Author Walree, F. van
Title Computational Methods For Hydrofoil Craft In Steady And Unsteady Flow
Conference/Journal PhD-thesis Delft University of Technology
Month March
Year 1999

Abstract
The objective of the study is to develop tools for hydrodynamic analysis of hydrofoil craft. Three main subjects in hydrodynamics are addressed: powering, seakeeping and manoeuvring. For the powering problem a steady flow method is developed while for the seakeeping and manoeuvring problem an unsteady flow method is developed.

Despite the fact that a substantial amount of knowledge on the characteristics of lifting surfaces and ship hull forms exists, no calculation method exists for predicting the powering performance of hydrofoil craft for the entire speed range, that has a wide range of applicability and that predicts at the same time the forces acting on the hull and foil system with an acceptable accuracy and computational efficiency. In order to develop a suitable tool the following approach is taken whereby most attention is paid to foil systems.

The lift and drag characteristics of the foil system are derived from a vortex lattice method. A Green's function formulation is used that accounts for linearized free surface conditions at the undisturbed water surface. Interaction between the forward and aft foil and between the foils and the hull is addressed. Hull force components are derived from experimental data on series of hydrofoil craft hull forms. Appendage force components are based on empirical formulations. Propulsor characteristics are based on open water diagrams of propeller series and on an existing calculation method for waterjet systems. The computational method is used within an iterative scheme to determine the equilibrium position of the craft relative to the water surface. An example of the application of the computer program Hydres, containing the steady flow computational method, shows that it's results can be used to minimize the required propulsion power to take-off.

Hydres is validated on basis of model test and full scale data for surface piercing and fully submerged hydrofoil craft. The uncertainty in model test results are assessed and found to be significant, mainly due to viscosity effects on the lift characteristics and foil system geometry errors. Nevertheless, the predictions are found to be in fair agreement with the model test data. The agreement with full scale data is satisfactory as well.

Similar to the steady flow method, no unsteady flow method exists that can be applied to arbitrary foil systems, that takes unsteady flow effects into account properly and that can be used in the time domain in six degrees of freedom. Therefore, such a time domain simulation method is developed for seakeeping and manoeuvring at cruise speed conditions. It is assumed that the hull has no contact with the water surface. The simulation method is based on an unsteady vortex lattice method. A time domain Green's function formulation is used to account for linearized free surface effects at the undisturbed free surface. Unsteady interaction between the forward and aft foil is addressed. A linear simulation method is developed that assumes small motions about a certain equilibrium position, a constant speed and a constant course. A non-linear simulation method that can handle large and transient motions is developed as well.

The unsteady vortex lattice method is validated numerically by means of comparisons with analytical results for basic foils. A validation study into damping and wave excitation forces on a tandem foil system is based on experimental data. A good agreement is found. Examples of the application of the time domain simulation method Hydsim, containing the unsteady vortex lattice method, are given for seakeeping and manoeuvring. These examples show the effects of linearization of the motions, unsteady flow effects and the significance of foil interaction and the wave part of the Green's function. Finally, Hydsim is validated on basis of full scale data for seakeeping and manoeuvring for surface piercing and fully submerged foil hydrofoil craft. A satisfactory agreement between simulation results and the full scale data is found.

The results of the study are thought to be valuable for use for investigations into the powering performance and the comfort and safety of operation of hydrofoil craft. The second part of the work may also serve as a basis for calculation tools for more conventional, fin stabilized craft types and propulsors utilizing hydrofoils.

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