PHYSICS OF TRAILING VORTICES BEHIND FLAPS-DOWN CONFIGURATIONS
vortex pairs. Commercial airplanes with flaps extended
produce multiple trailing vortices that remain distinct for some
distance behind the airplane. In the simplest case, the wing produces
tip vortices at the wingtips and flap vortices at the outboard edge
of the inboard flaps. These two pairs of co-rotating vortices and
a pair of counter-rotating vortices from the horizontal tail form
the basic flaps-down vortex system (fig.
2). Details of the airplane configuration determine how far
behind the airplane that the multiple vortex pairs remain as distinct
instabilities. In general, airplane trailing vortices
are weakly unstable. If the vortices are wavy (rather than straight
lines), the waviness may grow as a result of this natural instability.
In some cases, the waviness of the vortices can become so large
that the vortices from the left and right wings touch at different
points along their length. At these points, the vortices from the
left and right wings link together and produce a series of vortex
rings. This process can sometimes be observed in contrails, where
condensation provides a marker for the vortices. In this case, background
turbulence provides the initial waviness.
There are different instabilities
associated with a single vortex pair and with the multiple vortex
pairs of flaps-down configurations. The instability of a single
vortex pair has been found to amplify the waviness too slowly to
help break up the vortices in practice. The multiple-vortex system
has instabilities that can grow more rapidly. In particular, when
the waviness of the flap vortices is out of phase with the waviness
of the tip vortices, a strong amplification results. The active-control
system under development introduces this form of waviness to trigger
the multiple-vortex-pair instabilities. Growth of the instabilities
leads to the breakup of the vortices into a series of vortex rings.
Typical wavelengths used to break up the vortices are four to five
times the airplane wing span.