The stability of nonlinear tertiary solutions in rotating
plane Couette flow is examined
numerically. It is found that the tertiary flows, which bifurcate from
two-dimensional
streamwise vortex flows, are stable within a certain range of the rotation
rate when
the Reynolds number is relatively small. The stability boundary is determined
by
perturbations which are subharmonic in the streamwise direction. As the
Reynolds
number is increased, the rotation range for the stable tertiary motions
is destroyed
gradually by oscillatory instabilities. We expect that the tertiary flow
is overtaken by
time-dependent motions for large Reynolds numbers. The results are compared
with
the recent experimental observation by Tillmark & Alfredsson (1996).
