Large-scale motions in wall-bounded turbulent flows are frequently
interpreted as resulting from an aggregation process of smaller-scale
structures. Here, we explore the alternative possibility that such large-scale
motions are themselves self-sustained and do not draw their energy from
smaller-scale turbulent motions activated in buffer layers. To this end, it is
first shown that large-scale motions in turbulent Couette flow at Re=2150
self-sustain even when active processes at smaller scales are artificially
quenched by increasing the Smagorinsky constant Cs in large eddy simulations.
These results are in agreement with earlier results on pressure driven
turbulent channels. We further investigate the nature of the large-scale
coherent motions by computing upper and lower-branch nonlinear steady solutions
of the filtered (LES) equations with a Newton-Krylov solver,and find that they
are connected by a saddle-node bifurcation at large values of Cs. Upper branch
solutions for the filtered large scale motions are computed for Reynolds
numbers up to Re=2187 using specific paths in the Re-Cs parameter plane and
compared to large-scale coherent motions. Continuation to Cs = 0 reveals that
these large-scale steady solutions of the filtered equations are connected to
the Nagata-Clever-Busse-Waleffe branch of steady solutions of the Navier-Stokes
equations. In contrast, we find it impossible to connect the latter to buffer
layer motions through a continuation to higher Reynolds numbers in minimal flow
units