Advances in FloSim Mid-fidelity Simulations for Modern Yacht Performance Prediction and Analysis

Developing an accurate performance prediction for a modern racing yacht requires ever more detailed models of hydrodynamic forces. In many cases this data is presented as a force/moment response surface that is interrogated by the VPP for different yacht states. Properly characterizing these response surfaces can require 100’s or 1000’s of computation results covering both ranges in state variables (speed, heel, yaw) as well as different configuration options (control surface angles, keel cant etc.) For high level programs with suitable budgets the use of high-fidelity free-surface RANS solutions has become the technology of choice, but this requires significant investment in software licensing and computational time and resources that is beyond reach for many design programs. For those programs without access to these resources we propose a more practical approach using FloSim, a computationally efficient boundary element method (BEM) with a coupled boundary layer solver to populate the force response surfaces. These results can then be augmented with more refined RANS simulations at a reduced number of operating conditions that can act as verification points and/or be used in more complex mixed-fidelity response surface fitting methods. In an earlier study, where calculated results were compared against measured data for a racing yacht model, it was demonstrated that FloSim offers a 100-fold speed advantage over RANS in computational turnaround yet provides comparable accuracy. FloSim is an advanced BEM code; it has an unsteady time stepping procedure with coupled integral boundary layer analysis for viscous effects such as skin friction resistance and boundary layer displacement effects. The method offers free convection and rollup of vortex wake elements for non-linear lift characteristic evaluations and includes several modeling techniques for treating “real world” effects, such as flow separation, energy loss in breaking waves and wave generation. The present paper describes more recent developments in FloSim that further expand its scope for treating complex appended-yacht performance analyses problems with improvements in geometry preparation requirements and solution robustness and accuracy even in extreme orientations. The addition of a new free-surface trimming scheme “MCT” is described and demonstrated for dealing with hull-free surface intersections, this deals locally with intersected panels without re-gridding the surfaces. The strategy provides an improved and more robust treatment of free-surface/hull intersection in awkward-topology situations and a much-improved treatment of transom stern flows. With the much wider adoption of hydro-foiling in the premier yacht racing classes several specific features have been added to FloSim to better integrate into the design of hydrofoil configurations. This paper outlines computational improvements in the numerical treatment of the Kutta-Joukowski condition that improve solution times, and verification cases against experimental data for hydrofoils in proximity to a free surface. A new model has been implemented that captures the non-linear CL- characteristics of lifting surfaces and demonstrates improved CLmax prediction capabilities. The paper includes some comparisons of FloSim predictions against experimental data and against OpenFOAM RANS results; these further demonstrate its usefulness as a practical and reasonably accurate tool for rapidly populating force response surfaces.