Wave interference effects on two advancing ships

In this paper: Development of a method to predict the critical line between the quiescent and wake region. Validation of the theoretical estimation based on large amount of numerical calculations. Calculation of how much in percentage the wave interference effects are predicted

Analytical Solutions for Vertical Flow in Unsaturated, Rooted Soils with Variable Surface Fluxes

Analytical solutions to Richards\u27 equation have been derived to describe the distribution of pressure head, water content, and fluid flow for rooted, homogeneous soils with varying surface fluxes. The solutions assume that (i) the constitutive relations for the hydraulic conductivity and water content as function of the pressure head are exponential, (ii) the initial water content distribution is a steady-state distribution, and (iii) the root water uptake is a function of depth. Three simple forms of root water uptake are considered, that is, uniform, stepwise, and exponential functional forms. The lower boundary of the rooted soil profile studied is a water table, while at the upper boundary time-dependent surface fluxes are specified, either infiltration or evaporation. Application of the Kirchhoff transformation allows us to linearize Richards\u27 equation and derive exact solutions. The steady-state solution is given in a closed form and the transient solution has the form of an infinite series. The solutions are used to simulate the hydraulic behavior of the rooted soils under different conditions of root uptake and surface flux. The restricted assumptions for the solutions may limit the applicability, but the solutions are relatively flexible and easy to implement compared to other analytical and numerical schemes. The analytical solutions provide a reliable and convenient means for evaluating the accuracy of various numerical schemes, which usually require sophisticated algorithms to overcome convergence and mass balance problems

Generalized Expectation Maximization Framework for Blind Image Super Resolution

Learning-based methods for blind single image super resolution (SISR) conduct the restoration by a learned mapping between high-resolution (HR) images and their low-resolution (LR) counterparts degraded with arbitrary blur kernels. However, these methods mostly require an independent step to estimate the blur kernel, leading to error accumulation between steps. We propose an end-to-end learning framework for the blind SISR problem, which enables image restoration within a unified Bayesian framework with either full- or semi-supervision. The proposed method, namely SREMN, integrates learning techniques into the generalized expectation-maximization (GEM) algorithm and infers HR images from the maximum likelihood estimation (MLE). Extensive experiments show the superiority of the proposed method with comparison to existing work and novelty in semi-supervised learning

Ship hydrodynamics in confined waterways

The hydrodynamic performance of a vessel is highly dependent on its maneuvering waterways. The existence of the banks and bottom, as well as the presence of the other vessels, could have a significant influence on a ship’s hydrodynamic behavior. In confined waterways, many researchers suspect the applicability of the classical potential flow method because of its nonviscous and irrotational assumption. The main objective of the present article is to improve and develop the boundary value problem (BVP) of a potential flow method and validate its feasibility in predicting the hydrodynamic behavior of ships advancing in confined waterways. The methodology used in the present study is a 3D boundary element method based on a Rankine-type Green function. The numerical simulations are performed by using the in-house developed multibody hydrodynamic interaction program MHydro. The waves and forces (or moments) are calculated when ships are maneuvering in shallow and narrow channels, when ships are entering locks, or when two ships are encountering or passing each other. These calculations are compared with the benchmark test data published in MASHCON, and the published computational fluid dynamics results. It has been found that the free-surface elevation, lateral force, and roll moment can be well predicted in ship–bank and ship–bottom problems. However, the potential flow solver fails to predict the sign of the yaw moment because of the cross-flow effect. When a ship is entering a lock, the return-flow effect has to be considered. By adding a proper return-flow velocity to the BVP, the modified potential flow solver could predict the resistance and lateral forces very well. However, it fails to predict the yaw moment because of the flow separation at the lock entrance. The potential flow method is very reliable in predicting the ship–ship problem. The resistance and lateral force, as well as the yaw moment, can be predicted well by using the potential flow method

Wave force prediction effect on the energy absorption of a wave energy converter with real-time control

