Head Waves Research Articles

Validation of 6-DOF motion simulations for ship turning in regular waves

In this study, a six degrees of freedom (6-DOF) motion simulation method of a ship steering in regular waves is validated. The proposed simulation model is based on the two-time scale concept where the 6-DOF motions are expressed as the sum of the low-frequency maneuvering motions and high-frequency wave-induced motions. Turning simulations of a KCS container ship model with a rudder angle of $$\pm 35^\circ$$ in calm water and regular waves are performed and the obtained results are compared with the results of a free-running model test. The model tests were conducted using a ship model of length 3.057 m in a square tank at the National Research Institute of Fisheries Engineering, Japan. The wave conditions were as follows: the wave height was 3.6 m at full-scale, ratio of wavelength to ship length was 1.0, and the ship approached in the head wave direction before it was steered. The present method can simulate both the turning motion and wave-induced motions in regular waves with practical accuracy.

Acoustic fields for monopole logging while drilling with an eccentric collar

The acoustic problem of an eccentric drill collar in a fluid-filled borehole has been of interest in the field of acoustic logging while drilling (ALWD) in recent years. To reduce the effects of tool eccentricity on ALWD measurements, studies on acoustic responses under such conditions are essential. This study therefore has developed an analytical method to investigate borehole wavefields with an off-center monopole ALWD tool in fast and slow formations. By evaluating the contributions of compressional and shear branch points, the effects of the tool eccentricity on individual formation primary and shear head waves are investigated. Results illustrate that tool eccentricity only affects the excitation properties and has almost no effect on the extracted velocities. The joint analysis of synthetic full waveforms and dispersion diagrams with varying eccentricity degrees indicates that multipole modes are excited when the tool is off center, and their excitation amplitudes gradually increase with the increasing eccentricity, especially in the direction of tool movement. Moreover, the dispersion analysis reveals that the two modes with intersections in the centered case are coupled when the tool becomes eccentric. In particular, the coupling performance between the Stoneley and flexural modes is the most prominent. Furthermore, the effects of tool eccentricity on the monopole acquisition method, that is, the sum of waveforms received at four orthogonal azimuths, are evaluated. Results show that the above summation method can effectively reduce the effects of slight or moderate eccentricity. However, for large or extreme eccentricity, reducing or eliminating the effects of eccentricity on the Stoneley wave is a challenge for this method. Based on the above analyses, measurements may not be reliable for formation evaluation in the case of extreme eccentricity, especially for Stoneley wave applications.

Erratum: Lamb Waves and Adaptive Beamforming for Aberration Correction in Medical Ultrasound Imaging.

In our paper titled "Lamb Waves and Adaptive Beamforming for Aberration Correction in Medical Ultrasound Imaging" [1], we mentioned that the superposition of the different symmetric (S) modes in the frequency-wavenumber (f-k) domain results in a high intensity region where its slope corresponds to the longitudinal wave speed in the slab. However, we have recently understood that this high intensity region belongs to the propagation of a wave called lateral wave or head wave [2-5]. It is generated if the longitudinal sound speed of the aberrator (i.e. the PVC slab) is larger than that of water and if the incident wavefront is curved. When the incidence angle at the interface between water and PVC is near the critical angle, the refracted wave in PVC re-radiates a small part of its energy into the fluid (i.e. the head wave). As discussed in [4], if the thickness of the waveguide is larger than the wavelength, the first arriving signal is the head wave. This is also the case in our study [1] where the ultrasound wavelength of a compressional wave in PVC was close to 1 mm, and a PVC slab with a thickness of 8 mm was used.

Prediction of the aerodynamic behaviour of a full-scale naval ship in head waves using Detached Eddy Simulation

The airwake behaviour around a ship provides useful information for the safe operation of helicopters on naval ships as well as in helicopter pilot training. This study investigates the impact of ship motions on the airwake behind the superstructure of a naval ship using Detached Eddy Simulation. A full-scale simplified frigate geometry is analysed stationary and in head waves at three different wavelengths under a uniform wind field and in the presence of an atmospheric boundary layer. The results reveal that an atmospheric boundary layer impacts significantly the airwake, as well as the vertical wave-induced motions of the ship, which reduce in amplitude by between 20.9% and 22.39% in heave, and up to approximately 38% in pitch. Moreover, the results show that the presence of an atmospheric boundary layer impacts the ship's heave and pitch motion periods. The flow field is also significantly altered depending on the ambient wavelength and period of motion, particularly in the case where an atmospheric boundary layer is modelled.

