In this paper we explore essentially nonlinear disturbances produced i
n an incompressible boundary layer by a roughness on the wall. The sca
le of the stationary roughness is supposed to be large enough so that
generated waves are governed by the forced Benjamin-Davis-Acrivos (fBD
A) equation. The disturbance patterns for a wide range of roughness si
zes are analyzed revealing the remarkable phenomenon of bifurcations.
A very specific oscillation motion over the obstacle is found. It appe
ars to be the basic mechanism causing the periodic generation of solit
ary waves upstream and downstream. The general structure of disturbanc
es in space at different values of time is discussed. The asymptotic a
nalysis of the solution, when the intensity of the external agency Q b
ecomes a small parameter, is given. The quadratic term of an expansion
based on this parameter is responsible for the redistribution of solu
tion mass between regions located ahead and behind the obstacle, inevi
tably leading to the gradual growth of nonlinear effects. According to
the asymptotic consideration the time T(c) determining the onset of
the nonlinear stage of disturbance development is O(Q(-3)), this estim
ation correlates well with numerical results. (C) 1996 American Instit
ute of Physics.