Marine biofouling is a major concern in the operational performance of submerged floating tunnels (SFTs). The objective of this research is to extend hydrodynamic conditions in experiments and numerically investigate the effects of marine fouling on the hydrodynamic behavior of SFTs, including flow characteristics and forces on the SFT subject to waves. A sensitivity analysis of roughness parameters including different roughness heights and roughness coverage ratios is carried out. Additionally, the hydrodynamic forces of a roughened SFT with a circular shape and a newly designed parametric shape are compared.

@en

The effects of surface roughness as induced by marine fouling on the hydrodynamic forces on a submerged floating tunnel (SFT) are experimentally and numerically investigated in detail at Reynolds numbers Re = 8.125 × 10³–5.25 × 10⁴. A sensitivity analysis to different roughness parameters including roughness height, skewness, coverage ratio, and spatial arrangement is performed. In addition, an optimized parametric cross-section for an SFT is proposed, and the hydrodynamic performance of the parametric shape and circular SFT cross-section shape with roughness elements is compared. The pressure distribution along the SFT, flow separation and wake characteristics are analyzed to provide a systematic insight into the fundamental mechanism relating the roughness parameters and flow around an SFT. In order to better understand the nonlinear relationships among structural geometry, roughness parameters, flow states, and structural response, an artificial intelligence method using Random Forest (RF) for feature importance ranking is applied. The results show that with the parametric shape, the hydrodynamic forces on the fouled SFT can be effectively mitigated. The roughness height and coverage ratio affect the equivalent blockage and hence, change flow separation and recirculation length in the wake. Lower skewness of the roughness elements can increase the critical Re by changing the relative roughness parameter. Horizontal arrangement of the roughness elements on an SFT generally results in the largest hydrodynamic forces, compared to staggered and vertical distributions. Throughout the feature importance ranking, the flow regime is found to be the most important feature of the hydrodynamics of the SFT. In addition, the SFT cross-section shape and roughness coverage ratio play a dominant role.

@en

The modelling of complex free surface flows is challenging due to the mobility and deformability of the interface and air entrainment characteristics, which are highly affected by turbulence. With the framework of Reynolds averaged Navier–Stokes (RANS) models and the volume of fluid (VOF) method, turbulence quantities at the air–water interface tend to be over-estimated. In this study, interfacial turbulence treatment methods including the buoyancy modification model based on the simple gradient diffusion hypothesis (SGDH) and Egorov’s turbulence damping model are investigated. Furthermore, due to the unconditionally unstable characteristics of the standard k-ε turbulence model, the stabilized k-ε turbulence model is applied as a comparison. The turbulence attenuation performance using different interfacial turbulence treatment methods in the vicinity of the interface is compared and discussed for stratified flows and free overflow weirs for aerated and non-aerated nappe scenarios. The turbulence quantities and free surface profile under different flow conditions are validated against experimental data and an analytical model. The results show that for free surface waves, both the SGDH model and the turbulence damping model give strong improvements in turbulence production compared with the standard k-ε model. The SGDH model augments the turbulence kinetic energy (TKE) in the unstable stratification, leading to unphysical behaviour for the partially dispersed and separated flow.

@en

Marine biofouling is a major concern in the operational performance of submerged floating tunnels (SFTs). The objective of this research is to investigate the effects of marine fouling (represented by surface roughness) on the hydrodynamic behavior of SFTs, including the hydrodynamic forces on the SFT subject to current-only, wave-only, and combined current-wave flow conditions. The effects of increased surface roughness induced by marine fouling on the dynamic response of an SFT are characterized by hydrodynamic force coefficients, including drag and inertia coefficients. At the Water Lab of Delft University of Technology (TU Delft), experiments have been performed in a wave-current flume to compare the SFTs’ behaviors as affected by different roughness characteristics. In addition, a parametric cross-section for an SFT is presented, and the hydrodynamic performance associated with surface roughness effects on the parametric shape and circular SFT cross-section shape are compared. The results show that the parametric shape can effectively reduce the drag coefficient (C_d) under current-only conditions and lower the inertia coefficient (C_m) when waves are present. As roughness height and coverage ratio increase, C_d generally increases while C_m decreases. However, small differences in C_d and C_m can be observed with regard to roughness parameters for wave-only conditions. The Morison coefficients adapted for a marine-fouled SFT measured in the experiments are compared to predictions from engineering standards and are recommended for engineering practice.

