Subcritical transition of Taylor - Couette - Poiseuille flow at high radius ratio

We performed direct numerical simulations of Taylor-Couette-Poiseuille flows within an annular channel with a radius ratio of 0.883. A parametric study was conducted on subcritical transition processes of the wall-bounded combined shear flow with a torsional base-flow profile with three control parameters of F(P) representing the axial mean pressure gradient and two Reynolds numbers R e in and R e out, based on the inner cylinder and outer cylinder rotational velocities, respectively. In the set (R e in, R e out) = (400, - 1000), the laminar flow becomes turbulent via finite-length and infinite-length turbulent bands, called one-way helical turbulence, as F(P) increases. Two-way helical turbulence appeared in the counterpart of the annular Poiseuille flow without cylindrical rotations, suggesting that the azimuthal Couette flow broke the symmetry of the helical turbulence of the axial Poiseuille flow. In the set of (R e in, R e out) = (800, - 2000) and (1200, - 3000), we found a ring-shaped localized turbulence at F(P) that provided an axial friction Reynolds number comparable to the azimuthal one. The flow states were mapped in parameter space spanned by the axial and azimuthal friction Reynolds numbers. Eight different flow regimes, including the laminar state, were identified based on turbulent statistics during these flow visualizations.