Hydrofoils used on many water crafts and boats, operate in close proximity to the free surface. While the behavior of wings in a single-fluid medium is well known, the effects of the free surface on hydrofoils are not precisely known. The high computational cost of CFD simulations with the air-water interface makes them difficult to be used to build a full hydrodynamic surrogate model with many input parameters, as needed for a Velocity Prediction Program (VPP). For a small hydrofoil,it is possible to test the full-scale foil in a towing tank and measure the hydrodynamic forces. This work presents a test campaign of the main lifting wing of the Olympic Class windfoil (IQFoil) hydrofoil in the towing tank of Centrale Nantes. The measurements carried out in different configurations of immersion, heel and trim, allow for exploring the operation of the hydrofoil and the impact of theproximity of the free surface on its performances. It is shown that the lift force and slope with angle of attack are reduced when approaching the free surface and that the variations of drag are dominated by the induced drag. When the wing pierces the free surface, ventilation may occur resulting in a sharp drop in lift. These experimental results can be used as a validation benchmark for numerical simulations and are useful to build the hydrodynamic force model intended to supply a VPP.

As a first step toward a multi-fidelity optimization tool for hydrofoils, the present work assesses the ability of the in-house code PUFFIn to be used as a “low-fidelity” solver within the multi-fidelity framework. The code, based on the Boundary Element Method (BEM) and the potential flow theory, is used to study the performance of a typical windsurf hydrofoil operating near the free surface. The hydrofoil is composed of a front wing and a rear stabilizer in a plane-like configuration. Computations are performed for single body configurations (only one wing) and two-body configurations (front wing and stabilizer). First, three linearized models of the free surface are compared for the single front wing configuration with several values of the Froude number: the symmetry, anti-symmetry and Neumann-Kelvin conditions. The results show that for relatively high Froude number, the anti-symmetry and the Neumann-Kelvin conditions provide very similar forces. Then, the predictions of the BEM solver are compared with “high-fidelity” RANS computations, in terms of pressure drag and lift, pressure distribution on the hydrofoil and free surface elevation. Several Froude numbers and submergence depths are studied. The global lift and drag variations predicted by the BEM with the anti-symmetry and Neumann-Kelvin conditions on the single-body configurations are similar to the RANS predictions. For the two-body configurations, the Neumann-Kelvin condition outperforms the anti-symmetry condition. Based on the BEM/RANS comparison, the potential flow solver reveals to be a relevant tool for multi-fidelity optimization.

Lifting hydrofoils are gaining importance, since they drastically reduce the wetted surface area of a ship, thus decreasing resistance. To attain efficient hydrofoils, the geometries can be obtained from an automated optimisation process. However, hydrofoil simulations are computationally demanding, since fine meshes are needed to accurately capture the pressure field and the boundary layer on the hydrofoil. Simulation-based optimisation can therefore be very expensive. To speed up the fully automated hydrofoil optimisation procedure, we propose a multi-fidelity framework which takes advantage of both an efficient low-fidelity potential flow solver dedicated to hydrofoils and a high-fidelity RANS solver enhanced with adaptive grid refinement and dedicated foil-aligned overset meshes, to attain high accuracy with a limited computational budget. Both solvers are shown to be reliable for automatic simulation, and remarkable correlation between potential-flow and RANS results is obtained. Two different multi-fidelity frameworks are compared for a realistic hydrofoil: only RANS based and potential-RANS based. According to the optimisation results, the drag is able to be reduced by 17% and 8% in these frameworks, within a realistic time frame. Thus, industrial optimisation of hydrofoils appears possible. Finally, critical areas of future improvement regarding the robustness and efficiency of the optimisation procedure are discussed in this study.

