• Journal of the Society of Naval Architects of Korea
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• v.41 no.5
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• pp.8-13
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• 2004

In order to better understand the characteristics of the flow field around the submerged hydrofoil of finite span with high speed and shallow submergence. a numerical code which can solve the flow around a fast lifting body under the free surface was developed and used to obtain various interesting features of the flow. The code was based on the panel method of Hess( 1972), and the free surface condition was linearized to conform with the assumption of the high Froude number. It is shown that the effect of the change of submerged depth. angle of attack and aspect ratio upon the sectional lift coefficient is rather significant for the case of the chosen example wing, which has the rectangular planform. Since Lee(2002)'s theoretical results were for the wing of elliptical planform, the direct comparison of the two results was not possible. It seems that more computational results are in need to compare the theoretical and the numerical prediction in detail.

• Journal of Ocean Engineering and Technology
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• v.25 no.1
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• pp.1-7
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• 2011

The radiation of water waves by a heaving truncated circular cylinder with damping plate is solved in the frame of the three-dimensional linear potential theory. The damping plate has a distinct advantage in reducing the motion response of a floating circular cylinder by increasing the added mass and the damping coefficient. Using the matched eigenfunction expansion method, the characteristics of hydrodynamic added mass and the damping coefficient are investigated with various system parameters, such as the radius and submergence depth of the damping plate. It is found that both added mass and the damping coefficient are significantly increased due to the arranged features of the larger damping plate with shallow submergence, which are positive factors as a motion reduction device of the floating offshore platform. Also the numerical results for an oscillating submerged disk show that the added mass is negative and that the damping coefficient has a peak value at resonant frequency when submergence depth is sufficiently small.

• Journal of the Society of Naval Architects of Korea
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• v.33 no.1
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• pp.27-39
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• 1996

The effects of free surface and a strut on the lifting characteristics on a fin attached at the mid-chord of the strut are investigated experimentally in a circulating water channel. Variation of lift force on the fin is investigated with respect to free stream velocity(V), angle of attack of the fin(${\alpha}$) and ratio of the submergence depth of the fin to the chord of the fin(H/C). Attentions are focused on the lifting characteristics of the fin at shallow depths of submergence. Visualization of the free surface deformation along the strut and of the streamline about the fin is made in order to examine the inflow angle to the fin. Lift force on the fin alone i.e. in absence of the strut is also measured to investigate the difference in lifting characteristics of the fin caused by the strut. The results show that lift forces over the fin are largely affected by a free surface in the case of small submergence ratios(HiC<3.0). For HiC>5.0, the effects of the free surface are negligibly small. The inflow angle to the fin is significantly influenced by the strut and flow speed at the shallow depths of submergence. The deformation of the free surface is largely governed by the waves generated by the strut. However, for small submergence depths, the effects of the fin are found also significant.

• Journal of Ship and Ocean Technology
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• v.4 no.1
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• pp.11-20
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• 2000

Reynolds-averaged Navier-Stokes equations are numerically solved using a secondorder finite difference method for the analysis of turbulent flows around single and tandem hydrofoils advancing under the free surface. The location of the free surface, not known a priori, is computed from the kinematic free surface condition and the computational grid is conformed at each iteration to the free surface deformation. The eddy viscosity model of Baldwin-Lomax is employed for the turbulence closure. The method is validated through the comparision of the numerical results with the experimental data for a single hydrofoil of a Joukowski foil section. A computational study is also carried out to investigate the effect of the submergence depth and the Froude number on the lift and the drag of the hydrofoil. For tandem hydrofoils, computations are performed for several separation distances between the forward and aft foils to see the interference effect. The result shows clearly how the lift and drag change with the separation distance.

• Journal of the Society of Naval Architects of Korea
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• v.34 no.1
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• pp.50-59
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• 1997

Shipbuilder checks the status of main engine and propeller operation before sea trial. Generally these tests are carried out at a quay during fitting out of the ship. For these tests the operator has to estimate the maximum RPM with permissible torque and thrust to ensure the safety of the mooring line and ship. In this paper, the propeller characteristics according to the draft variation for a moored ship is inveatigated. From these tests, it is shown that shaft submergence is a dominant parameter in the propeller performance at shallow shaft submergence and that the propeller performance is dependent upon the propeller RPM when the shaft submergence is kept unchanged. In this study, a simple formula of the required thrust for a given propeller shaft submergence and propeller RPM is derived. 1be propeller thrust, which is calculated by another formula in case of dtep draft, is compared with results of bollard pull test for FPSO.

