State Key Laboratory of Coastal and Offshore Engineering

We develop an analytical model for the relationship between the strain measured by a fiber Bragg grating sensor and the actual structural strain. The values of the average strain transfer rates calculated from the analytical model agree well with available experiment data. Based on the analytical model, the critical adherence length of an optical fiber sensor can be calculated and is determined by a strain lag parameter, which contains both the effects of the geometry and the relative stiffness of the structural components. The analysis shows that the critical adherence length of a fiber sensing segment is the minimum length with which the fiber must be tightly bonded to a structure for adequate sensing. The strain transfer rate of an optical fiber sensor embedded in a multilayered structure is developed in a similar way, and the factors that influence the efficiency of optical fiber sensor strain transferring are discussed. It is concluded that the strain sensed by a fiber Bragg grating must be magnified by a factor (strain transfer rate) to be equal to the actual structural strain. This is of interest for the application of fiber Bragg grating sensors.

With the aid of direct numerical simulation, this paper presents a detailed investigation on the flow around a finite square cylinder at a fixed aspect ratio (AR) of 4 and six Reynolds numbers (Re = 50, 100, 150, 250, 500, and 1000). It is found that the mean streamwise vortex structure is also affected by Re, apart from the AR value. Three types of mean streamwise vortices have been identified and analyzed in detail, namely, “Quadrupole Type” at Re = 50 and Re = 100, “Six-Vortices Type” at Re = 150 and Re = 250, and “Dipole Type” at Re = 500 and Re = 1000. It is the first time that the “Six-Vortices Type” mean streamwise vortices are reported, which is considered as a transitional structure between the other two types. Besides, three kinds of spanwise vortex-shedding models have been observed in this study, namely, “Hairpin Vortex Model” at Re = 150, “C and Reverse-C and Hairpin Vortex Model (Symmetric Shedding)” at Re = 250, and “C and Reverse-C and Hairpin Vortex Model (Symmetric/Antisymmetric Shedding)” at Re = 500 and Re = 1000. The newly proposed “C and Reverse-C and Hairpin Vortex Model” shares some similarities with “Wang’s Model” [H. F. Wang and Y. Zhou, “The finite-length square cylinder near wake,” J. Fluid Mech. 638, 453–490 (2009)] but differs in aspects such as the absence of the connection line near the free-end and the “C-Shape” vortex structure in the early stage of the formation of the spanwise vortex.

Under the Chinese National Antarctic Research Expedition program in 2006, the annual thermal mass balance of landfast ice in the vicinity of Zhongshan Station, Prydz Bay, east Antarctica, was investigated. Sea ice formed from mid‐February onward, and maximum ice thickness occurred in late November. Snow cover remained thin, and blowing snow caused frequent redistribution of the snow. The vertical ice salinity showed a “question‐mark‐shaped” profile for most of the ice growth season, which only turned into an “I‐shaped” profile after the onset of ice melt. The oceanic heat flux as estimated from a flux balance at ice‐ocean interface using internal ice temperatures decreased from 11.8 (±3.5) W m −2 in April to an annual minimum of 1.9 (±2.4) W m −2 in September. It remained low through late November, in mid‐December it increased sharply to about 20.0 W m −2 . Simulations applying the modified versions of Stefan's law, taking account the oceanic heat flux and ice‐atmosphere coupling, compare well with observed ice growth. There was no obvious seasonal cycle for the thermal conductivity of snow cover, which was also derived from internal ice temperatures. Its annual mean was 0.20 (±0.04) W m ‐1 °C −1 .

