Asymmetry in Directional Spreading Function of Sea Waves Due to Refraction

2009 ◽  
Author(s):  
Changhoon Lee ◽  
Jae-Sang Jung ◽  
Merrick C. Haller
Keyword(s):  
Directional Asymmetry ◽  
Random Wave ◽  
Random Waves ◽  
Sea Waves ◽  
Wave Direction ◽  
Directional Distribution ◽  
Wave Refraction ◽  
Long Time ◽  
Spreading Function ◽  
Directional Distributions

In this study, a more general directional spreading function is developed that allows for asymmetric directional distributions. For multi-directional random waves that approach the shore obliquely over a planar slope, we demonstrate that directional asymmetry is generated due to wave refraction. The asymmetry created by refraction increases with the offshore peak wave direction. The present spreading function is compared to a pre-existing symmetric spreading function and is shown to better capture changes in the directional distribution that occur in a refracting, random wave field. Finally, the new asymmetric spreading function is compared to a long time series of wave directional spectra measured at a nearshore field site. The results demonstrate that refraction-induced asymmetry is common in the nearshore and the asymmetric spreading function gives an improved analytic representation of the overall directional distribution as compared to the symmetric function.

Hydrodynamic Analysis of Flapping-Foil Thruster Operating in Random Waves

2014 ◽  
Author(s):  
E. S. Filippas ◽  
K. A. Belibassakis
Keyword(s):  
Free Surface ◽  
Operating Conditions ◽  
Random Wave ◽  
Random Waves ◽  
Sea Waves ◽  
Thrust Coefficient ◽  
Design Assessment ◽  
Numerical Predictions ◽  
Marine Propulsion ◽  
Head Waves

Oscillating foils located beneath the ship’s hull are investigated an unsteady thrusters, augmenting the overall propulsion of the ship in rough seas and offering dynamic roll stabilization. The foil undergoes a combined oscillatory motion in the presence of waves. For the system in the horizontal arrangement the vertical heaving motion of the hydrofoil is induced by the motion of the ship in waves, essentially ship heave and pitch, while the rotational pitching motion of the foil about its pivot axis is set by an active control mechanism. In previous works, a potential-based panel method has been developed for the detailed investigation of the effects of free surface in harmonic waves, and the results are found to be in good agreement with numerical predictions from other methods and experimental data. Also, it has been demonstrated that significant energy can be extracted from the waves. In the present work we examine further the possibility of energy extraction under random wave conditions using active pitch control. More specifically, we consider operation of the foil in head waves characterized by a given frequency spectrum, corresponding to specific sea states. The effects of the wavy free surface are taken into account through the satisfaction of the corresponding boundary conditions. Numerical results concerning thrust coefficient are shown, indicating that significant efficiency can be obtained under optimal operating conditions. Thus, the present method can serve as a useful tool for the preliminary design, assessment and optimum control of such systems extracting energy from sea waves and augmenting marine propulsion.

scholarly journals DIFFRACTION DIAGRAMS FOR DIRECTIONAL RANDOM WAVES

1978 ◽  
Vol 1 (16) ◽  
pp. 35 ◽  
Author(s):  
Yoshimi Goda ◽  
Tomotsuka Takayama ◽  
Tasumasa Suzuki
Keyword(s):  
Wave Diffraction ◽  
Spectral Characteristics ◽  
Wave Spectrum ◽  
Standard Form ◽  
Random Wave ◽  
Random Waves ◽  
Directional Spectrum ◽  
Spreading Function ◽  
Wave Heights ◽  
Directional Wave

Conventional wave diffraction diagrams often yield erroneous estimation of wave heights behind breakwaters in the sea, because they are prepared for monochromatic waves while actual waves in the sea are random with directional spectral characteristics. A proposal is made for the standard form of directional wave spectrum on the basis of Mitsuyasu's formula for directional spreading function. A new set of diffraction diagrams have been constructed for random waves with the proposed directional spectrum. Problems of multi-diffraction and multi-reflection within a harbour can also be solved with serial applications of random wave diffraction.

