In this work, the flow over an elliptic cylinder near a moving wall is investigated for Reynolds numbers less than 150. Here, the ratio between the gap (i.e., the distance between the cylinder and the wall) and the length of the semi-major axis of the elliptic cylinder varies from 0.1 to 5. This ratio is hereafter denoted as the gap ratio. The resulting Kármán vortex street, the two-layered wake, and the secondary vortex street have been investigated and visualized. Numerical simulations show that for the steady flow, the wake is composed of two asymmetric recirculation vortices, while a decrease in the gap ratio suppresses the vortex shed from the lower part of the cylinder. For the unsteady flow, the wake can be classified into four different patterns based on the wake structures (the Kármán vortex street, the two-layered wake, and the secondary vortex street). The regions of these wake patterns are given in the gap ratio and Reynolds number space, showing that the critical Reynolds number for the transition between different patterns increases as the gap ratio decreases. An overall increase in the mean drag coefficient with increasing gap ratios is observed, except for a sudden drop that occurs within a small gap ratio range. Moreover, as the gap ratio increases, the onset location of the two-layered wake first decreases due to a decrease in flow velocity in the gap and then increases due to the weakening of the wall suppression effect.

This paper presents an engineering approach to study the effects of soil profile variation and scour on structural response of an offshore monopile wind turbine. A wind-wave model for finite water depth is proposed to obtain the corresponding sea-state based on the incident wind. Different wind, wave, and current loads on the wind turbine for the operational conditions are considered. The interaction between the foundation and the soil is simulated by nonlinear springs, for which stiffness properties are obtained from the axial load transfer curve, the tip load–displacement curve, and the lateral load–deflection curve. Four types of soil conditions are considered, i.e., 100% sand layer, 50% sand layer (top) and 50% clay layer (bottom), 50% clay layer (top), and 50% sand layer (bottom), as well as 100% clay layer. For a given current speed, the variations of the structural response of the wind turbine due to the effects of different wind–wave load combinations, soil conditions and scour have been investigated. Different wind–wave load combinations directly affect the mean internal bending moment and mean displacement vertically along the support structure. Different soil conditions change the eigenfrequency of the structure significantly. The top layer of the soil appears to have a strong influence on the mean internal bending moment and the mean shear force distribution along the foundation. Moreover, the effect of scour alters the eigenfrequency of the structure significantly. The maximum mean bending moment and displacement increase for the cases with a scour hole as compared to the cases with scour protection.

The average flow field inside and around the bottom of porous cylinders in a uniform flow is explored using particle image velocimetry (PIV). Tests were conducted on six cylinders with porosities of 0%, 30%, 60%, 75%, 82%, and 90% in a flume tank where the flow field inside and around the models is time averaged over 180 s. The models had a height-to-diameter ratio of 3 and were made from metal mesh. The Reynolds numbers ranged from 5000 to 20,000 based on the diameter of the models and from 75 to 300 based on the diameter of individual strands of the mesh, which corresponds to the Reynolds numbers occurring at salmon fish cage netting used along the Norwegian coast. The porosities of 82%, 75%, and 60% correspond to those of a fish cage netting in Norwegian salmon farming with no, light, and heavy biofouling, respectively. The results from this study are discussed with respect to the instantaneous flow field in and around the same cylinders at identical Reynolds numbers. The focus is on the effect of porosity on the ventilation inside the cages and the vertical transports within the near wake. It is shown that heavy fouling of aquacultural netting can lead to internal circulation inside fish cages and, therefore, has the potential to dramatically reduce the ventilation of the net pens. The description of the time-averaged flow field inside and around porous cylinders can be used as benchmarks to validate and adjust numerical models of the flow past porous cylinders. The results from this study can also be valuable for the fish farming industry, since bio-fouling and the reduced porosity of fish cages can be monitored and controlled directly by fish farmers.

The experimental wave paddle signal is unknown to the numerical modellers in many cases. This makes it quite challenging to numerically reproduce the time history of free surface elevation for irregular waves. In the present work, a numerical investigation is performed using a computational fluid dynamics (CFD) based model to validate and investigate a non-iterative free surface reconstruction technique for irregular waves. In the current approach, the free surface is reconstructed by spectrally composing the irregular wave train as a summation of the harmonic components coupled with the Dirichlet inlet boundary condition. The verification is performed by comparing the numerically reconstructed free surface elevation with theoretical input waves. The applicability of the present approach to generate irregular waves by reconstructing the free surface is investigated for different coastal and marine engineering problems. A numerical analysis is performed to validate the free surface reconstruction approach to generate breaking irregular waves over a submerged bar. The wave amplitudes, wave frequencies and wave phases are modelled with good accuracy in the time-domain during the higher-order energy transfers and complex processes like wave shoaling, wave breaking and wave decomposition. The present approach to generate irregular waves is also employed to model steep irregular waves in deep water. The free surface reconstruction method is able to simulate the irregular free surface profiles in deep water with low root mean square errors and high correlation coefficients. Furthermore, the irregular wave forces on a monopile are investigated in the time-domain. The amplitudes and phases of the force signal under irregular waves generated by using the current technique are modelled accurately in the time-domain. The proposed approach to numerically reproduce the free surface elevation in the time-domain provides promising and accurate results for all the benchmark cases.

This paper addresses the random wave-induced current in shallow water based on deep-water wind and wave statistics, where the wave-induced current is expressed in terms of the deep-water seastate wave parameters, significant wave height and mean zero-crossing wave period. The average statistical properties of the random wave-induced current in shallow water expressed in terms of the mean value and the standard deviation are presented. The results are exemplified by using long-term wind statistics from the northern North Sea and long-term wave statistics from the same ocean area. Overall, it appears that there is an agreement between the results based on these inputs from wind and wave statistics. The presented analytical method should be useful for making preliminary estimates of the random wave-induced drift in shallow water within seastates using either available deep-water wind statistics or deep-water wave statistics, which enhances the possibilities for assessing further the wave-induced current in, for example, near-coastal zones.