Numerical modeling of coupled thermo-mechanical response of a rock pillar

Understanding the rock mass response to excavation and thermal loading and improving the capability of the numerical models for simulating the progressive failure process of brittle rocks are important for safety assessment and optimization design of nuclear waste repositories.The international cooperative DECOVALEX-2011 project provides a platform for development,validation and comparison of numerical models,in which the sp pillar stability experiment（APSE） was selected as the modeling target for Task B.This paper presents the modeling results of Wuhan University（WHU） team for stages 1 and 2 of Task B by using a coupled thermo-mechanical model within the framework of continuum mechanics.The rock mass response to excavation is modeled with linear elastic,elastoplastic and brittle-plastic models,while the response to heating is modeled with a coupled thermo-elastic model.The capabilities and limitations of the model for representation of the thermo-mechanical responses of the rock pillar are discussed by comparing the modeling results with experimental observations.The results may provide a helpful reference for the stability and safety assessment of the hard granite host rock in China＇s Beishan preselected area for high-level radioactive waste disposal.

Shear behavior of intact granite under thermo-mechanical coupling and three-dimensional morphology of shear-formed fractures

The shear failure of intact rock under thermo-mechanical(TM)coupling conditions is common,such as in enhanced geothermal mining and deep mine construction.Under the effect of a continuous engineering disturbance,shear-formed fractures are prone to secondary instability,posing a severe threat to deep engineering.Although numerous studies regarding three-dimensional(3D)morphologies of fracture surfaces have been conducted,the understanding of shear-formed fractures under TM coupling conditions is limited.In this study,direct shear tests of intact granite under various TM coupling conditions were conducted,followed by 3D laser scanning tests of shear-formed fractures.Test results demonstrated that the peak shear strength of intact granite is positively correlated with the normal stress,whereas it is negatively correlated with the temperature.The internal friction angle and cohesion of intact granite significantly decrease with an increase in the temperature.The anisotropy,roughness value,and height of the asperities on the fracture surfaces are reduced as the normal stress increases,whereas their variation trends are the opposite as the temperature increases.The macroscopic failure mode of intact granite under TM coupling conditions is dominated by mixed tensileeshear and shear failures.As the normal stress increases,intragranular fractures are developed ranging from a local to a global distribution,and the macroscopic failure mode of intact granite changes from mixed tensileeshear to shear failure.Finally,3D morphological characteristics of the asperities on the shear-formed fracture surfaces were analyzed,and a quadrangular pyramid conceptual model representing these asperities was proposed and sufficiently verified.

Fully Coupled Fluid-Structure Interaction Model Based on Distributed Lagrange Multiplier/Fictitious Domain Method

This paper, with a finite element method, studies the interaction of a coupled incompressible fluid-rigid structure system with a free surface subjected to external wave excitations. With this fully coupled model, the rigid structure is taken as ＂fictitious＂ fluid with zero strain rate. Both fluid and structure are described by velocity and pressure. The whole domain, including fluid region and structure region, is modeled by the incompressible Navier-Stokes equations which are discretized with fixed Eulerian mesh. However, to keep the structure＇ s rigid body shape and behavior, a rigid body constraint is enforced on the ＂fictitious＂ fluid domain by use of the Distributed Lagrange Multipher/Fictitious Domain （DLM/ FD） method which is originally introduced to solve particulate flow problems by Glowinski et al. For the verification of the model presented herein, a 2D numerical wave tank is established to simulate small amplitude wave propagations, and then numerical results are compared with analytical solutions. Finally, a 2D example of fluid-structure interaction under wave dynamic forces provides convincing evidences for the method excellent solution quality and fidelity.

Hydrodynamic Response of A Fully Coupled TLP Hull-TTR System with Detailed Modeling of A Hydraulic Pneumatic Tensioner and Riser Joints

Tension Leg Platform(TLP)in deepwater oil and gas field development usually consists of a hull,tendons,and top tension risers(TTRs).To maintain its top tension,each TTR is connected with a tensioner system to the hull.Owing to the complicated configuration of the tensioners,the hull and TTRs form a strong coupled system.Traditionally,some simplified tensioner models are applied to analyze the TLP structures.There is a large discrepancy between their analysis results and the actual mechanism behaviors of a tensioner.It is very necessary to develop a more detailed tensioner model to consider the coupling effects between TLP and TTRs.In the present study,a fully coupled TLP hull-TTR system for hydrodynamic numerical simulation is established.A specific hydraulic pneumatic tensioner is modeled by considering 4 cylinders.The production TTR model is stacked up by specific riser joints.The simulation is also extended to analyze an array of TTRs.Different regular and irregular waves are considered.The behaviors of different cylinders are presented.The results show that it is important to consider the specific configurations of the tensioner and TTRs,which may lead to obviously different response behaviors,compared with those from a simplified model.