_ The use of hydrofoils to enhance ship performance raises the scientific issue of free-surface proximity, which is important to consider during the design stage, to feed velocity prediction programs, for instance. Typically, the flow over a shallowly submerged hydrofoil is characterized by the Froude number, the submergence depth-to-chord ratio, the angle of attack, and geometric parameters of the lifting surface. Among these parameters, the present paper investigates the influence of the wing aspect ratio on the lift and drag coefficients of hydrofoils operating near a free surface. For this purpose, rectangular wings with an H105 profile at 2° angle of attack and aspect ratios ranging from 4 to 20 are systematically analyzed using a 3D boundary element method. The free surface is modeled using a linearized Neumann-Kelvin boundary condition. Chord-based Froude numbers of 0.5, 1.1, and 6.3 are studied. The submersion depth is swept between 0.1 and 30 times the foil chord length. The evolution of the normalized lift and drag coefficients with respect to the foil submersion and the aspect ratio is discussed in detail. Flow velocity is shown to play a significant role in the evolution of the lift and drag coefficients with submersion depth, close to the free surface, for all the aspect ratios. Its influence gets reduced by moving away from the free surface. The critical submersion depth, where the free-surface effects cease, is found to increase with higher flow velocity and aspect ratio. Furthermore, both positive and negative correlations between the force coefficients and the aspect ratio are identified, depending on the operating conditions. It is found that when the proximity to the free surface either enhances or impairs a force coefficient relative to its value in unbounded flow, increasing the aspect ratio amplifies this effect. Overall, this study confirms the effectiveness of steady boundary element methods for simulating the flow around hydrofoil wings in the vicinity of a free surface and contributes to further understanding the influence of geometric parameters on hydrofoil performance. Keywords hydrofoil; free surface; aspect ratio; potential flow; boundary element method (BEM)

Lifting hydrofoils are gaining importance, since they drastically reduce the wetted surface area of a ship hull, thus decreasing resistance.To attain efficient hydrofoils, the geometries can be obtained from an automated optimization process, based on simulations.However, hydrofoil high-fidelity simulations are computationally demanding, since fine meshes are needed to accurately capture the pressure field and the boundary layer on the hydrofoil.Moreover, the immersed depth varies dynamically, which makes the simulation of hydrodynamic forces challenging.Simulation-based optimization can therefore be very expensive.Automated surrogate models, trained by a limited number of simulations, can reduce the required computational demand for the optimization process.Furthermore, if an efficient low-fidelity hydrofoil performance prediction tool is available, using surrogate models in a multi-fidelity framework [2] can provide a further reduction in the total required simulation cost, by combining the accuracy of a few high-fidelity simulations with the adequate exploration capability of a greater number of low-fidelity computations.In this study, we propose a hydrofoil optimization procedure based on two simulation methods, a dedicated hydrofoil potential flow solver [1] for low-fidelity and RANS for both medium-and highfidelity.The RANS solver uses adaptive grid refinement [2] to attain maximum accuracy with the lowest computational budget.Moreover, two distinctive improvements are provided within the surrogate modeling process.The first one aims to increase the accuracy of the uncertainty estimation when very few sample points are available and the second one provides better noise-canceling for the data in the sample points, with an estimation of the uncertainty due to the noise filtering.In this study, the proposed automated multi-fidelity surrogate model procedure will be tested for a parameterized geometric model of a realistic hydrofoil.The influence of the surrogate modeling technique and the effect of different combinations of fidelity levels on the efficiency of the optimization and the performance of the hydrofoil will be investigated.

Foiling yachts and crafts are both very sensitive to the flying height in terms of stability and performance, raising the scientific issue of the influence of the free-surface when the foil is at low submergence. This work presents numerical simulations of a 2D hydrofoil section NACA0012 at 5° angle of attack in the vicinity of the free surface, for different values of the submergence depth, for a chord-based Froude number of 0.571 and a Reynolds number of 159,000. URANS equations are solved with a mixture model to capture the free surface, and using an automatic grid refinement. Verification of the numerical model and validation with data from the literature are presented. Deformation of the free surface and alteration of the hydrodynamic forces compared to the deep immersion case are observed for a submergence depth-to-chord ratio h/c lower than 2. The foil drag increases up to more than three times the infinite-depth value at h/c ≈ 0.5. The lift force slightly increases until h/c around 1, and then decreases sharply. For h/c < 0.5, the pressure field around the foil is totally modified and the lift is swapped to downward. The study highlights the importance of considering the effect of finite submergence to compute foils’ hydrodynamic forces, for example to be used in Velocity Prediction Programs (VPP) of foiling crafts.

