地震下双曲线冷却塔的响应分析

借助ANSYS软件建立双曲线冷却塔的有限元分析模型,讨论自重、风荷载和地震作用下冷却塔的力学性能,通过计算、分析探讨塔筒局部开洞对冷却塔整体及洞口局部强度的影响,主要考虑洞口尺寸和开洞高度对冷却塔整体及洞口局部应力分布规律的影响。结果表明:在一定尺寸范围内,开设洞口对冷却塔的整体强度影响较小,但在洞口局部出现明显的应力集中效应;90 m高度处洞口的应力集中效应低于15 m高度处洞口的应力集中效应,但应力集中效应不容忽视。分析结果为冷却塔结构抗震的设计、加固和施工提供了理论依据和实用的参考建议。     

Upward infiltration–evaporation method to estimate soil hydraulic properties

Determination of saturated hydraulic conductivity, K_s, and the van Genuchten water retention curve θ(h) parameters is crucial in evaluating unsaturated soil water flow. The aim of this work is to present a method to estimate K_s, α and n from numerical analysis of an upward infiltration process at saturation (Cap0), with (Cap0 + h) and without (Cap0) an overpressure step (h) at the end of the wetting phase, followed by an evaporation process (Evap). The HYDRUS model as well as a brute-force search method were used for theoretical loam soil parameter estimation. The uniqueness and the accuracy of solutions from the response surfaces, K_s–n, α–n and K_s–α, were evaluated for different scenarios. Numerical experiments showed that only the Cap0 + Evap and Cap0 + h + Evap scenarios were univocally able to estimate the hydraulic properties. The method gave reliable results in sand, loam and clay-loam soils.     

Sediment detachment in piping-prone soils: Cohesion sources and potential weakening mechanisms

Soil piping is a widespread land degradation process that may lead to gully formation. However, the processes involved in sediment detachment from soil pipe walls have not been well studied, although their recognition is a crucial step to protect soils from piping erosion. This study aims to recognize the factors affecting cohesion and to identify the mechanisms which are likely to be responsible for the disintegration of soil. The study has been conducted in mid-altitude mountains under a temperate climate (the Bieszczady Mountains, the Carpathians, SE Poland). The research was based on the detailed field and laboratory analyses of morphology, and the physical and chemical properties of soil profiles with and without soil pipes. Moreover, experiments with flooding the undisturbed soil samples using different solutions (deionized water, ammonium oxalate, dithionate citrate, 35% hydrochloric acid and 30% hydrogen peroxide) were conducted in order to check the role of air slaking, the removal of soil organic carbon (SOC), and Fe and Al oxides on sediment detachment. The obtained results have confirmed that soil pipes develop in quite cohesive soils (silt loams), which allow the formation and maintenance of pipes with a diameter up to 30 cm. Soil cohesion, and thus susceptibility to piping, are impacted by the content of major oxides, soil particle size distribution, biological activity and porosity. The tested soils affected by piping erosion have a lower content of Al₂O₃ and Fe₂O₃, and free Fe (Fe_(DCB)), lower clay content, higher biological activity (more roots and animal burrows), higher porosity, and more and larger pores than the profile without soil pipes. The experiments have indicated that especially SOC along with Fe and Al oxides are an important cohesion source in the study area. This suggests that the removal of SOC, and Fe and Al oxides may weaken and disintegrate aggregates in soil pipes. Further study of soil leaching and tensile strength will broaden understanding of which chemical processes control where pipes will develop in other cohesive piping-prone soils. © 2020 John Wiley & Sons, Ltd.     

青藏高原东南缘壳幔力学耦合及其动力学意义

现代大陆动力学及构造学研究认为,壳内软弱带控制着地壳与地幔间相互作用的强弱,制约岩石圈层间的耦合关系,同时也影响地块的相互作用方式。这种耦合作用可以通过比较不同深度的形变和应力的相关特征来分析。青藏高原东南缘20～35 km的深度广泛存在低速带,有可能在现今地震及构造活动区内由壳内部分熔融承担了潜在的解耦功能。基于1999～2015年GPS坐标时间序列数据得到速度场,结合前人给出的SKS剪切波分裂数据,分析该区水平方向形变耦合特征;同时搜集莫霍面和岩石圈底面形变数据,给出垂向上的形变耦合特征。利用EGM2008重力模型的11～36阶球谐系数计算青藏高原东南缘岩石圈底面地幔对流应力场;搜集并整理2000年至今的1 131个震源机制解数据,采用区域应力张量阻尼反演法得到该区多震层应力场;进一步分析两种应力场的相关性。在ABAQUS软件中建立青藏高原东南缘岩石圈三维粘弹性有限元模型,同时加入上地壳的边界位移约束条件和底面地幔对流拖曳力加载条件,分别对水平方向和垂直方向的应力应变作分析,并给出强震剖面结果,探讨壳幔耦合强震孕育机制,据此分析青藏高原东南缘各个断层的地震危险性。研究认为,应力的不均匀发展是应变能积累的关键,同时垂向上圈层间的耦合关系极大地影响块体间的作用方式;壳内软弱带广泛发育的块体应力出现分层现象,边缘区域横向纵向应力变化明显,与之接触的上伏地壳地震危险性较高。     

