The evolution of localized three-dimensional disturbance in two- and t
hree-dimensional laminar boundary layers is examined. The linearized N
avier-Stokes equations for three-dimensional disturbances in a three-d
imensional parallel shear flow are solved numerically using Fourier tr
ansform Chebyshev collocation techniques. Modal analysis shows that su
bstantial short-term energy growth can be obtained even when all insta
bility waves are damped. This transient growth can increase the initia
l disturbance energy by two or three orders of magnitude, at which sta
ge nonlinear interactions might lead to a breakdown to turbulent flow,
bypassing the traditional Tollmien-Schlichting instability mechanism.
The dependence of the transient growth on wave number, Reynolds numbe
r, sweep angle and Hartree parameter is determined and a method for pr
edicting the maximum transient growth is proposed and found to be reas
onably accurate over a wide parameter range. Localized disturbances ar
e also examined and it is found that the bypass growth mechanism can e
nhance the formation of cross-flow vortices in a three-dimensional flo
w. Some implications are discussed, particularly with respect to the o
bserved effects of roughness on transition location.