. The performance of lifting bodies such as hydrofoils is determined by the flow state and particular attention should be paid to flow separation, as this greatly affects the generated lift and drag. Sailors are used to look at telltales (woolies) to trim their sails or steer their yacht. A French company developed an electronic telltale for sails based on a strain gauge activated by a silicon strand, with the appropriate signal processing to deliver the same information as a classical wool-made telltale, basically attached or separated flow. This new sensor proved useful when woolies are not visible or to deliver a signal to feed a control system, such as the autopilot for example. It was also applied to wind turbines to control the blade pitch. Mer Agit´ee is now developing an equivalent hydrodynamic e-Telltale to be used on hydrofoils and rudders to help trimming and controlling. The present work presents the investigation of a foil section fitted with this new sensor in a water tunnel, combining force and PIV measurements with the sensor signal, on a wide range of angle of attack. Results show that the hydro e-Telltale enables detecting the flow separation and anticipate stall, and possibly allows for detecting the boundary layer transition to turbulence. In many cases of fluid flow over a lifting body, it is interesting to get some real-time feedback from the flow in order to help optimizing performance and controlling the system. The feedback from this new sensor could be used in a closed-loop controlling system, for

The spinnaker is the most powerful and one of the most used sails both in racing and cruising - yet its complex aerodynamics governed by flow separation is still not fully understood. While the flow around a spinnaker is unsteady and highly tridimensional, locally the governing fluid mechanics may be represented by the quasi-steady bidimensional flow around a cambered circular arc with a sharp leading edge. The spinnaker is typically trimmed such that the stagnation point is at the leading edge with the sail streamline separating on the suction side and reattaching within the first 10% of the chord length, forming a leading-edge separation bubble (LESB). This flow feature sets the beginning of the boundary layer, whose separation further downstream is paramount for the global aerodynamic forces on the sail. This study investigates the effect of the LESB on the boundary layer regime and downstream flow separation through particle image velocimetry on a circular arc. The existence of the combination of a critical Reynolds number and a critical angle of attack to trigger turbulent separation is demonstrated. A turbulent LESB followed by a laminar boundary layer is observed in sub-critical regime. Conversely, in a post-critical condition, a turbulent LESB ensued by a turbulent boundary layer is detected, the latter continuing all the way to trailing-edge separation. This behaviour ultimately yields a sharp lift increase and drag reduction. These findings reveal the critical effect of the leading-edge vortical structures on the global flow field and forces experienced by cambered wings with leading-edge separation, including high performance spinnakers. It is envisaged that these results will contribute to improve the design and performance of downwind yacht sails.

The flow around a circular arc is governed by the effect of the sharp leading edge and the arc's curvature. There is a range of incidences where a leading-edge separation bubble (LESB) is formed on the convex side of the arc, and the reattached boundary layer separates further downstream. Akin to foils and cylinders, for increasing values of the Reynolds number, the boundary layer turns from laminar to turbulent resulting in a step change in the forces, here termed force crisis. This phenomenon is characterized experimentally for an arc with a camber-to-chord ratio of 0.22 and for a range of the Reynolds number from 53 530 to 218 000. Forces are measured both in a towing tank and in a water tunnel, and particle image velocimetry is undertaken in the water tunnel. In stark contrast to cylinders, where the force crisis is associated with the laminar-to-turbulent transition of the boundary layer, here, it is found to be associated with the suppressed relaminarization of the boundary layer. In fact, the LESB is always turbulent at the tested conditions, and relaminarization occurs up to a combination of critical angles of attack and critical Reynolds numbers. The critical angle of attack varies linearly with the Reynolds number. These results may contribute to the design of thin cambered wings, sails, and blades at a transitional Reynolds number such as the wings of micro aerial vehicles, swept wings in subsonic flight, turbomachinery blades, and the sails of autonomous sailing vessels.