@en

The submerged floating tunnel (SFT), also called an Archimedes Bridge, is a new type of infrastructure for wide and deep sea-crossings, regarded as one of the alternatives to bored and immersed tunnels and bridges. It is afloat in water employing its buoyancy and a support system to balance its self-weight. However, no prototype SFT has yet been built anywhere due to the immaturity of scientific research and engineering technology. The dynamic response of the SFT subject to operating and extreme environmental conditions, which determines structural safety and reliability, is a crucial issue that needs to be better understood. In order to better comprehend the response of the SFT to hydrodynamic forces, key points including hydrodynamic loads acting on the SFT and the structural dynamic response for various structural configurations, as well as the relations and interactions among these factors must be quantified. In this study, hydrodynamic loads on various SFT cross-sectional geometries were computed. The parametric cross-section shape described by a Bezier-PARSEC curve was optimized using a hybrid Artificial Neural Network (ANN) and Genetic Algorithm (GA). The practical range of aspect ratios of the SFT cross-section was determined by conducting a sensitivity analysis under tidal current conditions. It was found that an SFT cross-section with an aspect ratio of 0.47 using a leading-edge BP curve under the given clearance is optimal for a balanced consideration of hydrodynamic performance and construction cost. Furthermore, the machine learning method used is shown to be a reliable and effective tool for the SFT cross section optimization design. The hydrodynamic loads acting on the optimal cross-section shape were compared with simpler shapes under various environmental conditions of currents and waves, including the extreme environmental conditions of internal waves, tsunamis, and super typhoons. An internal solitary wave (ISW), described by the modified Korteweg de Vries (mKdV) theory, was adopted for the hydrodynamic loading analysis of the SFT based on field observations and high-resolution satellite images. It was found that the ISW can remarkably alter the buoyancy - weight ratio (BWR) of the SFT and hence, cause a large vertical hydrodynamic load on the SFT, threatening the safety and reliability of the SFT system. A worst-case tsunami and a hindcast typhoon in the Qiongzhou Strait were selected for extreme event hydrodynamic forcing analysis. It was found that extreme event hazards in the Qiongzhou Strait are rare due to the sheltering effect of Hainan Island. In terms of hydrodynamic forcing, the selected typhoon scenario is more devastating than the tsunami case for an SFT. The proposed parametric cross-sectional shape for the SFT shows better hydrodynamic performance than simpler shapes under all applied environmental conditions and is therefore recommended for the engineering design. After investigating different types of hydrodynamic loads acting on the SFT, the global dynamic response (including vibration) of the SFT was assessed. A numerical model of a prototype super-long coupled tube-mooring-joint SFT system based on Finite Element Method (FEM) was developed to better predict flow-induced vibration (FIV) and structural dynamic response. A pragmatic approach for structural dynamic response computation under realistic oceanic conditions was developed considering the spatial randomness of hydrodynamic loads. Multi-scale hydrodynamic models including a large-scale oceanographic model and a small-scale CFD model were developed for determination of hydrodynamic loads. It was found that the SFT tube is unlikely to experience severe resonance under steady current conditions, but the vibration of the SFT tube is dominated by wave conditions, where a single dominant mode excitation of the tube with a large wave height and period cause large amplitude motion. In order to give insight into structural dynamic response under extreme environmental conditions, internal forcing on the SFT and structural response of the SFT were computed subject to the ISW and super typhoon loads. This showed that the displacement and acceleration of the SFT under the ISW are far smaller than the structural serviceability requirements, and resonance of the tunnel tube becomes unlikely under the ISW condition due to its rather low intrinsic frequency. The dynamic response of the SFT subject to the typhoon scenario is much more severe than that of the ISW case, and the horizontal stiffness of the moored tube greatly affects its dynamic response. The maximum bending moment and torque on the SFT occur at its shore connections, where failure risk due to structural fatigue or buckling are substantial. The final aspect of this thesis aims to optimize the SFT structural configuration for minimization of hydraulic resonant loading. The core concept is to investigate the sensitivity of structural response to the structural fundamental frequencies outside the hydrodynamic frequency. It was found that natural frequencies of the SFT system are mainly affected by BWR, tunnel tube length, mooring configuration and stiffness, and joint and shore connection properties. A dynamic process for the SFT configuration optimization subject to different hydrodynamic loads can be established by smartly tuning the fundamental frequencies to mitigate structural dynamic response. @en

