Transition to the ultimate turbulent regime in a very wide gap Taylor-Couette flow (

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 $Re_s = 1.5 \times 10^5$ . 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 $2.5 \times 10^4 \leq Re_s \leq 3.7 \times 10^4$ . The dimensionless angular momentum flux showed an effective scaling of $Nu_\omega \sim Re_s^{0.76}$ for $Re_s \geq 2.5 \times 10^4$ 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