不同制样方式下含水合物粉细砂静力学特性研究

利用高压低温三轴仪对含水合物粉细砂进行剪切试验。分别用气饱和法与水饱和法制样,实现不同水合物饱和度和围压条件的三轴剪切,并分析含水合物砂的胶结作用对剪切特性的影响。试验结果表明：低饱和度时,气饱和与水饱和试样的偏应力差别不大;高饱和度时,制样方式对偏应力的影响较显著;水饱和试样的剪胀性大于气饱和试样,剪胀性随饱和度的升高和围压的降低而增大。峰值偏应力和稳态偏应力由黏聚力和摩擦力两部分组成,水合物的存在对稳态内摩擦角影响不大。     

Experimental study on the cyclic behavior of loose calcareous sand under linear stress paths

Abstract

Hollow cylinder torsional shear tests on loose isotropically and anisotropically consolidated calcareous sand were conducted to investigate the cyclic behavior under three different linear stress paths, including horizontal line, oblique line, and vertical line stress paths, in a coordinate system of the normal stress difference and the horizontal shear stress. The dominant strain components of the isotropically consolidated specimens are affected by the stress paths. With increasing consolidation stress ratio, axial strain gradually becomes the dominant strain component under the three different stress paths. The cyclic strength of the isotropically consolidated specimens under the three different stress paths are almost the same, while for the anisotropically consolidated specimens, the cyclic strengths are strongly affected by the stress paths. These results indicate that conventional cyclic triaxial tests may overestimate cyclic strength in some cases. Irrespective of the stress paths and cyclic stress ratios, the terminal residual excess pore pressure ratio decreases with increasing consolidation stress ratio. Moreover, an empirical equation is proposed to describe the relationship between the normalized shear work and the normalized residual excess pore pressure ratio. The comparative study reveals that the relationship proposed for silica sand is not suitable for the dynamic analyses of calcareous sand.     

Aspects of the measurement of the acoustic transfer function in a cavitation tunnel

Model scale experiments represent one of the most effective, and probably the most commonly used, approaches for the study of cavitating propeller noise. Despite this, the acquirement of consistent data of propeller noise from tests in model scale facilities is still a challenging task. Data collected with such an approach are usually affected by several scale effects and facility related issues which make such experiments still very complex. Among others, the effect of the facility itself as acoustical surrounding must be carefully addressed in order to properly estimate propeller source levels in ideal free field conditions. This is currently carried out, when possible, by measuring facility transfer functions and applying such functions as a correction to measured noise spectra. The development of robust procedures for the measurement of these transfer functions is thus very important. In the present work, this problem is addressed by discussing in details techniques, merits and possible problems related to this topic.     

An inverse problem methodology for multiple parameter estimation in bend stiffeners

The flexible riser top connection is a critical region for lifetime assessment due to large tension/curvature variations and modeling uncertainties. The bend stiffener polyurethane mechanical response not only presents a nonlinear loading rate and temperature dependency but is also subjected to weather ageing during operation, which may affect its mechanical behavior over time. The top tension, employed for riser local cross-section stress calculation, is usually obtained from global dynamic analyses performed under selected environmental conditions, if direct measurement is not available. As a consequence, both the bend stiffener effect on the curvature distribution and the top tension time series present inherent uncertainties for riser lifetime (re)assessment. In the present work, a proposed monitoring approach composed by gyrometers installed along flexible riser/bend stiffener top connection system length combined with an inverse problem methodology is numerically investigated to estimate the following parameters: (i) polyurethane hyperelastic response and (ii) effective top tension. The top connection system is modeled using a large deflection beam bending model and the parameters are estimated using a damped least-square minimization approach with the Levenberg–Marquardt algorithm. For the preliminary feasibility investigation, the gyrometer experimental data is numerically estimated through Monte Carlo simulations. A case study is carried out to investigate the influence that the number of sensors, sensors arrangement, loading conditions and top connection model have on the inverse parameters estimation. The results indicate that the proposed monitoring approach and inverse parameter estimation methodology may effectively reduce flexible riser lifetime calculation uncertainties.     

Effect of strain-dependent dynamic properties of backfill and foundation soil on the external stability of geosynthetic reinforced waterfront retaining structure subjected to harmonic motion

A simplified method to determine the minimum length of reinforcement required for the external stability of waterfront reinforced soil structures under seismic conditions is presented. In the present analysis, strain-dependent dynamic properties (shear modulus and damping ratio) are used. The results obtained from the present method are well compared with the results of pseudo-static method of analysis. For the set of input parameters, the estimated minimum length of reinforcement required against sliding failure is nearly 27–29% higher for an input normalized frequency of 1.06 and is nearly 22–25% lower for another input normalized frequency of 1.94 when compared with the results of pseudo-static approach. This can be attributed to the mode change behaviour of the waterfront structure. In addition, the effect of foundation type on the external stability of waterfront reinforced soil structures has also been presented and it is found that the foundation type has a significant effect on the same. For the given set of input parameters, the length of minimum reinforcement required for a slope and vertical wall having a flexible foundation are about 26–28% and 32–38% larger than that of a slope and vertical wall with rigid foundation, respectively.