The 12th International Conference on Hydrodynamics
18 – 23 september 2016, Egmond aan Zee, The Netherlands
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AN ITERATIVELY COUPLED SOLUTION METHOD FOR UNSTEADY SHEET CAVITATION PREDICTION ON MARINE PROPELLERS USING BEM


Go-down ichd2016 Tracking Number 147

Presentation:
Session: Ship hydromechanics resistance V
Room: Room 1
Session start: 14:00 Tue 20 Sep 2016

Joao Baltazar   joao.baltazar@tecnico.ulisboa.pt
Affifliation:

Falcao Campos   falcao.campos@tecnico.ulisboa.pt
Affifliation:


Topics: - Ship hydrodynamics resistance, propulsion, powering, seakeeping, manoeuvrability, slamming, sloshing, impact, green water, - Cavitation and cavitating flows

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.