Real-time control has been widely adopted to enlarge the energy extraction of a wave energy converter (WEC). In order to implement a real-time control, it is necessary to predict the wave excitation forces in the close future. In many previous studies, the wave forces over the prediction horizon were assumed to be already known, while the wave force prediction effect has been hardly examined. In this paper, we investigate the effect of wave force prediction on the energy absorption of a heaving point-absorber WEC with real-time latching control. The real-time control strategy is based on the combination of optimal command theory and first order-one variable grey model GM(1,1). It is shown that a long prediction horizon is beneficial to the energy absorption whereas the prediction deviation reduces extracting efficiency of the WEC. Further analysis indicates that deviation of wave force amplitude has little influence on the WEC performance. It is the phase deviation that leads to energy loss. Since the prediction deviation accumulates over the horizon, a moderate horizon is thus recommended

On wave diffraction of two-dimensional moonpools in a two-layer fluid with finite depth

This paper studies the wave diffraction problem of two-dimensional moonpools in a two-layer fluid by using domain decomposition scheme and the method of eigenfunction expansion. Wave exciting forces, free surface and internal wave elevations are computed and analyzed for both surface wave and internal wave modes. The present model is validated by comparing a limiting case with a single-layer fluid case. Both piston mode and sloshing mode resonances have been identified and analyzed. It is observed that, compared with the solutions in surface wave mode, the wave exciting forces in internal wave mode are much smaller, and show more peaks and valleys in low-frequency region. As the wave frequency increases, the bandwidth of sloshing mode resonances decreases. Extensive parametric studies have been performed to examine the effects of moonpool geometry and density stratification on the resonant wave motion and exciting forces. It is found that, for twin bodies with deep draft in surface wave mode, the decreasing density ratio has little effects on the sloshing mode resonance frequencies but can somehow suppress the horizontal wave exciting forces and surface wave elevations around piston mode resonance region. In addition, the presence of lower-layer fluid can lead to the reduction of piston mode resonance frequency

Resonant waves in the gap between two advancing barges

The gap resonance between two advancing rectangular barges in side-by-side arrangement is investigated using a 3-D Rankine source method. A modified Sommerfeld radiation condition accounting for Doppler shift is applied for the low forward speed problem when the scattered waves could propagate ahead of the barges. Numerical studies are conducted to investigate various factors which will influence the wave resonance in the narrow gap with particular attention paid on the forward speed effect and its coupling effects with gap width and draft. It is found that in the absence of forward speed, the trapped water surface oscillates like a flexible plate and the wave flow within the gap behaves like a standing wave. When the two barges are travelling ahead, the resonant wave patterns within the gap are reshaped. Additionally, the resonant frequencies shift to lower value and are compressed within a narrow range. Gap resonances are reduced by the augment of gap width. The effect of draft is shown to be associated with resonant modes. Draft effect becomes less pronounced at higher order resonant modes. Furthermore, both gap width and draft effects on gap resonance are found to be independent from forward speed

Dynamic and structural performances of offshore floating wind turbines in turbulent wind flow

A realistic turbulent wind field differs from a steady uniform one, in terms of the wind shear, the turbulence intensity and the coherence structure. Although it has been clear that an offshore floating wind turbine will behave differently in the turbulent wind, the individual effect of the above three items are not investigated sufficiently until now. The primary objective of the present research is to investigate in details how the wind shear, the turbulence intensity and the coherence influence the dynamic and structural responses of offshore floating wind turbines. Aero-hydro-servo-elastic coupled simulation of a semi-submersible floating wind turbine is run in time-domain. The wind shear has a limited effect on the global responses of the floating wind turbine although its influence on each individual blade is considerable. Comparatively, the floating wind turbine is quite sensitive to the turbulence intensity. In a wind field with high turbulence intensity, the platform motions become more violent and the structural loads are increased substantially. The proper orthogonal decomposition method is used to investigate the coherence quantitatively. A partial coherence structure helps to reduce the flow variation seen by the rotor and thereby beneficial to the safety of the floating wind turbine