Jisheng Zhang

Predictions of bridge scour: Application of a feed-forward neural network with an adaptive activation function

In this study, a new procedure to determine the optimum activation function for a neural network is proposed. Unlike previous methods of optimising activation functions, the proposed approach regards selection of the most suitable activation function as a discrete optimisation problem, which involves generating various combinations of function then evaluating their performance as activation functions in a neural network, returning the function or combination of functions which yields best result as the optimum. The efficacy of the proposed optimisation method is compared with conventional approaches using the data generated from several synthetic functions. Numerical results indicate that the network produced using the proposed method achieves a better accuracy with a smaller network size, compared to other approaches. Bridge scour problem is used to further demonstrate the performance of the proposed algorithm. Based on the training and validation results, a better estimation of both equilibrium and time dependent scour depth is produced by the neural network developed using the proposed optimisation method, compared to networks with a priori chosen activation functions. Furthermore, the performance of the proposed model is compared with predictions of empirical methods, with the former making more accurate predictions.

Two-Dimensional Model for Pore Pressure Accumulations in the Vicinity of a Buried Pipeline

In this paper, we presented an integrated numerical model for the wave-induced residual liquefaction around a buried offshore pipeline. In the present model, unlike previous investigations, two new features were added in the present model: (i) new definition of the source term for the residual pore pressure generations was proposed and extended from 1D to 2D; (ii) preconsolidation due to self-weight of the pipeline was considered. The present model was validated by comparing with the previous experimental data for the cases without a pipeline and with a buried pipeline. Based on the numerical model, first, we examined the effects of seabed, wave and pipeline characteristics on the pore pressure accumulations and residual liquefaction. The numerical results indicated a pipe with a deeper buried depth within the seabed with larger consolidation coefficient and relative density can reduce the risk of liquefaction around a pipeline. Second, we investigated the effects of a trench layer on the wave-induced seabed response. It is found that the geometry of the trench layer (thickness and width), as well as the backfill materials (permeability K and relative density Dr) have significant effect on the development of liquefaction zone around the buried pipeline. Furthermore, under certain conditions, partially backfill the trench layer up to one pipeline diameter is sufficient to protect the pipelines from the wave-induced liquefaction.

Three-dimensional numerical model for wave-induced seabed response around mono-pile

In this study, a new three-dimensional model was developed to provide a better understanding of the mechanism for wave-induced seabed response around the mono-pile. Based on the poro-elastic theory, the fully dynamic formulations were adopted in the present model to simulate the pore pressure, soil stresses and the displacements of both soil and mono-pile. Good agreement between the numerical simulation and experimental results was obtained. Based on the parametric study, the numerical results concluded that (1) the wave diffraction and reflection have significant effects on the pore pressure and soil displacements around the mono-pile; (2) the largest discrepancy of pore pressure due to the variation in seabed parameters appears in front of mono-pile, while the smallest discrepancy is at the position of angle 3π/4 with respect to the incident wave direction and (3) the increase in the mono-pile horizontal displacement corresponds to the increase in the wave height and the decrease in the seabed Youngs modulus.

A process-based model for sediment transport under various wave and current conditions

The purpose of this study is to investigate the capability of a newly developed process-based model for sediment transport under a wide variety of wave and current conditions. The model is based on the first-order boundary layer equation and the sediment advection-diffusion equation. In particular, a modified low Reynolds number k-ɛ model is coupled to provide the turbulence closure. Detailed model verifications have been performed by simulating a number of laboratory experiments, covering a considerable range of hydrodynamic conditions such as sinusoidal waves, asymmetric waves and wave-current interactions. The model provides satisfactory numerical results which agree well with the measured results, including the time-averaged/dependent sediment concentration profiles and sediment flux profiles, as well as the time series of concentration at given elevations. The observed influences of wave orbital velocity amplitude, wave period and sediment grain size are correctly reproduced, indicating that the fundamental physical mechanisms of those processes are properly represented in the model. It is revealed that the present model is capable of predicting sediment transport under a wide range of wave and current conditions, and can be used to further study the morphodynamic processes in real coastal regions.

Numerical Simulation of the Tidal Flow and Suspended Sediment Transport in the Qiantang Estuary

AbstractResults from a numerical modeling study are presented to investigate the tidal elevations, tidal current velocity, bed deformation, and suspended sediment concentration in the Qiantang Estuary, China. The Qiantang Estuary is well-known for its macrotide, which generates a hydrodynamically complex environment. This presents challenges for numerical modelers to accurately simulate the flow field and sediment transport in the region. This paper presents a mathematical model using finite volume method with unstructured mesh to simulate the tide-induced water elevation, current velocity, bed deformation, and suspended sediment transport in the Qiantang Estuary. The parameters in the model were determined using the long-term observed field data of the Qiantang Estuary. The simulated tidal elevations and current velocities agree well with the field observations. The numerical prediction of the bed deformation in 5 months is reasonably compared with the field measurements carried out in the same period. H...

