Boundary Element Method For Potential Flows Research Articles

Parametric Hull Form Optimization of Containerships for Minimum Resistance in Calm Water and in Waves

This paper described the process of generating the optimal parametric hull shape with a fully parametric modeling method for three containerships of different sizes. The newly created parametric ship hull was applied to another ship with a similar shape, which greatly saved time cost. A process of selecting design variables was developed, and during this process, the influence of these variables on calm water resistance was analyzed. After we obtained the optimal hulls, the wave added resistance and motions of original hulls and optimal hulls in regular head waves were analyzed and compared with experimental results. Computations of the flow around the hulls were obtained from a validated nonlinear potential flow boundary element method. Using the multi-objective optimization algorithm, surrogate-based global optimization (SBGO) reduced the computational effort. Compared with the original hull, wave resistance of the optimal hulls was significantly reduced for the two larger ships at Froude numbers corresponding to their design speeds. Optimizing the hull of the containerships slightly reduced their wave added resistance and total resistance in regular head waves, while optimization of their hulls hardly affected wave-induced motions.

Added resistance, heave and pitch for the KVLCC2 tanker using a fully nonlinear unsteady potential flow boundary element method

In this paper, a fully nonlinear unsteady potential flow method is used to predict added resistance, heave and pitch for the KVLCC2 hull in regular head waves at design speed. The method presents a nonlinear decomposition of the velocity potential and the wave field and an adaptive grid refinement. A formulation for the acceleration potential is used to obtain the pressure. To improve computational efficiency, a Barnes-Hut algorithm is introduced. A grid dependency study and a study on the impact of different time steps on the solution are performed. Numerical results have been compared with experimental data for the design speed. A general good agreement is found for added resistance, especially for longer waves. Heave and pitch are properly computed for all wave lengths in the range λ/Lpp=0.4 to 1.4.

The steering and course keeping qualities of high-speed craft and the inception of dynamic instabilities in the following sea

Small high-speed craft are the most vulnerable to the severity of the sea: achieving a design which pairs good performance and acceptable levels of safety is not a trivial task. The seakeeping and manoeuvrability of these vessels play a crucial role in following sea conditions: dynamic instabilities, namely broaching-to and surf-riding, are more than a rare eventuality and threaten the survivability of the vessel and the life of the mariners. This study investigates the effects of the steering qualities on the broaching-to behaviour of a high-speed craft when it is sailing in following and stern-quartering waves.The motions and loads of the vessel are simulated by means of a 3D time domain blended potential flow boundary element method (BEM), validated using captive model tests in regular waves carried out at the Seakeeping and Manoeuvring Basin (SMB) of MARIN. The hull directional stability and turning ability of the high-speed craft were artificially modified, showing that an increase in the directional stability as well as in the effectiveness of the steering can be beneficial to avoid the inception of broaching-to, but they have different consequences on the dynamics of the vessel's loss of control.

Hydrodynamic optimisation of a multi-purpose wind offshore supply vessel

ABSTRACTThe aim of this study was to reduce the total resistance of a multi-purpose wind offshore supply vessel by optimising its hull. Resistance was computed using a potential flow boundary element method and a Reynolds-averaged Navier–Stokes equations solver. Optimised hull forms were obtained for the ship advancing at different ship speeds under calm water conditions, employing the two multi-objective optimisation algorithms, Non-dominated Sorting Genetic Algorithm II (NSGA-II) and Multi-Objective Simulated Annealing (MOSA). Using NSGA-II yielded slightly larger reductions of total resistances than MOSA. The greatest reductions were achieved at ship speeds between 11 and 14knots. At these speeds, a thinner and longer bulbous bow reduced resistance. At speeds greater than 15knots, a bloated bulbous bow was more helpful to reduce resistance.

An Improved Numerical Computation of Hull Pressure Fluctuations Due to Unsteady Sheet Cavitation of a Propeller

