Weiguang Bao

Theoretical and Experimental Study on a Porous Cylinder Floating in Waves

In the present work, theoretical and experimental studies on the interaction of water waves with a truncated circular cylinder were performed. The cylinder, which is partly made of porous materials, possesses a porous sidewall and an impermeable bottom. A nondimensional parameter b is adopted in the theoretic formulation to describe the porosity, which is not directly related to the opening ratio τ of the porous materials. To validate the theoretical work and computed results, a series of model tests are carried out in a wave basin. Effort is made to establish an empirical relation between b and τ based on the comparison of the calculation and experimental data. The phenomenon of the sloshing mode that occurred at a certain wave number is observed, which might have an application in breakwaters. The validation of the Haskind relations is examined for the porous body. It is found that the damping coefficient consists of two parts. In addition to the component of conventional wave-radiating damping, there exists a second component caused by the porous effects.

The hydrodynamic forces acting on a cylinder array oscillating in waves and current

The problem of the interaction of multiple cylinders oscillating in waves and slow current is considered. The interaction is represented by waves emitted from adjacent cylinders towards the cylinder under consideration. Wave drift forces and moment in the horizontal plane are calculated by the far-field method based on the conservation of momentum or angular momentum. A semianalytical formula for the calculation of the wave drift damping is then deduced. The conservation of the integrals in these formulae is proved. Special treatments to improve the accuracy of results are discussed. Comparisons between calculated results and experimental measurements are made, showing satisfactory agreement. Effects of various combinations of current direction and incident wave angle on the wave drift damping and damping moment are also examined.

Hydrodynamic forces acting on a circular cylinder oscillating in waves and a small current

A complete theory to obtain semianalytical solutions of the wave drift damping for a circular cylinder freely oscillating in waves is developed. The wave drift damping can be significantly increased by heave and pitch motions. Effects of the draft of the cylinder and effects of the water depth are shown. The effective evaluation of the free-surface integral and the corner problem are also discussed, and the computed results of the far and near field formulations are compared. These semianalytical solutions may prove to be important in providing validation of results obtained by numerical techniques such as the higher-order boundary element method.

Interaction of a submerged elliptic plate with waves

With potential applications as a breakwater, an elliptic plate horizontally submerged in waves is investigated within the scope of linear wave theory. An elliptical coordinate system is adopted, which has an advantage to represent the solution in an analytical form, i.e. an expansion of eigen functions. By means of separation of variables, it turns out that the eigen functions in the elliptical coordinates consist of the Mathieu functions and the modified Mathieu functions. The interaction of the elliptic plate with the waves is studied. The wave loads, as well as the scattered wave field, are evaluated.

The Effect of Controlling Fins on the Motion of Resonance-Free SWATH (RFS) Platform

The speed reduction, additional resistance or slamming, which are caused by the large amplitude of ship motions, should be restricted completely for oceangoing large fast ship, because of the strict time-punctuality and high value of the cargo. In the present work, comparison of seaworthiness, such as the motion responses in head seas, is made among four types of ship hulls, i.e. Mono-hull, Ordinary SWATH, Trimaran and Resonance-Free SWATH (RFS). The last one can be regarded as a special type of catamaran. As a result of the extremely small water plane, the restoring moments are negative for RFS hull, which leads to free of resonance in the motion responses. Experiments in towing tank and theoretical calculations are carried out to examine the motion responses in waves. Calculations are generally based on potential theory modified by adding the viscous effects and including fin lifts. The results of the motion responses of RFS are compared with those of mono-hull ship, ordinary SWATH ship as well as trimaran ship. Where the motion of RFS is controlled using movable small underwater fins. The motion of ordinary SWATH is examined with and without fin control. It can be seen that the heave motion responses of RFS are very small in comparison with those of other ship hulls. Also, the pitch motion responses of RFS are much smaller than those of mono-hull or trimaran or ordinary SWATH without fin control, and are smaller than those of ordinary SWATH with fin control. Fin control actions, i.e. proportional and derivative control, are applied in the experiments and calculations. It is considered that the derivative control action is more effective on the motion responses of RFS than the proportional one as predicted. The larger the derivative gain value is, the smaller the motion response of RFS is.© 2010 ASME

