About

Resistance

The toolbox implements a state-of-the-art resistance solver, with included equilibrium solver for trim and sinkage. The solver separates the forces on the hull into the following components:

  • Wave-making pressure
  • Local-flow pressure
  • Friction
  • Appendages
  • Air drag

The wave-making component is the most important for (semi-)displacement hull forms, since commercially viable ships sail with speeds merely smaller where the wave-making resistance begins to increase toward its global maximum. Therefore, this area is of interest to the naval architect.

Michell's theory (1898) is often used in the preliminary design of slender ships, and due to its limitations, Holtrop-Mennen's method (1984) is often used for fuller hull forms. Our novel technique of including the displacement thickness of the boundary layer in potential flow theory, called the tangency correction, yields realistic pressure fields. Using the technique, Michell's theory is extended to include viscous effects, making it an universal tool for predicting resistance in the preliminary design stage.

The novel enhancements for the potential flow theory, which include viscosity effects, outperform RANS results and computation time. Averaged separated flows can be solved in milliseconds, as compared to needed LES solvers for separated flows, that require days of computing time.

The image above renders accurate streamlines and pressure field for the subcritical (top) and hypercritical (bottom) flow around a cylinder using the corrected potential–flow theory. The "simple" cylinder test is a stumbling block for original potential flow (d'Alembert's paradox) and for RANS solvers (cannot handle such separated flows). And everything here is computed in a fraction of a second, making it viable tool for preliminary optimisation.

Michell's original theory, named "thin-ship theory" can be applied only to slender hull forms. On the other hand, the introduced enhancements extend the theory to be applicable to almost any displacement and semi-displacement hull form.

Local-flow, without including effects of the free surface, is computed using the double-body technique. The wetted surface is mirrored about the waterplane and the modified potential flow solver computes the pressure. The obtained local-flow and wave-making pressure fields are then superposed.

The friction is estimated using the ITTC-1957 frictional correlation line, including surface roughness effects. Computation of appendages and air-flow forces are currently work in progress.

Catamarans with symmetrical hulls can be simulated, taking into account the hulls interaction.

Local flow around planing hulls is computed by Savitsky and Morabito methods. These solvers are currently in the testing phase, and will be introduced in the near future.

Propulsion

The solver for propellers in open water use regression equations that are created from extensive sets of experiments. Available propeller series are:

  • Wageningen-B
  • Gawn
  • Surface-piercing
  • KCA

The wake coefficient is computed directly using the computed pressure, and consequently, velocity fields. The thrust deduction coefficient is currently evaluated from the regression equation.

References

For references on used methods, and for list of parts that are implemented or in the process of implementation, please check out the roadmap.