An operating propeller is the main source of vibration on the ship hull, especially on the stern. In this article, an improved numerical scheme is presented to predict the pressure fluctuations on the ship hull due to the unsteady sheet cavitation of a propeller. The calculated results are compared with the published results and experimental data carried out in the hydrodynamics and cavitation tunnel of Hamburgische Schiffbau-Versuchsanstalt to verify the improvement. The present method is based on a two-cycle iterating scheme which satisfies the boundary integral equation in time domain. The hull pressure fluctuations calculated by the first-cycle iterations are treated as the initial values for the second-cycle iterations. The solid angles of the elements will deviate from the standard value, .5, as the dramatic variation in geometry appears, and accumulate numerical errors in the calculating process. During the second-cycle iterations, a filter based on the solid angles on hull elements is proposed to minimize the iterating error. A container ship is treated as the computing sample in this study, and evidence is offered regarding the 16% improvement achieved by the present method. 1. Introduction A propeller operating in a nonuniform wake field in the ocean is one of the prime sources of vibration and noise affecting the ship. The prediction of hull pressure fluctuations caused by propellers is an important consideration for many vessels and worthy of being researched. Hull pressure fluctuation caused by the marine propeller has been widely investigated since the 1980s. Huse and Guoqiang (1982) developed a semi-empirical prediction method, in which the cavitation volume on the propeller blade surface can be estimated using experimental data. Breslin et al. (1982) proposed a model based on the potential flow theory, which simulates the ship hull by means of a simple panel method and the propeller by means of a lifting surface vortex lattice method. Kinns and Bloor (2004) used an acoustic boundary element (BE) model to simplify the problem of hull vibration excitation due to propeller sources and dipoles. The convergence of results for a cruise liner model was demonstrated with different element distributions. Kehr and Kao (2004) derived the incident blade rate pressure induced by unsteady sheet cavitation (monopole) and unsteady forces (dipole) of an operating propeller, for calculating the hull pressure fluctuations. Brouwer (2005) applied a stationary set of rings of monopole and dipole sources in the frequency domain to solve the propeller-induced noise and vibrations. Lee et al. (2006) integrated computational fluid dynamics (CFD) and the finite difference method for the computation of propeller-induced hull vibration. It was recommended that the phase difference of the propeller-induced pressure should be considered for preventing overprediction. Kao and Kehr (2006) developed a time domain iteration method to calculate the hull pressure fluctuations induced by the operating propeller. The phase difference is included in the iterating scheme, and the computation is robustly convergent. Seol and Moon (2009) derived the governing equation of pressure fluctuation induced by sheet cavitation according to the Ffowcs Williams approach. In the study by van Wijngaarden (2011), a potential flow boundary element method (BEM) with measured hull pressure data as input was applied to solve the hull pressure fluctuations, and the model experiments were also carried out. It was concluded that the computed hull pressure fluctuations due to non-cavitating propellers is reasonably accurate compared with the experimental data. Kehr and Kao (2011) calculated the pressure fluctuations on ship hull due to propeller sheet cavitation by using the method presented in Kao and Kehr (2006). The numerical result is higher than the experimental data provided by the hydrodynamics and cavitation tunnel (HYKAT) of HSVA (Wiemer 1999) by approximately 27%. The pressure fluctuations on ship stern induced by cavitating propellers were solved by Kanemaru and Ando (2011) with a surface panel method. The maximum amplitude appears in the first blade frequency and overestimates the experimental data by about 30%. Kim et al. (2012) predicted the hull pressure fluctuations induced by marine propeller sheet cavitation, and the Doppler effect was considered at the same time. Wei and Wang (2013) employed CFD to simulate the propulsion of a submarine. The finite element method and BEM were then combined to solve the submarine's structure and acoustic responses under the propeller excitations. The matched-field inversion technique applied in Lee et al. (2014) uses the fluctuating hull pressure field measured by receivers and the acoustic field calculated by BEM to define the sheet cavitation noise, source strengths, source positions, and number of sources. The equivalent source model for the propeller can result in more accurately extrapolated hull pressure distribution. In Wei et al. (2016), the unsteady forces of a submarine propeller were predicted by CFD, and the hull pressure fluctuations due to the non-cavitation propeller were calculated by the method proposed in Kao and Kehr (2006).

Hydrodynamic Coefficients for a 3-D Uniform Flexible Barge Using Weakly Compressible Smoothed Particle Hydrodynamics

The numerical modelling of the interactions between water waves and floating structures is significant for different areas of the marine sector, especially seakeeping and prediction of wave-induced loads. Seakeeping analysis involving severe flow fluctuations is still quite challenging even for the conventional RANS method. Particle method has been viewed as alternative for such analysis especially those involving deformable boundary, wave breaking and fluid fragmentation around hull shapes. In this paper, the weakly compressible smoothed particle hydrodynamics (WCSPH), a fully Lagrangian particle method, is applied to simulate the symmetric radiation problem for a stationary barge treated as a flexible body. This is carried out by imposing prescribed forced simple harmonic oscillations in heave, pitch and the two- and three-node distortion modes. The resultant, radiation force predictions, namely added mass and fluid damping coefficients, are compared with results from 3-D potential flow boundary element method and 3-D RANS CFD predictions, in order to verify the adopted modelling techniques for WCSPH. WCSPH were found to be in agreement with most results and could predict the fluid actions equally well in most cases.