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Over the past two decades, the numerical and experimental progresses made in the field of downwind sail aerodynamics have contributed to a new understanding of their behaviour and improved designs. Contemporary advances include the numerical and experimental evidence of the leading-edge vortex, as well as greater correlation between model and full-scale testing. Nevertheless, much remains to be understood on the aerodynamics of downwind sails and their flow structures. In this paper, a detailed review of the different flow features of downwind sails, including the effect of separation bubbles and leading-edge vortices will be discussed. New experimental measurements of the flow field around a highly cambered thin circular arc geometry, representative of a bi-dimensional section of a spinnaker, will also be presented here for the first time. These results allow to interpret some inconsistent data from past experiments and simulations, and to provide guidance for future model testing and sail design
Phase 2C made enhancements to the Fluid-Structure Interaction (FSI) process, including a gradual change in pressure from one coupling to the next, which, when applied, introduces sub-iterations in between the FSI couplings. These sub- iterations can help keep the solution closer to a track of structural equilibriums between couplings.Converged FSI solutions in steady mode were achieved without resorting to running in the far more compute intensive unsteady mode. Unsteady FSI would require a far greater number of couplings than what now were required to reach the new steady solutions in 2C. Consequently, the steady approach made it possible to run additional simulations with four spinnaker sheet variations at each of the four AWA of 85, 105, 150 and 170 degrees. By themselves these sheet variations are educational with respect to spinnaker trimming. Additionally, they can be incorporated into a VPP as an active trim parameter.
Unsteady Aerodynamics of Downwind S-Turns in Small Boat Sailing
Unsteady aerodynamics and hydrodynamics play a significant role in small boat sailing. Highly dy- namic motions of the hull, appendages, and sail can be controlled through deliberate actions of the sailor. Kinetic sailing techniques involve coordinated changes in steering, sail trim, and bodyweight position, which can improve performance relative to steady sailing. We study the aerodynamics of the “S-Turning,” or “S-Curving,” kinetic technique, used to increase VMG while sailing downwind. Drag forces, which primarily propel the boat, can be increased by dynamics of the unsteady flow.
Full-scale, on-the-water tests are conducted in an Olympic Class Laser sailboat with a suite of on-board sensors: GPS, anemometer, inertial measurement units, and computer vision cameras. Laboratory tests use a representative 2D sail geometry in an XYθ-towing tank. Direct force measurement and particle image velocimetry are used to explore the fundamental vortex dynamics.
We measure characteristic motions of the boat during an S-Turn and show how boat performance is affected. Laboratory experiments show sail driving force is enhanced by an increase in the apparent wind speed over the sail, and by large scale vortex shedding produced the lateral motion of the sail in each turn. It is reasoned that this technique has in-part evolved to capture the beneficial effects of vortex shedding.
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%.
To Curl or Not to Curl: Wind Tunnel Investigations of Spinnaker Performance
This work presents a wind tunnel experimental study of the effect of curling on the spinnaker aerodynamic performance. Four spinnakers combining two different panellings and sail materials are tested at different wind speeds and wind angles in the Twisted Flow Wind Tunnel of the University of Auckland. Results show that the curling has a significant benefit on the propulsive force at an AWA ≥ 100◦ when this conclusion cannot be made at lower AWA where the best propulsive force is reached on the verge of curl- ing or before. Sail material and panelling have an effect on the sheet length where curling appears, stiffer material and cross cut panelling being the latest to curl. Finally, it is shown that the curling frequency increased linearly with the flow speed at AWA = 120◦.