Numerical models for evolution of extreme wave groups

The paper considers the application of two numerical models to simulate the evolution of steep breaking waves. The first one is a Lagrangian wave model based on equations of motion of an inviscid fluid in Lagrangian coordinates. A method for treating spilling breaking is introduced and includes dissipative suppression of the breaker and correction of crest shape to improve the post breaking behaviour. The model is used to create a Lagrangian numerical wave tank, to reproduce experimental results of wave group evolution. The same set of experiments is modelled using a novel VoF numerical wave tank created using OpenFOAM. Lagrangian numerical results are validated against experiments and VoF computations and good agreement is demonstrated. Differences are observed only for a small region around the breaking crest

The loading on a vertical cylinder in steep and breaking waves on sheared currents using smoothed particle hydrodynamics

Waves and currents coexist in a wide range of natural locations for the deployment of offshore structures and devices. This combined wave–current environment largely determines the loading of vertical surface piercing cylinders, which are the foundations typically used for offshore wind turbines along with many other offshore structures. The smoothed particle hydrodynamics (SPH) code DualSPHysics is used to simulate focused waves on sheared currents and assess subsequent loading on a vertical cylinder. Outputs from another numerical model are used to define the SPH inlet–outlet boundary conditions to generate the wave–current combinations. A modified damping zone is used to damp the waves, but allow the currents to exit the domain. Numerical results are validated against experimental measurements for surface elevation and associated loading on the cylinder. Four phase repeats are used in the SPH model to understand the harmonic structure of the surface elevation at the front face of the cylinder and associated loading. It is shown that the SPH model provides agreement with experimental measurements of harmonic components for both force and elevations. Taking advantage of the SPH method, wave amplitudes were increased up to, and beyond, the breaking threshold highlighting a complex relationship between peak force and wave phase, requiring detailed investigation. The numerical modeling of interactions of steep and breaking waves on sheared currents with the cylinder demonstrates the SPH model's capability for modeling highly nonlinear fluid–structure interaction problems

Study of dam break flow interaction with urban settlements over a sloping channel

This paper describes a dam break experiment on a sloped channel, carried out in a hydraulic flume at UCL for the purpose of computer model validations of extreme events, such as flash floods. An elevated reservoir was situated upstream followed by a 1/20 slope leading up to a flat floodplain. Plexiglas blocks were positioned on the floodplain constituting different urban settlements and creating different obstructions to the flow. The flume was instrumented along its length measuring the change in water depth in the reservoir; the water depth time histories in various locations; the flow patterns and flood front velocity; and lastly the pressure and load on the buildings. The experiments were repeated for different urban settlements, flood intensities (two different initial water depths in the reservoir) and roughness layers along the slope, representative of a vegetated and a non-vegetated hill. In the present study, the experimental results were described qualitatively and compared with theoretical processes and 2D numerical results obtained using OpenFOAM's RAS turbulent model. Water depth, velocity and load measurements were analysed for different cases and it was found that while the 2D model provided a good fit on the slope, the flows generated around the building were more complex 3D formations which lead to inaccuracies. All experiments were repeated multiple times to ensure repeatability and thus the procedure was validated successfully providing a complete dataset that can be used for the validation of computational models for extreme events

Experimental Study of Dispersion and Modulational Instability of Surface Gravity Waves on Constant Vorticity Currents

This paper examines experimentally the dispersion and stability of weakly nonlinear waves on opposing linearly vertically sheared current profiles (with constant vorticity). Measurements are compared against predictions from the unidirectional (1D + 1) constant vorticity nonlinear Schrödinger equation (the vor-NLSE) derived by Thomas et al. (Phys. Fluids, vol. 24, no. 12, 2012, 127102). The shear rate is negative in opposing currents when the magnitude of the current in the laboratory reference frame is negative (i.e. opposing the direction of wave propagation) and reduces with depth, as is most commonly encountered in nature. Compared to a uniform current with the same surface velocity, negative shear has the effect of increasing wavelength and enhancing stability. In experiments with a regular low-steepness wave, the dispersion relationship between wavelength and frequency is examined on five opposing current profiles with shear rates from 0 to −0.87 s−1. For all current profiles, the linear constant vorticity dispersion relation predicts the wavenumber to within the 95 % confidence bounds associated with estimates of shear rate and surface current velocity. The effect of shear on modulational instability was determined by the spectral evolution of a carrier wave seeded with spectral sidebands on opposing current profiles with shear rates between 0 and −0.48 s−1. Numerical solutions of the vor-NLSE are consistently found to predict sideband growth to within two standard deviations across repeated experiments, performing considerably better than its uniform-current NLSE counterpart. Similarly, the amplification of experimental wave envelopes is predicted well by numerical solutions of the vor-NLSE, and significantly over-predicted by the uniform-current NLSE