Dynamic Velocity Prediction Programs are taking an increasingly prominent role in high performance yacht design, as they allow to deal with seakeeping abilities and stability issues. Their validation is however often neglected for lack of time and data. This paper presents an experimental campaign carried out in the towing tank of the Ecole Centrale de Nantes, France, to validate the hull modeling in use in a previously presented Dynamic Velocity Prediction Program. Even though with foils, hulls are less frequently immersed, a reliable hull modeling is necessary to properly simulate the critical transient phases such as touchdowns and takeoffs. The model is a multihull float with a waterline length of 2.5 m. Measurements were made in head waves in both captive and semi-captive conditions (free to heave and pitch), with the model towed at constant yaw and speed. To get as close as possible to real sailing conditions, experiments were made at both zero and non-zero leeway angles, sweeping a wide range of speed values, with Froude numbers up to 1.2. Both linear and nonlinear wave conditions were studied in order to test the limits of the modeling approach, with wave steepness reaching up to 7% in captive conditions and 3.5% in semi-captive ones. The paper presents the design and methodology of the experiments, as well as comparisons of measured loads and motions with simulations. Loads are shown to be consistent, with a good representation of the sustained non-linearities. Pitch and heave motions depict an encouraging correlation which confirms that the modeling approach is valid.

Thanks to high lift-to-drag ratios, hydrofoils are of great interest for high-speed vessels. Modern sailing yachts fitted with foils have thus reached impressively high speeds on the water. But this hydrodynamic efficiency is achieved at the expense of stability. Accurate tradeoffs are therefore needed to ensure both performance and safety. While usual Velocity Prediction Programs (VPPs) are inadequate to assess dynamic stability, the varying nature of the offshore racing environment further complicates the task. Dynamic simulation in the time-domain is thus necessary to help architects assess their designs. This paper presents a system-based numerical tool which aims at predicting the dynamic behavior of offshore sailing yachts. A 6 degrees of freedom (DOF) algorithm is used, calculating loads as a superposition of several components (hull, appendage, sails). Part of them are computed at runtime while the others use pre-computed dataset, allowing a good compromise between efficiency and flexibility. Three 6DOF simulations of an existing offshore trimaran (a maneuver, unsteady wind conditions and quartering seas) are presented. They underline the interest of dynamic studies, demonstrating how important the yacht state history is to the understanding of her instantaneous behavior and showing that dynamic simulations open a different field of optimization than VPPs.

This research introduces a spatio-temporal planning framework whose objective is to simulate a sailing yacht match race. The race is a duel in which strategy and tactics play a major role as sailors continuously have to take decisions according to wind variations and opponent’s locations and actions. We introduce a decision-aid framework based on a stochastic game approach grounded on an action-oriented model that replicates yachts’ behaviors. The objective is to replicate as closely as possible the respective behaviors and navigation decisions taken by yachts competitors. The proposed formalism has been implemented and is illustrated by a sample race example.

The aim of this paper is to report and analyze experimental results on forces generated by a high-camber thin section with a sharp leading edge, namely, a curved plate with 22.3% relative camber, and measurements of the flowfield, in a nominally 2D flow. A wide range of angles of attack [-10 degrees; 32 degrees] is investigated for 3 values of the Reynolds number across the transitional range. Unusual polar curves are observed with sharp variations of lift with the angle of attack. At low incidence, the lift coefficient increases faster than 2 pi /radian. Moreover, very different behaviors occur for Reynolds numbers lower or higher than similar to 105. For small or negative angles of attack, the lift coefficient is negative (downward lift) at small Reynolds numbers, but positive (upward lift) at higher Reynolds numbers. The velocity field is investigated with particle image velocimetry. Variations of lift are related to changes in the topology of the flowfield around the section. These results help to better understand the flow around a high-camber thin section and provide a detailed benchmark database in a simple geometry for model validations. As strong Reynolds number effects are evidenced, this database would be particularly useful to assess the performance of transition models.

