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Fig. 9. The plots show the streamlines of fluid flow around a flapping wing for one
stroke cycle starting from rest, at an Re of 128. The arrow on the
wing shows the direction of the normalized aerodynamic forces acting on the
wing. The angle of attack during pure translation was 45° for both the
downstroke and the upstroke. The maximum translational velocity was 0.06 m
s-1. The colors of the streamline reflect the value of the stream
function, 
, along the streamlines. Red denotes the most positive values
and blue denotes the most negative values. During the downstroke, an attached
leading edge vortex (LEV) is initially formed while the trailing edge vortex
is shed (A,B). This corresponds to a growth in lift forces. In C, the LEV is
being shed while a new trailing edge vortex is formed. This corresponds to a
drop in lift. During rotation (D,E), the leading and trailing edge vortices
are shed. At the beginning of the upstroke (F), a new LEV is formed and a new
trailing edge vortex is formed and shed. This corresponds to an increase in
lift. In G, the LEV is shed and a new trailing edge vortex is formed. This
results in a drop in lift. Finally, a second leading edge vortex is formed and
the trailing edge vortex is shed, resulting in another lift peak (H,I).
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