RANSE investigation of sailing yacht appendages

Sailing yacht appendages are of major importance for the performance of a yacht. Consequently great attention is paid to the optimization of these appendages for the expected sailing conditions. Respective investigations are carried out experimentally as well as numerical using suitable CFD-methods. The YRU-Kiel uses RANSE methods for appendage optimization with supporting seakeeping methods for instationary phenomena. Modern RANSE codes provide results that can compete with towing tank and wind tunnel test results in terms of accuracy while easily outperforming experimental investigations when it comes to costs and turn-around times. A common approach for evaluation of an appendage design is to import RANSE results into a Velocity Prediction Program (VPP). Thus the properties of the hull and the sails as well as the hydrostatic properties of an appendage set are considered to evaluate the merits of its design. YRU-Kiel mainly uses the following approaches to optimise appendages: 1. Appendage set investigated in Wind tunnel setup in fully turbulent flow condition  To integrate RANSE results in subsequent VPP analysis, a suitable combination of flow velocities, heel-, yaw- and pitch angles as well as trim tab angles if applicable is investigated.  Within this method, the appendage set is investigated in a so called wind tunnel setup with a flat bottom at the fin-hull junction. The free surface effects of the canoe body are neglected and the appendage set is considered deeply immersed, which is an appropriate assumption for modern keel and bulb designs. A practical approach to appendage optimization is to systematically vary main dimensions and geometric parameters of an appendage set. The result of the entire set of RANSE runs is assembled in diagrams allowing to derive the hydrodynamic coefficients to be used by consecutive VPP analysis. While a speed sweep shows the viscous resistance as a function of velocity, the effective draft can be derived from a yaw angle sweep. Usually resistance deltas and deltas of the effective draft are calculated with respect to a benchmark. Together with hydrostatic information of the appendage set and additional data of the hydrodynamic and aerodynamic properties of the yacht and the sails, the VPP converts flow force deltas into VMG deltas.

A restriction of this method is the assumption of fully turbulent flow, which does not match reality. As a consequence the method is not suitable for design alternatives, where major variations of the laminar region can be expected.

2. Appendage set in Wind tunnel setup taking into account laminar turbulent transition
Recent developments in the modelling of turbulence allow to predict laminar to turbulent transition. A method available within the ANSYS CFX 11 flow code has been adapted, validated and successfully applied to high performance yacht appendages.

The method is based on a correlation between the empirically estimated transition onset momentum thickness Reynolds number and a vorticity based Reynolds number calculated using local variables available in a RANSE code. This is introduced into a transport equation of an intermittency function responsible for switching on or off the production and dissipation of turbulent kinetic energy.
A typical investigation taking advantage of the prediction of LTT is the determination of the optimum length of a ballast bulb. Here the change of the length of the laminar region at the leading edge of the bulb is of major impact on the change of resistance due to length of the bulb.

3. Appendage set combined with hull in towing tank setup  including free surface effects
The previously described methods neglect the influence of the canoe body and the wave generation due to a free water surface. In many cases inclusion of these phenomena is necessary, for example to study the impact of appendages on wave drag, the impact of wave pattern on the flow around appendages, the interaction of appendages and canoe body, etc..

Our approach to free surface simulations allows to take into account dynamic sinkage and trim of the yacht due to hydrodynamic and external forces. This is done by incorporating a solution method for the equation of motion in 2 DOF, analogous to a typical towing tank setup. YRU-Kiel has written a dedicated Technical White Paper on the subject of free surface simulations, see YRU-TWP: RANSE Investigations of fully appended yacht hulls including Free Surface and Dynamic Trim and Sinkage

4. An advanced strip method to determine the impact of the appendages on seakeeping behaviour and added resistance in sea ways
The performance of a sailing yacht in natural sea ways is strongly influenced by the motion of the yacht in waves, being in turn influenced by the appendage configurations. While generally possible to do seakeeping investigations with RANSE methods, the well known strip method offers a far more efficient approach to the investigation of seakeeping.

YRU-Kiel uses a proprietary strip method with enhancements to analyse fully appended yachts in sea ways. The main advantage of this method is, that it allows to predict the impact of appendages on motion, which is predicted in 6 DOF.
The standard approach of the strip method is based on a potential flow panel code calculation of the added mass and damping of sectional strips of the hull.
In addition the appendages are taken into account as lifting surfaces, which encounter a time dependent flow field as generated by the motion of the yacht and the velocity field of the incident waves.

A typical example, where the above method has been used successfully is the investigation of bulb length on yacht performance. It has been shown that the longitudinal buoyancy distribution as well as the change of gyradii due to a prolongation of the bulb has a major impact on the motion of the yacht and consequently on the performance. A recent enhancement of the method allows to separate the time averaged flow forces acting on foils from the total forces.

Conclusion
These approaches may be used as standalone methods or combined to gain the best result. All methods have in common that they can used very systematically and with full repeatability to determine the effect of even minor changes on overall boat performance.
RANSE simulations as described above are capable to supplement if not substitute experimental investigations. Its numerous advantages recommend itself for extensive appendage development programs carried out for professional sail sport campaigns as AC or VOR, but due the steep increase in available computational power this tool is also interesting for projects with smaller budgets like Olympic, TP52 or Open 60 or similar campaigns.