3D numerical modelling of scour around a jacket structure with dynamic free surface capturing

Local scour around pile-group foundations is a predominant cause of hydraulic instability in bridge engineering. This study employs a fully coupled three-dimensional computational fluid dynamics model to investigate local scour around a 2 × 2 inline pile group under steady flows. The model is validated against detailed laboratory measurements of flow and scour, demonstrating good agreement in both hydrodynamic and scour results, with scour depth simulations deviating by less than 15% from experimental data. Analysis of the flow fields reveal that scour evolution is accompanied by the descent of the horseshoe vortex, intensification of gap-flow, and acceleration around the side piles, while migration of bed shear stress from the pile flanks to the upstream slope dictates the equilibrium scour morphology. A systematic parametric study was conducted to evaluate the influence of the Froude number (Fr) and pile spacing (G/D) on scour depth. The results indicate that scour depth increases rapidly with Fr up to approximate 0.35, beyond which it plateaus as form-induced drag dissipates the incoming flow energy. Increasing G/D from 1 to 1.5 reduces the scour depth by about 12%, with smaller further reduction beyond G/D = 1.5, suggesting that this spacing offers a pragmatic compromise between structural footprint and scour resistance.

Numerical simulations are conducted to investigate the local scour below a sagging subsea pipeline vibrating in two degrees of freedom (2-DOF), covering a sagging ratio from −0.3 to 0 and a wide range of reduced velocities from 0.5 to 15. A negative sagging ratio means that the static balance position of the bottom surface of the pipeline is below the sand surface. The combined effects of the sagging and 2-DOF vibration of the pipeline on the scour are investigated. The maximum scour depth of the 2-DOF vibrating pipeline in the lock-in range of the vibration increases by 10% compared to the 1-DOF; however, the reduced velocity where the maximum scour depth occurs changes from 5 to 6. The variations of the vibration amplitude with the reduced velocity for all the sagging ratios follow a similar trend. For embedment ratios of −0.4 to −0.3, initial condition of scour simulation affects the equilibrium scour depth and vibration amplitudes. Simulations with a flat sand surface as initial condition prevent full scour development because the pipeline reaches its static balance position due to weak flow through the pipeline-to-bed gap. However, if the simulation starts with a sufficiently large initial scour depth, scour can reach its equilibrium.

The planform morphology of a river reach is the result of the combined actions of sediment motion (erosion, transport and deposition), hydrological regime, development and growth of vegetation. However, the interactions among these processes are still poorly understood and rarely investigated in laboratory flume experiments. In these experiments and also in numerical modelling, vegetation is usually represented by rigid cylinders, although it is widely recognized that this schematization cannot reproduce the effects of root stabilization and binding on riverbed sediment. In this work, we focus on the effects of added vegetation on morphological dynamics of alternate bars in a straight channel by means of flume experiments. We performed laboratory experiments reproducing hydraulic conditions that are typical of gravel bed rivers, in terms of water depth, bed slope and bed load; these conditions led to the formation of freely migrating alternate bars. We then employed rigid vegetation that was deployed on the reproduced alternate bars according to field observations. Various vegetation scenarios, in terms of density and spatial arrangement, were deployed in the flume experiments such to mimic different maintenance strategies. Results show the effects of rigid vegetation on the alternate bar configuration on the overall topographic pattern, the main alternate bar characteristics (such as amplitude and wavelength) and migration rate.

Estimation of breaking wave properties and their interaction with a jacket structure

The transportation of bank-collapsed materials is a key issue among river evolution processes. In this study, a series of flume experiments were conducted to monitor riverbank collapse processes and to explore the regularity of transportation for cohesive collapsed materials. The collapsed materials, both the bed and suspended loads, that transformed from collapsed materials were intensively evaluated under experimental conditions. The results showed that the collapsed materials contributed to 12～20 % sedimentation in situ, 8～14 % suspended loads and 70～80 % bed loads. In addition, the bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation of collapsed materials in terms of energy dissipation. This research provides theoretical and practical benefits for predicting channel evolution processes.

The transportation of bank-collapsed materials is a key issue among river evolution processes. In this study, a series of flume experiments were conducted to monitor riverbank collapse processes and to explore the regularity of transportation for cohesive collapsed materials. The collapsed materials, both the bed and suspended loads, that transformed from collapsed materials were intensively evaluated under experimental conditions. The results showed that the collapsed materials contributed to 12~20 % sedimentation in situ, 8~14 % suspended loads and 70~80 % bed loads. In addition, the bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation of collapsed materials in terms of energy dissipation. This research provides theoretical and practical benefits for predicting channel evolution processes.