The evolution of free and bound waves during dispersive focusing in a numerical and physical flume

publisher: Elsevier articletitle: The evolution of free and bound waves during dispersive focusing in a numerical and physical flume journaltitle: Coastal Engineering articlelink: http://dx.doi.org/10.1016/j.coastaleng.2017.11.003 content_type: article copyright: © 2017 The Authors. Published by Elsevier B.V

An experimental imaging method to study the evolution of bubble size distributions for different breaking wave crest geometries

Data for submitted paper. Data derived as part from a PhD program that measured breaking wave crests and bubble size evolution in laboratory.  Readme file has more info.  Bubble size distributions, bubble volume and wave crest profiles time series. </p

Study of dam break flow interaction with urban settlements over a sloping channel

This paper describes a dam break experiment on a sloped channel, carried out in a hydraulic flume at UCL for the purpose of computer model validations of extreme events, such as flash floods. An elevated reservoir was situated upstream followed by a 1/20 slope leading up to a flat floodplain. Plexiglas blocks were positioned on the floodplain constituting different urban settlements and creating different obstructions to the flow. The flume was instrumented along its length measuring the change in water depth in the reservoir; the water depth time histories in various locations; the flow patterns and flood front velocity; and lastly the pressure and load on the buildings. The experiments were repeated for different urban settlements, flood intensities (two different initial water depths in the reservoir) and roughness layers along the slope, representative of a vegetated and a non-vegetated hill. In the present study, the experimental results were described qualitatively and compared with theoretical processes and 2D numerical results obtained using OpenFOAM's RAS turbulent model. Water depth, velocity and load measurements were analysed for different cases and it was found that while the 2D model provided a good fit on the slope, the flows generated around the building were more complex 3D formations which lead to inaccuracies. All experiments were repeated multiple times to ensure repeatability and thus the procedure was validated successfully providing a complete dataset that can be used for the validation of computational models for extreme events

Experimental investigation of time-invariant eddy viscosity in wave-current interaction

An experimental study of the turbulent boundary layer, where waves propagate with a current, is presented in this paper. A wide range of test conditions have been covered, namely, flows over a rough bed and a smooth bed, combined flows in a large-scale oscillatory water tunnel and combined waves and currents in two flumes with different scales. Particle Image Velocimetry and Laser Doppler Velocimetry were employed to obtain the velocity field. Detailed analysis of eddy viscosity profiles calculated from the experiments leads to the conclusion that previous assumed profiles do not always accurately describe eddy viscosity distributions in a combined wave-current flow. The distributions of eddy viscosity are categorised into two types (two-layer or three-layer), based on the influence of wave motions superimposed. For those cases in the current-dominated regime, eddy viscosity profiles are similar to unidirectional turbulent currents. When combined flows are in the wave-dominated regime, three-layer eddy viscosity distributions are observed. For both types, a linear eddy viscosity profile is found to be present in the bottom 10 per cent of the turbulent boundary layer. Above this, the classic parabolic profile is observed, over the whole turbulent boundary layer for the first type and over 40 per cent of the turbulent boundary layer thickness for the second type. An empirical eddy viscosity distribution in the outer region is proposed for the second type. This newly developed eddy viscosity distribution provides guidance for numerical modellers in the field of wave-current interaction and for coastal engineers wishing to predict sediment transport

Numerical modelling of a vertical cylinder with dynamic response in steep and breaking waves using smoothed particle hydrodynamics

Highly nonlinear near-breaking and spilling breaking wave groups are common extreme events in the ocean. Accurate force prediction on offshore and ocean structures in these extreme wave conditions based on numerical approaches remains a problem of great practical importance. Most previous numerical studies have concentrated on non-breaking wave forces on rigid structures. Taking advantage of the smoothed particle hydrodynamics (SPH) method, this paper addresses this problem and presents the development and validation of a numerical model for highly nonlinear hydrodynamics of near-breaking and spilling breaking waves interacting with a vertical cylindrical structure using the SPH-based DualSPHysics solver. Open boundaries are applied for the generation of extreme wave conditions. The free-surface elevation and flow kinematics pre-computed within another numerical model are used as boundary conditions at the inlet of a smaller 3-D SPH-based numerical model to replicate the near-breaking and spilling breaking waves generated in a physical wave flume. A damping zone used for wave absorption is arranged at the end of the domain before the outlet. Numerical results are validated against experimental measurements of surface elevation and horizontal force on the vertical cylinder, demonstrating an agreement. After validation using a fixed model for the cylinder, a dynamic model is used to study the response to extreme wave events. Numerical results have also shown that the spilling breaking wave forces are significantly larger compared with near-breaking wave forces, and the secondary load cycle phenomenon becomes larger with dynamic response included in the present study