Numerical simulation of solitary-wave propagation over a steady current

AbstractA two-dimensional numerical model is developed to study the propagation of a solitary wave in the presence of a steady current flow. The numerical model is based on the Reynolds-averaged Navier-Stokes (RANS) equations with a k-e turbulence closure scheme and an internal wave-maker method. To capture the air-water interface, the volume of fluid (VOF) method is used in the numerical simulation. The current flow is initialized by imposing a steady inlet velocity on one computational domain end and a constant pressure outlet on the other end. The desired wave is generated by an internal wave maker. The propagation of a solitary wave traveling with a following/opposing current is simulated. The effects of the current velocity on the solitary-wave motion are investigated. The results show that the solitary wave has a smaller wave height, larger wave width, and higher traveling speed after interacting with a following current. Contrariwise, the solitary wave becomes higher with a smaller wave width and l...

Dynamic Response in a Porous Seabed of Finite Depth to Combined Wave and Current Loadings

ABSTRACT Liu, B.; Jeng, D.S., and Zhang, J.S., 2014. Dynamic response in a porous seabed of finite depth to combined wave and current loadings. In this study a u-p approximation for the dynamic response of a porous seabed to nature dynamic loadings is proposed. Unlike most previous research, the effects of currents are considered in the new analytical solution. Based on the numerical examples, the significant influence of combined wave and current loadings on seabed response in shallow water is concluded. This influence affects not only the amplitudes of the seabed response but also the distributions of the space region. The relative difference of the maximum pore pressure between different direction currents can reach 8% of the static water pressure. In addition, the relative difference of maximum liquefaction depth between the previous model (without currents) and the present model can reach 19% of the maximum liquefaction depth of the case without currents.

Numerical Study on Effects of the Embedded Monopile Foundation on Local Wave-Induced Porous Seabed Response

Effects of the embedded monopile foundation on the local distributions of pore water pressure, soil stresses, and liquefaction are investigated in this study using a three-dimensional integrated numerical model. The model is based on a Reynolds-Averaged Navier-Stokes wave module and a fully dynamic poroelastic seabed module and has been validated with the analytical solution and experimental data. Results show that, compared to the situation without an embedded foundation, the embedded monopile foundation increases and decreases the maximum pore water pressure in the seabed around and below the foundation, respectively. The embedded monopile foundation also significantly modifies the distributions of the maximum effective soil stress around the foundation and causes a local concentration of soil stress below the two lower corners of foundation. A parametric study reveals that the effects of embedded monopile foundation on pore water pressure increase as the degrees of saturation and soil permeability decrease. The embedded monopile foundation tends to decrease the liquefaction depth around the structure, and this effect is relatively more obvious for greater degrees of saturation, greater soil permeabilities, and smaller wave heights.

Morphodynamic response of Xiaomiaohong tidal channel to a coastal reclamation project in Jiangsu Coast, China

ABSTRACT Zhang, C., Zheng, J.H., Dong, X.W., Cao, K. and Zhang, J.S., 2013. Morphodynamic response of Xiaomiaohong tidal channel to a coastal reclamation project in Jiangsu Coast, China. Xiaomiaohong tidal channel is located at the southern part of the large-scale radial sand ridge system along Jiangsu Coast, China. A land reclamation project was carried out on the tidal flat at the south bank of Xiaomiaohong channel in 2003, which led to the considerable change of channel morphology. This study aims to investigate the morphodynamic response of the tidal channel to the reclamation project through field observation and numerical modeling. Measured data shows that bed deposition occurred at both the east and west sides of the project area while erosion is found between the project area and the channel axis, especially around the northwest corner of the project area. The relationship between the observed bathymetry change and the reclamation project is discussed. A two-dimensional numerical model is developed to investigate the project effects on the tidal current, tidal volume, sediment concentration and morphological evolution. It is indicated that the change of morphodynamic processes due to the reclamation project is only concentrated near the project area and the project does not significantly influence the overall channel evolution. While the nearby current direction and velocity were changed and the tidal volume was reduced after the reclamation, sediment concentration was little affected. The model is able to qualitatively simulate some important evolution trend of the channel under the impact of reclamation project but some difference is also found with respect to the erosion/deposition intensity and scale.

Numerical study on the interaction between waves and twin pipelines in sandy seabed

ABSTRACT Zhang, J., Zheng, J., Zhang, C., Jeng, D. and Guo, Y., 2013. Numerical study on the interaction between waves and twin pipelines in sandy seabed In offshore engineering practice, the construction of an identical cylinder close to the existing pipeline may largely affect the near flow field and hydrodynamic forces on the pipelines. The interaction between incident ocean wave and twin pipelines in sandy seabed plays an important role in the design of submarine pipelines. A mathematical model based on the Volume-Averaged Reynolds-Averaged Navier-Stokes (VARANS) equations, a two-equation k-ϵ turbulence closure and an internal source term wave maker is adopted to describe this complex phenomenon. The sandy bed where the pipelines are partly buried is treated as a rigid porous material, and its impact on wave-pipeline interaction is considered in the VARANS model in terms of porosity and equivalent mean diameter of porous material. The impacts of burial depth and distance between the centers of twin pipelines on the wave-seabed-twin pipelines interaction are numerically investigated. The research results from numerical simulations indicate that the burial depth and relative position of twin pipelines significantly affect the wave-averaged flow velocity and dynamic pressure distribution.