In this study, we discuss the observed flow regimes in Taylor–Couette flow of radius ratio η = 0.1 for various Reynolds numbers up to 1.5 × 10 4 . We investigate the flow using a visualization method. The flow states in the centrifugally unstable flow are investigated in the case of counter-rotating cylinders and pure inner cylinder rotation. Beside classical known flow states as Taylor-vortex flow and wavy vortex flow, we observe a variety of new flow structures in the cylindrical annulus, especially for the transition to turbulence. Coexisting turbulent and laminar regions inside the system are observed. Turbulent spots and turbulent bursts are observed, as well as an irregular Taylor-vortex flow and non-stationary turbulent vortices. Especially, a single axially aligned columnar vortex between the inner and outer cylinder is found. The principal regimes observed in flow between independently rotating cylinders are summarized in a flow-regime diagram. This article is part of the theme issue ‘Taylor–Couette and related flows on the centennial of Taylor’s seminal Philosophical Transactions paper (part 2)’.

Turbulent Taylor–Couette flow between two concentric independently rotating cylinders with a radius ratio of $\eta = 0.1$ is studied experimentally. While the scope is to study the counter-rotating cases between both cylinders, the radial and azimuthal velocity components are recorded at different horizontal planes with high-speed particle image velocimetry. The parametric study considered a set of different shear Reynolds numbers in the range of $20\,000 \leq Re_s \leq 1.31 \times 10^5$ , with different rotation ratios of $-0.06 \leq \mu \leq +0.008$ . The observed flow fields had a clear dependence on the rotation ratio, where flow patterns evolved with a more pronounced axial dependence. The angular momentum transport is computed as a result of the recorded flow fields and given by a quasi-Nusselt number. The dependence of the Nusselt number on the different rotation ratios shows a maximum for the low counter-rotating case and $\mu _{max}$ is found between $-0.011 < \mu _{max} < -0.0077$ . The Nusselt number decreases for stronger counter-rotation until a minimum is reached, where it tends to increase again for higher counter-rotation rates. The space–time behaviour of the turbulent flow showed the existence of patterns propagating from the inner region towards the outer region for all studied counter-rotating cases. In addition, patterns have been found that tend to propagate from the outer region towards the inner region with a novel character at high counter-rotation cases. These patterns enhance the angular momentum transport where a second maximum in the transport mechanism has to be expected.

In this study, we investigate the turbulent Taylor-Couette (TC) flow experimentally. First, in order to show the formation of the different patterns in the flow, the flow was seeded with kalliroscope particles and by applying a vertical laser sheet we were able to capture the different patterns in the flow. For a further detailed study, the flow field was measured in horizontal planes at different cylinder heights using Particle image velocimetry (PIV). The formation of patterns appears for the counter-rotation configuration where the outer cylinder rotates in the opposing direction of the inner cylinder rotation. For low shear Reynolds numbers, stable Taylor vortices fulfilling the whole gap appear for very low counter-rotation rates. While for the same rotation rates and higher shear Reynold number, large scale patterns with shorter length scale compared to the classical Taylor vortices appears, which can be described as interlaced fingers from in and outflow. The existence of these patterns leads to enhanced angular momentum transport.

The boundary layers in turbulent Taylor-Couette flow are exposed to transitions from&#xD;laminar to turbulent states if the flow is sufficiently sheared. The present study examines this&#xD;particular transition from the so-called ’classical’ to ’ultimate’ regime experimentally for a very&#xD;wide-gap Taylor-Couette flow with a radius ratio of η = 0.1 and shear Reynolds numbers of up&#xD;to Re_s = 1.5 × 10^5&#xD;. In order to determine the transition, the angular momentum transport&#xD;is measured by using torque sensors at the inner wall. This is complemented by measuring&#xD;the radial and azimuthal velocities via a time-resolved Particle-Image-Velocimetry (PIV). The&#xD;transition to the ultimate regime is found at 2.5 × 104 ≤ Re_s ≤ 3.7 × 104&#xD;. The dimensionless&#xD;angular momentum flux showed an effective scaling of Nu_ω ∼ Re^0.76&#xD;s for Re_s ≥ 2.5×10^4&#xD;and is in&#xD;agreement with the scaling laws used for the ultimate regime in narrow-gap Taylor-Couette flows.&#xD;In addition, a spectral analysis was performed showing the existence of highly energetic small-scale&#xD;and large-scale patterns in the classical regime whereas only highly energetic large-scale patterns&#xD;were observed in the ultimate regime.