Numerical Investigation Of Local Scour Research Articles

Numerical investigation of local scour around twin piles under steady current using CFD-DEM coupling method

In recent years, the local scour has developed into a major threat to the safety of offshore group piles. The numerical simulation methods are useful tools in investigating the key mechanisms behind local scour. However, conventional simulation methods often neglect the microscopic sediment behavior, thus impeding a more profound understanding of the scour mechanism. In this research, the CFD-DEM method was applied to investigate the local scour around twin piles. And the coarse grain method and porous media model were incorporated to enhance simulation efficiency. The CFD-DEM model was constructed and validated using experimental data. Based on the validated model, a series of local scour simulations around twin piles were conducted under different arrangements. The time-development curves of scour depth were analyzed, and the variation of the equilibrium scour depth with pile spacing exhibits good agreement with the dates from previous studies. Based on the simulation results, the flow field, scour pit morphology, sediment flux, and microscopic behavior of sediment around twin piles were analyzed under different arrangements, and four key mechanisms of twin piles' local scour were observed and analyzed during this process.

Numerical investigation of local scour around a vertically vibrating subsea pipeline under steady flow

Numerical simulations are conducted to investigate the local scour below a vertically vibrating subsea pipeline for gap ratios ranging from −0.3 to 0.1 and a wide range of reduced velocities ranging from 0.5 to 15. Positive and negative gap ratios represent the cases where the bottom of the pipeline is above and below the flat sand surface, respectively. The novelty of the work is the frequency analysis of the vibrating pipeline specially as a function of time as scour and erosion progression occurs. Both scour rate and the vibration amplitude are found to change suddenly at a critical time for reduced velocities inside the lock-in regime. The existence of the critical time is caused by the change of the response mode due to scour. Two types of critical time are found: Type 1 critical time where the vibration transitions from synchronisation to desynchronisation and Type 2 critical time where the flow transitions from steady flow to vortex shedding flow. The variations of the vibration amplitude with the reduced velocity for all the gap ratios follow the same trend and a distinct lock-in regime is found, but the width of the lock-in regime reduces with the reduction of the gap ratio. For all the studied gap ratios, the maximum scour depth consistently occurs at the reduced velocity of Ur = 5, which is the lower boundary of the lock-in regime.

Numerical investigation of local scour around tandem piers using proposed self-sustaining inlet turbulent boundary conditions

Turbulent characteristics significantly affect the numerical results of local scour in tandem piers. The novel self-sustaining model was used to quickly develop homogeneous isotropic turbulence to maintain the consistency of the numerical and physical flow fields. The Reynolds averaged N-S equations, which are close to the standard k−ε turbulence model, were employed to solve the complex flow field around the bridge piers. The sediment transport model, combined with dynamic mesh updating methods, simulated the evolution process of the local scour for tandem piers. The results indicated that the maximum scour depth occurred at the upstream pier at different distances between the centres of the two piers (L/D). The scour depth reached its largest value when L/D = 4, and the maximum scour depth reached its minimum value when L/D = 2. When L/D > 5, the shielding effect can be neglected for downstream piers. In addition, based on the numerical simulation results, a shielding coefficient was established, providing a method for quickly predicting the local scour depth based on the scour depth of the isolated pier and a reference for the scour resistance design of bridges, offshore fans, and other structural foundations.

Numerical investigation of local scour around a vibration pipeline under free surface wave condition

A two-dimensional finite element numerical model was developed to predict local scour around a vibration submarine pipeline under free surface wave condition. The flow governing equations are the two-dimensional Navier-Stokes equations which were closed by using the Shear-Stress Transport (SST) k-ω turbulence model. Both suspended load and bed load sediment transportations are considered in the scour model. The Arbitrary Lagrangian-Eulerian (ALE) method was used to track the moving boundaries induced by the vibration pipeline, the free wave surface, and the evolution of the seabed due to local scour. Satisfactory results were obtained after validations carried out. The numerical model is then used to investigate local scour around a vibration pipeline under free surface wave conditions. Present numerical results show that vibration makes the scour depth below the pipeline deeper, scour scope around the pipeline wider.

