Stratification effects on shoaling internal solitary waves

This combined numerical/laboratory study investigates the effect of stratification form on the shoaling characteristics of internal solitary waves propagating over a smooth, linear topographic slope. Three stratification types are investigated, namely (i) thin tanh (homogeneous upper and lower layers separated by a thin pycnocline), (ii) surface stratification (linearly stratified layer overlaying a homogeneous lower layer) and (iii) broad tanh (continuous density gradient throughout the water column). It is found that the form of stratification affects the breaking type associated with the shoaling wave. In the thin tanh stratification, good agreement is seen with past studies. Waves over the shallowest slopes undergo fission. Over steeper slopes, the breaking type changes from surging, through collapsing to plunging with increasing wave steepness $A_w/L_w$ for a given topographic slope, where $A_w$ and $L_w$ are incident wave amplitude and wavelength, respectively. In the surface stratification regime, the breaking classification differs from the thin tanh stratification. Plunging dynamics is inhibited by the density gradient throughout the upper layer, instead collapsing-type breakers form for the equivalent location in parameter space in the thin tanh stratification. In the broad tanh profile regime, plunging dynamics is likewise inhibited and the near-bottom density gradient prevents the collapsing dynamics. Instead, all waves either fission or form surging breakers. As wave steepness in the broad tanh stratification increases, the bolus formed by surging exhibits evidence of Kelvin–Helmholtz instabilities on its upper boundary. In both two- and three-dimensional simulations, billow size grows with increasing wave steepness, dynamics not previously observed in the literature.

Experimental investigation of the effects of intermediate gravity waves on the water evaporation rate

This experiment was done to predict the evaporation rate from the wavy water surface under the different convection regimes ( free, forced, and mixed) at turbulent airflow conditions over a wide range of the ratio(Gr/Re2 ). Evaporation rate from wavy water surface is strongly affected by combinations between wave steepness and main airflow velocity above the wavy water surface. Experimental results show that no pattern can be followed for which combinations of evaporation rate will increase. Thus, only two facts can be noticed: the evaporation rate is larger than that measured under the same airflow velocity conditions with no waves existing on evaporated water surface because the airflow is smooth and attached along the still water surface and when increasing the wave steepness(H/L,H/T), Airflow will separate at the lee side of wave crest near to the bottom of the wave trough. Thus, vortex will generate in the airflow separation region. These vortexes are unstable and cause an increase in turbulence, reducing the water surface's resistance to vertical transport water vapor and increasing the evaporation rate. Also, experimental results show that the evaporation rates are somewhat less than that measured under the same airflow velocity with smaller wave steepness due to air trapped region observed at the leeside of the wave crest near the bottom of the wave trough. Evaporation rate is increasing with increase airflow velocity under the same convection regime.

Influence of Sea State Parameters on the Accuracy of Wave Simulations of Different Complexity

The accurate description of the complex genesis and evolution of ocean waves as well as the associated kinematics and dynamics is indispensable for the design of offshore structures and assessment of marine operations. In the majority of cases, the water wave problem is reduced to potential flow theory on a somehow simplified level. However, the non-linear terms in the surface boundary conditions and the fact that they must be fulfilled on the unknown water surface make the boundary value problem considerably complex. On the one hand, the use of complex methods for solving the boundary value problem may give, at the expense of computational time, a very accurate representation of reality. On the other hand, simplified methods are numerically efficient but may only provide sufficient accuracy for a limited range of applications. This paper investigates the influence of different characteristic sea state parameters on the accuracy of irregular wave field simulations (based on a JONSWAP spectrum) by applying the high-order spectral method. Hereby, the underlying Taylor series expansion is truncated at different orders so that numerical simulations of different complexity can be investigated. The wave steepness, spectral-peak enhancement factor as well as directional spreading are systematically varied and truncation at fourth order serves as reference. It is shown that, for specific parameters in terms of wave steepness, enhancement factor and simulation time, the boundary value problem can be significantly reduced while providing sufficient accuracy.

