Display Method: |
A novel concept of wave attenuator is proposed for the defense of long waves, through integrating a flexible tail to the lee-side surface of a pile breakwater. The flexible tail works as a floating blanket made up of hinged blocks, whose scale and stiffness can be easily adjusted. A two-phase-flow numerical model is established based on the open-source computational fluid dynamics (CFD) code OpenFOAM to investigate its wave attenuation performance. Incompressible Navier−Stokes equations are solved in the fluid domain, where an additional computational solid mechanics (CSM) solver is embedded to describe the elastic deformation of the floating tail. The coupling of fluid dynamics and structural mechanics is solved in a full manner to allow assess of wave variation along the deforming body. The accuracy of the numerical model is validated through comparison with experimental data. Effects of the flexible tail on performance of the pile breakwater are investigated systematically. Dynamic behaviours of the tail are examined, and characteristics of its natural frequency are identified. For safety reasons, the wave loads impacting on the main body of the pile breakwater and the stress distribution over the tail are specially examined. It is found that both the length and stiffness of the tail can affect the wave-attenuation performance of the breakwater. A proper choice of the length and stiffness of the tail can greatly improve the long-wave defending capability of the pile breakwater. The maximum stress over the flexible tail can be restrained through optimising the deformation and stiffness of the tail.
The floating bridge bears the dead weight and live load with buoyancy, and has wide application prospect in deep-water transportation infrastructure. The structural analysis of floating bridge is challenging due to the complicated fluid-solid coupling effects of wind and wave. In this research, a novel time domain approach combining dynamic finite element method and state-space model (SSM) is established for the refined analysis of floating bridges. The dynamic coupled effects induced by wave excitation load, radiation load and buffeting load are carefully simulated. High-precision fitted SSMs for pontoons are established to enhance the calculation efficiency of hydrodynamic radiation forces in time domain. The dispersion relation is also introduced in the analysis model to appropriately consider the phase differences of wave loads on pontoons. The proposed approach is then employed to simulate the dynamic responses of a scaled floating bridge model which has been tested under real wind and wave loads in laboratory. The numerical results are found to agree well with the test data regarding the structural responses of floating bridge under the considered environmental conditions. The proposed time domain approach is considered to be accurate and effective in simulating the structural behaviors of floating bridge under typical environmental conditions.
Control parameter optimization is an efficient way to improve the endurance of underwater gliders (UGs), which influences their gliding efficiency and energy consumption. This paper analyzes the optimal matching between the net buoyancy and the pitching angle and proposes a segmented control strategy of Petrel-L. The optimization of this strategy is established based on the gliding range model of UG, which is solved based on the approximate model, and the variations of the optimal control parameters with the hotel load are obtained. The optimization results indicate that the segmented control strategy can significantly increase the gliding range when the optimal matching between the net buoyancy and the pitching angle is reached, and the increase rate is influenced by the hotel load. The gliding range of the underwater glider can be increased by 10.47% at a hotel load of 0.5 W. The optimal matching analysis adopted in this study can be applied to other UGs to realize endurance improvement.
Flow-induced vibration energy harvesting devices typically use an elastically supported body immersed in an oncoming flow to convert the sea and river current's hydrokinetic energy into electrical energy. The proportion of energy the device collects is greatly influenced by parameters such as the water flow velocity, spacing between device components, structure size, and damping coefficient. For parameter optimization and performance predictions of flow-induced vibration energy harvesting devices, we train a model of the power harvesting efficiency under different damping ratios, stiffnesses, spacing ratios, and reduced velocities based on experimental data. To improve the prediction accuracy, a feedforward network structure is optimized by using the topological evolutionary algorithm and a radial basis function network. Comparative analysis reveals that the radial basis function network model provides the best agreement with the experimental results and realizes accurate predictions of the power harvested by a dual-oscillator system in the vortex-induced vibration, transition region, and galloping. The prediction results show that the model's maximum power harvesting efficiency occurs in the vortex-induced vibration. The efficiency increases and then decreases with increasing stiffness and reduced velocity in this phase; an increase in the spacing ratio causes the efficiency to decrease and then increase; finally, increasing the damping ratio enhances the efficiency. The device achieves maximum power harvesting efficiency at a reduced velocity of Ur=4.11. The proposed model effectively predicts the maximum efficiency and the corresponding damping ratio and stiffness of the vortex-induced vibration and galloping, providing a new method for predicting tandem dual-oscillator hydrodynamic power conversion in flow-induced vibration.