Numerical investigation of local scour around submerged pipeline in shoaling conditions

A two-dimensional numerical model is employed to predict local scour around submarine pipelines under water waves in shoaling condition. The motion of water under waves is simulated by solving the Reynolds Averaged Navier-Stokes (RANS) equations. The wave surface is traced by the Arbitrary Lagrangian-Eulerian (ALE) method. The evolution of the seabed surface near the pipeline is predicted by solving the conservation of the sediment mass, which transport in the water in the forms of bed load and suspended load. The main aim of this study is to investigate the effect of the seabed slope on the scour profiles and scour depth. To achieve this aim, numerical simulations of scour around a pipeline on a flat seabed and on a slope seabed with a slope angle of 5°, 10° and 15° are conducted for various wave conditions. The numerical results show that present numerical results of equilibrium scour depth below pipeline agree the values calculated by the suggested formula quite well for β = 0° where β is the slope angle. As β≠0°, the equilibrium scour depth agrees with the empirical formula as KC < 8 and is greater than the latter for KC > 8.

Numerical investigation of local scour around a vibrating pipeline under steady currents

A two-dimensional finite element numerical model was developed to predict local scour around a vibration submarine pipeline in steady current. The numerical model was based on incompressible two-dimensional Navier-Stokes equations with a Shear-Stress Transport (SST) k-ω turbulence model closure. The transportations of suspended load and the bed load were taken into consideration in the present numerical simulation. The moving boundaries induced by the vibration pipeline and the evolution of the seabed due to local scour were tracked by using the Arbitrary Lagrangian Eulerian (ALE) method. Comparisons between the numerical results and available experimental data showed satisfactory agreements. The local scour around a vibrating pipeline has been investigated, including the maximum scour depth below the pipeline, the vortex shedding mode and the vibration amplitude of the pipeline. The numerical results showed that the maximum vibration amplitude of the pipeline could be about 1.2D, and the maximum scour depth occurred in the wake side of the vibration pipeline.

Numerical investigation of local scour around USAF with different hydraulic conditions under currents and waves

An umbrella suction anchor foundation (USAF) is a new foundation type used for offshore wind turbines. In this paper, a three-dimensional numerical model was established in FLOW-3D to study the local scour characteristics under the coupled effects of waves and currents. RANS equations, closed with an RNG k-ε turbulence model, were established for simulating the flow field around pile groups. The simulation results were compared with a 1:40 model test and showed good agreement. Then, the effects of different hydraulic conditions and dimensionless parameters such as water depth, wave height, velocity, Keulegan-Carpenter number (KC) and the ratio of relative wave-current velocity (Ucw) on the maximum scour depth were considered in detail, and compared with the results of other scholars. This study found that the local scour shape of a USAF is different from that of a monopile foundation. Its maximum scour depth appears downstream behind the USAF, and the anchoring structure effectively blocks the scour in front of the USAF. The shallower water depth, the higher wave height and the faster flow velocity result in a more intense scour. When KC < 7 and Ucw <0.7, the maximum scour depth of the USAF increases with increasing KC number and Ucw.

Experimental and Numerical Investigation of Local Scour for Suspended Square Caisson under Steady Flow

A caisson can be suspended in the river during its settlement construction. The water flows induces the local scour of sediments under the caisson, which increases the settlement difficulty and threatens the safety of construction. A numerical model using computational fluid dynamics (CFD) was introduced to simulate the local scour process around the caisson. The numerical model was validated with a set of experimental tests, which were conducted to investigate the local scour for a square caisson model with different clearances under steady flow. The evolutions and characteristics of local scour pits as a function of clearance were investigated based on the experimental and numerical results. The effect of grain size of sediment and the scour mechanism for suspended caisson were discussed as well. The main findings include: 1) the minimum inflow velocity for the scour of sediment depends on the clearance between the caisson bottom and sediment; 2) the numerical model provides reasonable evolution of local scour pit and flow field around the caisson; and 3) the scour depth of suspended caisson can be as high as 70% of that for the settled caisson and deserves attention for the safe settlement of caisson.