Dynamic Loads and Response of a Spar Buoy Wind Turbine with Pitch-Controlled Rotating Blades: An Experimental Study

New large-scale laboratory data are presented on a physical model of a spar buoy wind turbine with angular motion of control surfaces implemented (pitch control). The peculiarity of this type of rotating blade represents an essential aspect when studying floating offshore wind structures. Experiments were designed specifically to compare different operational environmental conditions in terms of wave steepness and wind speed. Results discussed here were derived from an analysis of only a part of the whole dataset. Consistent with recent small-scale experiments, data clearly show that the waves contributed to most of the model motions and mooring loads. A significant nonlinear behavior for sway, roll and yaw has been detected, whereas an increase in the wave period makes the wind speed less influential for surge, heave and pitch. In general, as the steepness increases, the oscillations decrease. However, higher wind speed does not mean greater platform motions. Data also indicate a significant role of the blade rotation in the turbine thrust, nacelle dynamic forces and power in six degrees of freedom. Certain pairs of wind speed-wave steepness are particularly unfavorable, since the first harmonic of the rotor (coupled to the first wave harmonic) causes the thrust force to be larger than that in more energetic sea states. The experiments suggest that the inclusion of pitch-controlled, variable-speed blades in physical (and numerical) tests on such types of structures is crucial, highlighting the importance of pitch motion as an important design factor.

Sustained Open Access Global Wave Observations for Science and Society

Studies of the generation and propagation of surface waves in the open ocean have been traditionally supported by sparse observations. Wave climatology is only known through data from expensive and heavy open ocean moorings, often not optimized for observing surface waves, coastal wave observing networks, or from satellites that can only measure the wave's amplitude. Yet, knowledge of wave physics is of fundamental importance to understand how the ocean and the atmosphere are coupled and to quantify, for example, exchanges of gas and momentum. Of similar importance is understanding how oceanic mesoscale, such as eddies and boundary currents, affect wave steepness and propagation; ultimately important to quantify, for example, hazards to navigation and to protect coastal communities from floods. Scientific advances in data assimilation and wave resolving models, which are supported by our visionary approach, are needed to improve coupled models to support extreme events modeling and forecasting and for improving climate assessment. In-situ global wave observations are one of the obviously missing key ingredients that are hampering progress in oceanography, meteorology, and climate sciences.

On the Evolution of Different Types of Green Water Events

Green water events may present different features in the initial stage of interaction with the deck of a structure. It is therefore important to investigate the evolution of different types of green water, since not all the events interact with the deck at the same time. In this paper, the evolution of three types of green water events (dam-break, plunging-dam-break, and hammer-fist) are studied. The water surface elevations and volumes over the deck in consecutive green water events, generated by incident [wave trains in a wave flume, were analyzed using image-based methods. The results show multiple-valued water surface elevations in the early stage of plunging-dam-break and hammer-fist type events. Detailed experimental measurements of this stage are shown for the first time. The effect of wave steepness on the duration of the events, maximum freeboard exceedance, and volumes were also investigated. Although the hammer-fist type showed high freeboard exceedances, the plunging-dam-break type presented the largest volumes over the deck. Some challenges for further assessments of green water propagation are reported.

Analysis of Dangerous Sea States in the Northwestern Mediterranean Area

Extreme sea waves, although rare, can be notably dangerous when associated with energetic sea states and can generate risks for the navigation. In the last few years, they have been the object of extensive research from the scientific community that helped with understanding the main physical aspects; however, the estimate of extreme waves probability in operational forecasts is still debated. In this study, we analyzed a number of sea-states that occurred in a precise area of the Mediterranean sea, near the location of a reported accident, with the objective of relating the probability of extreme events with different sea state conditions. For this purpose, we performed phase-resolving simulations of wave spectra obtained from a WaveWatch III hindcast, using a Higher Order Spectral Method. We produced statistics of the sea-surface elevation field, calculating crest distributions and the probability of extreme events from the analysis of a long time-series of the surface elevation. We found a good matching between the distributions of the numerically simulated field and theory, namely Tayfun second- and third- order ones, in contrast with a significant underestimate given by the Rayleigh distribution. We then related spectral quantities like angular spreading and wave steepness to the probability of occurrence of extreme events finding an enhanced probability for high mean steepness seas and narrow spectra, in accordance with literature results, finding also that the case study of the reported accident was not amongst the most dangerous. Finally, we related the skewness and kurtosis of the surface elevation to the wave steepness to explain the discrepancy between theoretical and numerical distributions.