A thermo-mechanical damage constitutive model for deep rock considering brittleness-ductility transition characteristics

This paper developed a statistical damage constitutive model for deep rock by considering the effects of external load and thermal treatment temperature based on the distortion energy.The model parameters were determined through the extremum features of stress−strain curve.Subsequently,the model predictions were compared with experimental results of marble samples.It is found that when the treatment temperature rises,the coupling damage evolution curve shows an S-shape and the slope of ascending branch gradually decreases during the coupling damage evolution process.At a constant temperature,confining pressure can suppress the expansion of micro-fractures.As the confining pressure increases the rock exhibits ductility characteristics,and the shape of coupling damage curve changes from an S-shape into a quasi-parabolic shape.This model can well characterize the influence of high temperature on the mechanical properties of deep rock and its brittleness-ductility transition characteristics under confining pressure.Also,it is suitable for sandstone and granite,especially in predicting the pre-peak stage and peak stress of stress−strain curve under the coupling action of confining pressure and high temperature.The relevant results can provide a reference for further research on the constitutive relationship of rock-like materials and their engineering applications.

Mathematical model of coupled thermo-mechanical behavior during plane strain compression of 06Cr19Ni9NbN steel

The process of plane strain compression of 06Cr19Ni9NbN steel was carried out in the temperature range of 1000-1200℃ and the reduction ratio range of 10-50%.Combining the finite element numerical simulation,a new constitutive model of thermal deformation was established, which provides the theoretical basis to optimize the plarie strain compression process of the steel.The temperature and grain size at different regions were achieved by experiment and simulation,respectively.According to the results,the mathematical models of stress and temperature during the plane strain compression were established by mathematical analysis method.The new temperature models were established in three regions,respectively,and the stress models took account of the variation of temperature and'st^rain rate.Finally,by comparing the results of calculation,numerical simulation and experiment,the accuracy and validity of these mathematical models were verified.

页岩气老井重复压裂时机优化方法

基于多孔介质弹性理论、嵌入式离散裂缝模型及有限体积法,考虑页岩气微观渗流机制,建立适用于裂缝性页岩气储集层的渗流-地质力学全耦合模型,并提出了重复压裂时机优化方法,采用四川盆地涪陵页岩气井资料分析了重复压裂时机的影响因素。研究表明:受地层压力衰竭影响,最大水平主应力反转面积占总面积的百分比随时间的延长先增加后减小,且距人工裂缝越近的区域,应力反转面积百分比曲线出现峰值的时间越短,最终归零(恢复到初始状态)的时间也越短。重复压裂的最佳时间受基质渗透率、初始应力差、天然裂缝逼近角的影响:基质渗透率、初始应力差越大,应力反转面积百分比曲线出现峰值、恢复到初始状态的时间越短,采取重复压裂措施的时机越早。天然裂缝逼近角越大,裂缝附近越难发生应力反转、重复压裂最佳时间越早,人工裂缝末端以远区域越易发生应力反转、重复压裂最佳时间越晚。对于基质渗透率很小的储集层,其单井产能递减快,为保证经济性,可采取关井或注气补能等措施恢复应力,提前实施重复压裂。

基于全耦合模型的海上吊装作业动力响应特征研究

针对现有海上吊装模拟方法不能有效考虑浮吊船舶—系泊系统—吊物系统之间的动态耦合,难以实现基座回转、吊臂变幅连续模拟问题,基于Matlab/Simulink、ADAMS联合仿真平台建立了海上吊装作业全耦合动力分析模型,探究了中长周期波作用下波浪周期、吊缆收放、基座回转、吊臂变幅等参数变化对吊装作业系统动力响应的影响规律。结果表明,波浪周期接近吊物系统固有周期时会引发共振,导致吊物摇摆幅值显著增加。基座回转、吊臂变幅等因改变了吊物的空间位置从而引发了浮吊船舶倾覆力矩的变化,进而对浮吊船舶运动响应产生显著影响。当基座回转90°后,浮吊船舶的横倾角增加约2.74°。

Multi-Fidelity Simulation of Gas Turbine Overall Performance by Directly Coupling High-Fidelity Models of Multiple Rotating Components

