14:00
Ship hydromechanics resistance V
Chair: Takuya Taniguchi
14:00
30 mins
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AN ITERATIVELY COUPLED SOLUTION METHOD FOR UNSTEADY SHEET CAVITATION PREDICTION ON MARINE PROPELLERS USING BEM
Joao Baltazar, Falcao Campos
Abstract: The potential flow solution with a Boundary Element Method (BEM) of the problem of a cavitating propeller operating in a ship wake field still provides a computationally efficient means to assess the extension of sheet cavitation and to base predictions of induced pressure fluctuations on the ship’s hull.
The modelling of sheet cavitation on marine propellers using BEM was first introduced by Fine [1]. The method is based on an integral equation for the velocity perturbation potential The presence of a cavity on the blades is modelled as a free boundary problem. A thin cavity is assumed so that the boundary conditions on the cavity are linearised with respect to the wetted flow. This implies that the dynamic and kinematic boundary conditions are applied on
the foil surface beneath the cavity, On the wetted surfaces only the kinematic boundary condition is applied. Dipoles and sources are placed on the body surfaces either on the wetted part or beneath the cavities. The problem is closed by suitable specification of cavity detachment and closure, and a Kutta condition at the blade trailing edge. The wake surfaces are modelled by dipoles and in the presence of cavities extending into the wake with additional sources.
The solution of the problem for a given cavitation number is to iterate on the cavity length. However, for each iteration step on the cavity extension the method solves a complete system of equations for the unknown potentials on the wetted panels of the blade and for the unknown sources on the panels beneath the cavity. This requires the solution of a new system of equations because some of the elements of the system matrix were changed with the modification of the cavity planform. An alternative iterative technique has been proposed [2] to solve the linear system of equations which avoids a new matrix inversion at each iteration step in the prediction of the cavity planform. In this paper the method is tested for the prediction of unsteady cavitation on the MARIN S-Propeller in non-uniform inflow conditions and results are compared with the cavitation observations and previous numerical results [3].
REFERENCES
[1] N.E. Fine, 1992. “Nonlinear Analysis of Cavitating Propellers in Nonuniform Flow”. Ph.D. Thesis, Massachusetts Institute of Technology, USA.
[2] J. Baltazar, J.A.C. Falcão de Campos, 2010. “An Iteratively Coupled Solution of the Cavitating Flow on Marine Propellers Using BEM”. Proceedings of the 9th International Conference on Hydrodynamics, pp. 838-843.
[3] G. Vaz, J. Bosschers. “Modelling Three Dimensional Sheet Cavitation on Marine Propellers Using a Boundary Element Method”. Proceedings of the Sixth International Symposium on Cavitation, Wageningen, The Netherlands, September 2006.
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14:30
30 mins
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APPLICATION OF CFD CALCULATIONS FOR THE IMPROVEMENT OF PLANING CRAFTS MANOEUVRABILITY MATHEMATICAL MODELS
Diego Villa, Michele Viviani, Elena Ferri
Abstract: The high performance requirements for propulsion systems and the increasing attention on manoeuvring features suggest the need for a thorough investigation about planing hull dynamics, already in the preliminary stages of the boat design. Despite during years a rather large number of studies has been carried out on the topic of planing hull dynamics, very limited data may be found in literature about planing hulls manoeuvrability [1][2]. As a matter of fact, most of studies have been concentrated on the topic of resistance prediction and boat powering, with some attention also on dynamic stability and seakeeping.
Considering this, in previous activities a time domain simulator, developed in Matlab-Simulink environment, has been developed at the University of Genoa, at first [3] adopting the 3+3 DOF approach proposed in [1]; in this approach, however, running attitude of the boat is given as a function of the drift angle only, partially preventing the possibility to consider its effect on the manoeuvrability characteristics of the boat. In a successive work [4], therefore, data from [2] have been used in order to extend the model to 4 DOF (adding roll to the usual motions in the horizontal plane) completely interacting with each other and to take into account the effect of trim and rise; results obtained showed a good agreement between numerical calculations and sea trials results.
Despite the promising results, discrepancies still exists, moreover the available database of experimental data in literature [2] is only limited to prismatic hulls, thus not considering the effect of more complex shapes. Having this in mind, in the present work a series of direct numerical calculations adopting a commercial RANS code have been carried out, with the aim of investigating the possibility of obtaining part of the data needed for the model, in order to avoid the necessity of previous experimental data. Results of this series of calculations are presented and discussed, allowing to get an insight into the problem and to track the way for the development of a fully numerical model for planing boats manoeuvrability.
REFERENCES
[1] Katayama T., Kimoto R., Ikeda Y. (2005): “ Effects on running attitudes on manoeuvring hydrodynamics forces for planning hulls”, Int.Conf. on Fast Sea Transp., FAST’2005, June 2005, St. Petersburg, Russia.
[2] Henry C.J. (1975):“Calm Water Equilibrium, Directional Stability and Steady Turning Conditions for Recreational Planning Craft”, Report SIT-DL-75-1851, Stevens Institute of Technology, Davidson Laboratory.
[3] Altosole, M., Figari, M., Viviani, M. (2009): “6 Dof Simulation of Maneuvering and Propulsive Performance of a Waterjet Propelled Mega Yacht”, 10th Int. Conf. on Fast Sea Transp. (FAST 2009), October 5-8, Athens, ISBN: 978-960-254-686-4.
[4] Ircani, A., Martelli, M., Viviani, M., Altosole, M,. Podenzana Bonvino, C., Grassi, D. “Propulsion and steering effects on dynamics of high speed craft”, 10th Symposium on High Speed Marine Vehicles (HSMV 2014), October 15-17, Naples
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15:00
30 mins
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EXPERIMENTAL STUDY ON ROLL MOTION OF DRILLSHIP WITH APPENDAGES
Jaehoon Lee, Yonghwan Kim, Je-Eun Choi, Chuel-Hyun Kim, Young-Bum Lee
Abstract: This paper considers the nonlinear roll characteristics of the ship shaped platform with and without appendages. Due to high nonlinearity in roll motion, experimental method, empirical formula or recently computational fluid dynamics have been adopted rather than potential based method. However, for non-typical hull shape or newly designed appendages, existing empirical formula or numerical methods are not fully validated. In this study, to effectively capture the roll characteristics with or without appendages in zero speed operational condition, experimental approach was applied.
In the towing tank test, roll motion was measured by inertia-gyro fusion sensor with lightly strained soft-spring system both in calm water and regular beam waves. Appendages which can induce nonlinear effect are analysed using various test conditions. Variation of initial roll amplitudes in free decay test in calm water and different wave amplitudes and wave frequencies in regular beam waves were considered to analyze the nonlinear nature of roll damping and restoring moment. Nonlinear characteristics in free roll decay test were examined using two different methods; high-order polynomial damping model proposed by Froude and Hilbert transform method. Hilbert transform has been studied in the signal processing field (Feldman, 1994) and adopted to ship and offshore research field by several authors (Lewandowski, 2011; Kim and Park, 2014). Effect of roll damping in motion responses were also examined by seakeeping test in regular beam waves. From experimental results, linear and nonlinear characteristics and related parameters of roll motion were discussed.
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