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Wind tunnel experiments on a 1:15th model-scale yacht were modelled with Detached Eddy Simulations, which allowed drawing the topology of the turbulent structures in the sail wake discovering new flow features. Simulations were performed with two different grids and three different time steps. It was found that a leading edge vortex grows from the foot to the head of the spinnaker (foresail), where it deflects downstream forming the tip vortex. The twist of the spinnaker leads to a mid-span helicoidal vortex, which has a horizontal axis almost parallel to the apparent wind and rotates in the same direction of the tip vortex. Vortical spanwise tubes are released from the trailing edge of the mainsail (aftsail) and the spinnaker and roll around the tip and the mid-span vortex of the spinnaker. Vortical tubes are also detached intermittently from the sails׳ feet and these break down into smaller and smaller structures while convecting downstream. For comparison, we also performed a Reynolds-averaged Navier–Stokes simulation. The comparison between forces and pressure distributions computed with different grids and time resolutions, different turbulence models, and measured with flexible and rigid sails showed similar trends; differences between numerical results were smaller than those between experimental results.
Delayed Detached Eddy Simulation of Sailing Yacht Sails
Wind tunnel experiments on a 1:15th model-scale AC33-class yacht were modeled with Reynolds-average Navier-Stokes simulations (RANS) and Delayed Detached Eddy Simulations (DDES). Numerical simulations were performed with two different grids, where the node distance was halved from the coarser to the finer grid, and with three different time steps, where the smallest one was 1/4th of the largest one. High-grid-resolution DDES allowed drawing the topology of the turbulent structures in the sail wake and discovering new flow features, which were hardly detectable with low-grid-resolution DDES and, particularly, with RANS. It was found that the span-wise twist of the spinnaker leads to a mid-span helicoidal vortex, which has a horizontal axis almost parallel to the apparent wind and rotates in the same direction of the tip vortex generated from the head of the sail. Vortical span-wise tubes are released from the trailing edges of the mainsail and the spinnaker and, while convecting downstream, these structures roll around the tip and mid-span vortices of the spinnaker. Vortical tubes are also detached intermittently from the sails’ feet and these break down into smaller and smaller structures while convecting downstream.
