Toward the development of a hydrofoil tailored to passively reduce its lift response to fluid load

Abstract

The objective of this research is to explore the possibility of using Passive Adaptive Composite (PAC) on structures to help control the lift generated by hydrofoils on boats such as the International Moth. Introducing composite fibres oriented at off-principal axis angles, allow a foil to passively control its pitch angle to reduce the lift generated at higher boat speeds helping to achieve a stable flight in a wide range of weather conditions. PAC utilises the inherent flexibility of a composite structure to induce a twist response under bending load which could be used to minimise the use of active control systems, or even improve the dynamic response of foils in waves. However, to design flexible foils requires numerical and experimental tools to assess the complex fluid structure interactions involved. This paper evaluates a simplified hydrofoil geometry designed to reduce its lift coefficient with increased flow speed. A coupled Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) methodology is presented to predict flexible foil performance. Validation of these numerical tools is achieved through the use of wind tunnel experiments including full field deformation measurements. Twist deformations resulted in a reduction in the effective angle of attack by approximately 30% at higher flow speeds reducing the foil lift and drag significantly.

Introduction

The International Moth is a single-handed ultra-lightweight foiling development class boat (International Moth Class, 2013). When foiling the mass of the boat and helm remains constant but the lift produced by the daggerboard and rudder T-foils increases with the square of the boat speed. The daggerboard T-foil is formed of two elements with an adjustable rear flap to modify the lift produced based on the vessel ride height. This constant movement of the rear foil changes the viscous drag of the whole foil section. Meanwhile, the angle of attack of the forward main element is slightly adjusted with dynamic body movements of the helm. The importance of being a lightweight sailing boat enhanced the use of composite materials. Using composite materials it is possible to design a structure tailored to a certain load, in this case the mass of the boat plus the crew. Introducing plies oriented at angles different than zero, 90 or 45° that are normally used in quasi-isotropic structures, it is possible to change the response of a composite structure under load.

Moreover, with the recent increase of foiling boats there is still a lack of accurate measures of structural response and shape of the hydrofoils. This gives rise to scientific questions on whether there is a manufacturing consistency between the port and starboard foil on a catamaran or different batches of foils on mono-hulls.

The aim of the current research is therefore to develop numerical and experimental tools capable of accurately describing the structural response of a foil under fluid-load and to design and develop a foil structure tailored to decrease its lift coefficient as the flow speed increases. In order to do so a coupled CFD and FEA methodology is developed and validated using full-field measurement techniques within a wind tunnel environment.

Those techniques are described in section 2 together with the advantages of developing a robust and repeatible Fluid Structure Interaction (FSI) experimental methodology. The FSI experimental technique was initially developed at the University of Southampton (Banks et al., 2015; Marimon Giovannetti et al., 2017; Marimon Giovannetti, 2017) and provides not only a measure of synchronised structural deformation and fluid response but also the uncertainty values associated with coupling the two optical systems.

Moreover, in section 3 two techniques that can be used to change the performance profile of an hydrofoil are described, namely Passive Adaptive Composites (PAC) that tailors the response of a structure by changing the orientation of the composite plies (Veers et al., 1998) and Differential Stiffness Bend-Twist (DSBT) that utilises the internal stiffness of a structure to change the aero-hydrodynamic response to fluid load (Raither et al., 2012). These two techniques, if used in parallel, can substantially change the effective angle of attack of a foil structure under load. The current research presents the first steps in investigating the possibilities of applying those techniques to the design of a daggerboard T-foil of the International Moth. PAC have been researched in applications for wind-turbine blades (De Goeij et al., 1999; Lin and Lai, 2010; Maheri and Isikveren, 2009), tidal turbines (Nicholls-Lee et al., 2009; Barber, 2017), propeller blades (Khan et al., 2000; Young and Motley, 2011) and Micro Air Vehicles (Hu et al., 2008; Tamai et al., 2008), so it is now possible to use the knowledge from other applications for high performance sailing boats.

After the two background sections, an idealised section that can be adapted in future research to high speeds boats such as the International Moth is presented. Utilising the inherent flexibility of PAC at high speeds and relatively large tip deflections the angle of attack can be passively reduced to decrease the induced drag. Twisting an aerofoil section toward feather indeed reduces the effective angle of attack of the foil. The equations of an analytical model that relates the lift force to the plies orientation within the structure are also described.

The design of the flexible aerofoil is presented in section 5. The position and layup of the internal stiffener are presented as well as the manufacturing techniques.

Finally, the full-scale experimental and numerical setup as well as the results from an idealised section are presented to demonstrate the passive-adaptive response to fluid load. This research merely represents the findings on FSI of a full-scale flexible model. Those results can be used in future projects as base to build a main element foil of the International Moth.