Scaling Up by Aerodynamic Porosity
Its well known that a porous kite fabric adds stability to
flight in high wind. Designers have learned to use a range of materials &
constructions to build-in the effect, from unsized cloth, to meshes, open
ribbon weaves, & holes in the cloth. Last year, in hurricane force winds
on the US Pacific NW Coast, i noted that my string hammock was flying out
almost horizontal, but weirdly stable, when everything else was in tumult.
Pondering the effect, some of the principles emerged.
A porous wing does not develop as much raw lift as a non-porous one of the
same overall plan, so in light wind a porous wing is disadvantaged; the
stability effect is also less needed. In higher wind a porous kite has
adequate lift and begins to excel by enhanced stability. For equivalent
lift, a larger porous kite can match a smaller non-porous kite, but
several dynamic effects differ. Letting air thru the wing blocks formation
of the separation bubbles that cause violent stall. Depending on the pore
or hole size, vortical lines are formed that act like soft ropes to damp
flutter. The overall porous kite creates a diffuse wind-field obstacle,
with a softer more diffuse wake. A larger porous kite has a slower
weaker dynamic response to turbulence by increased moment of inertia, by
magnified dimensionless time.
To create megascale kite structure a fractal scale of porosity is proposed
for what is dubbed a "metakite", a super kite made of a dense array of
smaller kite elements. The smaller kites naturally operate at lower Re by
a reduced characteristic dimension. A sound principle is to increase
porosity progressively toward the rear of the kite, keeping high-lift at
the leading edge, with drogue force toward the trailing edge.
~Dave Santos 2011
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