Rolling is a dangerous motion for navigation, and has been one of the focuses in the investigation of ship hydrodynamics. Large eddy simulation is adopted for the numerical simulation of the viscous ...flow field around the cross section of Series-60 in the condition of forced rolling. A numerical simulation scheme of high precision is established based on the systematic analysis of spatial and temporal discretization. The integral process of the vortex from generation to dissipation is analyzed in detail by the scheme established, it is demonstrated that a vortex falling off from the bilge keel is not separated but connected to the bilge keel until a reverse motion arise. The influence of roll amplitude and period on the vortex structure is investigated, and it is found that the vortex structure is susceptible to roll amplitude rather than period.
•The parameters of spatial and temporal discretization in LES are obtained by the systematical numerical calculation.•The vortex generated at the tip of the bilge keel is the primary contributor of the rolling moment.•The vortex generated is not shedding off until a shear force caused by the inverse motion arise.•The vortex structure is susceptible to roll amplitude rather than period.
On ship roll resonance frequency Wawrzyński, Wojciech; Krata, Przemysław
Ocean engineering,
11/2016, Letnik:
126
Journal Article
Recenzirano
The paper deals with the problem of modeling of rolling motion under a variety of excitation parameters. Special emphasis is put on the analysis and prediction of the frequency of the resonant mode ...of rolling, since it is often an essential issue in terms of motion of a ship related to her safety against capsizing or excessive amplitudes of roll. The research is performed for both free rolling and excited rolling and it is based on the one-degree-of-freedom equation. The conducted simulations were carried out paying special attention to the effects related to non-linear characteristics of stiffness represented by the GZ-curve and also the non-linearity of damping. A group of ships were considered in order to research and define the key factors governing rolling characteristics. The novel method for rolling period prediction was elaborated and tested revealing significant discrepancies between the results of the pending GM-based IMO-recommended method and the results of numerical simulations performed for a wide range of ships operational loading conditions. Since the research shows a drawback of the IMO formula for rolling period prediction a new formula is proposed instead.
•Resonance roll frequency may significantly vary with the amplitude.•Variations of resonance roll frequency depend mainly on the shape of the GZ curve.•Roll damping has limited impact on resonance roll frequency.•The method for a resonance roll period estimation is proposed.•The proposed method takes into account the nonlinearity of the GZ curve.
Recently, the IMO (International Maritime Organization) has reviewed technical issues considering the second generation intact stability criteria in the design stage of ships. In this paper, the ...evaluation procedure for Lv1 (Level 1) and Lv2 (Level 2) was introduced by focusing on the excessive acceleration mode. Based on real ship data, the calculation process has been explained in detail to make it easy to understand. When the Lv1 criteria considering simple hydrostatic calculations are not satisfied, the computational results of the Lv2 criteria based on mathematical modeling and the hydrodynamics are presented. The relatively low ship roll periods and large changes in the hull shape in the vertical direction make the ships potentially vulnerable to excessive acceleration phenomena. Therefore, the minimum value of height KG that satisfies the stability criteria evaluation in consideration of loading conditions for actually navigating of the ship in the sea should be estimated and operated. In particular, roll damping coefficients using the Ikeda’s method, which are essential for Lv2 vulnerability calculation, were obtained and verified by comparing them with other ship results.
Heeling moments can be caused by wind, by the centrifugal force in turning, by crowding of passengers on one side, by towing, or by the tension in the cable that links two vessels during operations ...at sea. Dividing a heeling moment by the displacement force we obtain a heeling arm. Heeling arms intersect the curve of statical stability in two points corresponding to angles of statical stability; only the first one is stable. Certain loads can reduce the stability and endanger the ship; they include laterally displaced loads, hanging loads, free-surfaces of liquids and shifting loads. When a ship is grounded or docked, and the water level descends, there is a critical point beyond which the sip capsizes. A ship with negative metacentric height can find a position of equilibrium if the curve of statical stability lies in its beginning above the tangent in origin. The ship lies then at an angle that can be corrected only by lowering the centre of gravity.
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