Experimental evidence of streamwise vortices as finite amplitude solutions in transitional plane Couette flow

Elongated streamwise structures are considered as a key element of the transition to turbulence in various wall flows. In pure plane Couette flow (pCf), longitudinal streaks originating from pairs of streamwise counter-rotating vortices are clearly identified surrounding growing turbulent spots or at late stages of spot relaxation. The same structures bifurcate subcritically from a slightly modified Couette flow where a thin spanwise wire has been introduced in the zero-velocity plane. The basic flow profile, as measured by laser Doppler velocimetry, is shown to approach continuously the original linear velocity profile as the radius of the wire is decreased. On the other hand, the vortices remain almost unchanged and the bifurcation threshold remains bounded from above by the global stability threshold below which turbulent spots relax spontaneously. This supports the conjecture that a related nontrivial nonlinear solution exists in the pure pCf limit. These observations are compared to numerical stability calculations of the modified flow and to finite amplitude solutions to pCf problems with a different tunable modification.