Abstract:
The effect of transpiration on the boundary layer near the leading edge of a swept wing
has been investigated using a large swept cylinder model with a laser-drilled titanium
leading edge to model the leading edge of a swept wing.
In the region near the leading edge, boundary layer transition due to crossflow
instability has been examined. Natural transition on a porous surface was compared
with that on a non-porous surface, and it was found that transition occurred at lower
R ’s on the porous surface (ie there was a performance penalty due to the porous
surface). The effect of suction on transition due to crossflow instability was then
studied. It was found that only moderate amounts of suction were required to delay the
onset of crossflow-induced transition and a simple algebraic model has been derived, in
terms of R , Rex, and Cq, to describe transition on the porous surface with or without
suction. It was also found that two-dimensional trip wires had a negligible effect on
crossflow transition, except where they caused attachment-line contamination.
On the attachment-line, several subjects were addressed. The effect of attachment-line
blowing was considered, and good agreement was obtained with previous work. The
effect of spanwise blowing length was also addressed, and a simple algebraic model was
derived, in terms of R , s/rj, and Cq, to describe attachment-line transition due to
blowing. A comparison has also been made with linear stability theory.
The effect of suction at the wing-fiiselage junction was examined as an alternative to
suction on the attachment-line. However, it was found that applying suction on the
attachment-line when the boundary layer had attained infinite swept conditions was
much more efficient than applying suction in the junction region.
Suction was successfully used to relaminarise a turbulent attachment-line at R values
between 600 and 950, the magnitude predicted for the next generation of large transport
aircraft. During the experiments, no sign of critical oversuction was found.
Finally, the behaviour of a relaminarised attachment-line flowing onto a non-porous
surface was studied. The conditions for natural transition on the non-porous surface
were measured, and it was found that they were the same as those predicted by previous
work on an entirely non-porous attachment-line.
