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Numerical Simulations of a Surface Piercing A- Class Catamaran Hydrofoil and Comparison Against Model Tests

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Numerical Simulations of a Surface Piercing A- Class Catamaran Hydrofoil and Comparison Against Model Tests Journal of Sailing Technology, Article 2017-04. © 2017, The Society of Naval Architects and Marine Engineers. NUMERICAL SIMULATIONS OF A SURFACE PIERCING A- CLASS CATAMARAN HYDROFOIL AND COMPARISON AGAINST MODEL TESTS Thilo Keller TU Berlin, Berlin, Germany Juryk Henrichs DNV GL SE, Potsdam, Germany Dr. Karsten Hochkirch Downloaded from http://onepetro.org/jst/article-pdf/2/01/1/2205567/sname-jst-2017-04.pdf by guest on 27 September 2021 DNV GL SE, Potsdam, Germany Dr. Andrés Cura Hochbaum TU Berlin, Berlin, Germany Manuscript received March 1, 2017; accepted April 4, 2017. Abstract: Hydrofoil supported sailing vessels gained more and more importance within the last years. Due to new processes of manufacturing, it is possible to build slender section foils with low drag coefficients and heave stable hydrofoil geometries are becoming possible to construct. These surface piercing foils often tend to ventilate and cause cavitation at high speeds. The aim of this work is to define a setup to calculate the hydrodynamic forces on such foils with RANS CFD and to investigate whether the onset of ventilation and cavitation can be predicted with sufficient accuracy. Therefore, a surface piercing hydrofoil of an A-Class catamaran is simulated by using the RANS software FineMarine with its volume of fluid method. The C-shaped hydrofoil is analyzed for one speed at Froude Number 7.9 and various angles of attack (AoA) by varying rake and leeway angle in ranges actually used while sailing. In addition, model tests were carried out in the K27 cavitation tunnel of TU-Berlin, for the given hydrofoil and in the same conditions as simulated with CFD to provide data for validation. Based on the CFD calculations this paper presents how the rake and leeway angles influence the foil’s lift to drag ratio. The simulations have been verified by extensive analyses, including domain size verification for unrestricted water, mesh refinement and y+ verification. The influence of the dimensions of the K27 (cavitation tunnel of the Technical University of Berlin) on the flow around the hydrofoil and the wave system is also considered, as the test section of the K27 significantly influences the flow around the foil, the forces and the wave elevation. Finally the CFD results are compared against the experiments conducted in the K27. Keywords: hydrofoil, surface piercing, CFD, ventilation, experiment 1 NOMENCLATURE � Chord length [m] � Tank blockage coefficient [-] � Span [m] � Acceleration time [s] � Speed [m/s] � Projected foil area [m²] �' Area of tank cross section [m²] �� Froude number [-] �+ Lift coefficient [-] �, Drag coefficient [-] �./0 Reference length [m] �� Reynolds number [-] Downloaded from http://onepetro.org/jst/article-pdf/2/01/1/2205567/sname-jst-2017-04.pdf by guest on 27 September 2021 � Immersion of the foil tip [m] �5 Non-dimensional wall distance of first cell layer [-] ∝7, Angle of attack in the 2D plane of the profile section [°] �9+:; Zero lift angle [°] β Leeway angle (in figures: L) [°] φ Rake angle [°] ω Cant angle [°] ρ Density of water [kg/m³] Λ Aspect ratio [-] AoA Angle of Attack CFD Computational Fluid Dynamics EFD Experimental Fluid Dynamics FV Fully ventilated FW Fully wetted ITTC International Towing Tank Conference LE Leading edge PV Partially ventilated TE Trailing edge INTRODUCTION Within the last years we have seen more and more sailing vessels using the technology of hydrofoils, either to support the hull or to use them as a fully foiling system. Especially the 34th America´s Cup has shown how much potential can be gained with this technology. It shall be noted that there are several different systems developed to get boats fully foiling. The widely used Moth´s technology with a T-foil setup and an adjustable flap is probably the most often built foiling system. But on catamarans like the A-Class the rule book prohibits foils like the T-foils on the Moth. The A-Class rules (IACA, 2013) only allow foils to be inserted from top and with a minimum distance between the foils of 1.5m under the waterline. That is why only surface piercing lifting foils are seen nowadays in this class. This type foils show some additional challenges in the design process. First of all they show a significant amount of wave making which is causing extra drag compared to deeply submerged foils. Secondly, ventilation and cavitation are likely to occur at increasing speeds and larger AoA’s. Ventilation means that air is sucked down from the free surface 2 towards the tip of the foil. This leads to a