Vortex-induced vibration (VIV) of a rotating circular cylinder at a low Reynolds number of 150 and a low mass ratio of 2 is studied numerically. Simulations are conducted at three rotation rates of α = 0, 0.5, and 1 and reduced velocities in the range of 1–13 with an interval of 0.2. The numerical results show that the rotation of the cylinder increases the response amplitude and widens the lock-in regime for the one-degree-of-freedom (1-dof) VIV in the cross-flow direction. The two-degree-of-freedom (2-dof) responses of the cylinder at α = 0.5 and 1 are significantly different from that at α = 0. For the 2-dof VIV, the response amplitude in the inline direction, which is much smaller than that in the cross-flow direction at α = 0, is increased significantly at α = 0.5 and 1. One initial branch is found at α = 0.5 and two initial branches are found at α = 1. In the initial branches, the response frequency locks onto a frequency that is smaller than the natural frequency of the cylinder and the response amplitude increases with the reduced velocity. The vortex shedding is found to be in the P+S mode for reduced velocities near the higher boundary of the initial branches and 2S mode in all other reduced velocity ranges for the 2-dof VIV. Simulations are conducted under both the increasing and decreasing reduced velocity conditions. A hysteresis region is found near the higher boundary of the lower branch for α = 0, 0.5, and 1 in the 1-dof of VIV and for α = 0 in the 2-dof VIV. The hysteresis region occurs near the higher boundary of the initial branches for α = 0.5 and 1 in the 2-dof VIV. By analysing the component of the force coefficient that is in phase with the velocity of the cylinder, it is found that pressure force excites the vibration and the viscous force damps the vibration in both the inline and the cross-flow directions in the 2-dof VIV. The magnitude of the time averaged pressure and viscous force coefficients that are in phase with the velocities of the cylinder in the lock-in regime are found to be much greater than their counterparts outside the lock-in regime.

Numerical results of fluid flow over a rotationally oscillating circular cylinder with splitter plate are presented here. Different from the previous examinations with freely rotatable assembly, the fluid and structure interactions are treated as a coupled dynamic system by fully considering the structural inertia, stiffness, and damping. The hydrodynamic characteristics are examined in terms of reduced velocity Ur at a relatively low Reynolds number Re = 100 for different plate lengths of L/D = 0.5, 1.0, and 1.5, where Ur = U/(Dfn), Re = UD/υ and fn = (κ/J)0.5/2π with U the free stream velocity, D the diameter of the circular cylinder, υ the fluid kinematic viscosity, fn the natural frequency, J the inertial moment, κ the torsional stiffness, and L the plate length. Contrast to the freely rotating cylinder/plate body, that is, in the limit of κ → 0 or Ur →∞, remarkable rotary oscillation is observed at relatively low reduced velocities. For the typical case with L/D = 1.0, the maximum amplitude may reach five times that at the highest reduced velocity of Ur = 15.0 considered in this work. At the critical reduced velocity Ur = 4.2, notable hydrodynamic jumps are identified for the rotation amplitude, response frequency, mean drag coefficient, lift amplitude, and vortex shedding frequency. Moreover, the phase angle between the fluid moment and rotary oscillation abruptly changes from 0 to π at Ur = 6.5. Due to the combined effect of fluid moment, rotation response, and phase difference, the natural frequency of the rotating body varies in flow, leading to a wide regime of lock-in/synchronization (Ur ≥4.2, for L/D = 1.0). The phenomenon of rotation bifurcation, i.e., the equilibrium position of the rotary oscillation deflects to a position which is not parallel to the free stream, is found to only occur at higher reduced velocities. The longer splitter plate has the lower critical reduced velocity. The occurrence of bifurcation is attributed to the anti-symmetry breaking of the wake flow evolution. The resultant asymmetric mean pressure distribution on the splitter plate gives rise to the net lift force and the deviated moment on the assembly, leading to the offset mean position of splitter plate. The global vortex shedding is identified to be the classic 2S mode for both cases with and without the bifurcation, although the second vortex formation and the shedding pattern in the near wake for the bifurcate case are different from the non-bifurcate case with lower reduced velocities.

Fiber Bragg grating (FBG) sensors show superior potential for structural health monitoring of civil structures to ensure their structural integrity, durability, and reliability. In this work, FBG sensors, including strain and temperature sensors, are applied for health monitoring of the oil production offshore platform number CB271, which is located in the Bohai Sea, East China. The procedure of FBG sensor installation during platform construction, as well as model validation in a laboratory under a variety of loading conditions on a seismic simulation shaking table, is also presented. In the tests, FBG strain sensors are placed as a strain rosette on the surface of the platform central pillar, and an FBG temperature sensor is installed close to those strain sensors for temperature compensation. The FBG sensors have been in operation for one year without any significant reduction of working performance. Strain responses induced by the impacts of ocean waves and the ship's hundred tons of weight are monitored on site successfully. The fundamental frequency of the platform identified by the results of the FBG sensors agrees well with that obtained by theoretical analysis. In the monitoring, FBG sensors exhibit excellent performance and higher tolerance to harsh environments in the long-term real-time health monitoring of ocean offshore platforms.

