Amin Ghadirian

Steep wave passage around vertical circular cylinders is associated with an additional force peak occurring after the main peak: the secondary load cycle. The secondary load cycle for a focused wave group typical of offshore wind turbine foundations at 33 m full-scale water depth is investigated in scale 1 : 50. Ensemble-averaged force, front face pressures and free surface elevation measurements are used as the basis for the investigation. A two-phase free-surface Reynolds-averaged Navier–Stokes solver is validated against generic cases of turbulent flow over a wall, wave-boundary layer flow for a Reynolds number, . The suction occurs due to the rapid decrease of water level below the generated water column at the back of the cylinder, which at this time has only just begun its downward motion. The first force local minimum in the secondary load cycle is aided by the hydrostatic pressure from the water column while the second local minimum of the secondary load cycle is aided by wash-down effects on the front side. Finally, the role of the observed vortices behind the cylinder is discussed and compared to reference computations with slip conditions. The results confirm findings from earlier slip boundary studies that the global force history through the secondary load cycle is not strongly affected by the boundary layer. The source of vortices behind the cylinder, observed in both sets of computations is discussed.

A pressure impulse model is presented for wave impact on vertical circular cylinders. Pressure impulse is the time integral of the pressure during an impact of short time scale. The model is derived for a simplistic geometry and has relative impact height, crest length and cylinder radius as effective variables. The last parameter, the maximum angle of impact, is free and can be calibrated to yield the right force impulse. A progression of simpler pressure impulse models are derived in terms of a three-dimensional box generalization of the two-dimensional wall model and an axisymmetric model for vertical cylinders. The dependence on the model parameters is investigated in the simpler models and linked to the behaviour of the three-dimensional cylinder model. The model is next validated against numerical results for a wave impact for a phase- and direction-focused wave group. The maximum impact angle is determined by calibration against the force impulse. A good match of the pressure impulse fields is found. Further comparison to the force impulse of two common models in marine engineering reveals improved consistency for the present model. The model is found to provide a promising representation of the pressure impulse field, based on a limited number of input parameters. Its further validation and potential as a robust tool in force and response prediction is discussed.

This paper investigates the flow field in the rotor plane of a full‐scale operating wind turbine using full‐scale light detection and ranging (LiDAR) measurements for the first time. Comparison of the measured flow field with results from large eddy simulations (LES) combined with an actuator line approach is also presented for in‐depth study of the induction field in the rotor plane. The measurements include data from two synchronized LiDAR systems—one scanning the undisturbed upstream inflow field and one measuring in the rotor plane. The standard deviation of the mean of velocity time series are and presented as a measure of reliability. The method for calculating the axial velocity based on the line‐of‐sight velocity is explained and the uncertainty of such method is presented. The process of calculating the yaw misalignment is described.The time‐averaged and phase‐averaged axial velocity and induction factors are presented relative to radius and azimuth, and the general behavior is described relative to the flow regimes around the blades, tower and nacelle. Simulations and measurements are compared with special emphasis on the flow structures in the vicinity of the individual rotor blades. A convincing agreement between measurements and simulations is demonstrated. The uncertainties originated from the imprecise positions and angles of the measurement instruments are shown. The uncertainties are limited to the middle parts of the blades between 15 m to 25 m from the root. In addition, longer selected time series show smaller uncertainties. This proves the reliability of the application of the methodology for even longer time series.

Two different numerical models, OceanWave3D and a coupled solver, OceanWave3D-OpenFOAM (Waves2Foam), are used to reproduce extreme events in one sea state. The events are chosen as, the measured event that generates the largest peak moment (exceedance probability of 0.05%) and one event with a slightly smaller peak moment (exceedance probability of 0.3%). Time series of free surface elevation, depth integrated forces, bending moment at the sea bed and pressure time series at 5 different heights on the cylinder are compared for two events between the measurements and the numerical models. The numerical pressure field on the monopile at impact is analyzed and stagnation pressures at the back side of the cylinder, in addition to the main impact pressure at the front side are observed. There is a good agreement between the OceanWave3D results and the measurements in the reproduction of the first selected event. However, for the larger selected event, OceanWave3D results in the peaks of time series are smaller than the measurements. This illustrates the sensitivity of the strong impact loads to the state of wave breaking. For small values of the inline force, the OpeanFOAM results provided good agreement with the measurements. The secondary load cycles are observed in the measured force and bending moment time series and the reproduced times series using OpenFOAM.

Experiments with both uni- and multidirectional wave realizations with a stiff pile subjected to extreme wave forces are considered. Differences in crest heights and force peaks resulting from directional spread waves are analysed. The wave realizations are reproduced numerically in the fully nonlinear wave model OceanWave3D. The numerical reproductions compare well to the experiments. Only for the largest forces significant differences are seen, which is due to a very simple breaking filter applied in OceanWave3D. In the wave spectra, the higher harmonics occur for smaller frequencies than the straight multiples of the peak frequency. Further, the higher harmonics of the multidirectional wave spectra contain less energy. Both effects can be explained by the second order wave theory. Finally, the computed wave kinematics are used to investigate the dynamic response of an offshore wind turbine. The excitation of the first natural frequency is largest for the unidirectional wave realizations, as the higher harmonics are largest for these realizations.