Study the Effect of Wave Directionality on Dynamic Nonlinear Behavior of Jack-Up Subjected to Wave and Earthquake Loading

2007 ◽  
Author(s):  
M. Abbasi ◽  
A. R. M. Gharabaghi
Keyword(s):  
Time History ◽  
Field Measurements ◽  
Earthquake Loading ◽  
Wave Loading ◽  
Random Waves ◽  
Sea Waves ◽  
Wave Direction ◽  
Maximum Displacement ◽  
Wave Directionality ◽  
Jack Up

Sea waves are random in nature as they propagate with different frequencies and in different directions. In literature, there are several studies about dynamic behavior of marine structures under random waves in frequency domain, but little or nothing has been done about wave directionality. In this paper, the behavior of a typical jack-up platform operating in Caspian Sea is studied. In order to model the interaction between spudcan and surrounding soil, it was modeled separately by PLAXIS software. The applied soil properties are based on the field measurements. The results from plastic analyses show that they can differ up to 18 percent compared with those obtained based on the recommendations of API. A 3D model of Iran-Khazar Jack-up was studied using ANSYS software. All elements of triangular legs were modeled by PIPE59 elements and the results obtained from previous step were used to model nonlinear interaction of spudcan footings. The wave and current characteristics are based on the field data. The time-history records of earthquake used in this research are based on Manjil earthquake related to the same area. Nonlinear dynamic analyses including large deflection and material non-linearity was performed. The temporal variation of displacement at deck level was compared under solely wave or earthquake loading and simultaneously acting wave and earthquake loading assuming to be in the same direction and in different directions. It is found that when earthquake loading is applied simultaneously with wave loading in the same direction, displacements are less than wave loading alone. However, when they are applied in different directions, especially when the direction of applied earthquake differs about 30 or 90 degrees with respect to wave direction, displacements become larger. In addition, the effect of wave directionality on the maximum displacement of structure was considered.

scholarly journals RANDOM WAVE SIMULATION IN A LABORATORY WAVE TANK

1976 ◽  
Vol 1 (15) ◽  
pp. 20 ◽  
Author(s):  
Akira Kimura ◽  
Yuichi Iwagaki
Keyword(s):  
Simulation System ◽  
Random Wave ◽  
New Wave ◽  
Random Waves ◽  
Sea Waves ◽  
Coastal Structures ◽  
Wave Tank ◽  
Wave Simulation ◽  
Simulation Techniques ◽  
Engineering Problems

Most of coastal engineering problems have been studied with monocromatic waves. However, sea waves which arrive at the coast are random. It is very difficult to estimate exactly the influence of these random waves to coastal structures. Then the model tests in a laboratory wave tank using random wave simulation techniques seem to be most desirable way to estimate the influence of randomness of sea waves. For this purpose, the accomplishment of random wave simulation system, which make possible generating random waves having statistically same properties as those of sea waves, has long been desired. The authors achieved to establish such a new wave simulation system. In this paper, the characteristics of this system are demonstrated experimentally through several cases of random wave simulations.

scholarly journals Time Scale for Scour Beneath Pipelines Due to Long-Crested and Short-Crested Nonlinear Random Waves Plus Current

2021 ◽  
Vol 9 (2) ◽  
pp. 114
Author(s):  
Dag Myrhaug ◽  
Muk Chen Ong
Keyword(s):  
Time Scale ◽  
Current Flow ◽  
Irregular Waves ◽  
Wave Crest ◽  
Random Wave ◽  
Random Waves ◽  
Flow Conditions ◽  
Regular Waves ◽  
Nonlinear Random Waves ◽  
2D And 3D

This article derives the time scale of pipeline scour caused by 2D (long-crested) and 3D (short-crested) nonlinear irregular waves and current for wave-dominant flow. The motivation is to provide a simple engineering tool suitable to use when assessing the time scale of equilibrium pipeline scour for these flow conditions. The method assumes the random wave process to be stationary and narrow banded adopting a distribution of the wave crest height representing 2D and 3D nonlinear irregular waves and a time scale formula for regular waves plus current. The presented results cover a range of random waves plus current flow conditions for which the method is valid. Results for typical field conditions are also presented. A possible application of the outcome of this study is that, e.g., consulting engineers can use it as part of assessing the on-bottom stability of seabed pipelines.

scholarly journals Analysis of Dangerous Sea States in the Northwestern Mediterranean Area

2021 ◽  
Vol 9 (4) ◽  
pp. 422
Author(s):  
Alessio Innocenti ◽  
Miguel Onorato ◽  
Carlo Brandini
Keyword(s):  
Extreme Events ◽  
Mediterranean Area ◽  
Rayleigh Distribution ◽  
Surface Elevation ◽  
Wave Steepness ◽  
Sea Waves ◽  
Operational Forecasts ◽  
Long Time ◽  
Simulated Field

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.

