To show the effects of different sail profiles on the air flow, I built a “sheet” of small wood bars and light fabric, which are backed with a rubber band. A profile attached to a servo controller can be pressed against the sheet to bend it more or less, thus changing the sheet’s profile.
At the beginning of the video, the flow stalls. But if we give the sheet a slight profile, the airflow is deflected and becomes laminar on both sides (after 10 seconds into the video). If we change the angle of the sail, the flow stalls once again (after 20 seconds into the video). It also stalls when we increase the sail’s profile.
Depending on the profile and the approach angle of the sail an optimum range exists, in which the air flows on both sides of the sail are nicely laminar. This optimum range is very small, and small changes to the profile and/or the approach angle have a great impact on the flows.