The motion of particle clouds (i.e., sediment clouds) usually can be found in engineering applications such as wastewater discharge, land reclamation, and marine bed capping. In this paper, a series of laboratory tests are conducted on coral sand to investigate the shape feature of the single particle and the mixing processes of the coral sand particle clouds. The shape of coral sand particle is measured and quantified. The experimental results demonstrate that the shape of coral sand particles tends to be spherical as the particle size decreases, and empirical equations were established to explain the variation of D50 and fS,50 of coral sand. Compared with the silica sand, the evolution of the coral sand particle cloud still experiences three stages, but the threshold for the Reynolds number of particle clouds entering the next stage changes. Further, the normalized axial distance of the coral sand particle clouds is 58% smaller. The frontal velocity exhibits similar varying tendency for the coral sand particle cloud. Considering the difference in shape between coral sand particles and silica sand particles, a semi-empirical formula was proposed based on the original silica sand prediction formula by adding the shape factor and the experimental data of 122 μm≤D50≤842 μm. It can predict the frontal velocity of the coral sand particle clouds.
When a high-speed body with cavity passes through water-air free surface and exits water, its mechanical environment and dynamic characteristics change significantly due to the great difference in density and viscosity between water and air. With focusing on this problem, the Computational Fluid Dynamics (CFD) method is applied to perform numerical calculation on the process of this vapor-liquid-gas flow during the water exit of a high-speed cylinder, with the Volume of Fraction (VOF) multiphase flow interface-capturing techniques and the overset grid technology. After the verification and validation of the CFD model through mesh convergence study and a water-entry experiment, cavity evolution and flow characteristics including pressure and velocity distribution during the water exit are analyzed. The effects of different initial velocities on the pressure distribution and drag characteristics of the cylinder are investigated. Calculated results show that the cavity collapse during water exit causes strong pressure fluctuation on the cylinder; when the cylinder exits water enveloped in a supercavity, the pressure distribution on its wall surface and surrounding water region is relatively uniform, and the drag changes gently, and thus the cylinder has good motion stability.
During the self-weight penetration process of the suction foundation on the dense sand seabed, due to the shallow penetration depth, the excess seepage seawater from the outside to the inside of the foundation may cause the negative pressure penetration process failure. Increasing the self-weight penetration depth has become an important problem for the safe construction of the suction foundation. The new suction anchor foundation has been proposed, and the self-weight penetration characteristics of the traditional suction foundation and the new suction anchor foundation are studied and compared through laboratory experiments and analysis. For the above two foundation types, by considering five foundation diameters and two bottom shapes, 20 models are tested with the same penetration energy. The effects of different foundation diameters on the penetration depth, the soil plug characteristics, and the surrounding sand layer are studied. The results show that the penetration depth of the new suction foundation is smaller than that of the traditional suction foundation. With the same penetration energy, the penetration depth of the suction foundation becomes shallower as the diameter increases. The smaller the diameter of the suction foundation, the more likely it is to be fully plugged, and the smaller the height of the soil plug will be. In the stage of self-weight penetration, the impact cavity appears around the foundation, which may affect the stability of the suction foundation.
