by Louis Vittoz, R&D Engineer

 

Vendée-Arctique has just finished with an exciting final leg and the victory of Jérémie Beyou on Charal. It has been an undoubtedly foretaste of the incoming sailing races. 2020 and 2021 are going to be awesome years with two others major sailing events, the Vendée Globe (departure expected in November 2020, from Les Sables d’Olonne, France) and the 36th America’s Cup (in March 2021, in Auckland, New Zealand), if the coronavirus outbreak does not disturb the schedule. These sailing races are totally distinct: a solo race around the world for the Vendée Globle, and match races between two crews for the America’s Cup. But both races are definitely technological challenges. The best illustration of this is undoubtedly the widespread use of hydrofoils that allow sailing boats to fly over the sea surface and to exceed the wind speed.

Although the concept of the foil is pretty old, the first newsworthy demonstration of its supremacy occurred during the 34th America’s Cup in 2012. This event represented a significant turning point in the use of hydrofoils with the AC72 class that proved the full potential of these appendages.

Team New Zealand AC72, San Francisco Bay

Figure 1: Team New Zealand AC72, San Francisco Bay (by DutchTreat, Some rights reserved)

After that, oceanic races implemented this technology in their own classes such as the IMOCA 60. During the last edition of the Vendée Globe 2016/2017, the first four yachts crossing the finish line were foiler boats.

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Figure 2: Foil of Charal (photo by Abujoy, took before the start of Route du Rhum in 2018. Somes rights reserved)

Such a disruption from Archimedean boats to flying boats has been accelerated by the takeoff of Computational Fluid Dynamics (CFD). The short period of time between two editions of the same sailing event does not allow innovations only based on full-scale trials. Model-scale tests can improve designs but are still too long while the design window for an entire yacht is less than half a year. In contrast CFD gives the opportunity to test hundreds or thousands designs at the same time, mastering external parameters and uncertainty with a high level of confidence and accuracy. An optimization impacting the performance of the sailing yacht can be tested in few hours, leveraging thousands of cores to run the computations, and a decision can be made in a short time. And in a race with tiny time advantages between competitors, even a minor optimization can make the difference (fun fact, during the last Vendée Globe, the arrival time difference between the fourth and sixth skippers was only 3 hours after 80 days of sailing!). Concerning the foils, CFD helps better understand how the yacht is stable, give the hydrodynamic loads on the appendages (for the fatigue design) and so on. Besides, knowing that the price of a single appendage can easily exceed $150k helps realize the performance requirements expected by the design team.

As a complex fluid-structure interaction (FSI) problem involving the free-surface, the comprehension of all the phenomena surrounding the mechanics of the foil is far from being exhaustive. Among others, one hot topic deals with the ventilation phenomenon. Roughly speaking, the ventilation affects appendages of propellers that are close to the free surface or that are piercing it. It might happen when the dynamic pressure on the extrados of the foil is lower that the atmospheric pressure, so that air can enter. Since the density of air and water differ by a factor of 1000, a significant drop in the lift force can occur, decreasing the stability of the yacht. The understanding and the identification of the risk of the ventilation is essential to ensure a good flight stability.

From a numerical point of view, the reproduction of the ventilation is still difficult as it requires very fine mesh and accurate solver to catch the interface between air and water. A solver addressing these issues is not straightforward to develop and at Nextflow, we are convinced that some key features such as the adaptive mesh refinement (AMR), Cartesian grid, and immersed boundary (IB) methods can contribute to better understand the physics related to some specific marine applications like the hydrofoil.

We have recently achieved a new step in the development of such a solver, coupling a wall function of the Spalart-Allmaras model with an immersed boundary method.

Watch here the vorticity field on the free surface plane (monophasic simulation, double model assumption, 5M Reynolds number, Spalart-Allmaras turbulence model, adaptive mesh refinement based on the vorticity criterion).

Waiting for the Vendée Globe on November, we congratulate all skippers finishing the Vendée-Arctique these days, an undoubtedly foretaste of the incoming race.

Thanks to VPLP for providing the hydrofoil geometry and to BVSMO for helping us in setting-up simulations with representative operating conditions.