Optimisation of planing hulls (short: OptiGleit)
This project aims at developing tools which can be used to optimise the hydrodynamics, in particular the wave resistance, of planing vessels. The hull form under investigation is the so-called parametric fast hull (pfh). The pfh has been developed and patented by DW-ShipConsult and is intended for fast vessels and especially for planing vessels. Possible applications include pleasure boats, patrol boats, save-and-rescue vessels and navy vessels. Further information can be found on www.pfh-ships.com.
The tools will be used in the following steps in the ongoing development of the pfh:
- Only 2D-developable surfaces will be used to build the hull form of the pfh in order to reduce production costs. In this process, a CAD tool supplied by FRIENDSHIP SYSTEMS is used. MFH-Emden will build a first 8.5m prototype with 2D-developable surfaces. The hydrodynamic behaviour of the developed hull will be investigated experimentally and the results of the full-scale model tests will be used to validate the applied numerical methods.
- The use of 2D-developable surfaces leads to certain restrictions regarding the hull form. In order to find the best configuration with respect to ship resistance, manoeuvrability and seakeeping, an optimisation of the hull form is carried out. Evolutionary algorithms, embedded in the FRIENDSHIP FRAMEWORK, are used to modify several hull parameters. In optimisation processes it is important to keep the computational effort as low as possible. At the same time, results have to be precise enough to allow for the right design decisions to be made. Therefore, a potential flow method using the 2D+t theory is developed. The experiments and viscous flow methods are used for validation.
Viscous flow methods (RANS)
To achieve high quality results, viscous flow methods are used to calculate the wave pattern, the pressure and friction resistance, and the dynamic heave and trim of the planing hull. The software used is STAR-CCM+ from CD-adapco. Unstructured meshes with prismatic layers at body boundaries are implemented and combined with local refinements at the body boundaries and the free surface. The implicit unsteady solver and k-epsilon turbulence model is applied. Because the dynamic floating position is highly sensitive due to discretisation errors, dependency studies regarding to mesh size, solver settings and time discretisation are carried out with respect to drag, pressure distribution, volume fraction under the boat and wave pattern resolution.
Potential flow methods (2D+t)
The 2D+t theory is able to predict the free surface flow around a ship, including nonlinear effects such as splash, which are of importance when dealing with planing vessels. Its simplifications come from treating the actual three-dimensional problem of the flow around the planing vessel as a set of two-dimensional problems. Therefore, transverse waves cannot be accounted for. This is acceptable at higher speeds as transverse waves become negligible in this velocity range. Viscous effects have little influence on the actual wave propagation and are basically restricted to the boundary layer near the hull, so potential flow theory, i.e. a panel method, can be used to predict the flow around the two-dimensional sections. This simplifies the grid generation and discretisation of the hull form and the free surface, and reduces the computational effort dramatically. As a result, the non-linear free surface flow behaviour at higher Froude numbers around the planing vessel can be predicted with low computational effort.