Wave Fields Diffracted by an Array of Porous Circular Cylinders

In the present work, the diffraction of waves by an array of porous circular cylinders with or without an inner porous plate is considered in detail based on the linear wave theory. For the sake of designing more effective floating breakwaters, the transmission rate of waves propagating through the array is also calculated. Each cylinder is partly made of porous materials. Namely, it possesses a porous sidewall and an impermeable bottom. In addition, an inner porous plate is fixed horizontally inside the cylinders, which works as obstruction to make wave dissipate more effectively and eliminates the phenomenon of sloshing mode. To deal with hydrodynamic interactions among a great number of bodies, a hierarchical interaction theory, which saves computation time efficiently, is adopted to compare with the approach of Kagemoto & Yue (1986). Consequently, a series of model test of an array of porous cylinder are carried out in the basin to validate the theoretical work and the calculation results. The draft of the cylinders, the location of the inner porous plate and the spacing between adjacent cylinders are adjusted to investigate its effect on the wave dissipation.Copyright

A PMM Experimental Research on Ship Maneuverability in Waves

Recently, much attention has been paid on the maneuvering characters of ships in waves. Apparently, ships steering in waves are subjected to linear wave loads at the encountering frequency. Nevertheless, the maneuvering characters of ships are more affected by the nonlinear wave drift forces at low frequencies. In the present work, to examine the wave effects on the ship maneuverability, a serial of towing tests with a Planer Motion Mechanism system (PMM) is carried out in waves. Forces and horizontal displacements of the model are measured throughout the tests. The measured data are analyzed by the Fourier analysis to obtain components at different frequencies. Discussion on the importance of the force components is made.Copyright

Non-Linear Wave Forces Acting on a Body of Arbitrary Shape Slowly Oscillating in Waves

In the present study, non-linear wave loads such as the wave drift force, wave drift damping and wave drift added mass, acting on a moored body is evaluated based on the potential theory. The body is oscillating at a low frequency under the non-linear excitation of waves. The problem of interaction between the low-frequency oscillation of the body and ambient wave fields is considered. A moving coordinate frame following the low frequency motion is adopted. Two small parameters, which measure the wave slope and the frequency of slow oscillations (compared with the wave frequency) respectively, are used in the perturbation analysis. So obtained boundary value problems for each order of potentials are solved by means of the hybrid method. The fluid domain is divided into two regions by an virtual circular cylinder surrounding the body. Different approaches, i.e. boundary element method and eigen-function expansion, are applied to these two regions. Calculated nonlinear wave loads are compared to the semi-analytical results to validate the present method.Copyright

Wave-Drift Added Mass of a Cylinder Array Free to Respond to the Incident Waves

Conventional linear added mass and damping can be obtained when a floating body is forced to oscillate in the calm water. However, with the presence of the incident waves, there exists an alternative source of added mass and damping caused by the nonlinear interactions between waves and low-frequency oscillations. Proportional to the square of the wave amplitude, they are called the wave drift added mass and the wave drift damping. The problem of a circular cylinder array slowly oscillating in both diffraction and radiation wave fields is considered in the present work. The frequency of the low-frequency oscillation is assumed to be much smaller than the wave frequency. Perturbation expansion based on two time scales is performed to simplify the problem. Wave loads including the wave drift added mass are formulated by integration of the hydrodynamic pressure over the instantaneous wetted body surface.Copyright

Hydrodynamic behaviours of elastic net sheets in waves

From previous experimental investigations on a heaving sea cage, it has been observed that the hydrodynamic forces and motions of the sea cage are strongly affected by the deformation of the cage net. As a fundamental study, the hydrodynamic behaviours of elastic net sheets with a circular or a square shape are studied in the present work. The net sheet is regarded as a piece of elastic porous membrane. The deformation of the net sheet is expanded into a series of natural modes deduced from the free vibration of the membrane (dry mode). The boundary value problem of fluid motion is formulated on the basis of the linear wave theory. A Darcy-type law is applied to the porous boundary. The problem is solved either by the eigenfunction expansion (for the circular shape only) or by the hybrid method with the Rankine source as the Green’s function. To validate the theoretical work and the numerical calculation, experiments are carried out in a wave basin. The models used in the tests are square shaped with a metal frame. The net part of the models is made of elastic cords with pre-tension. Both diffraction tests and forced heaving tests are performed. In addition to the vertical force, the deformation of the net surface is recorded by video cameras and analysed with a video image analysis system.