the'exposed' graphene step edge, friction increases in magnitude and exhibits more complicated behaviors. During the step-down motion of the tip over the exposed step edge, both resistive and assistive components can be detected in the lateral force signal of AFM if the scan resolution is sufficiently high. The resistive component is attributed to chemical interactions between the functional groups at the tip and step-edge surfaces, and the assistive component is due to the topographic effect, same as the case of buried step edge. If a blunt tip is used, the distinct effects of these two components become more prominent. In the step-up scan direction, the blunt tip appears to have two separate topographic effects–elastic deformation of the contact region at the bottom of the tip due to the substrate height change at the step edge and tilting of the tip while the vertical position of the cantilever (the end of the tip) ascends from the lower terrace to the upper terrace. The high-resolution measurement of friction behaviors at graphene step edges will further enrich understanding of interfacial friction behaviors on graphene-covered surfaces.

关键词： friction graphene step edge atomic force microscopy tip bluntness

NONLINEAR STOCHASTIC HEAT EQUATION DRIVEN BY SPATIALLY COLORED NOISE:MOMENTS AND INTERMITTENCY

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Acta Mathematica Scientia 2019年第3期39卷 645-668页

作者： Le chen Kunwoo kim Department of Mathematical Sciences University of NevadaLas Vegas4505 S.Maryland PkwyLas VegasNevada89154-4020USA Department of Mathematics Pohang University of Science and Technology77 Cheongam-RoNam-GuPohangGyeongbuk37673Korea

In this article, we study the nonlinear stochastic heat equation in the spatial domain R^d subject to a Gaussian noise which is white in time and colored in space. The spatial correlation can be any symmetric, nonnegative and nonnegative-definite function that satisfies Dalang's condition. We establish the existence and uniqueness of a random field solution starting from measure-valued initial data. We find the upper and lower bounds for the second moment. With these moment bounds, we first establish some necessary and sufficient conditions for the phase transition of the moment Lyapunov exponents, which extends the classical results from the stochastic heat equation on Z^d to that on R^***, we prove a localization result for the intermittency fronts, which extends results by Conus and Khoshnevisan [9] from one space dimension to higher space dimension. The linear case has been recently proved by Huang et al [17] using different techniques.

关键词： Stochastic heat equation moment estimates phase transition intermittency intermittency front measure-valued initial data moment Lyapunov exponents

Modified analogue forecasting in the hidden Markov frameworkfor meteorological droughts

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Science China(Technological Sciences) 2019年第1期62卷 151-162页

作者： chen Si CHUNG GunHui kim Byung Sik kim Tae-Woong Department of Civil and Environmental Engineering Hanyang University Department of Civil Engineering Hoseo University Department of Urban Environmental Disaster Prevention Engineering Kangwon National University

An analogue method(AM) is a nonparametric approach that has been applied to predict the future states of a dynamic system by following the evolution of the analogues in the historical archive. In this study, we proposed a hidden Markov model(HMM)framework for a modified analogue forecasting(MAF) approach for meteorological droughts in Korea. The unobservable(hidden) state process in the framework aims to model the underlying drought state, while the observation process was formed from the time series of the standardized precipitation index(SPI) as a drought index. Within the framework, the likelihood estimator was used as the measure of similarity between past SPI analogues and current data. The MAF approach was conducted on the selected analogues to make forecasts at lead times of one and three months. The proposed model was applied to five selected stations in Korea using the SPI data from 1973 to 2016. The forecasting performance of the proposed model was tested during the validation period(2003–2016) using several statistical criteria and it was compared to a persistence-based benchmark model. The results showed significant improvement in the forecasting capacity, and satisfactory performance for numerical SPI forecasting and categorical drought forecasting. The results also suggested that the proposed model was able to provide useful information for determining future drought categories for early drought warning with a lead time of up to three months.

关键词： modified analogue forecasting hidden Markov model meteorological drought standardized precipitation index

Composition design of high-entropy alloys with deep sets learning

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npj Computational Materials 2022年第1期8卷 834-844页

作者： Jie Zhang chen Cai George kim Yusu Wang Wei chen Department of Mechanical Materialsand Aerospace EngineeringIllinois Institute of TechnologyChicagoIL60616USA Halıcıoğlu Data Science Institute University of California San DiegoLa JollaCA92093USA

High entropy alloys(HEAs)are an important material class in the development of next-generation structural materials,but the astronomically large composition space cannot be efficiently explored by experiments or first-principles *** learning(ML)methods might address this challenge,but ML of HEAs has been hindered by the scarcity of HEA property *** this work,the EMTO-CPA method was used to generate a large HEA dataset(spanning a composition space of 14 elements)containing 7086 cubic HEA structures with structural properties,1911 of which have the complete elastic tensor *** elastic property dataset was used to train a ML model with the Deep Sets *** Deep Sets model has better predictive performance and generalizability compared to other ML *** rule mining was applied to the model predictions to describe the compositional dependence of HEA elastic properties and to demonstrate the potential for data-driven alloy design.

关键词： alloys alloy entropy

Constructing confidence intervals of extreme rainfall quantiles using Bayesian,bootstrap,and profile likelihood approaches

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Science China(Technological Sciences) 2016年第4期59卷 573-585页

作者： chen Si LI YaXing SHIN JiYae kim TaeWoong department of civil and environmental engineering hanyang universitySeoul 133-791Korea department of electronics&communication engineering hanyang universityAnsan 426-791Korea department of civil and environmental engineering hanyang universityAnsan 426-791Korea

Hydrological risk is highly dependent on the occurrence of extreme *** fact has led to a wide range of studies on the estimation and uncertainty analysis of the *** most cases,confidence intervals(CIs)are constructed to represent the uncertainty of the *** the accuracy of CIs depends on the asymptotic normality of the data and is questionable with limited observations in practice,a Bayesian highest posterior density(HPD)interval,bootstrap percentile interval,and profile likelihood(PL)interval have been introduced to analyze the uncertainty that does not depend on the normality ***,comparison studies to investigate their performances in terms of the accuracy and uncertainty of the estimates are *** addition,the strengths,weakness,and conditions necessary for performing each method also must be ***,in this study,test experiments with simulations from varying parent distributions and different sample sizes were ***,applications to the annual maximum rainfall(AMR)time series data in South Korea were *** districts with 38-year(1973–2010)AMR observations were fitted by the three aforementioned methods in the *** both the experimental and application results,the Bayesian method is found to provide the lowest uncertainty of the design level while the PL estimates generally have the highest accuracy but also the largest *** bootstrap estimates are usually inferior to the other two methods,but can perform adequately when the distribution model is not heavy-tailed and the sample size is *** distribution tail behavior and the sample size are clearly found to affect the estimation accuracy and *** study presents a comparative result,which can help researchers make decisions in the context of assessing extreme rainfall uncertainties.

关键词： Bayesian bootstrap profile likelihood confidence interval frequency analysis