Turbulence and jet-driven zonal flows: Secondary circulation in rotating fluids due to asymmetric forcing

We report on experiments and modeling on a rotating confined liquid that is forced by circumferential jets coaxial with the rotation axis, wherein system-scale secondary flows are observed to emerge. The jets are evenly divided in number between inlets and outlets and have zero net mass transport. For low forcing strengths the sign of this flow depends on the sign of a sloped end cap, which simulates a planetary β plane. For increased forcing strengths the secondary flow direction is insensitive to the slope sign, and instead appears to be dominated by an asymmetry in the forcing mechanism, namely, the difference in radial divergence between the inlet and outlet jet profiles. This asymmetry yields a net radial velocity that is affected by the Coriolis force, inducing secondary zonal flow

Turbulence and jet-driven zonal flows: Secondary circulation in rotating fluids due to asymmetric forcing

We report on experiments and modeling on a rotating confined liquid that is forced by circumferential jets coaxial with the rotation axis, wherein system-scale secondary flows are observed to emerge. The jets are evenly divided in number between inlets and outlets and have zero net mass transport. For low forcing strengths the sign of this flow depends on the sign of a sloped end cap, which simulates a planetary β plane. For increased forcing strengths the secondary flow direction is insensitive to the slope sign, and instead appears to be dominated by an asymmetry in the forcing mechanism, namely, the difference in radial divergence between the inlet and outlet jet profiles. This asymmetry yields a net radial velocity that is affected by the Coriolis force, inducing secondary zonal flow

O mandarová da mandioca.

Biologia do inseto; Ovo; Larva; Pré-pupa; Pupa; Adulto; Época de ocorrência; Controle; Controle cultural; Controle biológico; Controle químico.bitstream/item/81297/1/Mandarova-ALBA-FARIAS-Circular-Tecnica-5-1980.pdfMemória

Effect of axially varying sandpaper roughness on bubbly drag reduction in Taylor–Couette turbulence

We experimentally investigate the influence of alternating rough and smooth walls on bubbly drag reduction (DR). To this end, we apply rough sandpaper bands of width s between 48.4mm and 148.5mm, and roughness height around the smooth inner cylinder of the Twente Turbulent Taylor–Couette facility. Between two sandpaper bands, the inner cylinder is left uncovered over similar width s, resulting in alternating rough and smooth bands, forming a constant pattern in axial direction. We measure the DR in water that originates from introducing air bubbles to the fluid at (shear) Reynolds numbers Res ranging from 0.5 × 106 to 1.8 × 106. Results are compared to bubbly DR measurements with a completely smooth inner cylinder and an inner cylinder that is completely covered with sandpaper of the same roughness k. The outer cylinder is left smooth for all variations. The results are also compared to bubbly DR measurements where a smooth outer cylinder is rotating in opposite direction to the smooth inner cylinder. This counter rotation induces secondary flow structures that are very similar to those observed when the inner cylinder is composed of alternating rough and smooth bands. For the measurements with roughness, the bubbly DR is found to initially increase more strongly with Res, before levelling off to reach a value that no longer depends on Res. This is attributed to a more even axial distribution of the air bubbles, resulting from the increased turbulence intensity of the flow compared to flow over a completely smooth wall at the same Res. The air bubbles are seen to accumulate at the rough wall sections in the flow. Here, locally, the drag is largest and so the drag reducing effect of the bubbles is felt strongest. Therefore, a larger maximum value of bubbly DR is found for the alternating rough and smooth walls compared to the completely rough wall

Statistics of rigid fibers in strongly sheared turbulence

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 effects of the particles on such flows and vice versa are highly nontrivial 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 -0.38 pi +/- 0.05 pi (-68 +/- 9 degrees) 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

Double maxima of angular momentum transport in small gap Taylor–Couette turbulence

We use experiments and direct numerical simulations to probe the phase space of low-curvature Taylor–Couette flow in the vicinity of the ultimate regime. The cylinder radius ratio is fixed at η = ri/ro = 0.91, where ri (ro) is the inner (outer) cylinder radius. Non-dimensional shear drivings (Taylor numbers Ta) in the range 107 ≤ Ta ≤ 1011 are explored for both co- and counter-rotating configurations. In the Ta range 108 ≤ Ta ≤ 1010, we observe two local maxima of the angular momentum transport as a function of the cylinder rotation ratio, which can be described as either ‘co-’ or ‘counter-rotating’ due to their location or as ‘broad’ or ‘narrow’ due to their shape. We confirm that the broad peak is accompanied by the strengthening of the large-scale structures, and that the narrow peak appears once the driving (Ta) is strong enough. As first evidenced in numerical simulations by Brauckmann et al. (J. Fluid Mech., vol. 790, 2016, pp. 419–452), the broad peak is produced by centrifugal instabilities and that the narrow peak is a consequence of shear instabilities. We describe how the peaks change with Ta as the flow becomes more turbulent. Close to the transition to the ultimate regime when the boundary layers (BLs) become turbulent, the usual structure of counter-rotating Taylor vortex pairs breaks down and stable unpaired rolls appear locally. We attribute this state to changes in the underlying roll characteristics during the transition to the ultimate regime. Further changes in the flow structure around Ta ≈ 1010 cause the broad peak to disappear completely and the narrow peak to move. This second transition is caused when the regions inside the BLs which are locally smooth regions disappear and the whole boundary layer becomes active

Controlling secondary flow in Taylor-Couette turbulence through spanwise-varying roughness

Highly turbulent Taylor-Couette flow with spanwise-varying roughness is investigated experimentally and numerically (direct numerical simulations with an immersed boundary method) to determine the effects of the spacing and spanwise 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 in the range 0.47 <= (s) over tilde = s/d <= 1.23, where d is the gap width. Experimentally, we explore Ta = O(10(12)) and 0.61 <= (s) over tilde <= 3.74. For both approaches the radius ratio is fixed at eta = 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 spacing (s) over tilde. Both numerically and experimentally, we find a maximum in the angular momentum transport as a function of (s) over tilde. This can be attributed to the re-arrangement of the large-scale structures triggered by the presence of the rough stripes, leading to correspondingly large-scale turbulent vortices

Synchrotron-based structural and spectroscopic studies of ball milled RuSeMo and RuSnMo particles as oxygen reduction electrocatalyst for PEM fuel cells

Particles of RuSeMo and RuSnMo have been produced by ball milling; they present catalytic activity towards the oxygen reduction reaction (ORR) in acid media. A Tafel slope close to 120 mV/dec was found for both materials. Their morphology was first characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images reveal particles in the sub-micrometer range. The structure of the materials was further probed with synchrotron radiation powder X-ray diffraction (SR-PXD) and X-ray absorption spectroscopy (XAS). SR-PXD reveals the existence of metallic Ru as the main phase and the formation of phases such as RuSe2 in RuSeMo and Ru3Sn7 in RuSnMo. Mo was found to form solid solution into the RuSe2 phase in ball milled RuSeMo. Finally, The Ru L-3-edge and Mo L-3-edge XAS fingerprints were correlated with the catalytic activity towards ORR. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved