Nonlinear effects on the free evolution of three-dimensional disturban
ces are discussed, these disturbances having a spot-like character suf
ficiently far downstream of the initial disturbance. The inviscid init
ial-value formulation taken involving the three-dimensional unsteady E
uler equations offers hope of considerable analytical progress on the
nonlinear side, as well as being suggested by some of the experimental
evidence on turbulent spots and by engineering modelling and previous
related theory. The large-time large-distance behaviour is associated
with the two major length scales, proportional to (time)1/2 and to (t
ime), in the evolving spot; within the former scale the Euler flow exh
ibits a three-dimensional triple-deck-like structure; within the latte
r scale, in contrast, there are additional time-independent scales in
operation. As the typical disturbance amplitude increases, nonlinear e
ffects first enter the reckoning in edge layers near the spot's wing-t
ips. The nonlinearity is mostly due to interplay between the fluctuati
ons present and the three-dimensional mean-flow correction which varie
s relatively slowly. The resulting amplitude interaction points to a s
ubsequent flooding of nonlinear effects into the middle of the spot. T
here it is suggested that the fluctuation/mean-flow interaction become
s strongly nonlinear, substantially altering the mean properties in pa
rticular. A new global viscous-inviscid interaction between the short
and long scales present, involving Reynolds stresses, is also identifi
ed. The additional significance of viscous sublayer bursts is also not
ed, along with comments on links with experiments and direct numerical
simulations, on channel flows and jets, and on further research.