Multi-fidelity simulations incorporate computational fluid dynamics(CFD) models into a thermodynamic model,enabling the simulation of the overall performance of an entire gas turbine with high-fidelity components.Traditional iterative coupled methods rely on characteristic maps,while fully coupled methods directly incorporate high-fidelity simulations.However,fully coupled methods face challenges in simulating rotating components,including weak convergence and complex implementation.To address these challenges,a fully coupled method with logarithmic transformations was developed to directly integrate high-fidelity CFD models of multiple rotating components.The developed fully coupled method was then applied to evaluate the overall performance of a KJ66 micro gas turbine across various off-design simulations.The developed fully coupled method was also compared with the traditional iterative coupled method.Furthermore,experimental data from ground tests were conducted to verify its effectiveness.The convergence history indicated that the proposed fully coupled method exhibited stable convergence,even under far-off-design simulations.The experimental verification demonstrated that the multi-fidelity simulation with the fully coupled method achieved high accuracy in off-design conditions.Further analysis revealed inherent differences in the coupling methods of CFD models between the developed fully coupled and traditional iterative coupled methods.These inherent differences provide valuable insights for reducing errors between the component-level model and CFD models in different coupling methods.The developed fully coupled method,introducing logarithmic transformations,offers more realistic support for the detailed and optimal design of high-fidelity rotating components within the overall performance platform of gas turbines.

Optimization method of refracturing timing for old shale gas wells

Based on the elastic theory of porous media,embedded discrete fracture model and finite volume method,and considering the micro-seepage mechanism of shale gas,a fully coupled seepage-geomechanical model suitable for fractured shale gas reservoirs is established,the optimization method of refracturing timing is proposed,and the influencing factors of refracturing timing are analyzed based on the data from shale gas well in Fuling of Sichuan Basin.The results show that due to the depletion of formation pressure,the percentage of the maximum horizontal principal stress reversal area in the total area increases and then decreases with time.The closer the area is to the hydraulic fracture,the shorter the time for the peak of the stress reversal area percentage curve to appear,and the shorter the time for the final zero return(to the initial state).The optimum time of refracturing is affected by matrix permeability,initial stress difference and natural fracture approach angle.The larger the matrix permeability and initial stress difference is,the shorter the time for stress reversal area percentage curve to reach peak and return to the initial state,and the earlier the time to take refracturing measures.The larger the natural fracture approach angle is,the more difficult it is for stress reversal to occur near the fracture,and the earlier the optimum refracturing time is.The more likely the stress reversal occurs at the far end of the artificial fracture,the later the optimal time of refracturing is.Reservoirs with low matrix permeability have a rapid decrease in single well productivity.To ensure economic efficiency,measures such as shut-in or gas injection can be taken to restore the stress,and refracturing can be implemented in advance.

Studying the performance of fully encapsulated rock bolts with modified structural elements

Numerical simulation is a useful tool in investigating the loading performance of rock bolts.The cable structural elements(cableSELs)in FLAC3D are commonly adopted to simulate rock bolts to solve geotechnical issues.In this study,the bonding performance of the interface between the rock bolt and the grout material was simulated with a two-stage shearing coupling model.Furthermore,the FISH language was used to incorporate this two-stage shear coupling model into FLAC3D to modify the current cableSELs.Comparison was performed between numerical and experimental results to confirm that the numerical approach can properly simulate the loading performance of rock bolts.Based on the modified cableSELs,the influence of the bolt diameter on the performance of rock bolts and the shear stress propagation along the interface between the bolt and the grout were studied.The simulation results indicated that the load transfer capacity of rock bolts rose with the rock bolt diameter apparently.With the bolt diameter increasing,the performance of the rock bolting system was likely to change from the ductile behaviour to the brittle behaviour.Moreover,after the rock bolt was loaded,the position where the maximum shear stress occurred was variable.Specifically,with the continuous loading,it shifted from the rock bolt loaded end to the other end.

Research on Robotized Advance Support and Supporting Time for Deep Fully Mechanized Excavation Roadway

To keep coal workers away from the hazardous area with frequent accidents such as the roof fall and rib spalling in an underground coalmine,we put forward the solution with robotized self-moving anchor-supporting unit.The existing research shows that the surrounding rock of the roadway has self-stability,and the early or late support is not conducive to the safe and reliable support of the roadway,so there is a problem of support opportunity.In order to study the supporting effect and the optimal supporting time of the above solution,we established the mechanical coupling model of surrounding rock and advance support,and investigated the surrounding rock deformation and advance support pressure distribution under different reserved roof subsidence by using the numerical simulation software FLAC3D.The results show that the deformation of surrounding rock increases and finally tends to a stable level with the increase of pre settlement of roadway roof,and when the pre settlement of roof is between 8-15 mm,the vertical pressure of the top beam of advance support reaches the minimum value,about 0.58 MPa.Based on the above research,we put forward the optimum supporting time in roadway excavation,and summarized the evaluation method based on the mechanical coupling model of surrounding rock-advance support.

