Advancements in free surface RANSE simulations for sailing yacht applications

• We traced occurrence numerical ventilation to implications of convection scheme.
• From there we derived a remedy to circumvent the problem.
• Successfully tested remedy using an academical test case.
• Applied results to numerical towing tank testing of ACCV5 yachts.
• Formal verification and validation of results against towing tank data.

The analysis of yacht hulls performance using RANSE based free surface simulations has become an accepted approach over the last decade. Access to this technology has been eased by the development of user-friendly software and by the increase of computational power. Results are widely accepted as superior to previous non-viscous approaches and have to compete with towing tank results in terms of accuracy. However, many practical applications suffer from a numerical smearing of the free surface interface between air and water which can be described as numerical ventilation. This problem occurs when the intersection between bow and calm water surface forms an acute angle and is further pronounced if the stem is rounded or blunt. It is therefore especially linked to sailing yacht applications. The problem manifests itself as a non-physical suction of the air–water mixture under the yacht hull, causing a significant under-prediction of viscous resistance. While this is the easily observable appearance of the problem, a second issue is its effect on wave resistance. It can be shown that wave damping is significantly increased, causing a prediction of wave resistance which is also too low. The paper provides a review of the Volume-of-Fluid method. It discusses the resultant implications for practical applications. A remedy to circumvent the problem is described and its impact on the accuracy of the result is shown. Simulations on an identical appended hull with and without interface smearing are compared. Effects on free surface visualization and numerical accuracy are shown. The paper finishes with a thorough verification and validation of a fully appended yacht in accordance with ITTC standards.