An Integral Boundary Element Method to Predict Hydrodynamic Characteristics of Cavitating Marine Propellers
by
Stefano Brizzolara
DINAV - University of Genova
Coauthors: Stefano Gaggero

A method for the solution of the potential flow around a marine propellers subject to sheet cavitation developed by the Marine CFD group of the University of Genova are described in its main theoretical and numerical aspects in the paper.

The mathematical problem is defined by the Laplace equation for the perturbation velocity potential around the hub, propeller surfaces and the boundaries of the attached sheet cavities, which can arise on propeller blades in particular working conditions. Equations at the boundaries are expressed through a mixed Dirichlet / Neumann condition: in fact, the potential is known on the growing part of the sheet cavity, while the gradient of the potential is known on the ‘wetted’ portion of the blade. The problem is closed by the Kutta condition to be enforced at the trailing edge of the propeller blades for which a vortical sheet trailed by each blade is introduced in the propeller wake.

Since the three dimensional geometries of the attached cavities (extension and thickness on each blade), as well as that of the trailed vortical wake, is not know ‘a priori’, the problem becomes also a geometrically non-linear problem and requires an iterative solution for these unknown surfaces.

The numerical solution of the problem is based on the integral boundary element methods, for which the propeller, hub and vortical wake surfaces are discretized into a large number of quadrilateral panels each one having a constant distribution of sources and doublets.

The method requires a particular description of the numerical derivatives of the unknown potential over the curvilinear coordinates of the surfaces at the boundary (propeller and cavities) as well as the iterative algorithms to search for the three dimensional shapes of the wake and the cavities.

Mathematical details of the method are outlined in the paper.

Finally, the validation of the method, as applied to a typical modern propeller geometry, is presented in the paper. The hydrodynamic characteristics of this propeller were measured at the cavitation tunnel of the Department of Naval Architecture of the University of Genova, together with the observation of its cavitation patterns during an extensive experimental campaign. So, experimental propeller thrust, torque and sheet cavity extension are compared with the corresponding numerical predictions, at different advance coefficients and cavitation numbers. Good correlations are in general achieved for what regards not only cavitation patterns, but also thrust and torque breakdown due to the cavity inception and development on propeller blades.

Date received: March 24, 2008

Copyright © 2008 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cawz-76.