Extreme wave impact due to tsunamis and storm surge create large wave heights causing destruction to coastal and offshore structures. These extreme waves are represented by focused waves in the present study and the impact on offshore deck is studied. Numerical wave tank used is modelled using open-source software REE3D, where the level set method is used to capture the air-water interface. Vertical impact force on offshore deck is computed and compared with the experimental results to validate the numerical model. Focused wave is generated by phase focusing a group of waves at a particular position and time. The nonlinearity of focused wave and its effect on the vertical impact force is quantified for different airgap and increasing wave heights. The steepness of this focused wave is increased to initiate phase focused breaking in the numerical wave tank, which is validated with experimental results of Ghadirian et al., 2016. The main purpose of this paper is to examine breaking focused wave group loads on the offshore deck and to study the impact on deck at different breaking locations. The positioning of the deck with respect to breaker location have shown that the maximum horizontal impact force due to breaking wave occurs when the plunging crest hits the deck side.

Computational Fluid Dynamics modelling of hydrodynamic characteristics of oscillatory flow past a square cylinder using the level set method

Investigating the structure of flow turbulence and bed load sediment transport is crucial as it provides insights into the functioning of aquatic environments, where such variations can lead to changes in ecosystem dynamics. This study focuses on the impact of sediments on the hydraulic characteristics of flow at self‐cleansing without deposition conditions of sediment transport. The self‐cleansing without deposition is not only a mode of sediment transport in alluvial channels, but it also serves as a criterion for the design of lined channels. Among the various design concepts for lined open channels, such as sewers and drainage channels, self‐cleansing without deposition condition is implemented as the most conservative and reliable approach. However, most of the conducted experimental studies on self‐cleansing without deposition have focused on measuring the basic flow and sediment characteristics for modelling purposes and neglected the effect of bed load, sediment size, flow discharge, and channel bed slope on turbulence characteristics. This study addresses this gap by examining the impact of bed load, sediment size, bed slope, and discharge on turbulence characteristics through a series of experiments conducted in a 12.5 m flume with a rectangular cross‐section, equipped with an automatic control system (ACS) at the Hydraulic Laboratory of Yaşar University. The channel bed slope, sediment discharge, flow discharge, and depth were adjusted and measured using ACS. Discharge and flow depth were measured using an ultrasonic flow‐meter and depth sensors, respectively. Flow characteristics were measured using a Vectrino profiler device. The study reveals that bed load sediment transport reduces streamwise velocity, especially for coarse particles. Additionally, at a constant bed slope, velocity differences remain small at lower discharges but become more significant as discharge increases. Turbulence intensity rises with bed load motion, more in the streamwise direction than vertically. At a constant bed slope, increasing discharge enhances turbulence, but the effect is more pronounced at lower slopes and less significant at steeper slopes. Reynolds shear stress increases with particle size and steeper slopes, indicating greater shear production. These observations suggest critical implications for the design and optimization of open‐channel systems, emphasizing the need for detailed consideration of particle sizes and bed conditions in engineering practices.

On the local scour around a jacket foundation under bidirectional flow loading

Computational fluid dynamics modeling of abutment scour under steady current using the level set method

Grain-size distributions of bed load: Inferences from flume experiments using heterogeneous sediment beds

A high-accuracy numerical solution, coupling one-dimensional shallow water and bed-evolution equations, with, for the first time, a suspended sediment advection equation, thereby including bed and/or suspended load, is used to examine two swash events on an initially plane erodible beach: the event of Peregrine & Williams (J. Fluid Mech., vol. 440, 2001, pp. 391–399) and that of a solitary wave approaching the beach. Equations are solved by the method of characteristics, and the numerical model is verified. Full coupling of suspended load to beach change for Peregrine & Williams (J. Fluid Mech., vol. 440, 2001, pp. 391–399) yields only slightly altered swash flows, depending on beach mobility and sediment response time; a series of similar final beach change patterns results for different beach mobilities. Suspended- and bed-load transport have distinct morphodynamical signatures. For the solitary wave a backwash bore is created (Hibberd & Peregrine, J. Fluid Mech., vol. 95, 1979, pp. 323–345). This morphodynamical bore propagates offshore initially, and leads to the creation of a beach bed step (Larson & Sunamura, J. Sedimentary Petrology, vol. 63, 1993, pp. 495–500), primarily due to bed-load transport. Its height is directly related to bed-load mobility, and also depends strongly on the bed friction coefficient. The shock dynamics of this bed step is explained and illustrated. Bed- and suspended-load mobilities are quantified using field data, and an attempt is made to relate predictions to measurements of single swash events on a natural beach. Average predicted bed change magnitudes across the swash are of the order of 2 mm, with maximum bed changes of up to approximately 10 cm at the bed step.

Numerical investigate of the effects of surface roughness on local scour around pipeline under steady current condition

Bedload and suspended load contributions to breaker bar morphodynamics