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

This paper presents a numerical investigation of local scour with dynamic free surface capturing around a jacket structure. The hydrodynamics and resulting scour are calculated using the open-source CFD model REEF3D. The model solves the Reynolds-averaged Navier–Stokes equations with k−ω turbulence closure. The free surface is captured with the level set method. The simulated flow hydrodynamics are coupled with sediment transport algorithms. The modelling of the local scouring process is based on the bed load and the suspended load while accounting for the bed slope for the calculation of the critical bed shear stress. In order to obtain a more realistic prediction of the scour hole, a sand-slide algorithm is implemented to correct the bed slope when it exceeds the angle of repose. The grid and time step size convergence tests are performed to ensure the quality of the simulated hydrodynamics in the numerical wave tank (NWT). The model is validated for local scour around a pile in an experimental flume using different values of Keulegan–Carpenter number (KC). The results show a good agreement with the experimental data. The validated model is applied to simulate scour around a jacket structure. The key findings from the study are the modelling of flow hydrodynamics and the time development of local scour around a jacket exposed to waves and steady current.

Numerical Investigation of Local Scour with Inclined Piles

Du, S.; Liang, B., and Lee, D.Y, 2019. Numerical investigation of local scour with inclined piles. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 161-165. Coconut Creek (Florida), ISSN 0749-0208.In coastal and offshore engineering applications, foundations of breakwaters, offshore wind turbine foundations and piles are often damaged by local scour induced by currents or waves, which will threaten people's property and life security. The present study focuses on the local scour around cylinder piles, which are inclined toward upstream, downstream, and lateral. A vertical pile together with the three types of inclined piles was investigated by a numerical model in clear-water scour condition. Each type of the inclined pile was simulated for five inclination angles ranging from 15° to 45°. The angle of inclination is between the pile axis and the bed normal. By solving the Reynolds Averaged Navier–Stokes (RANS) equations, steady currents around the piles are simulated with the validated model. The results show significant differences in scour and deposition for the vertical and inclined piles. Sediments transport induced by the horseshoe vortex, contracted streamlines and the wake vortices were analyzed. Upward flow and downward flow in front of the pile were seen quite contrary between downstream inclined pile and upstream inclined pile. Local scour in lateral inclined pile was found moving toward to the inclined side. Maximum scour depth for inclined piles decreases in different degrees compared to the vertical pile.

Experimental and Numerical Investigation of Local Scour Around Submarine Piggyback Pipeline Under Steady Current

As a new type of submarine pipeline, the piggyback pipeline has been gradually adopted in engineering practice to enhance the performance and safety of submarine pipelines. However, limited simulation work and few experimental studies have been published on the scour around the piggyback pipeline under steady current. This study numerically and experimentally investigates the local scour of the piggyback pipe under steady current. The influence of prominent factors such as pipe diameter, inflow Reynolds number, and gap between the main and small pipes, on the maximum scour depth have been examined and discussed in detail. Furthermore, one formula to predict the maximum scour depth under the piggyback pipeline has been derived based on the theoretical analysis of scour equilibrium. The feasibility of the proposed formula has been effectively calibrated by both experimental data and numerical results. The findings drawn from this study are instructive in the future design and application of the piggyback pipeline.

Numerical investigation of local scour beneath a sagging subsea pipeline in steady currents

Two dimensional (2-D) numerical simulations are conducted to predict scour below the midsection of the suspended span of a pipeline that sags naturally into a scour pit. The deflection of the pipeline is calculated based on the suspension length (or span length) of the pipeline, which is predicted by an empirical formula for the length of the scour pit. The general properties of the scour process are investigated for various pipeline properties, pipeline embedment depths and Shields parameters. The effect of each of these parameters on pipeline sagging are combined into one non-dimensional sagging parameter. A touchdown time is defined as the time when the pipeline touches the bottom of the sand pit. Empirical formulae are proposed for predicting the final scour depth at touchdown and the time scale required for touchdown. It is found that the final sagging depth is a function of the sagging parameter and the Shields parameter, whilst the touchdown time is only a function of the sagging parameter. A formula is also developed for predicting the time history of scour depth prior to touchdown. Collectively, the empirical formulae and numerical simulations are found to be in reasonable agreement with an existing theoretical model available in the literature.