Numerical Study on Unsteady Pressure Distribution on Bulk Carrier in Head Waves with Forward Speed

This study deals with wave-induced unsteady pressure on a ship moving with a constant forward speed in regular head waves. Two different numerical methods are applied to solve wave–ship interaction problems: a Rankine panel method which adopts velocity potential, and a Cartesian-grid method which solves the momentum and mass conservation equations under the assumption of inviscid and incompressible fluids. Before comparing l1ocal pressure distributions, the computational methods are validated for global quantities, such as ship motion responses and added resistance, by comparison with available experimental data. Then, the computational results and experimental data are compared for hydrodynamic pressure, particularly focusing on the magnitude of the first-harmonic component in different sections and vertical locations. Furthermore, the Cartesian-grid method is used to simulate the various wave-amplitude conditions, and the characteristics of the zeroth-, first-, and second-harmonic components of wave-induced pressure are investigated. The nonlinearity of pressure distribution is observed mostly from the pressure near the still-water-level of the ship bow and the normalized first-harmonic component of wave-induced pressure decreases as the wave steepness increases. Lastly, to understand the local characteristics of wave-induced unsteady pressure, the time-averaged added pressure and added local force are analyzed. It is found that the major contribution of the time-averaged added local force that occurs around the ship stem above the design waterline.

The effects of tool eccentricity on individual P and S head waves in monopole acoustic LWD

Abstract Velocities of P and S waves are main goals of downhole acoustic logging. In this work, we study the effects of an off-center acoustic tool on formation P and S head waves in monopole logging while drilling (LWD), which will be helpful for accurate interpretation of recorded logs. We first develop an analytic method to solve the wavefields of this asymmetric LWD model. Then using a branch-cut integration technique, we evaluate the contributions of branch points associated with P and S waves, and further investigate the effects of tool eccentricity on their characteristics of excitation, attenuation and waveforms. The analyses reveal that the variation of the excitation and attenuation of both P and S head waves with eccentricity depends on frequencies and receiver azimuths strongly. Besides, new resonance peaks appear in excitation spectra due to influences of poles of multipole modes near branch points when the monopole tool is off-center. According to semblance results of individual compressional and shear waveforms, extracted velocities are not affected by tool eccentricity in both fast and slow formations. In fast formations, spectra analyses indicate that S-wave excitation is more sensitive to tool eccentricity than P-wave. Moreover, resonance peaks in P-wave excitation spectra increase with the increasing eccentricity in all directions. In slow formations, off-center tools almost have no influence on both P and S waves at low frequencies, which suggests that the effects of tool eccentricity can be reduced by adjusting the source's operating frequency.

Effect of Pitch Moment of Inertia on Added Resistance of Container Ship in Waves

Owing to the compelling demand for CO2 reduction in recent years, various attempts have been expended to improve the resistance performance of ships in actual seas. In this study, the effect of the pitch moment of inertia on the added resistance in waves is investigated via tank tests and strip-method-based calculations for an S-175 container ship measuring a length (L) of 175 m. Consequently, when the pitch radius of gyration is reduced from 0.27L to 0.23L, it is discovered that the added resistance in waves reduced by approximately 30% for a ship moving with Froude number 0.15 in irregular head waves with a significant wave height of 4 m. In addition, the effect on the added resistance is significant in the head wave direction from 0° to 75° but insignificant in the beam and following wave direction from 90° to 180°. Hence, it is effective to reduce the pitch moment of inertia to improve the propulsive performance of a ship in waves.

