Transition to the ultimate turbulent regime in a very wide gap Taylor-Couette flow (=0.1) with a stationary outer cylinder

The boundary layers in turbulent Taylor-Couette flow are exposed to transitions from laminar to turbulent states if the flow is sufficiently sheared. The present study examines this particular transition from the so-called "classical" to "ultimate" regime experimentally for a very wide-gap Taylor-Couette flow with a radius ratio of eta = 0.1 and shear Reynolds numbers of up to Re-s = 1.5 x 10(5). In order to determine the transition, the angular momentum transport is measured by using torque sensors at the inner wall. This is complemented by measuring the radial and azimuthal velocities via a time-resolved Particle-Image-Velocimetry (PIV). The transition to the ultimate regime is found at 2.5 x 10(4) <= Re-s <= 3.7 x 10(4). The dimensionless angular momentum flux showed an effective scaling of Nu(omega) similar to Re-s(0.76) for Re-s >= 2.5 x 10(4) and is in agreement with the scaling laws used for the ultimate regime in narrow- gap Taylor-Couette flows. In addition, a spectral analysis was performed showing the existence of highly energetic small-scale and large-scale patterns in the classical regime whereas only highly energetic large-scale patterns were observed in the ultimate regime. Copyright (c) 2023 The author(s)