Free Surface Calculations - Evaluation of boat performance in the Numerical towing tank
Hydrodynamic properties of sailing yachts are traditionally evaluated by conventional towing tank testing. However, progress has been made recently towards realizing a reliable numerical towing tank by using RANSE methods with free surface capabilities. Solving the RANS equation is a numerical method capable of taking into account effects like viscosity, turbulence and flow separation.
In presence of a free surface an additional transport equation has to be introduced to capture the free surface interface and its deformation due to the investigated body’s wave pattern. Within the method used the free surface is calculated by using an interface-capturing scheme based on a Volume-of-Fluid (VOF) approach. One of the main problems in numerical simulation of free-floating bodies is that these bodies have to be allowed to trim sink dynamically to get reasonable results, whilst RANSE solvers usually use a rigid grid.
To cope with this problem, YRU-Kiel has developed a body motion module which allows to simulate the sailing yacht in free-floating condition taking into account any combination of hull weight and sail forces and balancing them with the flow forces resulting from the simulation.
The translation and rotation of the body is determined by integrating the equation of linear and angular momentum with an first order Euler approach which has been modified for numerical stability. The resulting displacement and rotation angle are returned to the RANSE code to deform the computational grid. The flow forces are then updated and the resulting forces again used to solve the rigid body equation. This iterative procedure is carried out until convergence is achieved.
Generally said this method generates the same type of results as can be derived from towing tank testing. Accuracy of results achieved from towing tank and RANSE simulation is on equal level. One of the main advantages of the RANSE-simulation is that it can be conducted in full scale.
This way the main disadvantage of towing tank testing is avoided: RANSE simulation can satisfy Reynolds' as well as Froude's rules of similarity simultaneously, relieving the results of any uncertainties arising from the model to full scale transformation. A further advantage is that RANSE simulation allows to use exact dyamic sailing forces and moments whilst in the towing tank this values can only be estimated. This further enhances the accuracy of the simulation.
RANSE simulations as described above are capable to supplement if not substitute towing tank testing. It’s numerous advantages recommend itself for extensive hull development programs carried out with professional sail sport as AC or VOR, but due to the steep increase in available computational power this tool is also of interest for projects with smaller budgets like Olympic, TP52 or Open 60 or similar campaigns.