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This work presents an investigation into the aerodynamics of downwind sailing using different meth- ods for modelling turbulence, comparing Large Eddy Simulation (LES) and Reynolds-Averaged Navier- Stokes (RANS) methods. Pubblished experiments on a downwind sail plan [2] [8] showed areas of flow separation and vortex structures at the leading and trailing edge of the gennaker. The ability of LES to model the transition to turbulence within the shear layer leads to an accurate prediction of the leading edge separation bubble, which significantly influences the flow field around the top half of the sail. The tran- sient nature of the LES solution allows the computation of the creation and shedding of unsteady vortices at the leading edge and downstream of the sail draft. The effect of the vortex rolls being convected towards the trailing edge is to generate a boundary layer which is more resistent to separation. Comparison with the experimental pressure distribution shows the correct prediction of the separation by LES, while the RANS result shows a large area of stalled flow which limits the suction on the sail. As a result, the overall drive and side forces computed by the LES are in good agreement with the experiments with less than 3.5% error, while RANS underestimates their magnitude by more than 14%.
New Methodology of Bending Fatigue Test and Slamming Test on PVC Foam Core Sandwich with GFRP Faces
The purpose of this study is to investigate the influence of a spatially moving load and edge effects on the fatigue life of the foam-cored sandwich structures. A spatially moving load can be observed in structures subjected to slamming. A new geometry of specimen is developed to reduce the influence of edge effects in the test specimen. Numerical model results of the new geometry are presented. This study confirms that edge effects are leading to early failures and shear stress concentrations are significantly reduced near the edges, improving ASTM C393 standard.
Evaluation of Multi-Element Wing Sail Aerodynamics from Two-Dimensional Wind Tunnel Investigations
Following the 33rd America's Cup which featured a trimaran versus a catamaran, and the recent 34th America's Cup in 2013 featuring AC72 catamarans with multi-element wing sail yachts sailing at unprecedented speeds, interest in wing sail technology has increased substantially. Unfortunately there is currently very little open peer-reviewed literature available with a focus on multi-element wing design for yachts. The limited available literature focuses primarily on the structures of wings and their control, rather than on the aerodynamic design. While there is substantial available literature on the aerodynamic properties of aircraft wings, the differences in the flow domains between aeroplanes and yachts is significant. A yacht sail will operate in aReynolds number range of 0.2 to 8 million while aircraft operate regularly in excess of 10 million. Furthermore, yachts operate in the turbulent atmospheric boundary layer and require high maximum lift coefficients at many apparent wind angles, and minimising drag is not so critical. This paper reviews the literature on wing sail design for high performance yachts and discusses the results of wind tunnel testing at the Yacht Research Unit at the University of Auckland. Two wings with different symmetrical profiles have been tested at low Reynolds numbers with surface pressure measurements to measure the effect of gap geometry, angle of attack and camber on a wing sail’s performance characteristic. It has been found that for the two element wing studied, the gap size and pivot point of the rear element have only a weak influence on the lift and drag coefficients. Reynolds number has a strong effect on separation for highly cambered foils.
Slamming Induced Loads on a Rigid Cylinder and Comparison with Rigid Wedges
Slamming is a pervasive issue for marine vehicles, few of which are composed entirely of flat panels. Experimental and numerical data from flat panels and wedges are often extrapolated to handle other shapes because much previous work has focused on flat panels and wedges. Though correction factors and other strategies have been employed to achieve satisfactory correlation, curved shapes behave in a fundamentally different way. The current work seeks to investigate the effect of curvature on slamming loads through the use of constant velocity experimental testing and coupled Finite Element-Smoothed Particle Hydrodynamics numerical simulations. Experiments and numerical modelling conducted in the current work show that curved bodies experience a much higher initial loading than rigid wedges, which then abates to a quasi-constant residual load. This load profile varies significantly from that found in flat panels. Peak impact force and impulse plots generated from simulating a range of geometries indicate rigid cylinder slamming represents a more severe load case than rigid wedge slamming. Correlation between experiments and simulations is evaluated by comparison of the time-frequency domain representations of the respective transient signals which results in a single goodness of fit value. Once correlated, numerical simulation techniques can be used to further investigate the influence of curvature on slamming loads. Understanding this relationship between curvature and slamming loads has the potential to increase design optimization resulting in more failure resistant structures.
Polymeric foam materials are widely used as cores for sandwich composite hull structures in high performance marine vessels. Designers are faced with the challenge of selecting the most appropriate material type and density from the many different formulations of foams available on the market. Transient hydrodynamic pressures from slamming generate local regions of high transverse shear forces in the vicinity of panel boundaries and are hence a key load case for hull panel design of high-speed craft. The transient nature of the loading can generate stress rates that are high enough to affect the strength of the core material, particularly for polymeric foams. However material properties for foams are typically characterised by quasi-static, or in a few cases elevated rate, loading of coupon scale specimens, and there is very limited information available about how different polymeric cores behave in actual slamming events. The aim of this paper is to evaluate the strength of a range of polymeric core materials in controlled laboratory slam testing, and compare these to strengths measured by static and dynamic loading of coupon scale specimens. Core materials studied included Cross-linked and Linear PVC, PET and SAN Foams. This combination of materials provided a range of different levels of ductility from the low-elongation PET cores through to the high-elongation linear PVC and SAN foams. Results of the slam testing provided a quantitative ranking of the core materials, supporting empirical experience that high-elongation materials can perform better in slamming situations than predicted by their quasi-static strengths.