scholarly journals SHORELINE AT JETTY DUE TO CYCLIC AND RANDOM WAVES

1988 ◽  
Vol 1 (21) ◽  
pp. 141
Author(s):  
Todd L. Walton ◽  
Philip L.F. Liu ◽  
Edward B. Hands
Keyword(s):  
Time Series ◽  
Sediment Transport ◽  
Numerical Model ◽  
Analytical Solution ◽  
Numerical Models ◽  
Random Waves ◽  
Dimensionless Form ◽  
Wave Direction ◽  
Continuous Cycling ◽  
Small Wave

This paper examines the effects of random and deterministic cycling of wave direction on the updrift beach planform adjacent to a jetty. Results provided using a simplified numerical model cast in dimensionless form indicate the importance of the time series of wave direction in determining design jetty length for a given net sediment transport. Continuous cycling of • wave direction leads to the expected analytical solution. Simplications in the numerical model used restrict the applications to small wave angles, no diffraction, no reflection of waves off structure, no refraction, and no sand bypassing at jetty. The concept can be extended to more sophisticated numerical models.

scholarly journals Effects of waves and currents on the siltation problem of Damietta harbour, Nile Delta coast, Egypt

2007 ◽  
Vol 8 (2) ◽  
pp. 33 ◽  
Author(s):  
ABO BAKER.I. ABO ZED
Keyword(s):  
Sediment Transport ◽  
Nile Delta ◽  
Monitoring Program ◽  
Wave Direction ◽  
Littoral Drift ◽  
Wave Refraction ◽  
The North ◽  
Dynamic Factors ◽  
Navigation Channel ◽  
Nile Delta Coast

This study evaluates the effect of prevailing dynamic factors on the sedimentation process in Damietta Harbour along the Nile delta coast of Egypt. The monitoring program spanned the period between 1978 and 1999 and included measurements of waves, currents and bathymetric profiles. The evaluation was based on determination of erosion and accretion rates, current regime, sediment transport, wave characteristics and wave refraction. Results revealed that the predominant wave direction from N-NW sector (86 %) throughout the year is responsible for generation of a longshore eastward current. Less frequent waves from the N-NE sector generate an opposing longshore westward current. The refraction pattern for the prevailing wave direction indicates that the harbour and its navigation channel are located within a divergence of wave orthogonal and in an accretion sediment sink area. The annual net rate of littoral drift on the western side of the harbour is about 1.43 * 105 m3 (accretion), while the annual net rate of littoral drift on the eastern side is about 2.54 * 105 m3 (erosion). Currents fluctuate tremendously in speed and direction, especially during the winter months. Hence, sediment transport takes place in offshore, eastward, and onshore directions. Progressive vector diagrams show that the largest near bottom offshore, onshore and easterly net drift occurs during summer, spring and winter respectively. The onshore sediment transport generated during spring and summer plays an important role in the redistribution of eroded sediments during the winter. The overall study of dynamic factors indicated that the harbour site is characterized by eastern, western, offshore and onshore sediment movements. Therefore, the north-south orientation of the navigation channel, with its depth greater than the surrounding area, interrupts sediment drift from different directions and reduces the current speed. Consequently, the sediments sink within the navigation channel from different directions. The sources of sediments contributing to the siltation process of the harbour and its navigation channels are mainly derived from the Rosetta promontory, Burullus beaches, Damietta promontory and from offshore and the dumping area.

Experimental Study of Slow-Drift Ship Motions in Shallow Water Random Waves

2011 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Trygve Kristiansen
Keyword(s):  
Spectral Analysis ◽  
Shallow Water ◽  
Transfer Functions ◽  
Second Order ◽  
Ship Motions ◽  
Random Wave ◽  
Random Waves ◽  
Low Frequencies ◽  
Wave Basin ◽  
Drift Forces

Slowly varying motions and drift forces of a large moored ship in random waves at 35m water depth are investigated by an experimental wave basin study in scale 1:50. A simple horizontal mooring set-up is used. A second-order wave correction is applied to minimize “parasitic” long waves. The effect on the ship motion from the correction is clearly seen, although less in random wave spectra than in pure bi-chromatic waves. Empirical quadratic transfer functions (QTFs) of the surge drift force are found by use of cross-bi-spectral analysis, in two different spectra have been obtained. The QTF levels increase significantly with lower wave frequencies (except at the diagonal), which is special for finite and shallow water. Furthermore, the QTF levels frequencies at low frequencies increase significantly out from the QTF diagonal. Thus Newman’s approximation should preferrably not be used in these cases. Using the LF waves as a direct excitation in a “linear” ship force analysis gives random records that compare reasonably well with those from the cross-bi-spectral analysis. This confirms the idea that the drift forces in shallow water are closely correlated to the second-order potential, and thereby by the second-order LF waves.

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