A twisted flow wind tunnel for testing yacht sails

The paper outlines the requirements for wind tunnel testing model yachts for sail aerodynamics investigations. It is shown that the apparent wind onto a yacht is ''twisted'' over the mast height, due to the vector addition of the yacht and wind velocities. The design features of a special wind tunnel built in New Zealand which can produce twisted flow in the test section which is suitable for testing model yachts are described. This wind tunnel is basically a conventional open-circuit long test-section boundary-layer wind tunnel, with the addition of some vertical vanes at the outlet. These can be twisted to desired orientations to achieve how in the model test region which has the velocity and direction varying with height in the required manner. Some commissioning results from one twist configuration are presented which illustrate the success of the method. Some of the drawbacks of this approach which are difficult to overcome are also outlined.