Journal of Waterway, Port, Coastal, and Ocean Engineering

This paper investigates fluid and floating object interaction using a novel adaptation of the weakly compressible smoothed particle hydrodynamics (WCSPH) method by incorporating a floating object model. In particular, this paper examines the water impact, hydrodynamic forces, fluid motions, and movement of objects in the conventional case studies of object entry and exit from still water. A two-dimensional wedge drop analysis was examined, and the hydrodynamic forces show acceptable agreement with published experimental and numerical results. The movement of the object is well predicted. The velocity field of the fluid domain is also captured. Simulations for water entry and exit of a buoyant and neutral density cylinder compares well with previous experimental, numerical, and empirical studies in penetration, free surface comparisons, and object movement. These results provide a good foundation to evaluate the accuracy and stability of WCSPH for modeling the interaction between free surface flow and free moving floating objects.

The complex coupled interactions between fluid and sandy sediment on the seafloor are simulated with a three-dimensional bottom boundary layer model (SedMix3D) developed from mixture theory. SedMix3D solves the unfiltered Navier-Stokes equations for a fluid-sediment mixture treated as a single continuum with effective properties that parameterize the fluid-sediment and sediment-sediment interactions including a variable mixture viscosity, a bulk hindered settling velocity, and a shear-induced, empirically calibrated, mixture diffusion term. A sediment flux equation models the concentration of sediment by describing the balance of sediment flux by advection, gravity, and shear-induced diffusion. The grid spacing is on the order of a sediment grain diameter, and simulated flows had maximum free-stream velocities between 20 and $120\mathrm{cm}/\mathrm{s}$ and periods between 2 and 4 s. Modeled ripple geometries ranged from a single ripple to multiple ripples with varying heights, lengths, and steepness. Only noncohesive sediments ($d=0.04\mathrm{cm}$) with the material properties of quartz in water were considered. The model predicted ripple heights and lengths that compare reasonably to existing ripple predictor formulae. SedMix3D also predicts the merging and separation of ripples as they transition from an initial state to an equilibrium state.

An accurate estimation of the modal parameters of a structure, including modal frequency and damping ratio, is crucial for many practical engineering problems. This article introduces Prony’s method for identifying multiple modal parameters of a jacket-type platform from the measured free-vibration data. In contrast to the Fourier analysis for the decomposition of a periodic signal, Prony’s method involves the decomposition of a transient signal into a number of damped sinusoidal components. By using a finite-element model of a realistic offshore jacket-type platform, free-vibration data, corresponding to two different initial displacement patterns, were extracted and used in a numerical study. From the sampled response acceleration signals associated with each of the initial displacement patterns, the modal parameters of different participating modes were accurately determined. This numerical study demonstrates that the modal parameters, of at least the first nine modes of the jacket-type platform, including six translation modes and three torsion modes, can be accurately estimated by using the proposed method.

Vessel-generated waves have the potential to erode the shoreline where there is regular commercial and recreational boat traffic in otherwise low-energy estuarine and fetch-limited environments. Whereas it is well established that vessel-generated waves are capable of resuspending significant quantities of bottom and bank sediment, there are limited field data showing how and when sediment is resuspended by the wake and the direction of net transport. This paper presents the results of an instrumented field study to measure sediment resuspension and transport in response to wakes generated by supercritical pilot boats and subcritical container ships at the mouth of the Savannah River in Georgia. Suspended sediment concentrations increase with the turbulent kinetic energy of the supercritical pilot-boat wake. The amount of sediment resuspended dependent on the available supply of sediment on the upper-foreshore. Whereas sediment is transported landward by the individual waves of the group, net transport is offshore in response to a low-frequency oscillation similar to a second-order group-forced current. In some cases, the direction of net transport is reinforced or reversed depending on the timing of the pilot-boat wake with the seiche forced by a passing container ship. In contrast to the pilot boats, sediment transport by subcritical container ships tends to be landward, but can also be weakly offshore depending on the timing of the wave group with the low-frequency drawdown and surge.

An experiment was conducted in a wave flume to examine the processes of gravel beach profile evolution under irregular waves. Two erosion tests were performed on a steep slope of $1/2$ with two different significant wave heights. An accretion test was conducted on a mild slope of $1/5$ until its profile became quasi-equilibrium. Another test was initiated with a gravel bar constructed at the toe of the equilibrium profile. The bar migrated onshore and resulted in the seaward shift of the equilibrium profile. Quasi-equilibrium profiles observed after 18 or 36 bursts of 400-s wave action were similar but affected by the differences in the wave height and initial profile. A numerical model developed for damage progression of stone armor layers is shown to predict the profile changes of the two erosion tests. The bed load formula used in the model is adjusted to better predict the onshore bar migration and the accretional change on the mild slope. More extensive comparisons of the numerical model with data will be required before it can be applied to gravel and cobble beaches.

This study examined the validity of using an artificial neural network (ANN) to predict tsunami water levels at several locations in Osaka Bay. The metropolitan areas of Osaka Bay have short warning times for tsunamis; a real-time tsunami forecast will allow for improved evacuation plans and will reduce the effect of these coastal disasters. Different tsunami conditions changing the relative strength of the asperities and background sources, such as fault displacement, fault length, fault width, fault slope, depth from sea bottom, and strike, were used for training the ANN; the data sets were generated by applying the nonlinear shallow water wave equations assuming different earthquake fault models. The linear activation function produced optimal results for the ANN output units, and the tangent-sigmoid function yielded good results for the ANN hidden layer units. The Levenberg-Marquardt method with Bayesian regulation was employed for the training of the ANN. Output from the trained ANN was the preliminary and secondary tsunami waves; these ANN output data agreed well with numerically obtained tsunami simulation results.