Fenfang Zhao

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.

Study on the hydrodynamic characteristics of the rectangular V-type otter board using computational fluid dynamics

The hydrodynamic performance of the rectangular V-type otter board is studied by two kinds of numerical simulation methods. The model tests are conducted in the flume tank as well as a comparison to the numerical results. It is found that the CFX analysis is better at simulating the forces, especially for predicting the maximum lift coefficient and the maximum lift-drag ratio. On the other hand the FLUENT analysis is better at simulating the velocity field. The full-scale otter board is then studied by means of CFX simulation to investigate their hydrodynamic performance. The results show that the otter board will be optimized when the aspect ratio is set to 0.490 with a dihedral angle (Г) of 17°. It is confirmed that the numerical simulation is a powerful tool in studying the hydrodynamic characteristics of the otter board and is useful in designing and optimizing otter boards.

Experimental study on the effect of horizontal waves on netting panels

The effect of horizontal waves on flexible netting panels is examined in this study, to evaluate the testing method of pretensioned mooring, radial systems, and flexible netting structures. The netting was suspended at a specific hanging ratio for six polyethylene panels with different characteristics. The aim was to evaluate the calculation method for horizontal wave forces on flexible netting panels. A regular wave was used in the experiment, with wave period 0.8–2.0 s and wave height 50–250 mm. The force on the netting structure was recorded by a tension transducer and digital signal recorder, and was simulated by a cubic spline in terms of the wave experiment under different wave conditions. The horizontal wave force on the netting panel changed periodically and asymmetrically, which was similar to the surface wave elevation. The horizontal wave force was related to the netting panel height and width (l), wave height (H) and wave length (L), twine diameter (d) and bar length (a) of the mesh. Using dual series relations, least-squares approximation, and multiple stepwise regression analysis, the following formula was obtained for the horizontal wave average apex value (F) on the netting panel: F=0.13ρgl(H/2)2(d/a)(H0.64L0.77/0.9a0.44).

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

Calculation of Wave Forces Acting on a Cylinder With a Porous Plate Fixed Inside

To evaluate wave forces and to estimate the motion of breakwater, a circular cylinder is investigated based on the linear wave theory in the present work. The cylinder possesses a porous sidewall, an impermeable bottom and a horizontal porous plate inside that is fixed in the cylinder to work as obstruct and make wave dissipation more effectively. To simplify the problem, the Darcy’s fine-pore model is applied to the boundary condition on the porous body surface. The boundary value problem is solved by means of the eigen-function expansion approach. The fluid domain is divided into three regions and different eigen-function series are used. The so-called dispersion relation for the region inside the cylinder is quite different from a conventional one due to the existence of the porous plate. It leads to eigen values of complex number. To obtain solutions for the radiation problems, particular solution should be constructed to take account of the normal velocity appearing on the porous boundary. The wave loads are evaluated by integrating the pressure difference on two sides of the wetted body surface. The theoretical works are in good consistence with the experimental results. The Haskind relations are 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, exists a second component caused by the porous effects.© 2009 ASME

Wave Fields Diffracted by an Array of Truncated Circular Cylinders

Along with the continual expanding of sea cage culturing recently, the floating breakwaters that protect the seawater fish cages from the incoming waves are more and more popularly utilized. In addition to the advantage of low cost and movable, it has fewer blockages to the normal seawater exchange compared to the fixed ones. As a first step for the purpose of designing more effective floating breakwaters, the diffraction of waves by the arrays of bottom-mounted and truncated circular cylinders, based on the linear wave theory, is considered in detail in this paper. The transmission of waves propagating through the arrays is also evaluated. The phenomenon of near-trapping mode at certain wavenumber is investigated, where it has peak forces and low transmission rate. The arrays with unevenly spaced arrangement of circular cylinders are studied, which has an expected efficiency on reducing the peak forces.Copyright

A Comparative Study on Hydrodynamic Performance of Double Deflector Rectangular Cambered Otter Board

In the present work, the hydrodynamic performance of the double deflector rectangular cambered otter board was studied using wind tunnel experiment, flume tank experiment and numerical simulation. Results showed that the otter board had a good hydrodynamic performance with the maximum lift-to-drag ratio (KMAX = 3.70). The flow separation occurred when the angle of attack (AOA) was at 45°, which revealed that the double deflector structure of the otter board can delay the flow separation. Numerical simulation results showed a good agreement with experiment ones, and could predict the critical AOA, which showed that it can be used to study the hydrodynamic performance of the otter board with the advantage of flow visualization. However, the drag coefficient in flume tank was much higher than that in wind tunnel, which resulted in a lower lift-to-drag ratio. These may be due to different fluid media between flume tank and wind tunnel, which result in the big difference of the vortexes around the otter board. Given the otter boards are operated in water, it was suggested to apply both flume tank experiment and numerical simulation to study the hydrodynamic performance of otter board.

Two-dimensional numerical simulation of flow around three-stranded rope

Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-ε model and Shear-Stress Transport κ-ω (SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.