Abstract
In this paper, we experimentally study the influence of large-scale Taylor rolls on the small-scale statistics and the flow organization in fully turbulent Taylor–Couette flow for Reynolds numbers up to $Re_{S}=3\times 10^{5}$. The velocity field in the gap confined by coaxial and independently rotating cylinders at a radius ratio of $\unicode[STIX]{x1D702}=0.714$ is measured using planar particle image velocimetry in horizontal planes at different cylinder heights. Flow regions with and without prominent Taylor vortices are compared. We show that the local angular momentum transport (expressed in terms of a Nusselt number) mainly takes place in the regions of the vortex in- and outflow, where the radial and azimuthal velocity components are highly correlated. The efficient momentum transfer is reflected in intermittent bursts, which becomes visible in the exponential tails of the probability density functions of the local Nusselt number. In addition, by calculating azimuthal energy co-spectra, small-scale plumes are revealed to be the underlying structure of these bursts. These flow features are very similar to the one observed in Rayleigh–Bénard convection, which emphasizes the analogies of these systems. By performing a complex proper orthogonal decomposition, we remarkably detect azimuthally travelling waves superimposed on the turbulent Taylor vortices, not only in the classical but also in the ultimate regime. This very large-scale flow pattern, which is most pronounced at the axial location of the vortex centre, is similar to the well-known wavy Taylor vortex flow, which has comparable wave speeds, but much larger azimuthal wavenumbers.
Highlights
The flow in between two independently rotating cylinders, known as Taylor–Couette (TC) flow, is a commonly used model for general rotating shear flows
We experimentally study the influence of large-scale Taylor rolls on the small-scale statistics and the flow organization in fully turbulent Taylor–Couette flow for Reynolds numbers up to ReS = 3 × 105
Based on this literature review, the following open questions are addressed within this manuscript: how do turbulence-dominated and vortex-dominated flow states differ with respect to their velocity field statistics, how important are the vortex inflow, vortex centre and vortex outflow regions for the momentum transport, how do small-scale structures affect this transport, what is the length scale of these structures and do wavy-vortex-like turbulent Taylor vortices exist in the ultimate turbulent regime? To answer these questions, we make use of particle image velocimetry (PIV) measurements in horizontal planes at different cylinder heights for η = 0.714, in the range of ReS ∈ [9.3 × 103, 3.5 × 105] and μ ∈ [0, −0.36]
Summary
The flow in between two independently rotating cylinders, known as Taylor–Couette (TC) flow, is a commonly used model for general rotating shear flows. The DNSs of Ostilla-Mónico et al (2016) in the boundary layer regions reveal a peak in the azimuthal and axial spectra of the radial velocity component at large wavenumbers, which indicates the existence of small-scale plumes Their simulations were done for η = 0.909 and μ = 0 at ReS 105. Based on this literature review, the following open questions are addressed within this manuscript: how do turbulence-dominated and vortex-dominated flow states differ with respect to their velocity field statistics, how important are the vortex inflow, vortex centre and vortex outflow regions for the momentum transport, how do small-scale structures affect this transport, what is the length scale of these structures and do wavy-vortex-like turbulent Taylor vortices exist in the ultimate turbulent regime?
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