Numerical benchmark studies on drag and lift coefficients of a marine riser at high Reynolds numbers

Numerical benchmark studies on drag and lift coefficients of a marine riser have been organized by the 27th ITTC Ocean Engineering Committee. The purpose of the studies was to benchmark the capabilities of CFD methods through quantitative comparisons and validation studies against the model test results of a circular cylinder by MARIN. Studies were focused on the drag crisis phenomenon for the stationary smooth cylinder in the critical Reynolds number regime. Eight organizations have participated in the studies by using RANS, DES and LES methods. An overview of the model test results, test cases, submissions and comparison results are presented in this paper. Conclusions and recommendations are made for future studies.     

涡激振动实验中的流速增大装置研发与性能研究

拖曳水池中进行立管涡激振动实验时,为了保证采样时间长度,难以达到较高的Re数。流速增大装置可以在不提高拖车车速的情况下增大立管外的流速。利用这种流速增大装置还可实现流速分层流场中细长柔性立管涡激振动实验。经过对流速增大装置中的进流段曲线进行优选,发现Witozinsky曲线的总体性能最好。在对流速增大装置进行水池实验和数值模拟后,发现流速增大区域的流速增大倍数接近进流段收缩比,流速增大区域流场比较稳定、均匀,流速增大装置对其外的流场影响很小。此流速增大装置不但可应用于拖曳水池中的立管涡激振动实验,还可以应用于对流速要求较高的水下航行体的水池试验,如鱼雷、水下机器人等。     

Two Improvements on Numerical Simulation of 2-DOF Vortex-Induced Vibration with Low Mass Ratio

Till now, there have been lots of researches on numerical simulation of vortex-induced vibration. Acceptable results have been obtained for fixed cylinders with low Reynolds number. However, for responses of 2-DOF vortex-induced vibration with low mass ratio, the accuracy is not satisfactory, especially for the maximum amplitudes. In Jauvtis and Williamson''s work, the maximum amplitude of the cylinder with low mass ratio m*=2.6 can reach as large as 1.5D to be called as the “super-upper branch”, but from current literatures, few simulation results can achieve such value, even fail to capture the upper branch. Besides, it is found that the amplitude decays too fast in the lower branch with the RANS-based turbulence model. The reason is likely to be the defects of the turbulence model itself in the prediction of unsteady separated flows as well as the unreasonable setting of the numerical simulation parameters. Aiming at above issues, a modified turbulence model is proposed in this paper, and the effect of the acceleration of flow field on the response of vortex-induced vibration is studied based on OpenFOAM. By analyzing the responses of amplitude, phase and trajectory, frequency and vortex mode, it is proved that the vortex-induced vibration can be predicted accurately with the modified turbulence model under appropriate flow field acceleration.     

Experimental and theoretical methods on determination of stress-dependent natural frequency of marine sedimentary clay

Excited by the vibration sources in dynamic engineering, the natural frequency and damping factor of the saturated marine sedimentary clay are key dynamic parameters that influence the responses under cyclic loads. Experimental and theoretical methods are proposed in this paper to analyze the natural frequency and the stress-dependent nonlinearity. The experimental method shows that the natural frequency of soils with specific stress state subjected to large cyclic shear strain can be estimated from the data of dynamic triaxial tests based on the amplitude–frequency response curve. Trial and error by the criterion from the half-power bandwidth method is used to determine the optimal fitting. The results of a theoretical study on the free vibration of soil layers are then presented to derive the analytic solution of natural frequency. In addition to the two frequency-independent elements (a lumped mass matrix and a stiffness matrix), the system’s equivalent damping coefficient matrix is iteratively determined based upon the forced vibration experimentally. Finally, the impacts of the resonance phenomenon on the dynamic shear modulus and hysteretic loop are discussed.     

考虑挤土效应时楔形桩纵向振动阻抗研究

基于复刚度传递多圈层平面应变模型,研究考虑桩周土挤土效应时成层地基中楔形桩的纵向振动问题。首先根据桩周土体的纵向成层情况并考虑楔形桩的变截面特性,将桩土系统沿纵向划分为有限个微元段,对每个微元段的桩周土体建立复刚度传递多圈层平面应变模型,并通过剪切复刚度递推方法求得桩周土作用在桩身的剪切复刚度;然后将求得的剪切复刚度代入桩身纵向振动控制方程,运用Laplace变换技术和阻抗函数递推方法,推导得到考虑桩周土挤土效应时成层地基中楔形桩纵向振动时桩顶复阻抗的解析解;最后,采用参数研究方法在低频范围内分析挤土效应对桩顶复阻抗的影响及其规律。     

Using an inerter‐based device for structural vibration suppression

This paper proposes the use of a novel type of passive vibration control system to reduce vibrations in civil engineering structures subject to base excitation. The new system is based on the inerter, a device that was initially developed for high‐performance suspensions in Formula 1 racing cars. The principal advantage of the inerter is that a high level of vibration isolation can be achieved with low amounts of added mass. This feature makes it an attractive potential alternative to traditional tuned mass dampers (TMDs). In this paper, the inerter system is modelled inside a multi‐storey building and is located on braces between adjacent storeys. Numerical results show that an excellent level of vibration reduction is achieved, potentially offering improvement over TMDs. The inerter‐based system is compared to a TMD system by using a range of base excitation inputs, including an earthquake signal, to demonstrate how the performance could potentially be improved by using an inerter instead of a TMD. Copyright © 2013 John Wiley & Sons, Ltd.     

