Upwind sail aerodynamics: A RANS numerical investigation validated with wind tunnel pressure measurements

Abstract

The aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using Computational Fluid Dynamics. A Reynolds-averaged Navier-Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where the pressure distributions on the sails were measured. An original approach was employed by using two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered.

Experimental analysis of a strong fluid–structure interaction on a soft membrane—Application to the flapping of a yacht downwind sail

“…Wind tunnel is commonly used to control the aerodynamic and motion environment of yacht. It has been subject to a large number of experimental studies to measure both the aerodynamic loads on the boat frame using a balance (Flay, 1996;Viola and Flay, 2009;Fossati and Muggiasca, 2011;Campbell, 2014) and the pressure and 3D shape deformation of the structure (Graf and Müller, 2009;Viola and Flay, 2011;Viola et al, 2013). Recent studies have highlighted the non-trivial effect of dynamic oscillation on the aerodynamic forces (Gerhardt et al, 2011;Fossati and Muggiasca, 2011;Augier et al, 2013Augier et al, , 2014a.…”

Inviscid approach for upwind sails aerodynamics. How far can we go?

“…Nevertheless, the inviscid approach is limited to cases where the flow can be considered fully attached, a consequential hypothesis that needs to be checked. In upwind sailing with flat and twisted sails such as the presented backstay ON case, this is mostly the case, but it is known that there are situations where flow separation is significant even in upwind sailing, particularly in light wind conditions when the sails are fuller and not twisted much (Sacher et al, 2015;Viola et al, 2013). The sails top sections are particularly prone to flow separation.…”

“…It is now well known that flow separation may be nonnegligible also in upwind sailing (e.g. Fluck et al, 2010;Fossati and Muggiasca, 2012;Viola et al, 2013). Some studies have already compared the inviscid flow FSI model ARAVANTI to the viscous flow FSI model ARA-ISIS, coupling the same structural code ARA to the RANS solver ISIS-CFD (commercialized as FINE™/Marine), and have highlighted situations where the potential flow approach is incorrect (Sacher et al, 2015).…”

A numerical method for the design of ships with wind-assisted propulsion

“…10 shows the lift and drag coefficients based on the sum of the two chord lengths for different AWA w and φ w ¼ 51. The error bars show the numerical uncertainty at 95% confidence level due to the grid and time resolution that was estimated using the guidelines for uncertainty quantification in Viola et al (2013).…”