Journal of Hydrospace Technology
- Machinery ＞ Naval Architecture/Ocean Engineering
Volume 1 Issue 2
Hydrodynamic Coefficients of an Oscillating Cylinder in Steady Horizontal Translation on the Free SurfaceHong, Do-Chun 1
An integral equation associated with a mixed distribution of source and normal doublet on the wetted surface of a two dimensional body has been presented for calculating the hydrodynamic coefficients of an oscillating cylinder in steady horizontal translation on the free surface. This equation contains two dimensional equivalent of the line integral for a three dimensional surface-piercing body. It also contains an integral relation which eliminate the occurrence of irrelevant solution. The resulting overdetermined linear system is solved by the method of Householder. Hydrodynamic coefficients of a half immersed circular cylinder have been calculated for various Froude numbers up to 0.35. The present numerical results fur a very small Froude number agree well with those for zero Froude number. It seems that the present method yields reasonable numerical results for all frequencies without restriction on the magnitude of Froude number in the context of linear wave theory.
Lee, G.J. 13
The motion response of a high speed SES in waves is important because the ride quality of passengers is mainly affected by it. The pitch motion has a large influence on the vertical motion at the bow. But the pitch motion of SES does not have been analyzed properly. The reason for that is the absence of proper mathematical model for the stem bag, the bow seal, and the inherent non-linearity. In this paper, the heave and pitch motion of high speed SES in waves have been treated. For doing it, the mathematical model for the stern bag was set up, and the hydrodynamic forces on the side hulls were obtained by using the principle of momentum change. The motion responses in waves were calculated, and the analysis of the motions was done.
Chun, Ho-Hwan 36
This paper discusses the way in which the SWATH concept was applied to a 21 tonne fishing vessel making use of various advantages over a monohull of similar size; low responses to waves, large deck area, good stability characteristics, outstanding maneuverability at low speeds, working efficiency as well as safety and lower speed loss in waves. The hydrodynamic characteristics have been rigorously examined by tank testing a one seventh scale model and comparing the results with computational results based on several computer programs. A 21 tonne SWATH boat fishery of shellfish by traps on the North West coast of Scotland was designed and constructed. The full scale measurements of resistance and powering, stability trials and rough sea trials were performed and reported in this paper.
Lee, Seung-Joon 59
For treating a hydrofoil moving with nonzero angle of attack, first, Tulin's theory is extended, and some computational results are given. Then a newly developed source-vortex panel code, which employs a nonlinear free surface boundary condition proposed by Lee and satisfies the Kutta condition, is explained, and its results are compared with the existing experimental data and other numerical results. Overall performance of the developed code is shown as satisfactory.
Kim, Wu-Joan;Van, Suak-Ho 69
An experimental and computational study is carried out to investigate the flow characteristics around a hydrofoil when free surface presents. In the experimental study a NACA 0012 section is towed in a 2-D flume to document the wave profiles and surface pressure distribution. In the computational study a finite-difference scheme is employed to solve the Wavier-Stokes equations and free surface profiles are obtained directly from the kinematic boundary condition. The calculated results are compared with those of the present and other two experiments to confirm the capability of the present method in free surface problems. The agreement between calculation and measurement is very good. It is found that the pressure on the upper(suction) side of the fail does not drop as much as in deeply submerged case and the lift deceases as a result.
Application of a Potential-Based Panel Method for Analysis of a Two-Dimensional Cavitating Hydrofoils Advancing Beneath a Free SurfaceLew, Jae-Moon;Lee, Chang-Sup;Kim, Young-Gi 83
A potential-based panel method is presented for the analysis of a partially or supercavitating two-dimensional hydrofoil at a finite submergence beneath a free surface, treating without approximation the effects of the finite Froude number and the hydrostatic pressure. Free surface sources and normal dipoles are distributed on the foil and cavity surfaces, their strength being determined by satisfying the kinematic and dynamic boundary conditions on the foil-cavity boundary. The cavity surface is determined iteratively as a part of the solution. Numerical results show that the wave profile is altered significantly due to the presence of the cavity. The buoyancy effect due to the hydrostatic pressure, which has usually been neglected in most of the cavitating flow analysis, is found playing an important role, especially for the supercavitating hydrofoil; the gravity field increases the cavity size in shallow submergence, but decreases when deeply submerged, while the lift is reduced at all depth.