• Journal of the Society of Naval Architects of Korea
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• v.51 no.5
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• pp.435-449
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• 2014

The effects of the gravity field and the free surface on the cavity shape and the drag are investigated through a numerical analysis for the steady supercavitating flow past a simple two-dimensional body underneath the free surface. The continuity and the RANS equations are numerically solved for an incompressible fluid using a $k-{\epsilon}$ turbulence model and a mixture fluid model has been applied for calculating the multiphase flow of air, water and vapor using the method of volume of fluid and the Schnerr-Sauer cavitation model. Numerical solutions have been obtained for the supercavitating flow about a two-dimensional $30^{\circ}$ wedge in wide range of depths of submergence and inflow velocities. The results are presented for the cavity shape, especially the length and the width, and the drag of the wedge in comparison with those of the case for the infinite fluid flow neglecting the gravity and the free surface. The influences of the gravity field and the free surface on the aforementioned quantities are discussed. The length and the width of the supercavity are reduced and the centerline of the cavity rises toward the free surface due to the effects of the gravity field and the free surface. The drag coefficient of the wedge, however, is about the same except for shallow depths of submergence. As the supercavitating wedge is approaching very close to the free surface, it is found the length and the width of a cavity are shorten even though the cavitation number is reduced. Also the present result suggests that, under the influence of the gravity field and the free surface, the length of the supercavity for a certain cavitation number varies and moreover is proportional to the inverse of the submergence depth Froude number.

• Journal of the Society of Naval Architects of Korea
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• v.30 no.2
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• pp.112-122
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• 1993

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 it when deeply submerged, while the lift reduces at all submergence depth.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.5
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• pp.484-491
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• 2007

Numerical experiments using a numerical wave tank have been performed to verier the nonlinear interaction between monochromatic waves and a submerged horizontal plate. As a model for numerical wave tank, we used a higher-order Boundary Element Method(BEM) based on fully nonlinear potential flow theory and CADMAS-SURF for solving Navier Stokes equations and exact free surface conditions. Both nonlinear models are able to predict the higher harmonic generation in the shallow water region over a submerged horizontal plate. CADMAS-SURF, which involves the viscous effect, can evaluate the higher harmonic generation by flow separation and vortices at the each ends of plate. The comparison of reflection and transmission coefficients with experimental results(Patarapanich and Cheong, 1989) at different lengths and submergence depths of a horizontal plate are presented with a good agreement. It is found that the transfer of energy from the incident fundamental waves to higher harmonics becomes larger as the submergence depth ratio decreases and the length ratio increases.

• Journal of Hydrospace Technology
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• v.2 no.1
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• pp.1-9
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• 1996

We consider the following two questions mainly in this study. First one is how the free surface hayes affect the lift of a shallowly submerged moving body. For this matte., we reinterpret the theoretical results of Kochin(1936), and point out that the high Froude number approximation is not always on the safer side. Second one is what sort of dimensionless parameters determine the occurrence of wave breaking behind a moving submerged body. Temporarily before getting a better answer, we propose that the two-parameter-plane, namely, the plane of the Froude number and the square root of the ratio of the submerged depth and the body length, may be used for predicting the possibility of wave breaking behind the submerged body. A region in the parameter plane is put forth as that of wave breaking, and the validity of this proposal is shown by its agreement with the existing experimental data of Parkin et al(1955) and those of Duncan(1983). Finally, linear and nonlinear numerical results are compared with the existing experimental data to see in what range of the parameters the linear and nonlinear theory case predict the wave field and the pressure on the body with reasonable accuracy. However, since the experimental data, which offer both the pressure and wave elevation for a submerged moving body, are very scarce, much cannot be attained through this comparative study. Hence, it is strongly recommended to carry out well planned experiments to get such data.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.2 s.140
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• pp.80-87
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• 2005

A set of experiments was carried out for obtaining the velocity field around the hydrofoil of finite span, using a wing of the NACA 0012 section in a circulating water channel. DPIV technique was used to measure the velocity field, and the velocity measurements along the span were done for 3 speeds, 3 submerged depths, and 4 angles of attack. Experimental data are compared with the theoretical assumptions, as well as the numerical findings by Lee and Lee(2004). Special care is given to the flow near the tips and in the region close to the leading edge. Though indirect, using the measured data of the velocity, it is now possible to compare the aerodynamic and the hydrodynamic strength of the circulation distribution of a wing in the framework of the lifting-line theory.