Control of viscous fingering and mixing in miscible displacements with time-dependent rates

In miscible displacements encountered in enhanced oil recovery processes, the unfavorable viscosity contrast between injected solvent and oil usually leads to viscous fingering (VF), a hydrodynamic instability which may result in a lower sweep efficiency and oil recovery. This phenomenon can be observed in a wide range of flows in subsurface porous media. This study examined a simple cyclic time-dependent displacement rate and its effects on the onset and longer development of VF. It is found that such varying displacement rate can either stabilize or destabilize VF, depending on the cycle period, amplitude, and displacement scenarios. The most important mechanism is that such time-dependent rate can effectively change the competition between convection (destabilizing effect) and dispersion (stabilizing effect). This is different from the widely used constant injection rate where the flow instability is actually determined by the Peclet number and mobility contrast for a given scenario. This study therefore provided a new aspect to control VF, either enhance or reduce, with low additional costs. It is therefore both scientifically and practically important for a wide range of flows in subsurface porous media. © 2017 American Institute of Chemical Engineers AIChE J, 65: 360–371, 2019     

Unconditional stability and error estimates of the modified characteristics FEMs for the time-dependent incompressible MHD equations

This paper focuses on the unconditional stability and convergence of characteristics type methods for the time-dependent incompressible MHD equations. For this purpose, we introduce a new characteristics time-discrete system. The optimal error estimates in $L^2$ and $H^1$ norms for the typical modified characteristics finite element method unconditionally can be deduced, while the whole previous works require certain time-step restrictions. Some numerical experiments document performance of the characteristics type methods for the time-dependent incompressible MHD equations.     

Seismic life-cycle cost analysis of ageing highway bridges under chloride exposure conditions: modelling and recommendations

Chloride-induced corrosion of highway bridges constitutes a critical form of environmental deterioration and may result in significant escalation of seismic life-cycle costs due to increased fragility during earthquake events. Most of existing literature tends to adopt simplistic uniform area loss assumptions in lieu of potentially complex, yet realistic and more detrimental, pitting corrosion models for seismic vulnerability analysis. Since the degree of deterioration depends on the severity and duration of exposure, there exists a need to investigate the influence of uniform vs. pitting corrosion assumption on seismic life-cycle costs for varied chloride exposure conditions. A case-study example of a highway bridge in Central and Southeastern US reveals consideration of pitting corrosion as critical for extreme exposures compared to relatively minor settings. Subsequently this study provides recommendations to aid bridge engineers and stakeholders to balance between computational cost and accuracy of results to aid prompt decisions on rehabilitation of ageing bridges in different exposure conditions. A framework is also included to compute seismic life-cycle costs from generic measures of corrosion, independent of assumed exposure scenario. This framework is particularly helpful for seismic loss assessment of highway bridges in chloride exposure zones with periodic field measurements to estimate the extent of structural deterioration.     

Long-range Coulomb interaction effects on polarons in conjugated polymers

Combining the Su–Schrieffer–Heeger and Pariser–Parr–Pople model (SSH+PPP), we have studied the influence of electron–electron interactions on the motion and dissociation of a polaron in the presence of an electric field, with emphasis on the long-range Coulomb interactions. The multiconfigurational time-dependent Hartree–Fock (MCTDHF) formalism was used to compute the electron–electron interactions. How the saturated velocity and the critical dissociation electric field of the polaron are related to the on-site Coulomb repulsion and long-range Coulomb interactions has been investigated. It was found that the on-site Coulomb interaction does not favor the motion of a polaron. There is a critical strength of the long-range Coulomb interaction for which the polaron takes on the most localized configuration. Comparing with the results obtained using the extended Hubbard model, we found that competition between the long-range Coulomb interactions and the on-site Coulomb interaction leads to a non-monotonic dependence of both the saturated velocity and the critical dissociation electric field on the long-range Coulomb interactions.     

Transient thermoelastic analysis of infinite plates with time-periodic heat transfer coefficient: Frequency response of thermal stress

An analytical method is presented for one-dimensional transient heat conduction and associated thermal stress problems of a layered infinite plate whose heat transfer coe?cient (HTC) on one external boundary is an arbitrary function of time. A shifting function method and an orthogonal expansion technique are used to obtain analytical solutions to the problems. Using the solutions, the structural response to sinusoidal HTC oscillation is investigated in single and bilayered plates, the other boundary of which is kept at constant temperature. Numerical results demonstrate the effects of average HTC (measured by the Biot number) and mechanical boundary conditions on the frequency responses of temperature and thermal stress. It is found that the range of stress oscillation decreases with an increase in the frequency of HTC oscillation except for single-layered plates without surface tractions.     