Kim, Ki-Sung;Park, Yeong-Ho 100
The plastic collapse equations are obtained for regular grids under a central point load with two types of boundaries, i.e. simply-supported ends and fixed ends. General plastic collapse mechanisms are derived by applying upper bound theorem for assumed collapse mechanisms. The possible collapse mechanisms are divided into three types which allows to examine all possible collapse mechanisms systematically. Then, those formulae are applied to the pontoon deck problem to find the minimum weight design.
Han, Sung-Kon;Lee, Kwang-Min;Lee, Hwa-Lyong;Bae, Yeong-Soo 116
This paper deals with general procedure of the direct structural analysis of a container vessel and some difficulties encountered during the application of wave induced loads in real projects. Direct structural analysis requires a design wave representing the most probable extreme wave condition during its lifetime and an appropriate application scheme of wale loads into FE model of a hull. A design wave condition is determined by spectral analysis using the transfer function of a dominant load component, based on a given probability level. Dynamic and static load components are combined to get real condition of a vessel under waves. The effects of wave crest and trough are generated by modification of pressure distribution along the side shell. Counter accelerations are introduced to make the system in a state of equilibrium. FE model of the hull is also adjusted considering the realistic positions of center of gravity. Several idealization schemes are adopted to apply inertia forces induced from the mass of a vessel combined with applied accelerations. Main attention is paid to keep global effects obtained in wave load analysis throughout structural analysis. The results of rigorous application of wave loads into FE model are compared with those of simplified method in which only global effects such as vertical and horizontal bending moment and torsional moment are applied. It is shown that the proposed method gives more reliable results than the other methods, and is useful for practical uses.
Chung, Kie-Tae;Kim, Young-Bok;Shin, Hyoung-Chol;Han, Sung-Ryong;Huh, Young-Chol 131
In the vibration analysis of today's weight-optimized ship structures, it is very easy to find the non-beam like modes of vibration such as the double bottom vibration or the local vibration in ships with large openings. However, it is not easy to find the added mass and the corresponding three dimensional reduction factors. It is well known that the hydroelastic vibration analysis of structures in contact with fluid can be solved by applying the finite element method to structures and the boundary element method to fluid domain. However, such an approach is impractical due to the characteristics of the fluid added mass matrix fully coupled on the whole wetted surface. To overcome this difficulty, an efficient program MOFSIA(Modal Fluid-Structure Interaction Analysis) based on the modal coupling of fluid-structure interaction using reanalysis scheme is developed fur the free and forced vibration analysis of the arbitrary wetted structures with various fluid boundary conditions such as side wall, sea bottom or dock bottom, free surface and radiating boundary. The efficiency and accuracy of calculated results using the program MOFSIA developed by this project has been proved through the vibration experimental measurements of the plate model and the ship model, and also through the numerical experiments for the same models and 3200 TEU container ship.
A Simplified Approach to the Analysis of the Ultimate Compressive Strength of Welded Stiffened PlatesJoo, Chang-Doo;Seo, Seung-Il 145
In this paper, a method to calculate the ultimate compressive strength of welded one-sided stiffened plates simply supported along all edges is proposed. At first, initial imperfections such as distortions and residual stresses due to welding are predicted by using simplified methods. Then, the collapse modes of the stiffened plate are assumed and collapse load for each mode is calculated. Among these loads, the lower value is selected as the ultimate strength of the plate. Collapse modes are assumed as follows: 1. Overall buckling of the stiffened plate
$\rightarrow$Overall collapse due to stiffener bending 2. Local buckling of the plate $\rightarrow$Local collapse of the plate $\rightarrow$Overall collapse due to stiffener yielding 3. Local buckling of the plate $\rightarrow$Overall collapse due to stiffener bending 4. Local buckling of the plate $\rightarrow$Local collapse of the plate $\rightarrow$Overall collapse due to stiffener tripping. The elastic large deformation analysis based on the Rayleigh-Ritz method is carried out, and plastic analysis assuming hinge lines is also carried out. Collapse load is defined as the crossing point of the two analysis curves. This method enables the ultimate strength to be calculated with small computing time and good accuracy. Using the present method, oharacteristics of the stiffener including bending and tripping can also be clarified.
Song, Jae-Young;Chon, Young-Kee;Ha, Tae-Bum 167
In this study, the unified requirements of IACS on longitudinal strength of ships are verified through the nonlinear time domain analyses in irregular waves. The formula for the horizontal shear force, bending moment and torsional moment for ships of large deck openings are proposed based on the calculation results for existing ships. Also, external hydrodynamic sea pressure, accelerations and motions are calculated using linear strip theory and the corresponding design formulae are proposed. The calculated results are compared with the existing classification rules and will be incorporated into the rules in the near future after more detailed verification.