Offshore wind turbines (OWTs) might be subjected to seismic loads with different peak accelerations during operation in the actively seismic regions. The earthquakes might be a potential risk for the OWTs due to its stochastic nature. Earthquake with wind and wave loads could act on OWT at the same time; thus, the structural responses of such OWTs should be analyzed taking into consideration the reasonable load combinations. Based on the hydro-elastic similarity, an integrated model of the combined National Renewable Energy Laboratory (NREL) 5 MW wind turbine and a practical pentapod substructure is designed for testing. The governing equations of motion of the integrated OWT are established. The dynamic tests and numerical analysis of the OWT model are performed under different combinations of seismic, wind, and sea load conditions. The El Centro and American Petroleum Institute (API)-based synthesized seismic waves with different peak ground accelerations (PGAs) are considered in this study. The numerical results are in good agreement with the experimental ones. The coupling effect of the OWT structure under the combined load conditions is demonstrated from the experimental and numerical results. The results indicate that the interaction of earthquake, wind, wave, and current should be taken into account in order to obtain proper structural response, especially with small PGA.

Physical experiments focusing on the propagation of gravity waves of finite depth on adverse currents were implemented to examine their effect on the development of the modulational instability and to study the geometric characteristics of extreme waves. A series of wave trains with varying initial steepness, perturbation frequency, and initial perturbed strength were mechanically generated in a wave-current flume. The present results show that opposing currents can speed the growth of the modulational instability, verifying the previous theory qualitatively. A current-modified nonlinear Schrödinger equation can predict the measured sideband growth rates well for wave trains with lower perturbation frequencies, but overestimated those with higher perturbation frequencies. On the other hand, the limiting steepness of extreme waves measured in the presence of opposing currents was smaller than that measured in quiescent water. Additionally, current strength was found to have limited influence on the geometric properties of extreme waves as well as on their limiting steepness.

When a fiber Bragg grating (FBG) sensor is embedded in a structure to sense its strain, a portion of strain is absorbed by the protective interlayer of the fiber optical sensor. If an angle exists between the FBG sensor axis and external principal stress direction of the host material, the strain transfer from the host material to the sensor will be much different than that of the external stress parallel to the sensor axis. A suitable strain transfer model is developed for evaluating the interaction between the surrounding matrix and a length of optical fiber under nonaxial stress. A number of realistic assumptions are introduced to simplify the process of the mathematics rigor. Strain transfer rate is introduced to describe the level of strain loss within the protective interlayer and the amount transferred to the optical fiber core. Theoretical results show that the angle of the optical fiber sensor plays an important role in strain transferring from the surrounding materials to the optical fiber core. The theoretical findings are verified through a series of experiments with FBG sensors. The evaluation error of average strain transfer rate is discussed because of sensor-located angle deviation.

The brittle fracture of rock with an angled crack under combined tensile and compressive loading conditions is studied using linear elastic fracture mechanics (LEFM). The modified maximum tangential stress criterion (MTSC) and the maximum shear stress criterion (MSSC) are used to check crack initiations in the tensile and shear modes, respectively. The effects of the friction coefficient of the crack surfaces and the nonsingular stresses (T stresses) on the crack initiation are studied for the cases of both low and high compressive confining pressure coefficients. The T stresses include those both parallel (T x ) and perpendicular (T y ) to the crack plane. The type of crack initiation under the combined tensile and compressive loading conditions is found to remain tensile dominated when the compressive confining pressure coefficient is small. However, shear crack extension becomes possible with the compressive confining pressure coefficient and friction coefficient increasing if the crack orientation angle is small. Moreover, the high compressive confining pressure and substantial friction are found to increase the possibility of shear crack extension. The theoretical predictions presented in this study move one step forward than the available analytical solutions for the angled crack subjected to general biaxial load and agree well with those from experimental tests.