On mathematical modeling of a hypersonic flow past a thin wing with variable shape

The present work is devoted to a further study of the spatial flowing of a thin wing of variable shape by a hypersonic flow of non-viscous gas. The head shock wave is considered to be attached to the leading edge of the wing. The use of the thin shock layer method for solving the system of gas dynamics equations makes it possible to build a mathematical model of the flow in question. It should also be noted that the analysis of boundary conditions makes it possible to determine the structure of the expansion of the quantities sought in a series and to construct approximate analytical solutions. In this case, in determining the first approximation corrections, two equations are integrated independently of the other equations. The application of the Euler — Ampere transform allows to construct a solution depending on two arbitrary functions and an unknown of the shock front. To determine these functions, previously it was obtained an integro-differential system of equations. This paper proposes one of the variants of the semi-inverse method for constructing the solution of this system, in which the form of one of arbitrary function is given. This approach allows you to additionally set the equation for the leading edge of the wing, and in the case when the head wave is attached along the entire leading edge and the inclination of the wing surface on it. The variant of the semi-inverse method presented in the work for the non-stationary spatial problem of the flow has made it possible to obtain a particular solution, which is a model for various flow regimes around the wing. Formulas are obtained to determine: the shape of the front of the shock wave, the shape of the surface of the streamlined body, the distance between the shock wave and the surface of the body, and the flow parameters on the wing surface.

Modified thrust empirical formula of a flapping foil by including the effects of azimuth angles

Wave energy is one of the most available sources of renewable energy in the world. It has been previously proven that the flapping foil can generate thrust forces using energy from the surface waves and an empirical formula was proposed to predict the thrust forces generated by a flapping foil consist of NACA0015 section (Kumar and Shin, 2019a). However, the proposed empirical formula was restricted to the head waves i.e. 0° azimuth angle which was not useful for the flapping foils encountering with oblique and following waves. Therefore, in this study, the thrust empirical formula was modified to include the effects of azimuth angles based on the experimentally obtained data. And the modified empirical equations were validated by the combination of foils experimentally.

Asymptotics of the Head Wave in the Cauchy Problem for a Difference Scheme Corresponding to the Two-Dimensional Wave Equation with Localized Initial Data

В работе строится асимптотика головной волны задачи Коши для псевдодифференциального уравнения, отвечающего разностной схеме для волнового уравнения с локализованными начальными данными. В зависимости от соотношения параметров задачи у головной волны возникают дисперсионные эффекты. В случае сильных дисперсионнных эффектов строится равномерная асимптотика головной волны в достаточно большой окрестности переднего фронта. Более того, такую асимптотику можно представить с помощью тэта-функций Якоби в случае, когда начальная функция имеет вид гауссовой экспоненты. Библиография: 14 названий.

Improvement of prediction methods of power increase in regular head waves using calm-water and resistance tests in waves

Abstract This paper applies load variation method to predict speed-power-rpm relationship along with propulsive performances in regular head waves, and to derive overload factors (ITTC, 2018). ‘Calm-water tests’ and ‘resistance test in waves’ are used. The modified overload factors are proposed taking non-linearity into consideration, and applied to the direct powering, and resistance and thrust identity method. These indirect methods are evaluated through comparing the speed-power-rpm relationships with those obtained from the resistance and self-propulsion tests in calm water and in waves. The objective ship is KVLCC2. The load variation method predicts well the speed-power-rpm relationship and propulsion performances in waves. The direct powering method with modified overload factors also predicts well. The resistance and thrust identity method with modified overload factor predicts with a little difference. The direct powering method with overload factors predicts with a relatively larger difference.

Comparative study on the prediction of speed-power-rpm of the KVLCC2 in regular head waves using model tests

This paper predicts the speed-power-rpm relationship in regular head waves using various indirect methods: load variation, direct powering, resistance and thrust identity, torque and revolution, thrust and revolution, and Taylor expansion methods. The subject ship is KVLCC2. The wave conditions are the regular head waves of λ/LPP = 0.6 and 1.0 with three wave steepness ratios at three ship speeds of 13.5, 14.5 and 15.5 knots (design speed). In the case of λ/LPP = 0.6 at design speed, two more wave steepness ratios have been taken into consideration. The indirect methods have been evaluated through comparing the speed-power-rpm relationships with those obtained from the resistance and self-propulsion tests in calm water and in waves. The load variation method has been applied to predict propulsive performances in waves, and to derive overload factors (ITTC, 2018). The overload factors have been applied to obtain propulsive efficiency and propeller revolution. The thrust and revolution method (ITTC, 2014) has been modified.

