14.9: Wake Turbulence
With flying wings one often gets along with the usual rigid wake turbulence,
since behind the wing no moments affecting surfaces are present. With
multi-fixed-wing aircraft (tail, ducks, biplane) it is advisable, often even
necessary to work with variable wake turbulence.
A lift-producing surface diverts the air flow,
and at the wing tips vortices are formed. Surfaces located behind the wing are
very strongly affected by this diverted flow. With the following functions one
can start an iterative computation with variable wake turbulence.
To start an iterative computation with variable wake turbulence one of the
following check-boxes must be selected:
'Wake Turbulence Corr. Single alignment'
or
'Wake Turbulence Corr. Complete alignment'.
The flow lines of wake turbulence consist of
discrete pieces, which are one behind the other arranged as on a pearl necklet.
Close behind the trailing edge these pieces have a small length, which becomes
ever longer with increasing distance from trailing edge.
With the function 'Wake Turbulence Corr. Complete alignment' first all pieces
of a flow line are aligned and than a new vortex computation is started.
With the function 'Wake Turbulence Corr. Single alignment' first the piece of a
flow line near the trailing edge is aligned, than a new vortex computation is
started. Afterwards the next piece of a flow line is aligned and again a new
vortex computation is started and so on. The final result of the single
alignment function is little more precise, on the other hand computing time
increases significantly. In most cases the complete alignment function is
sufficient.
Also a combination of the two functions is permitted, thus becomes the
computation most exactly, results in however intolerable long computing times.
Select the marked button to make the wake turbulence visible.
The picture below shows rigid wake turbulence.
One can however see here only a small part of the wake turbulence, in reality
the flow lines continues to the infinite. The program does well without a
complete visualisation, since the graphic becomes otherwise very unclear.
The length of displayed wake turbulence can be
entered manually into the input field 'Total length of elements [m]' or with
the Up/Down-buttons near the input field. The dimensional unit for entered data
is 'meter' [m].
This visible part of wake turbulence is divided into small elements, which
become finer to the wing (sinusoidally). The number of visible elements can be
set in the input field 'No. Elements Wake Turbulence'
The picture below shows the above wing computed with wake turbulence iteration.
In the wing centre, the flow is diverted downwards and at the wing tips are
vertices formed.