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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