In the present study we investigate the flapping instability of a light, soft, highly cambered membrane subject to wind loading. An original in-situ experiment is developed where time-resolved pressures and forces are measured on a full-scale yacht downwind sail called a spinnaker. Particular features of this aero-elastic problem are that the membrane is weakly constrained - held only at three corners-, has a free leading edge, and has no proper shape in the absence of wind loading. In usual operating conditions, the soft structure is subject to a flapping instability giving rise to almost periodic folding and unfolding of the fore part of the sail, associated to strong variations of pressures and forces. This dynamic behavior is analyzed in detail and the space-time evolution of pressures on the membrane is linked to the flapping phenomenon. A peak in forces is observed when the membrane recovers its full shape. Thanks to the Bi-Orthogonal Decomposition (BOD) applied to the pressure fields, the dynamic behavior is reasonably well represented by the two first modes where mode 1 mostly carries the global aerodynamic force behavior and mode 2 mostly represents the effects of the membrane flapping. A physical mechanism of the flapping process is proposed based on the discussion of aerodynamic pressures and strains in the membrane. (C) 2018 Elsevier Ltd. All rights reserved.

This work presents a wind tunnel experimental study on the effect of the leading edge flapping on the aerodynamic performance of a spinnaker. Four J80-class spinnaker models, combining two different assembling structures (panel layout) and two different sail materials are tested at various wind speeds and wind angles in a wind tunnel. Results show that, for the wind angle range the spinnaker is designed for, the sustained periodic flapping of the sail leading edge has a significant benefit on performance, with 10% increase in drive force. In these model-scale tests, the sail structural properties did not show significant differences in performance, but affect the point where flapping sets in: a model with a stiffer material and a cross-cut panel layout starts flapping for a longer sheet length, compared to a lighter cloth and a tri-radial layout. Finally, it is shown that the non-dimensional flapping frequency is rather constant ≃0.4 in the design range of wind angle, but it varies with the wind speed and sail structural properties on a smaller wind angle where the spinnaker is more stretched.

While the aerodynamics of upwind sails are relatively well understood, flows past downwind sails are still very challenging. Indeed, downwind sails which can be considered as highly cambered thin wing profiles, are well known for their massive separations and complex wake flows. Therefore the aim of this study was to examine a very simple highly curved thin wing profile in order to elucidate features of real flow behaviours past such sails. Therefore, a two-dimensional thin circular arc has been investigated. The studied model had a camber of 21 - 22% comparable to downwind sails. The wind tunnel pressure measurements have enabled us to understand why the sudden transition in the lift force exists at low incidences but not at higher incidences. At low incidences the flow stagnates on the top face and a laminar boundary layer develops first. If the Reynolds number is too low, the laminar boundary layer is not able to transition to turbulent. This laminar boundary layer separates very early leading to low lift and high drag. However, when the Reynolds number is high enough, the boundary layer transitions to turbulent creating a laminar separation bubble. This more robust boundary layer can withstand the adverse pressure gradient and stay attached much longer, creating a sudden significant increase in lift and a drop in drag. At high incidences, a leading edge bubble forces the flow to transition to turbulent. Therefore, the boundary layer is fully turbulent irrespective of the Reynolds number and a unique flow regime exists at these high incidences.

An experiment was performed in the Yacht Research Unit's Twisted Flow Wind Tunnel (University of Auckland) to test the effect of dynamic trimming on three IMOCA 60 inspired mainsail models in an upwind (apparent wind angle βAW = 60°) unheeled configuration. This study presents dynamic fluid structure interaction results in well controlled conditions (wind, sheet length) with a dynamic trimming system. Trimming oscillations are done around an optimum value of the optimization target coefficient CFobj previously found with a static trim. Different oscillation amplitudes and frequencies of trimming are investigated. Measurements are done with a 6 component force balance and a load sensor giving access to the unsteady mainsail sheet load. The driving force coefficient CFx and CFobj first decrease at low reduced frequency fr for quasi-steady state then increase, becoming higher than the static state situation. CFx and CFobj show an optimum for the three different design sail shapes located at fr = 0.255. This optimum is linked to the power transmitted to the rig and sail system by the trimming device. The effect of the camber of the design shape is also investigated. The flat mainsail design benefits more than the other mainsail designs from the dynamic trimming compared to their respective static situation. This study presents dynamic results that cannot be accurately predicted with a quasi-static approach. These results are therefore valuable for future fluid-structure interaction numerical tools validations in unsteady conditions.