Ray path of head waves with irregular interfaces

Head waves are usually considered to be the refracted waves propagating along flat interfaces with an underlying higher velocity.However,the path that the rays travel along in media with irregular interfaces is not clear.Here we study the problem by simulation using a new approach of the spectral-element method with some overlapped elements（SEMO） that can accurately evaluate waves traveling along an irregular interface.Consequently,the head waves are separated from interface waves by a time window.Thus,their energy and arrival time changes can be analyzed independently.These analyses demonstrate that,contrary to the case for head waves propagating along a flat interface,there are two mechanisms for head waves traveling along an irregular interface：a refraction mechanism and transmission mechanism.That is,the head waves may be refracted waves propagating along the interface or transmitted waves induced by the waves propagating in the higher-velocity media.Such knowledge will be helpful in constructing a more accurate inversion method,such as head wave travel-time tomography,and in obtaining a more accurate model of subsurface structure which is very important for understanding the formation mechanism of some special areas,such as the Tibetan Plateau.

Effect of Ship Bow Overhang on Water Shipping for Ship Advancing in Regular Head Waves

This paper presents the results of an experimental investigation dealing with the effect of bow overhang extensions on the quantity of shipping water over the foredeck in case of ships advancing in regular head waves. To perform this investigation, a series of free-running tests was conducted in regular waves using an experimental model of a multipurpose cargo ship to quantify the amount of shipping water. The tests were performed on five bow overhang variants with several combinations of wavelength and ship speed conditions. It was observed that the quantity of shipping water was affected by some parameters such as wavelength, ship speed, and bow shape in terms of an overhang extension. The results show the significant influence of an overhang extension, which is associated with the bow flare shape, on the occurrence of water shipping. These results involve the combined incoming regular waves and model speed.

A Practical Speed Loss Prediction Model at Arbitrary Wave Heading for Ship Voyage Optimization

This paper proposes a semi-empirical model to predict a ship’s speed loss at arbitrary wave heading.In the model,the formulas that estimate a ship’s added resistance due to waves attacking from different heading angles have been further developed.A correction factor is proposed to consider the nonlinear effect due to large waves in power estimation.The formulas are developed and verified by model tests of 5 ships in regular waves with various heading angles.The full-scale measurements from three different types of ships,i.e.,a PCTC,a container ship,and a chemical tanker,are used to validate the proposed model for speed loss prediction in irregular waves.The effect of the improved model for speed loss prediction on a ship’s voyage optimization is also investigated.The results indicate that a ship’s voyage optimization solutions can be significantly affected by the prediction accuracy of speed loss caused by waves.

Investigation of the Effect of Bulbuos Bow of Containership in Head Waves Using CFD

Reynold＇s averaged Navier-Stokes based CFD （computational fluid dynamics） technique WISDAM developed at The University of Tokyo is used to investigate and compare the hydrodynamic loads on container ship models SR108 and KCS in 120 degree regular oblique wave conditions. WISDAM has the capability of handling 6DOF （six degrees of freedom） in ship motions. Finite volume method with structured and overlapping grid system is employed. The flow variables are described in staggered manner, i.e., velocity components arc defined at the face center while pressure is at the cell center. Computational results agree favorably well with existing towing tank results especially for ship motions. Computational results also show that ship with bulbous bow experience higher hydrodynamic loads on bow section.

Prospects and Current Studies on the Fault Zone Seismic Waves

Deep structure and material properties of faults can be understood by observing and simulating the particular phase in a fault fracture zone. This paper reviews the development of fault-zone seismic waves in the seismological domain. The present research status of fault-zone head wave and trapped wave are summarized systematically. Based on recent progress in this field,the paper discusses the prospect on the utilization of seismic wave in fault structure research.

Improved Formula for Estimating Added Resistance of Ships in Engineering Applications

The authors previously introduced a semi-empirical formula that enabled fast estimation of the added resistance of ships in head waves, and in this study the formula is further refined for easy use in engineering applications. It includes an alternative ship draft correction coefficient, which better accounts for the wave pressure decay with ship’s draft. In addition, it only uses the speed and main characteristics of the ship and wave environment as input, and has been simplified to the extent that it can be readily processed using a pocket calculator. Extensive validations are conducted for different ship types at low to moderate speeds in various typical irregular sea conditions, and encouraging results are obtained. This relevant and topical research lies within the framework of the recent IMO MEPC.232（65） （2013） EEDI guidelines for estimating the minimum powering of ships in adverse weather conditions, which specify for the use of simple methods in current Level 2 assessment within engineering applications.Keywords： added resistance, minimum power, IMO regulation, EEDI regulation, weather coefficient, semi-empirical formulas, ships, head waves

Analytical Solution for the Transient Response of A Sloping Seabed Induced by A P-Wave Line Source

Many offshore marine structures are built on the seabed that are slightly or considerably sloping.To study the sloping seabed transient response during marine earthquakes,an analytical solution induced by a P-wave line source embedded in the solid is presented.During the derivation,the wave fields in the fluid layer and the semi-infinite solid are firstly constructed by using the generalized ray method and the fluid-solid interface reflection and transmission coefficients.Then,the analytical solution in the transformed domain is obtained by superposing these wave fields,and the analytical solution in the time domain by applying the analytical inverse Laplace transform method.The the head wave generation conditions and arrival times at the fluid-solid interface are derived through this solution.Through the use of numerical examples,the analytical solution is proved right and the impacts of the sloping angle on the hydrodynamic pressure in the sea,the seismic wave propagation in the seabed,the head wave,and the Scholte wave at the seawater-seabed interface are also addressed.

Characteristics of head wave in multi-layered half-space

In this article, we analyze the dynamic characteristics of head wave in multi-layered half-space media models with high-velocity layer or low-velocity layer, and the model with a continuous transition-zone between the crust and the mantle by using synthetic seismogram. It is concluded that the dynamic characteristics of head wave are sensitive to the thickness and velocity of the high-velocity layer. There is obvious diffraction phenomenon of seismic wave if the thickness of high-velocity layer is very small compared with the characteristic wavelength. In this case, the high-velocity layer cannot shield the head wave propagating along the upper interface of the media below it, and the amplitude of this head wave is proportional to the thickness or the velocity of the high-velocity layer. When the thickness of high-velocity layer is nearly identical to the characteristic wavelength of seismic wave, the wave phases reflected from the bottom of the high-velocity layer and the head wave phase may have very close arrival and weaken each other because of destructive interference. As to low-velocity layer, the amplitude of the head wave is weak and decreases with the velocity of this layer. It is also found that if a continuous transition-zone between the crust and the mantle is introduced, we can get a strong apparent head wave phase in synthetic seismogram and the amplitude of this phase increases with the thickness or velocity gradient of the transition-zone.