Marine current turbine (MCT), which is designed for the power supply of underwater mooring platform (UMP), is investigated in this article. To reduce its flow noise, the microgrooved surface is applied at the suction surface of the turbine blades. Comprehensive analyses of the effects of the UMP on MCT with microgrooved surface in different working conditions are presented. The transient turbulent flow field is obtained by incompressible large eddy simulation (LES), and then the Ffowcs Williams and Hawkings (FW—H) acoustic analogy is adopted to forecast the flow noise generated from the pressure fluctuations and loadings of the UMP shell and MCT blade surfaces. The numerical methods are first validated with experimental data and good agreements are obtained. Then, the influence of several key parameters on the performance of the MCT is then systematically studied, including interval distance, angle of pitch and angle of sideslip. For each case, the hydrodynamic parameters (thrust coefficient, torque coefficient and power coefficient), the vortical structures behind the model and the overall sound pressure level (OASPL) directionality are analyzed. Additionally, the noise reduction effect of the microgrooved surface is also presented. The present investigation could provide an overall understanding for the performance of MCT combined with UMP.
Offshore wind energy resources are operational in cold regions, while offshore wind turbines will face the threat of icing. Therefore, it is necessary to study icing of offshore wind turbines under different icing conditions. In this study, icing sensitivity of offshore wind turbine blades are performed using a combination of FLUENT and FENSAP-ICE software, and the effects of liquid water content (LWC), medium volume diameter (MVD), wind speed and air temperature on blade icing shape are analyzed by two types of ice, namely rime ice and glaze ice. The results show that the increase of LWC and MVD will increase the amount of ice that forms on the blade surface for either glaze ice or rime ice, and an increase of MVD will expand the adhesion surface between ice and blade. Before reaching the rated wind speed of 11.4 m/s, it does not directly affect the icing shape. However, after reaching the rated wind speed, the attack angle of the incoming flow decreases obviously, and the amount of ice increases markedly. When the ambient air temperature meets the icing conditions of glaze ice (i.e., −5ºC to 0ºC), the lower the temperature, the more glaze ice freezes, whereas air temperature has no impact on the icing of rime ice. Compared with onshore wind turbines, offshore wind turbines might face extreme meteorological conditions, and the wind speed has no impact on the amount of ice that forms on the blade surface for most wind speeds
The shear strength and dilatancy of typical uncemented calcareous sand from the South China Sea are investigated by soil lab tests. According to drained triaxial tests at various relative densities and confining stresses, it is found that the constant volume friction angle is approximated as 39° and the traditional Bolton’s equations can be modified to estimate the peak friction angle and dilation angle. The reliability of the equation proposed for the peak friction angle is verified in terms of calcareous sands from more onshore and offshore sites worldwide, while the errors of the predicted dilation angles scatter in a relatively large range. Totally, the dilation angles of sands in the South China Sea are estimated by the equation presented with an error of ±30%. The peak friction angle measured by the undrained is similar to that by the drained tests as the relative density smaller than 60%, while the former is slightly lower for denser samples.
In this paper, an improvement has been made to the approximation technique of a complex domain through the stair-step approach to have a considerable accuracy, minimize computational cost, and avoid the hardship of manual work. A novel stair-step representation algorithm is used in this regard, where the entire procedure is carried out through our developed MATLAB routine. Arakawa C-grid is used in our approximation with (1/120)° grid resolution. As a test case, the method is applied to approximate the domain covering the area between 15°–23°N latitudes and 85°–95°E longitudes in the Bay of Bengal. Along with the approximation of the land-sea interface, coastal stations are also identified. Approximated land-sea interfaces and coastal stations are found to be in good agreement with the actual ones based on the similarity index, overlap fraction, and extra fraction criteria. The method can be used for approximating an irregular geometric domain to employ the finite difference method in solving problems related to long waves. As a test case, shallow water equations in Cartesian coordinates are solved on the domain of interest for simulating water levels due to the nonlinear tide-surge interaction associated with the storms April 1991 and AILA, 2009 along the coast of Bangladesh. The same input except for the discretized domain and bathymetry as that of Paul et al. (2016) is used in our simulation. The results are found to be in reasonable agreement with the observed data procured from Bangladesh Inland Water Transport Authority.