Numerical Simulation of Ship Sailing In Regular Head Waves Using CFD Method

Numerical Simulation of Ship Sailing In Regular Head Waves Using CFD Method

Ship motion-sloshing interaction using a field method

Even in relatively calm waters, low amplitude wave-induced motions of an LNG carrier may induce large amplitude liquid sloshing inside the ship's partially filled tanks, and the interaction between ship motions and sloshing may affect the ship's seekeeping properties. A computational procedure, here referred to as the RANS-RANS method, was developed to account for this interaction, and this method was then employed to predict the free surface flow inside the tanks and the corresponding motions of the ship in regular head and beam waves. This method coupled a compressible VoF technique with a generic wave generation and absorption scheme to obtain wave-induced ship motions with and without considering the effects of sloshing in the ship's tanks. Systematic grid studies were performed to obtain a sufficiently fine grid needed to yield converging predictions. The resulting wave patterns, ship motions, and internal sloshing elevations were compared with results obtained from a computational method, here referred to as the RANS-BEM method, that relied on a boundary element method to obtain ship motions. This RANS-RANS method was validated against model test measurements.

Experimental and Numerical Investigation of Green Water Occurrence for KRISO Container Ship

The occurrence of green water on the deck of Korea Research Institute of Ships & Ocean Engineering (KRISO) container ship is investigated using model test experiments and a fully coupled impulse response function (IRF)–computational fluid dynamics (CFD)–based numerical approach. In the experimental study, green water pressure over the deck and superstructure is investigated for different regular head wave conditions (wavelength/ship length ratio: .8–1.5) and vessel speeds (Froude number: .055–.166). The impact pressure on the deck is found to be highest at a wavelength/ ship length ratio of 1.2 and increases drastically with the increase in Froude number. The variation of green water pressure with wave steepness is linear for points on the forward deck and quadratic for the superstructure. In the second part, a coupled IRF-CFD–based numerical method is developed in which the global hydrodynamic forces such as radiation–diffraction and Froude–Krylov force are computed using a potential flow solver, whereas the local pressure due to the shipping water impact is computed using CFD and added as an external force. Comparisons of vessel motions and green water pressures with experiments indicate that the coupled IRF-CFD method can be a robust and efficient tool to predict shipping water loads on ships. Introduction The prediction of green water shipped on the deck and the resulting impact on ships and offshore structures have been an important research topic in the last couple of decades because it is directly related to the safety of the structure at sea. Historically, shipping of water on the deck had caused many serious damages to the bow and superstructure, and some of them have even capsized; e.g., the Schiehallion Floating Production Storage and Offloading (FPSO) was damaged severely in November 1998 because of steep waves. These incidents resulted in considerable research on this topic in the past two decades. The primary focus has been on the understanding of critical aspects and the accurate prediction of green water loading which requires proper modeling of green water occurrence. Also, accurate calculation of the green water loads on the deck and superstructure is very important for structural design load calculation. However, this is a nontrivial task because it depends on many physical and geometrical factors such as wave steepness, the wavelength- to-ship length ratio, hull shape around the bow region, and vessel speed. Hence, efforts have been made to develop suitable numerical models that may consider most of these aspects under necessary underlying assumptions.

Locating microseismic events using both head and direct waves travelling in vertical transverse isotropic media

SUMMARYMicroseismic monitoring is widely used to detect hydraulic fractures. Accurate mapping of microseismic events is essential to detect such fractures enhancing productivity. The eikonal solver is an efficient forward-modelling method used to map microseismic events. However, traditional eikonal solvers do not distinguish between head and direct waves, computing only the traveltimes of the waves that arrive first. We developed a new eikonal solver that computes the traveltimes of direct waves by imposing new constraints on the conventional, vertical transverse isotropy solver. We then performed numerical experiments exploiting the traveltimes of direct waves. We used the traveltimes of only the first arrivals, and those of both first and direct arrivals, when performing inverted event mapping. The results showed that the uncertainties of event locations were minimized when both head and direct waves were analysed due to the increased both the number of available data and the travelling path diversity. Also, we found that the use of only direct-arrival traveltimes was valuable when head-wave first arrivals were difficult to detect because the signal-to-noise ratio was low.