磁致伸缩振动能量收集器的全耦合非线性等效电路模型

磁致伸缩材料具有应变大、响应速度快、稳定性好、频带宽的特点,是制作振动能量收集器的理想材料。当前磁致伸缩振动能量收集器建模主要利用线性压磁方程,此模型未能从材料自身耦合和磁路结构进行输出分析,导致输出预测误差较大。该文首先搭建了磁致伸缩材料磁特性测试装置,测试分析了磁致伸缩材料Galfenol合金在不同压应力尤其是大幅值应力下的磁特性;然后基于Gibbs自由能推导了Galfenol材料的全耦合非线性本构方程,进而构建了考虑漏磁、非线性、机磁耦合及饱和效应的振动能量收集器的等效电路模型,并对等效电路模型进行了非线性数学表征和参数识别;最后基于Galfenol材料设计了一个可以承受大幅值振动力的双棒型振动能量收集器样机,通过实验研究了收集器输出电压在不同力幅值、力频率、负载阻值等工况下的变化规律。实验结果与模型的计算结果对比分析表明,所建立的全耦合非线性等效电路模型可以准确预测振动能量收集器的输出电压特性。

考虑桩基础柔性的固定式海上风力机调谐液柱阻尼器振动控制研究

采用线性耦合弹簧模拟风力机桩基础柔性,基于时域整体耦合分析方法,建立考虑桩基础柔性的海上风力机-调谐液柱阻尼器(TLCD)的耦合数值仿真模型,开展风浪联合作用下固定式海上风力机结构振动控制研究,探讨TLCD对于减振效果的影响,并结合频域响应揭示了TLCD的减振机理。研究表明,所设计TLCD通过调谐支撑结构一阶模态有效降低了固定式海上风力机结构自由衰减时程和耦合运动响应。与此同时,验证了不同设计工况下海上风力结构减振效果的差异,以及环境荷载与海上风力结构耦合效应对于TLCD减振效果的影响,进一步,依据海上风力机结构耦合运动响应控制率对TLCD的适用性和有效性进行了评价。

考虑桩基础柔性的海上风机结构可靠度安全评价研究

为了解决海上风机结构可靠性评价模型普遍存在的不能准确模拟荷载间的耦合效应的问题,文章基于OpenFAST开发了一种全耦合海上风机支撑结构可靠性评估模型。考虑到桩土相互作用对风机结构响应的显著影响,文章基于耦合弹簧模型建立了具有柔性单桩基础的NREL5MW海上风机模型。选取海上风机结构的叶尖塔筒间隙及桩基泥面平动位移作为典型失效模式,分别开展了考虑柔性桩基的海上风机结构在正常运行及停机工况下的结构安全评价。研究结果不仅验证了该模型能够高效准确地开展海上风机结构可靠度安全评价,还揭示了波浪荷载是导致桩基发生平动位移的主导荷载。

考虑桨叶伺服控制的浮式风机多刚体动力学建模与验证

浮式风机是深远海域风力发电的关键结构,分析其在复杂海况下的动力学响应特征对保障远海风能开发具有重要意义。在初步设计和方案比选阶段,需要开发能够定量把握浮式风机动力学主要特征、且分析高效的一体化分析模型。为此,针对大型Spar式海上浮式风机,建立了多刚体动力学全耦合分析模型。基于Lagrange方程,推导了考虑桨叶转动与桨距控制的8自由度刚体运动方程。结合所建议的多刚体模型,基于Spar式浮式风机1∶50缩尺模型试验实测数据,建立了与试验物理模型相应的一体化多刚体数值模型,并进行了静力、纯风、纯浪以及风-浪联合条件下数值分析结果与试验观测结果的对比分析。采用本文建模理论,建立了OC3-Spar式浮式风机足尺结构数值分析模型,并与常用的风机结构分析软件FAST的计算结果进行了对比分析。通过与上述缩尺物理模型试验和足尺数值模型软件分析对比,验证了浮式风机多刚体动力学分析模型的有效性。