In order to assess the dynamic performance of a submerged floating tunnel (SFT) subject to flow-induced vibration (FIV) conditions in a practical engineering application, a one-way fluid–structure interaction (FSI) model consisting of multi-scale hydrodynamic solvers combined with the finite element method (FEM) is established. A typical long, large aspect ratio SFT is modeled by coupling tube, joint, and mooring components. The SFT is simulated in the time domain under currents, waves, and extreme events. FIV of SFTs with different cross-section shapes is investigated by analyzing each structure's natural frequencies, hydraulic loading frequency, and dominant modes. The results show that FIV of the SFT tube is dominated by wave conditions. The excitation of the SFT's first dominant mode by a large wave height and period should be avoided. Standing and traveling wave patterns and multi-mode response are observed during extreme events. The hydrodynamic forcing and structural dynamic response of the SFT can be effectively reduced by adopting a parametric cross-section.

@en

The interaction between an oceanic internal solitary wave (ISW) and a prototype submerged floating tunnel (SFT) is numerically investigated. Effect of oceanic internal solitary wave amplitude, the relative distance of the SFT to the pycnocline, cross-sectional geometry of the SFT, and the density ratio of the two fluid layers are analyzed. At a potential application site, the dynamic response of an SFT composed of a tube-joint-mooring system forced by an oceanic ISW is studied using Finite Element Method (FEM) modeling. The numerical results show that the ISW-induced force can be effectively reduced by adopting a parametric SFT cross section instead of a circle or ellipse. The influence of the relative distance of the SFT to the ISW pycnocline is crucial, and can remarkably alter the vertical force and buoyancy-weight ratio (BWR) of the SFT during ISW propagation. Large shear forces and bending moments on the SFT can occur, affecting the tension in the mooring lines, and threatening the safety and reliability of the SFT system. However, the deflections and accelerations of the SFT under the applied ISW are within structural serviceability requirements due to the low frequency of the ISW compared to the natural frequency of the SFT tube.

@en

Concepts of a submerged floating tunnel (SFT) for novel sea-crossings have been researched in recent years. An SFT tube should be moored afloat by tensioned mooring systems to maintain the tube position under complex hydrodynamic loads. In-line force is amongst the dominant hydrodynamic parameters in the SFT cross-section design and the mooring system reliability evaluation. Selecting a suitable in-line force computation method is crucial to successful and accurate SFT cross-section optimization. The transition SST model is an effective turbulence transition prediction tool in the boundary layer computation subjected to tidal flow at both low and high Reynolds numbers. Two types of parametric Bézier curves applied in airfoil optimization are used to describe the SFT cross-section. We show that an SFT cross-section described by a leading-edge Bézier-PARSEC (BP) curve has better hydrodynamic performance than a trailing-edge BP curve of equal aspect ratio (AR). To avoid large flow separation, an AR not exceeding 0.47 is recommended. An SFT cross-section design should balance hydrodynamic performance and construction cost. The SFT cross-section with AR = 0.47 using the leading-edge BP curve with fixed clearance has a comparatively small in-line force and a minimum material cost.

