  Plane Couette flow perturbed by a spanwise oriented ribbon, similar to a
configuration investigated experimentally at the Centre d'Etudes de Saclay, is
investigated numerically using a spectral-element code. 2D steady states are
computed for the perturbed configuration; these differ from the unperturbed
flows mainly by a region of counter-circulation surrounding the ribbon. The 2D
steady flow loses stability to 3D eigenmodes at Re = 230, beta = 1.3 for rho =
0.086 and Re = 550, beta = 1.5 for rho = 0.043, where Re is the Reynolds
number, beta is the spanwise wavenumber and rho is the half-height of the
ribbon. For rho = 0.086, the bifurcation is determined to be subcritical by
calculating the cubic term in the normal form equation from the timeseries of a
single nonlinear simulation; steady 3D flows are found for Re as low as 200.
The critical eigenmode and nonlinear 3D states contain streamwise vortices
localized near the ribbon, whose streamwise extent increases with Re. All of
these results agree well with experimental observations.