An empirical formula to predict overtopping discharge of vertical wall is presented, in which an expression similar to the solitary wave function is proposed to describe the rule of the influence of relative water depth. The formula is derived from performing an investigation to the well-known overtopping graphs of Goda, and for the sake of interest, the process of the derivation is detailed. To make clear the formula’s performance, relevant test datasets in the CLASH database are extracted to examine the error levels of the formula. As a result, an overall good agreement has been found between the predictions of the formula and the extracted datasets in a wide range outside the extreme shallow region, i.e., the range of relative water depth smaller than 0.6, in which the water depth near the wall is so shallow that nearly all the incoming waves have been broken before reaching the wall.
In this paper, we investigate a (3+1)-dimensional generalized variable-coefficient shallow water wave equation, which can be used to describe the flow below a pressure surface in oceanography and atmospheric science. Employing the Kadomtsev−Petviashvili hierarchy reduction, we obtain the semi-rational solutions which describe the lumps and rogue waves interacting with the kink solitons. We find that the lump appears from one kink soliton and fuses into the other on the x−y and x−t planes. However, on the x−z plane, the localized waves in the middle of the parallel kink solitons are in two forms: lumps and line rogue waves. The effects of the variable coefficients on the two forms are discussed. The dispersion coefficient influences the speed of solitons, while the background coefficient influences the background’s height.
Sandwich panel is commonly used in ship and marine engineering equipment, such as side structure and superstructure deck of a ship, which is of good anti-explosion performance. This paper addresses a study on the dynamic response of the U-typed sandwich panel under explosion load through the numerical simulation and theoretical methods. Based on the orthotropic plate theory, the U-typed sandwich panel is simplified and transformed into a single degree of freedom (SDOF) spring system, the equivalent motion equation of the SDOF system and the expression of triangular explosion load function are established based on the SDOF theory, and the maximum response spectrum of the SDOF system is obtained. Then, the response of the equivalent SDOF system of the U-typed sandwich panel under explosion load is analyzed, and the theoretical results match well with the numerical simulation results, which verifies the accuracy of the theoretical method proposed in this paper. The theoretical method proposed in this paper could have good engineering applications for the structural anti-explosion design, and provide a reference for the evaluation of the anti-explosion performance of ship and offshore platform structures.
Tidal bores are a unique hydrodynamic phenomenon during flood tide in the Qiantang Estuary. The tidal bore propagation around the similar right-angle shoreline is rarely documented in tidal estuaries. To investigate tidal bores around this shoreline, a hydrodynamic model combined with a theoretical method is employed to reveal the characteristics of the bore propagation. The theoretical solution of the tidal bore intensity is deduced to illustrate the relationship of the incident tidal bores and the back-flow bores during the propagation. The hydrodynamic model based on shallow water equation is employed to perform the simulation of tidal bores in the estuary. Model results with respect to the bore height and the propagation speed of tidal bores have a favourable agreement with field data. The tidal bore dynamics in the neighborhood of the similar right-angle shoreline are elucidated. The characteristics of tidal bores in terms of water surface, velocity, bore steepness and the intensity are illustrated and the back-flow bore is analyzed by numerical and theoretical methods around the similar right-angle shoreline. The height of the back-flow bore relative to the incident tidal bore ranges from 1.05 to 1.77. Model result reveals that the ambient water depth and the shape of the similar right-angle shoreline are contributed to the back-flow bore formation.
ScholarOne Manuscripts Log In
- Volume 36
- Issue 5
- September 2022
- Superintended by:
CHINA ASSOCIATION FOR SCIENCE AND TECHNOLOGY
- Sponsored by:
Chinese Ocean Engineering Society （COES）
- Edited by:
Nanjing Hydraulic Research Institute