@en

Submerged floating tunnel (SFT) is a new type of promising traffic structure for the crossing of strait, ocean and fjord. This paper used a CFD method to investigate the hydrodynamic response of a 2D coupled tube-mooring system SFT under regular wave impacts. The influence of some key parameters was discussed, including submerged depth, buoyancy weight ratio (BWR), inclined mooring angle (IMA), mooring stiffness, mooring system arrangement, mooring pre-tension distribution andwave conditions. The results showthat themotion amplitudes and external forces of the SFT decrease with increasing submerged depth, but the SFT motion responses increase with increasing wave height and IMA. The influence of wave period is complicated, and the resonance of the SFT system must be avoided. Reasonable range of BWR exist, which leads to less motion response and more structural stability. Pre-tension distribution is not a key factor, and the mooring arrangement lacking vertical lines is not recommended. The influence factors on dynamic response was investigated, which provides references for the engineering practice.

@en

The cross-section geometry of a submerged floating tunnel (SFT) has a large effect on hydrodynamic characteristics, structural behavior and service level, making the tunnel cross section the primary factor in optimizing efficiency. Minimizing the mean drag and the dynamic variability in the lift of the SFT cross section under bi-directional (i.e., tidal) flow has a dramatic impact on the reduction of structural displacements and mooring loads. Based on a parametric Bézier curve dynamically comprising the leading-edge radius, tunnel height and width to define the SFT geometry, a sensitivity analysis of the Bézier curve parameters for a fixed aspect ratio with prototype dimensions under uniform flow conditions was conducted by applying Computational Fluid Dynamics (CFD), and the pressure distribution around the SFT cross-section surface was analyzed. A theoretical method comprising the Kármán vortex street parameters was employed to verify the CFD simulation results. In order to determine the SFT cross section with optimal hydrodynamic properties, the mean drag and Root Mean Square (RMS) lift coefficients were selected as optimization objectives, and four Bézier curve parameters were the input variables, in a neural network and genetic algorithm optimization process (a hybrid BP-GA structure), which is less likely to become trapped in local minima. The results show the optimal tunnel cross section has a mean drag and a RMS lift coefficient reduced by 0.9% and 6.3%, respectively, compared to the original CFD dataset.

@en

A submerged floating tunnel (SFT) is a promising alternative to conventional bridges and tunnels, and can be potentially built in the Qiongzhou Strait in China. However, this area is under the threat of disasters including mega tsunamis and severe storm surges. To evaluate the hydrodynamic loads of the SFT in this hazardous zone subject to severe tsunami and typhoon impacts, the Delft3D-FLOW hydrodynamic model and SWAN wave model are coupled, and a Computational Fluid Dynamics (CFD) method is adopted. The maximum probable tsunami and typhoon Rammasun (July 2014) are selected as hazard assessment conditions in the Qiongzhou Strait. Whether the tsunami and hindcast storm surge cause extreme forcing and bring challenges to the SFT engineering design, operation, and maintenance in the Qiongzhou Strait are discussed in this study. We reveal that the typhoon impacts are more devastating than tsunami for an SFT in the Qiongzhou Strait. In order to determine the optimal SFT cross-section under extreme events, we use a parametric Bezier curve profile compared with two simpler shapes including circular and elliptical cross sections. In-line force and lift are respectively applied to evaluate the SFT's hydrodynamic behaviour. Our results reveal that the gross horizontal force on the parametric Bezier curve shape is more sensitive to flow acceleration, while the circular cross-section is dominated by current speed. The parametric Bezier curve cross-section shape has the preferable property of reducing the in-line force and postponing serious vortex shedding compared with the two simpler shapes.

@en

A submerged floating tunnel’s (SFT) cross-sectional geometry is one of the main factors influencing its hydrodynamic characteristics. Minimizing the drag and lift is important for reducing the displacement of the structure and load on the mooring system. In this study, a parametric cross-section design method based on the Bézier curve is used to define the geometry of the SFT cross-section. A sensitivity analysis of the Bézier curve parameters is conducted with Computational Fluid Dynamics (CFD), and compared with simpler common cross-sections. The results show the parametric design method noticeably reduces the mean drag and RMS lift coefficient, compared with simpler common sections.@en