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The Quest for the Perfect Shape
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Note on the effect of side bend
Anatomy of a Mini-Transat
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Mini boat - Maxi challenge
SailPowerCalc
Mini boat - Maxi challenge
470 Aerodynamics
Lifting bows with foresails
StressMapper
Telling tales ...
MacSail
The scientific Finn
Wind tunnel images >
 
 

Wind tunnel images

Computer enhanced images from smoke tests on a sailprofile in the Helsinki University of Technology low speed wind tunnel. The tests were performed to verify computer predictions of the aerodynamics of a thin, sail like airfoil, as well as to study separation bubbles behind the mast and evaluate the effect of mast shape on the performance of the sail.

The clips are captured from video and enhanced in Photoshop to visualize the flow in detail.

Sail at stall. Sail with NACA shaped small wing mast at an angle of attack well beyond stall (alfa = 27°). On the video, you can see how a large vortex is shed intermittently from the luff and the leech. In this still picture, the "starting vortex" at the leach is clearly visible. This vortex attempts to suck the air on the leeward side from the luff towards the leech, to satisfy the Kutta condition and maintain an attached flow on the leeward side - in vain, because the angle of attack is too large. This 2-dimensional profile stalled at an angle of attack between 14°-15°.

Luff separation bubble behind the mast on leeward side. The red yarn tuft at 5% of chord length is clearly showing the reversed flow inside the separation bubble. The flow reattaches at 8-9% of the chord as witnessed by the other tuft. Inside the bubble, the thinned-out smoke is almost inert or slowly creeping forward against the flow. The angle of attack of the sail is close to ideal.


Trailing edge separation. The flow is separating at about 75% of the chord length. The flow separation on a thin, sail-like aerofoil appears to start approximately simultaneously at the luff and the leech. The disturbance caused by the leeward side separation bubble at the luff ("leeward telltails flying") is strong enough to trigger the flow separation at the leech. As angle of attack is increased, separation progresses from both ends towards the middle, but at a much higher rate from the leech. Just before stall, the flow luff flow reattaches behind the separation bubble at 18%, and then reseparates at 60%.

In these tests, the luff separation bubble on the leeward side never reached more than 20% of the chord. Thus there was no evidence of the very long separation bubble (up to 70-80%) typical on certain thin-nosed, but thick airfoil profiles ("the thin airfoil stall") . Alfa = 12°.

The tests were done with a 2-dimensional, carbon fibre profile equipped with pressure taps on both sides. The max camber of the profile is 12,5 % at 45% from the luff. The length of the mast is 2% of the chord. The tests were performed mostly at a flow speed of 25 m/s, corresponding to a 10 kn breeze in full scale (for a Finn sail). Flow speeds ranging from 10 to 40 m/s were tried, and it was found that Reynolds Number had no influence on the aerodynamic characteristics of the thin, sail-like airfoil.

Copyright © 1995 WB-Sails Ltd. All rights reserved.

 

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