Numerical investigation of local scour around three adjacent piles with different arrangements under current

A set of three-dimensional numerical models is established to investigate the mechanisms of local scour around three adjacent piles with different arrangements under steady currents. The RANS equations, closed with a RNGk−ε turbulence model, are established for simulating the flow field around pile groups. A sediment scour model is adopted to estimate the motion of the sediment by predicting the erosion, advection, and deposition of the sediment. The simulation results of the bed elevation contours and the time evolution of scour depth around a single pile model are verified with the previous experimental results and show good agreement. Then, the effects of pile spacing for three adjacent piles with different arrangements on the flow field, bed elevation contours, and scour depth are discussed in detail. The results reveal that the dimensionless pile spacing has a significant effect on the flow field and local scour around the three adjacent piles. The scour hole patterns and scour depth around each pile with different arrangements are totally different.

Numerical investigation of local scour at two adjacent cylinders

Local scour around cylinders in a side-by-side or tandem arrangement under clear-water conditions is investigated numerically. Large eddy simulations with a Smagorinsky subgrid model are combined with a ghost-cell immersed boundary method, and details of the bed scouring are realized with sophisticated sediment and morphodynamic models. The scour patterns and depths in the two-cylinder cases are shown to be significantly influenced by the cylinder spacing. The features of the scour evolution, depth, and flow fields for a range of cylinder spacings are discussed. The maximum scour depth in the side-by-side cylinder cases increases as the distance between the cylinders decreases, whereas in the tandem cases, it tends to initially increase with increasing distance between the cylinders, after which it gradually decreases beyond the peak point. The maximum scour depths and trends computed using the present model show good agreement with the measured data in the literature.

Experimental and numerical investigation of local scour around a submerged vertical circular cylinder in steady currents

Local scour around a submerged vertical circular cylinder in steady currents was studied both experimentally and numerically. The physical experiments were conducted for two different cylinder diameters with a range of cylinder height-to-diameter ratios. Transient scour depth at the stagnation point (upstream edge) of the cylinder was measured using the so-called conductivity scour probes. Three-dimensional (3D) seabed topography around each model cylinder was measured using a laser profiler. The effect of the height-to-diameter ratio on the scour depth was investigated. The experimental results show that the scour depth at the stagnation point is independent on cylinder height-to-diameter ratio when the later is smaller than 2. The increase rate of equilibrium scour depth with cylinder height increases with an increase in Shields parameter. A three-dimensional finite element numerical model is developed for simulating local scour around submerged vertical cylinders. Steady flow around the cylinder is simulated by solving the Reynolds-Averaged Navier–Stokes (RANS) equations with a k– ω turbulence closure. Both suspended load and bed load sediment transport rates were included in the model. Efforts are made to reduce the computational time associated with three-dimensional morphological modelling including the use of wall-function to avoid resolving the near-wall boundary layers, large morphological time step and parallel computing techniques. Scour around a submerged vertical circular cylinder founded on the seabed is simulated and the numerical results are validated against the test data. The scour mechanisms around the submerged cylinder are investigated using the numerical model.

Numerical investigation of local scour below a vibrating pipeline under steady currents

Local scour below a vibrating pipeline under steady current is investigated by a finite element numerical model. The flow, sediment transport and pipeline response are coupled in the numerical model. The numerical results of scour depths and pipeline vibration amplitudes are compared with measured data available in literature. Good agreement is obtained. It is found that pipeline vibrations cause increases of scour depth below the pipeline. The scour pit underneath a two-degree-of-freedom vibrating pipeline is deeper than that under a pipeline vibrating only in the transverse flow direction. The effects of water depth are also investigated. The present numerical result shows that water depth has weak effect on the scour depth. However it does affect the time scale of the scour. The shallower the water depth is, the less time it requires to reaches the equilibrium state of scour. It is found that the vibration forces vortices to be shed from the bottom side of the pipeline. Then vortex shedding around a vibrating pipeline is closer to the seabed than vortex shedding around a fixed pipeline. This contributes to the increase of the scour depth.