An innovative approach is developed to design equivalent truncated mooring system for hybrid model testing. Considering the gravity, tension, current force and mooring line extension, the piecewise extrapolating method is employed to the static analysis of the multi-component mooring line. In addition, the quasi-static approach and dissipated energy model are used to figure out the mooring-induced damping of mooring system. Employing genetic algorithm, an optimised design program is developed to design equivalent truncated mooring system based on the similarity of both static and damping characteristics. Considering the catenary, semi-taut and taut mooring systems used for some semi-submersible platform of 1500 m water depths, equivalent truncated mooring system is designed by using the design program in this paper, respectively. The results prove that the design program is available for all common types of deep water mooring system and can be used for hybrid model testing.

A stable three-dimensional (3D) wake structure for flow past a circular cylinder has been discovered through 3D direct numerical simulations (DNS). The stable 3D wake structure occurs over a small range of Reynolds number (Re) below the critical Re for Mode A∗ (i.e., Mode A with large-scale vortex dislocations, where Mode A is the first 3D instability mode which will evolve into Mode A∗) instability. It is believed that the stable 3D wake structure discovered in this study is the stable state of Mode A wake structure inferred by Williamson [“Three-dimensional wake transition,” J. Fluid Mech. 328, 345–407 (1996)]. This confirms the wake transition sequence of 2D → A → A∗ → B suggested by Williamson. Compared with conventional Mode A structure, the stable state of Mode A structure has much weaker amplitude and does not evolve into large-scale vortex dislocations. The stable state of Mode A structure is triggered by small-scale spanwise disturbance introduced upstream of the cylinder, due to energy amplification through convective instability of the flow. The stable state of Mode A is transient and is damped out eventually under a transient initial disturbance condition, but is sustained throughout under a persistent disturbance condition. The emergence of the stable state of Mode A structure is correlated with both Re and the disturbance level. With the decrease of Re, the stable state of Mode A structure gradually becomes less well-defined and eventually disappears. With the decrease of the disturbance level, the stable state of Mode A structure emerges at a higher Re.

In order to monitor the deflection of bored piles, BOTDA sensing technology is employed to measure the strain and temperature profiles along three bored piles adjacent to a deep excavation. Based on the measured strain, the deflections can be calculated according to the mechanics of materials. Comparison between the deflections measured by traditional inclinometer and strain based measurement method has been fully conducted. A two-dimensional FEM model was constructed to simulate the interaction between excavation and subway tunnel and examine the deflection of the three bored piles during progressive excavation. Finally, the deflection profiles calculated from measured strains and simulated by FEM are comprehensively summarized and analysed. It can be found that the deflection changing trend at the top of the bored piles are consistent with the simulation results with acceptable errors while the deflection calculated from the measure strains has a significant discrepancy with the simulation results.

Flow past two cylinders of different diameters in close proximity is simulated numerically for a constant diameter ratio of 0.45, a gap ratio of 0.0625, and a Reynolds number of 1000 (defined using the diameter of the main cylinder). The effect of the position angle α of the small cylinder relative to the large one on force coefficients and wake flow patterns are studied. Depending on the position angle α of the small cylinder, four wake flow modes are identified: the upstream interference mode for α = 0°, 22.5°, and 45°, the intermittent attached gap flow mode for α = 67.5° and 90°, the attached gap flow mode for α = 112.5° and 135°, and the wake interference mode for α = 157.5° and 180°. The RMS lift coefficients of both cylinders are reduced significantly compared with that of a single cylinder, regardless of the position angle of the small cylinder. Although the variation trends of the mean drag and lift coefficients with the position angle of the small cylinder obtained from the two-dimensional (2D) and three-dimensional (3D) simulations are similar, the 2D simulations overestimate the mean drag coefficient, the RMS drag and lift coefficients compared with those obtained from the 3D simulations.

In this paper, to fully explore the mechanical characteristic of hollow glass microspheres (HGM) composites in deep-sea environment, the compressive behavior of solid buoyancy materials under different temperature gradients and various stress states were studied by a triaxial test system. Through the analysis of experimental results, the glass transition temperature and energy absorption of the material were revealed. Such a content, will further be useful to construct the yield criterion and specialize the sensitivity of each parameter.

