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Streamlines
& swirls |
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| Flow
simulation: streamlines & leech vortices
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This little study in CFD
focuses on air flow around the sails, visualized
with the help of streamlines. The streamlines
are colored by the local flow speed to give
an idea about the flow acceleration and
deceleration. |
The airflow around sails
is complex. On the windward side air is
decelerated so there is a higher pressure,
and on the leeward side vice versa. This
creates a problem at the ends, the top
and the foot of the sail where the pressures
tend to equalize. In consequence, towards
the top of the sail air is bending upwards
on the windward side and downwards on
the leeward side. At the foot the opposite
is true: air tends to escape or leak under
the boom and/or the foot of the sail.
This results in crossflows
at the leech where air is leaving the
sails. The crossflows roll into larger
vortices or swirls behind the sail. These
vortices consume energy, forming easily
50% of the total air drag of a sailboat.
The vortices also induce velocities over
the sails themselves, much like the apparent
wind is formed through boat movement and
true wind - we are in a chicken-egg situation,
where the vortices can be thought to bend
the air flow, or we can also think that
the vortices are formed because of the
bent air. On the windward side of the
vortice core, an upward component blends
into the air flow, and on the leeward
side a downward component influences the
local flow.
If air wasn't invisible...
The computer simulation
on the right shows how complex the airflow
around sails can be. In this view from
behind you can discern the vortices towards
the top of the sails and those at the
foot behind the clew of each sail.
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| Flow
simulation on the surface of the sails,
seen from the leeward side (left) and the
windward side (right), colored with pressure
(Cp).
On the lee side, see how the air is flowing
up from the foot. In the upper part air
flows almost perpendicular to the luff of
the sail, with a little descending tendency.
Red tones correspond to accelerated flow
(diminished pressure, negative numbers on
the scale). Acceleration is strongest in
the front-top part of the genoa.
On the windward side, air is escaping under
the boom in the bottom part and bending
up towards the top part of the sail plan.
Blue tones indicate higher pressure, turquoise
means free wind pressure (zero on the pressure
scale to the right.)
Telltales near the luff of the genoa witness
about the theory: The one on the windward
side is flowing higher up than the one behind
the sail on the leeward side.
The very efficient sail plan is that of
a 46-foot IMS racer-cruiser, apparent wind
is 16 knots/22 degrees and heel 17 degrees.
Take a look at the
mainsail and the genoa,
as photographed in the analyzed situation.
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Smoke tests
Smoke tests in the windtunnel reveal
the tip vortex behind the top of this
Europe dinghy sail model. The sail is
painted black to show the smoke more clearly
(look at the
video).
... at full scale
Look at this amazing
shot courtesy Daniel
Forster/Seahorse Magazine.
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| CFD
simulations
These simulations reveal how the air flow
is bent in the vicinity of sails. Why is
the bending of the air of interest to us?
It turns out that's how sails generate their
power - the more air is bent, the more power.
The power transferred to the boat through
this bending follows Newton's law of action-reaction.
If you want to read
more, Tom Speer offers a clear and compact
explanation.
In the
first
simulation
a vertical layer
of air is moved from the windward side to
the leeward of the boat (will open in a
new window).
In the
second simulation a horizontal layer
of air is moved up from deck level towards
the top of the sail plan (will open in a
new window).
In the third
simulation
you can see how
the tip and the foot vortices develop behind
a Europe dinghy sail. This is the "bad
air zone" familiar to any racing sailor.
In the fourth
simulation you can see how
the wake develops behind the mainsail and
the jib.
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CFD
= Computer Fluid
Dynamics. CFD is a great
tool for visualizing and explaining flow
phenomena. While the latest flow software
is very powerful and capable of calculating
amazing things at astonishing accuracy,
the old saying "garbage in, garbage
out" is more true than ever. Besides
of presenting the problem in a meaningful
way, one needs lots of knowledge and experience
to interpret the results correctly. Simulation
through CFD is especially useful at giving
qualitative information - when it comes
to quantitative results or hard numbers,
you have to be even more cautious when drawing
conclusions about the merits of one design
over another. Wind tunnel tests are needed
to calibrate and validate the CFD code before
reliable results are obtained.
With the power of modern CFD at the desktop,
it is too easy to produce beautiful pictures
with little connection to reality. Often
these pictures are produced by flow experts
with little sail-specific knowledge, and
then interpreted by sail designers without
sufficient understanding of the CFD tool,
and as a result you get just that - pretty
pictures. WB-Sails has used CFD as a part
of sail development since late '80s and
we have done many a wind tunnel test to
take our CFD codes further and closer to
reality. [back to top] |
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