Highly turbulent Taylor-Couette flow with spanwise-varying roughness is investigated experimentally and numerically (direct numerical simulations (DNS) with an immersed boundary method (IBM)) to determine the effects of the spacing and axial width s of the spanwise varying roughness on the total drag and on the flow structures. We apply sandgrain roughness, in the form of alternating rough and smooth bands to the inner cylinder. Numerically, the Taylor number is O(10 9 ) and the roughness width is varied between 0.47 s = s/d 1.23, where d is the gap width. Experimentally, we explore Ta = O(10 12 ) and 0.61 s 3.74. For both approaches the radius ratio is fixed at η = r i /r o = 0.716, with r i and r o the radius of the inner and outer cylinder respectively. We present how the global transport properties and the local flow structures depend on the boundary conditions set by the roughness spacings. Both numerically and experimentally, we find a maximum in the angular momentum transport as function ofs. This can be atributed to the re-arrangement of the large-scale structures triggered by the presence of the rough stripes, leading to correspondingly large-scale turbulent vortices.
Practically all flows are turbulent in nature and contain some kind of irregularly-shaped particles, e.g. dirt, pollen, or life forms such as bacteria or insects. The effect of the particles on such flows and vice-versa are highly non-trivial and are not completely understood, particularly when the particles are finite-sized. Here we report an experimental study of millimetric fibers in a strongly sheared turbulent flow. We find that the fibers show a preferred orientation of −0.38π ± 0.05π (−68 ± 9 • ) with respect to the mean flow direction in high-Reynolds number Taylor-Couette turbulence, for all studied Reynolds numbers, fiber concentrations, and locations. Despite the finite-size of the anisotropic particles, we can explain the preferential alignment by using Jefferey's equation, which provides evidence of the benefit of a simplified point-particle approach. Furthermore, the fiber angular velocity is strongly intermittent, again indicative of point-particle-like behavior in turbulence. Thus large anisotropic particles still can retain signatures of the local flow despite classical spatial and temporal filtering effects.
We provide experimental measurements for the effective scaling of the TaylorReynolds number within the bulk Re λ,bulk , based on local flow quantities as a function of the driving strength (expressed as the Taylor number Ta), in the ultimate regime of Taylor-Couette flow. We define Re λ,bulk = (σ bulk (u θ )) 2 (15/(ν bulk )) 1/2 , where σ bulk (u θ ) is the bulk-averaged standard deviation of the azimuthal velocity, bulk is the bulk-averaged local dissipation rate and ν is the liquid kinematic viscosity. The data are obtained through flow velocity field measurements using particle image velocimetry. We estimate the value of the local dissipation rate (r) using the scaling of the second-order velocity structure functions in the longitudinal and transverse directions within the inertial range -without invoking Taylor's hypothesis. We find an effective scaling of bulk /(ν 3 d −4 ) ∼ Ta 1.40 , (corresponding to Nu ω,bulk ∼ Ta 0.40 for the dimensionless local angular velocity transfer), which is nearly the same as for the global energy dissipation rate obtained from both torque measurements (Nu ω ∼ Ta 0.40 ) and direct numerical simulations (Nu ω ∼ Ta 0.38 ). The resulting Kolmogorov length scale is then found to scale as η bulk /d ∼ Ta −0.35 and the turbulence intensity as I θ,bulk ∼ Ta −0.061 . With both the local dissipation rate and the local fluctuations available we finally find that the Taylor-Reynolds number effectively scales as Re λ,bulk ∼ Ta 0.18 in the present parameter regime of 4.0 × 10 8 < Ta < 9.0 × 10 10 .
Emulsions are omnipresent in the food industry, health care, and chemical synthesis. In this Letter the dynamics of metastable oil-water emulsions in highly turbulent (10 11 ≤ Ta ≤ 3 × 10 13 ) Taylor-Couette flow, far from equilibrium, is investigated. By varying the oil-in-water void fraction, catastrophic phase inversion between oil-in-water and water-in-oil emulsions can be triggered, changing the morphology, including droplet sizes, and rheological properties of the mixture, dramatically. The manifestation of these different states is exemplified by combining global torque measurements and local in situ laser induced fluorescence microscopy imaging. Despite the turbulent state of the flow and the dynamic equilibrium of the oil-water mixture, the global torque response of the system is found to be as if the fluid were Newtonian, and the effective viscosity of the mixture was found to be several times bigger or smaller than either of its constituents.
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