  In order to obtain insights into dynamics of developed plane Couette
turbulence, this paper considers bifurcation structure of unstable periodic
orbits (UPOs) in the large-eddy-simulation (LES) system with the
Smagorinsky-type eddy viscosity model. Treating the Smagorinsky constant as a
bifurcation parameter, we detect the bifurcation points connecting two known
UPOs which were separately discovered in the Navier--Stokes system [G. Kawahara
and S. Kida, J. Fluid Mech., 449, 291--300 (2001)]. At the moderately high
Reynolds number, the LES UPO of the present study possesses the spanwise
vortices which seem to be caused by the streak instability and appear in the
central region of the channel. We note that to our knowledge a spanwise vortex
has not been reported through the UPO analysis of wall flows. The stretched
spanwise vortices of the LES UPO enhance transfer of the streamwise turbulent
momentum as in developed near-wall turbulence.