考虑桩基础柔性的地震风浪作用下海上风力机结构耦合响应机理研究

基于FAST海上风力机整体耦合分析理论和桩基线性化理论,建立包含桩基础柔性的海上风力机基础结构的整体耦合运动方程。进而,通过对FAST v8进行二次开发,同时考虑桩基础柔性,建立包括转子机舱组件-风力机塔筒-基础结构的海上风力机在地震、风、浪荷载作用下的结构耦合仿真模型。根据建立的整体耦合数值仿真模型,开展地震、风、浪荷载联合作用下海上风力机动力响应研究,着重探讨桩基础柔性对于海上风力机结构在地震组合工况下的动力特性及耦合响应机理的影响。结果表明,桩基础柔性对于海上风力机结构动力特性有显著影响。与耦合弹簧边界相比,当忽略桩基础柔性时,会低估整体结构二阶频率对于地震作用下塔顶位移响应的影响,并在基底倾覆力矩响应中激发高阶模态,造成海上风力机结构动力响应变化规律的显著差异。因此,在海上风力机结构抗震设计与研究中必须考虑桩基础柔性的影响。

海上漂浮式风机气动阻尼效应全耦合数值分析

海上漂浮式风机的风轮气动阻尼效应对基础运动有重要影响。建立了基于气动-水动-弹性-伺服的全耦合漂浮式风机数值模型,通过自由衰减动态模拟,利用轮毂单一自由度运动模型评估漂浮式风机的纵摇阻尼水平范围;根据初步评估的纵摇阻尼水平,设置风浪联合工况矩阵对漂浮式风机进行气动阻尼的动态评估。结果表明,动态工况下的漂浮式风机气动阻尼效应与简单评估模型中通过闭环方法计算获得的阻尼曲线变化趋势保持一致;气动阻尼效应对纵摇运动功率谱密度(PSD)曲线影响不显著,能够减缓纵摇运动幅度;与纵摇运动耦合的纵荡固有频率PSD峰值显著体现气动阻尼效应;相同波浪工况下,气动阻尼效应抑制浮体纵摇运动平均值,纵摇PSD曲线及极值统计结果也得出相似结论;小于额定风速时,气动阻尼效应减小纵摇角极大值与平均值之差;高于额定风速时,气动阻尼效应减小浮体纵摇幅值。研究结果为海上漂浮式风机气动阻尼效应评估提供了参考。

完全非线性波浪与二维固定结构物作用的IGN-BEM耦合分析模型

为高效准确地对完全非线性波浪与二维固定结构物的相互作用进行模拟分析,建立了二维完全非线性时域耦合模型。耦合模型将计算域划分为靠近结构物的内域和远离结构物的外域,每个区域均采用满足完全非线性自由水面边界条件的波浪模型进行求解。在内域使用Laplace方程描述流体运动并采用高阶边界元法(BEM)对其进行求解;而在没有结构物的外域,波浪运动的控制方程为Irrotational Green-Naghdi(IGN)方程并采用有限元法(FEM)对其进行求解。内域和外域通过一段重叠区域进行耦合,从而实现模型间变量的传递。首先利用耦合模型分别对规则波的传播、直墙前立波的生成以及相关物理模型试验进行模拟,数值结果与精确解和试验结果的良好吻合验证了耦合模型耦合方式的合理性以及处理非线性问题的准确性;然后使用耦合模型模拟分析了波浪与固定结构物间的相互作用,并将结果与线性解析解以及完全非线性BEM模型的结果进行了对比分析,进一步证明了耦合模型的正确性与高效性。

考虑桩基础柔性的固定式海上风力机调谐质量阻尼器参数优化研究

将被动调谐质量阻尼器(TMD)应用于单桩基础海上风力机,以降低风、浪联合作用下风力机塔筒结构动力响应。利用等效桩长法模拟桩基础柔性,将该方法引入气动-水动-伺服-弹性-振动控制风力机仿真程序FAST-SC,建立能考虑桩基础柔性影响的固定式风力机-TMD耦合数值仿真模型。同时,选取固定式海上风力机支撑结构一阶弯曲模态为主模态,并将TMD简化为单自由度体系,基于拉格朗日方程建立简化的单桩海上风力机-TMD耦合模型。基于该简化模型,采用响应面法分别对刚性桩基础和柔性桩基础条件下的海上风力机的TMD参数进行优化。将优化的TMD参数代入所建立的能考虑桩基础柔性影响的海上风力机-TMD耦合数值模型中,对比不同风、浪作用下TMD对采用不同桩基础边界条件的海上风力机减振效果的差异。通过对比得出:不同桩基础边界使得单桩基础海上风力机结构频率不同,所得的最优TMD设计参数也不相同,优化设计后的TMD对风、浪联合作用下柔性桩基础海上风力机的减振效果明显优于刚性桩基础。