An efficient method for estimating time-dependent failure possibility by combining adaptive Kriging with adaptive truncated fuzzy simulation

Time-dependent failure possibility (TDFP) can measure the structural safety level for a time interval of interest under fuzzy uncertainty, but its calculational cost is unaffordable by using fuzzy simulation (FS) due to a required large size of FS candidate sampling pool (CSP). Although time-dependent adaptive Kriging model (T-AK) combined with FS (T-AK-FS) was presented to reduce the number of calling performance function, a large FS CSP still makes training T-AK time-consuming. To improve its efficiency, an adaptive truncated FS (ATFS) with T-AK (T-AK-ATFS) is proposed by CSP size reduction approach. By T-AK-ATFS, the largest safety hypercube in fuzzy standard space is adaptively searched, in which the samples are in safety states and can be removed from the FS CSP. Moreover, T-AK is adaptively trained to search the largest safety hypercube and estimate TDFP simultaneously. In adaptively searching process, the FS CSP is divided into several sub-CSPs, on which training T-AK is more time-saving. Overall, strategies of T-AK-ATFS include proposing ATFS to reduce the FS CSP, adaptively searching the largest safety hypercube, estimating the TDFP with the same T-AK and training T-AK in the sub-CSPs sequentially. Verified by examples, these strategies make T-AK-ATFS more efficient than existing FS and T-AK-FS.     

基于概率密度演化的锈蚀混凝土梁时变可靠性分析

建立了锈蚀钢筋混凝土梁的极限状态函数，其中引入了锈蚀不均匀系数考虑钢筋锈蚀不均匀性和随机性。根据概率守恒原理引出锈蚀钢筋混凝土梁极限状态函数的广义概率密度演化方程，并介绍了其TVD(total variation diminishing)差分格式的有限差分方法。运用“吸收边界条件”提出了基于概率密度演化理论的锈蚀钢筋混凝土梁时变可靠度计算方法。以三个锈蚀钢筋混凝土梁为例，展示广义概率密度演化方程以及时变可靠度的计算结果。通过100万次Monte Carlo模拟方法以及2范数误差指标考察了概率密度演化及时变可靠度计算结果的精准度。二阶矩信息和可靠度的对比结果显示：概率密度演化法能够以较小的计算代价，较为精确地捕捉锈蚀钢筋混凝土梁在服役期间的概率密度演化信息，较为准确地预测其时变可靠度。     

韩江河底隧道预灌浆区的强度时间相关特性

作为防水、防腐和地基加固等多种目的和手段之一,灌浆技术可应用于河底隧道或者海底隧道。水泥作为灌浆的主要材料具有强度时间相关特性。可是目前在大部分河底隧道在设计中不考虑水泥的这种强度时间相关特性。据此,提出以弹性波测试方法获得河底隧道施工中预注浆的强度时间相关特性,并采用数值模拟方法将获得的强度时间相关特性用于评价河底隧道的施工。为了获得预注浆区域的强度时间相关特性,进行了弹性波试验。然后把试验获得的预注浆区域的强度时间相关特性用于隧道施工仿真。计算结果表明,预注浆区域的强度时间相关特性不会对河底隧道的安全产生太大的影响。在模拟河底隧道施工时,发现其强度可以使用2～7d的预注浆区域的强度。分析结果也表明,通过强度和刚度的折减可模拟其强度时间相关特性。通过试验和数值模拟相结合的方式,提出预注浆区域的强度时间相关特性对河底隧道影响的方法,可为隧道工程的施工提供参考。     

变荷载作用下未打穿竖井地基固结分析

利用已有瞬时荷载作用下未打穿竖井地基固结模型,根据线性偏微分方程的叠加原理,将变荷载作用问题分解为瞬时荷载作用问题,给出了变荷载作用下未打穿竖井地基固结解答。利用该求解方法,对一未打穿竖井地基在单级线性加载和多级加载情况下的孔压消散进行了分析。分析表明,受加载历时影响,竖井打设区平均固结度减小幅度大于下卧层,但下卧层受影响时间要长得多。另外,在粗略设计中,可以将多级加载简化为单级线性加载,这样对后期平均固结度的影响不大。     

变荷载下双曲线模型修正土体一维固结理论

 假定土体本构关系满足双曲线模型，把利用该模型得出的孔隙比e和有效应力s ¢的双曲线关系应用到一维单层均质地基固结理论的研究中，通过假定土体的初始有效应力沿深度均匀分布和渗透系数的降低与压缩系数的减小成正比，推导出任意荷载作用下一维非线性固结理论的方程。对于所得到的非线性偏微分方程，通过x-s ¢代换化成线性偏微分方程，并按照具体的初始和边界条件采用分离变量法求出土体有效应力s ¢和超静孔隙水压力u的解，由此分别推导出按沉降定义的平均固结度Us和按平均孔隙水压力定义的平均固结度Up的表达式。采用MATLAB语言编制相应的计算程序，结合具体算例与已有的修正非线性固结理论解的结果进行对比来论证本修正的正确性。计算结果表明：固结度随着时间的增长而增长，其增长速率逐渐减小；Us值稍大于Up值；2种修正的计算结果中存在微小的差异，其原因是在推导固结方程时采用了不同的本构模型以及不同的对渗透系数变化规律的假定。