Design,simulation, and large‐scale testing of an innovative vibration mitigation device employing essentially nonlinear elastomeric springs

This study proposes an innovative passive vibration mitigation device employing essentially nonlinear elastomeric springs as its most critical component. Essential nonlinearity denotes the absence (or near absence) of a linear component in the stiffness characteristics of these elastomeric springs. These devices were implemented and tested on a large‐scale nine‐story model building structure. The main focus of these devices is to mitigate structural response under impulse‐like and seismic loading when the structure remains elastic. During the design process of the device, numerical simulations, optimizations, and parametric studies of the structure‐device system were performed to obtain stiffness parameters for the devices so that they can maximize the apparent damping of the fundamental mode of the structure. Pyramidal elastomeric springs were employed to physically realize the optimized essentially nonlinear spring components. Component‐level finite element analyses and experiments were conducted to design the nonlinear springs. Finally, shake table tests using impulse‐like and seismic excitation with different loading levels were performed to experimentally evaluate the performance of the device. Experimental results demonstrate that the properly designed devices can mitigate structural vibration responses, including floor acceleration, displacement, and column strain in an effective, rapid, and robust fashion. Comparison between numerical and experimental results verified the computational model of the nonlinear system and provided a comprehensive verification for the proposed device. Copyright © 2014 John Wiley & Sons, Ltd.     

Full-scale experimental modal analysis of an arch dam: The first experience in Iran

Forced vibration field tests and finite-element studies were conducted on the Shahid Rajaee concrete arch dam in Northern Iran to determine the dynamic properties of the dam–reservoir–foundation system. The first forced vibration tests on the dam were performed with two different types of exciter units, with a limited maximum force, bolted on the dam crest for alternative in-phase and out-of-phase sequencing. Because of an insufficient number of recording sensors, two arrangements of sensors were used to cover sufficient points on the dam crest and one gallery during tests. Two kinds of vibration tests, on–off and frequency sweeping, were carried out on the dam. The primary natural frequencies of the coupled system for both symmetric and anti-symmetric vibration modes were approximated during on–off tests in two types of sequencing of exciters, in phase and out-of-phase, with a maximum frequency of 14 Hz. The principal forced vibration tests were performed at precise resonant frequencies based on the results of the on–off tests in which sweeping around the approximated frequencies at 0.1 Hz increments was performed. Baseline correction and suitable bandpass filtering were applied to the test records and then signal processing was carried out to compute the auto power, cross power and coherence spectra. Nine middle modes of vibration of the coupled system and corresponding damping ratios were estimated. The empirical results are compared against the results from calibrated finite-element modeling of the system using former ambient vibration tests, considering the dam–reservoir–foundation interaction effects. Good agreement is obtained between experimental and numerical results for eight middle modes of the dam–reservoir–foundation system.     

Full-scale model testing on a ballastless high-speed railway under simulated train moving loads

Model testing in laboratory, as an effective alternative to field measurement, provides valuable data to understand railway׳s dynamic behaviors under train moving loads. This paper presents comprehensive experimental results on track vibration and soil response of a ballastless high-speed railway from a full-scale model testing with simulated train moving loads at various speeds. A portion of a realistic ballastless railway comprising slab track, roadbed, subgrade, and subsoil was constructed in a larger steel box. A computer-controlled sequential loading system was developed to generate equivalent vertical loadings at the track structure for simulating the dynamic excitations due to train׳s movements. Comparisons with the field measurements show that the proposed model testing can accurately reproduce dynamic behaviors of the track structure and underlying soils under train moving loads. The attenuation characteristics of dynamic soil stresses in a ballastless slab track is found to have distinct differences from that in a ballasted track. The model testing results provide better understanding of the influence of dynamic soil–structure interaction and train speed on the response of track structure and soils.     

Prediction of ground motion due to the collapse of a large-scale cooling tower under strong earthquakes

The ground motion owing to the collapse of a large-scale cooling tower under strong earthquakes was appropriately predicted using a comprehensive approach. The predicted results can be used for the safety evaluation of nuclear-related facilities adjacent to the cooling tower as well as in the planning of nuclear power plant construction in China. In this study, a cooling tower–soil model was first developed based on a falling weight–soil model, which the authors verified by falling weight tests. Then the collapse process of a cooling tower was simulated, and the collapse-induced ground vibrations were assessed by using the proposed model. Finally, the ground motion, which was a combination of the earthquake-induced ground motion and the collapse-induced ground vibrations, was estimated based on the superposition principle of waves. It was found that the cooling tower may collapse under strong earthquakes with the peak ground accelerations (PGAs) in the range of 0.35–0.45 g in x (EW) and y (NS) directions, respectively. These PGAs are far beyond the PGA range of major earthquakes in the common seismic design in China. The types of the site geologies of towers can significantly affect the collapse-induced ground vibrations. For a typical hard soil consisting of strongly weathered sandy slate, moderate ground vibrations may occur in the considered region. The collapse-induced PGAs were in the range of 0.017–0.046 g for the observed points at distances of 350 m in radial direction. For a rock-like foundation, the collapse-induced radial PGAs may be as high as 0.08 g at distances of 350 m, indicating that the effect of the collapse-induced ground vibrations on the nuclear-related facilities should be seriously assessed in certain scenarios.