Fluid Structure Interaction in High Performance Catamaran C-foils under Load

An experimental technique to accurately quantify the deformation and the bend-twist coupling of high performance composite foils under fluid loading is presented. The experimental results are reproduced in a Computational Fluid Dynamic (CFD) environment to assess the impact of board deflection and changes in pitch angle on vertical force generated in the C-foils while sailing under increased hydrodynamic pressure. A three dimensional Digital Image Correlation (DIC) methodology suitable for use within a wind tunnel is developed. The technique allows for the measurement of full-field deflection during fluid-structure interaction (FSI) experiments. Combined with DIC technique, the C-foil tip vortex is investigated using Particle Image Velocimetry (PIV) to correlate the variation of the vortex position and strength to the deflection of the board. These techniques, combined with CFD investigations allow potential changes in structural behaviour to be assessed with regard to improving the performances of the foils in sailing conditions. Experimental results are presented for a high performance curved foil from a NACRA F20 catamaran tested within the University of Southampton RJ Mitchell wind tunnel. The fluid regime is chosen to have a Reynolds number equivalent to light upwind sailing conditions (Rn=6.66x10: boat speed of 6 knots) with a fifth of the fluid loading experienced in the water. Curved foils provide both a hydrodynamic side-force to counteract the aerodynamic forces of the sails and a vertical lift force to reduce the wetted surface area and hence the resistance. It is therefore necessary to investigate from a sailor point of view the influences of the side force and vertical coefficients that the change in effective angle of attack and of pitch will give to the stability and the performances of the catamaran. 1 PhD Student, University of Southampton, Fluid Structures Interaction Group 2 Research Fellow, University of Southampton, Fluid Structures Interaction Group 3 Associate Professor, University of Southampton, Fluid Structures Interaction Group 4 Professor, University of Southampton, Fluid Structures Interaction Group NOMENCLATURE AoA Angle of Attack [deg] CD Drag Coefficient CFD Computational Fluid Dynamics CL Side Force Coefficient CZ Vertical Coefficient DIC Digital Image Correlation FEA Finite Element Analysis FSI Fluid Structure Interaction FL Lift force in wind tunnel axis [N] Fx Drag force in board axis [N] Fy Sideforce in board axis [N] PIV Particle Image Velocimetry r radius [m] Rn Reynolds number VS Wind speed [m.s ] δ Deflection [mm] θ Twist angle [deg] ν Kinematic viscosity of fluid [m.s] ρ Density of fluid [kg.m] φ Pitch angle [deg] y Stereo angle [deg]