On the basis of a clear understanding of dynamic stress distributions in face slabs of concrete-faced rockfill dams (CFRDs), the cracking failure of a face slab can be avoided using antiseismic design measures. In this study, the distribution of tensile overstresses in face slabs during earthquakes was analyzed while the effects of dam geometries and variable nature of input motions were taken into account. The area of face slab vulnerable to cracking was identified. Furthermore, the installation of a horizontal joint is suggested to mitigate the tensile stress in face slabs when subjected to earthquakes. Three-dimensional (3D) dynamic analyses of CFRDs with various geometries were conducted, and the dynamic slope-direction tensile stresses in face slabs with joints at various positions were compared. The results indicate that peak dynamic slope-direction tensile stress can be significantly decreased by installing the joint where the maximum tensile stress occurs.

Abstract A green procedure for the one-pot three-component synthesis of 1-amidoalkyl-2-naphthol and 3-amino-1-phenyl-1 H benzo[ f ]chromene-2-carbonitrile derivatives from the reaction of 2-naphtol, aldehydes, and malononitrile/acetamide in the presence of a catalytic amount of Fe 3 O 4 @enamine-B(OSO 3 H) 2 as an efficient and novel heterogeneous magnetic nanostructure catalyst is described. The catalyst was characterized using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). These strategies possess some merits such as simple work-up method, easy preparation of the catalyst, short reaction times, good-to-high yields, and non-use of hazardous solvents during all steps of the reactions. Moreover, due to the magnetic nature of the catalyst, it was readily recovered by magnetic decantation and can be recycled at least six runs with no considerable decrease in catalytic activity.

Abstract Wave energy converters (WECs) are built to extract wave energy. However, this kind of device is still expensive for commercial utilization. To cut down the cost of WECs by sharing the construction cost with breakwaters, an integrated cylindrical WEC-type breakwater system that includes a cylindrical WEC array in front of a very long breakwater is proposed to extract wave energy and attenuate incident waves. This paper aims to optimize the performance of the integrated cylindrical WEC-type breakwater system. A computational fluid dynamics tool, openfoam®, and a potential flow theory-based solver, HAMS®, are utilized. openfoam® provides viscosity corrections to a modified version of HAMS® in order to accurately and efficiently predict the integrated system’s performance. Parametric studies are conducted to optimize the integrated system, and a novel setup with an extra arc structure is found to significantly improve the performance of the integrated system.

This paper evaluates potential of a novel, low-cost, and environmental friendly cement-based composite for wind energy harvesting. The composite is fabricated of cementitious matrix. Fibers and mesh reinforcements were introduced into the matrix for enhanced flexural performance. The composite was used to fabricate wind turbine blades which were tested for suitability under varying wind loadings. Wind turbine blades fabricated of the proposed cementitious composite successfully yielded power output when tested in wind tunnel; the maximum power coefficient being 0.182. The developed composite was also tested for mechanical performance and early-age shrinkage, the results of which indicated satisfactory performance. The proposed composite, if used for fabricating wind turbine blades, can significantly reduce the wind power generation cost. However, the lower power coefficient attained in this study can be improved by using hollow cross sections which are anticipated to reduce the corresponding unit weight and augment aerodynamic behavior of blades.

This study compiled a database called CYCU/RockTip/51 consisting of 51 rock-socketed drilled shafts installed at different sites worldwide, covering a wide variety of rock properties and shaft geometries. The tip resistances from seven representative prediction models were compared with the measured values. These measured values were obtained from the load–displacement curves of field load tests using three interpretation criteria. It was found that the prediction models of Teng, Coates, Rowe and Armitage, and ARGEMA overpredicted the measured capacity, while Zhang and Einstein, Vipulanandan et al. and Zhang are less biased. These tip prediction models also could be classified according to displacement requirements. The proposed tip prediction models of this study are presented based on different interpretation methods and the displacement ranges that they are mobilized. Finally, the normalized load–displacement curve was fitted to the hyperbolic curve with two model parameters ( a and b). The parameters a and b define the reciprocal of the initial slope and the asymptotic resistance, respectively. The statistics of ( b, a) for rock-socketed drilled shafts in CYCU/RockTip/51 are mean = (0.76, 1.09), COV = (0.13, 0.59), and correlation coefficient = −0.79. These statistics are useful for reliability-based design.