Nonlinear dynamic characteristics of marine rotor-bearing system under coupled heave and pitch ship motions

The coupled heave and pitch motions of a ship sailing in head waves affect the stability of the marine rotor-bearing system. Based on the theory of analytical mechanics, this study establishes a dynamic model of the rotor-bearing system subjected to the coupled motions of heave and pitch, considering nonlinear oil film moments produced by the tilting of the rotor in the bearings. The nonlinear dynamic behaviours of the system are analysed using numerical methods to obtain Poincaré sections, bifurcation diagrams, and the largest Lyapunov exponents. The results show that dynamic bifurcation characteristics reveal complex quasi-periodic motion of upper and lower branches after the initial instability of the system, and the speed of second instability increase markedly. At high speeds, the amplitude of the rotor system increases sharply, which can cause the rotor to touch the inner wall of the bearings in the quasi-periodic state and a failure to transition to the chaotic state. Additionally, the effects of heave and pitch amplitude variations on the dynamic characteristics of the system are also discussed.

Virtual head waves in ocean ambient noise: Theory and modeling.

The Green's function retrieval in media with horizontal boundaries usually only considers the extraction of direct and reflected waves but ignores the virtual head waves, which have been observed experimentally from ocean ambient noise and used to invert for geometric and environmental parameters. This paper derives the extraction of virtual head waves from ocean ambient noise using a vertically spaced sensor pair in a Pekeris waveguide. Ocean ambient noise in the water column is a superposition of direct, reflected, and head waves. The virtual head waves are produced by the cross-correlations between head waves and either reflected waves or other head waves. The locations of sources that contribute to the virtual head waves are derived based on the method of stationary phase. It is the integration over time of contributions from these sources that makes the virtual head waves observable. The estimation of seabed sound speed with virtual head waves using a vertical line array is also demonstrated. The slope of the virtual head waves is different from that of direct and reflected waves in the virtual source gather; it is therefore possible to constructively stack the virtual head waves. The predictions are verified with simulations.

Determination of Seakeeping Performance for a Case Study Vessel by the Strip Theory Method

Abstract The increase of seakeeping performance is of particular importance for car and passenger ferries, service ships in the gas and oil extraction industry and offshore wind power farm industry, as well as for special purpose ships (including military applications). In the water areas of the Baltic Sea, North Sea, and Mediterranean Sea, which are characterised by a short and steep wave, the hull shape has a substantial impact on the operational capacity and propulsion efficiency of the ship, as well as on comfort and safety of navigation. The article analyses selected aspects of seakeeping for four variants of a selected case study vessel, indicating practical limitations of the strip method. The analysed aspects included hull heaving and pitching, added resistance, Motion Thickness Indicator (MSI), and Subjective Magnitude (SM). Experimental tests were also performed in the towing tank. Their comparison with the numerical results has indicated high inaccuracy of the strip method. What is more, the simplified representation of hull shape used in the strip method makes it impossible to analyse the effect of hull shape changes on the predicted seakeeping characteristics. Especially for the case of head wave, neglecting highly non-linear phenomena, such as slamming or head wave breaking, in strip method-based computer simulations will significantly decrease the reliability of the obtained results. When using the strip method, the seakeeping analysis should be complemented with model tests in a towing tank, or by another more complex numerical analysis, such as CFD for instance.

Effect of Waves on the Behavior of Emergent Buoyantly Rising Submarines Using CFD

Emergent buoyantly rising submarines encounter excess roll problems, partially owing to waves that significantly affect their behavior. This study predicts the behavior of a submarine, including when it rises in static water, beam sea, head wave, following wave, 30∘ bow wave, 60∘ bow wave, 30∘ quartering wave, and 60∘ quartering wave, using the computational fluid dynamics method. The beam sea has a slight effect on pitch prior to the submarine rising to the water surface, but the maximum roll angle in the beam sea is 4.43 times that in static water. After a submarine submerges in water, the pitching oscillation does not decay quickly owing to the yaw angle. The head wave and the following wave have a continuous significant effect on the pitch; the submarine sail remains under the water surface after it submerges from the highest position. The head wave and the following wave have a slight effect on the roll and yaw before the submarine rises to the water surface; however, the roll angle suddenly increases after the submarine submerges from the highest position. As the initial angle between the submarine centerline and wave direction increases, the effect of waves on the longitudinal motion decreases. The amplitude of the pitching oscillation decreases with an increase in the initial angle between the submarine centerline and wave direction, and the waterline when the submarine oscillates on the water surface decreases. The difference in the maximum roll angle between when a submarine rises in an oblique wave and when it rises in beam sea is below 6.3∘. Submarines should try to avoid rising in a head wave and the following wave.