significant and sudden loss of lift, which typically cannot be compensated fast enough to continue a controlled flight. Within this work RANS CFD calculations have been carried out for a representative segment of an A-Class catamaran lifting foil, to investigate if the hydrodynamic forces on such foils can be correctly predicted with a RANS CFD code in a computationally cost effective setup and if the onset of cavitation and ventilation can be predicted with acceptable accuracy. To validate the calculations experimental investigations have been carried out on the same geometry and for the same conditions using the K27 cavitation tank facility of TU Berlin. All computations had been done blind, i.e. they have been carried out prior to the model tests and were preceded by intensive studies on the numerical effects of e.g. domain and grid size as well as time step size. To allow for the most straight forward comparison between experiment and CFD and to avoid any uncertainties due to scale effects, a segment of the A-Cat foil small enough to fit Downloaded from http://onepetro.org/jst/article-pdf/2/01/1/2205567/sname-jst-2017-04.pdf by guest on 27 September 2021 in the test section of the cavitation tunnel was used. For the numerical simulations the well-known software package FineMarine was chosen. This software package includes the meshing tool Hexpress to generate unstructured grids. The aim of this paper is to present the methods used as well as the results derived from experiment and CFD in order to find a feasible approach for this kind of calculations. TEST OBJECT AND CASES As test object, an A-Class catamaran hydrofoil had been chosen. Typically these boats sail with both foils immersed at the same time. Due to the large distance between the two foils no direct interaction is expected and therefore only the starboard foil has been investigated. The foil itself has a span wise bend with a constant radius which makes it a so called C- foil. The chord and section is constant within that bended span region. Over the last 0.3 m towards the tip, the foil is straight and shows an elliptical taper on the leading edge with a straight trailing edge (cf. Figure 1). The straight part of the foil is angled 30° (cant angle) to the vertical. The chord length � of the foil is 0.16 m tapering to 0.06 m at the tip. In order to fit the foil in the measuring section of the K27 cavitation tank only a relatively small immersion � of 0.3 m could be used, which represents a limiting operational case. As a result the aspect ratio � is 2.645 which is very low and significantly changes the overall performance and behavior of the foil as well as its stall characteristics. At an immersion of � = 0.3 m the resulting span � is 0.34 m and projected foil area � is 0.0437 m². 3 Downloaded from http://onepetro.org/jst/article-pdf/2/01/1/2205567/sname-jst-2017-04.pdf by guest on 27 September 2021 Figure 1 - Hydrofoil views, left: perspective, middle: front, right: side The hydrofoil section used is a Selig/Donovan SD7032-099-88 and is kept the same over the whole span. It is designed for low Reynolds numbers. The main characteristics of the foil are: Thickness 10% with the maximum thickness at 26.6% of � and camber 3.4% with the maximum camber at 45.1% of � (cf. Figure 2). The zero lift angle is �9+:; is -4.08°. Figure 2 – Foil section [2] With the intention to investigate the same matrix in the cavitation tank and considering the spatial limitations in the test section, all CFD simulations were carried out with a tip immersion of � = 0.3 m, a cant angle � = 30° and at a speed � =10 m/s. The rake was varied in the range 2°< � < -8° in steps of 2° resembling the rake adjustment in the center board case of the boat. The leeway angle was varied in the range -2°< � < 20° in steps of 4°. The results, however, are presented with respect to the resulting 2-dimensional angle of attack ∝7, the straight part of the foil will experience and a cross-flow-angle indicating the amount of flow along the span. Force reference point The intersection of the trailing edge with the free surface was chosen as reference point for the force vector in all numerical and experimental investigations, as well as the 4 geometry origin for the CFD computations. The x-axis is facing forward against the free stream, the y-axis is facing to port and the z-axis is facing upwards (see Figure 3). NUMERICAL INVESTIGATIONS The numerical simulation was performed using the commercial software package FineMarine Version 3.1-3 which comprises the RANS CFD solver ISIS, the grid generator Hexpress and the post-processing tool CFView. Downloaded from http://onepetro.org/jst/article-pdf/2/01/1/2205567/sname-jst-2017-04.pdf by guest on 27 September 2021 Figure 3 – Foil in the test section of the K27 and reference system.
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