Examining the inertial subrange with nanoscale cross-wire measurements of turbulent pipe flow at high Reynolds number near the centreline [post-print]

Highly resolved, two-component velocity measurements were made near the centreline of turbulent pipe flow for Reynolds numbers in the range . These unique data were obtained with a nanoscale cross-wire probe and used to examine the inertial subrange scaling of the longitudinal and transverse velocity components. Classical dissipation rate estimates were made using both the integration of one-dimensional dissipation spectra for each velocity component and the third-order moment of the longitudinal structure function. Although the second-order moments and one-dimensional spectra for each component showed behaviour consistent with local isotropy, clear inertial range similarity and behaviour were not exhibited in the third-order structure functions at these Reynolds numbers. When corrected for the effects of radial inhomogeneities at the centreline following the generalized expression of Danaila et al. (J. Fluid Mech., vol. 430, 2001, pp. 87-109), re-derived for the pipe flow domain, the third-order moments of the longitudinal structure function exhibited a clearer plateau per the classical Kolmogorov \u27four-fifths law\u27. Similar corrections described by Danaila et al. (J. Fluid Mech., vol. 430, 2001, pp. 87-109) applied to the analogous equation for the mixed structure functions (i.e. the \u27four-thirds law\u27) also yielded improvement over all ranges of scale, improving with increasing Reynolds number. The rate at which the \u27four-fifths\u27 law and \u27four-thirds\u27 law were approached by the third-order structure functions was found to be more gradual than decaying isotropic turbulence for the same Reynolds numbers

Flow in a Commercial Steel Pipe

Fully-developed turbulent flow in a commercial steel pipe is studied using single component hot-wire probes in both oneand two-point experiments. The streamwise turbulence component was measured over a Reynolds number range from 7.6× 104 to 8.3×106, covering the smooth to fully rough regimes. The experiments were conducted in the Princeton/ONR Superpipe facility that uses compressed air at pressures up to 200 atm as the working fluid. For Reynolds numbers less than about 8 ×105 the surface was hydraulically-smooth, and the results agreed closely with the smooth-wall turbulence intensity and spectral data obtained by Morrison et al. [10] and Zhao & Smits [14]. An assessment was performed of probe resolution and results indicate that the turbulence statistics of the large-scale motions were unaffected by the sensing wire length even at high Reynolds numbers. Transitionally-rough and fully-rough data showed deviation from the smooth-wall data as roughness effects became more prominent. In particular, the outer peak in the turbulence intensity observed at high Reynolds numbers in smooth pipe flow decreased in magnitude or stayed constant for transitionally rough and fully rough flow. The two-point azimuthal correlations were found to be consistent with the presence of very large scale coherent regions of low-wavenumber, low-momentum fluid observed in previous studies of wall-bounded flows. The correlations indicated that the azimuthal scale of these regions is Reynolds number independent

Origins of the open-circuit voltage in ternary organic solar cells and design rules for minimized voltage losses

The power conversion efficiency of ternary organic solar cells (TOSCs), consisting of one host binary blend and one guest component, remains limited by large voltage losses. The fundamental understanding of the open-circuit voltage (V OC) in TOSCs is controversial, limiting rational design of the guest component. In this study, we systematically investigate how the guest component affects the radiative and non-radiative related parts of V OC of a series of TOSCs using the detailed balanced principle. We highlight that the thermal population of charge-transfer and local exciton states provided by the guest binary blend (that is, the guest-component-based binary blend) has a significant influence on the non-radiative voltage losses. Ultimately, we provide two design rules for enhancing the V OC in TOSCs: high emission yield for the guest binary blend and similar charge-transfer-state energies for host/guest binary blends; high miscibility of the guest component with the low gap component in the host binary blend

Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes

This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from Re_θ = 11 100 to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here

Wall roughness induces asymptotic ultimate turbulence

Turbulence is omnipresent in Nature and technology, governing the transport of heat, mass, and momentum on multiple scales. For real-world applications of wall-bounded turbulence, the underlying surfaces are virtually always rough; yet characterizing and understanding the effects of wall roughness for turbulence remains a challenge, especially for rotating and thermally driven turbulence. By combining extensive experiments and numerical simulations, here, taking as example the paradigmatic Taylor-Couette system (the closed flow between two independently rotating coaxial cylinders), we show how wall roughness greatly enhances the overall transport properties and the corresponding scaling exponents. If only one of the walls is rough, we reveal that the bulk velocity is slaved to the rough side, due to the much stronger coupling to that wall by the detaching flow structures. If both walls are rough, the viscosity dependence is thoroughly eliminated in the boundary layers and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of transport, whose existence had been predicted by Robert Kraichnan in 1962 (Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be extrapolated to arbitrarily large Reynolds numbers

Dynamics of Immune System Gene Expression upon Bacterial Challenge and Wounding in a Social Insect (Bombus terrestris)

The innate immune system which helps individuals to combat pathogens comprises a set of genes representing four immune system pathways (Toll, Imd, JNK and JAK/STAT). There is a lack of immune genes in social insects (e.g. honeybees) when compared to Diptera. Potentially, this might be compensated by an advanced system of social immunity (synergistic action of several individuals). The bumble bee, Bombus terrestris, is a primitively eusocial species with an annual life cycle and colonies headed by a single queen. We used this key pollinator to study the temporal dynamics of immune system gene expression in response to wounding and bacterial challenge

Immune pathways and defence mechanisms in honey bees Apis mellifera

Social insects are able to mount both group-level and individual defences against pathogens. Here we focus on individual defences, by presenting a genome-wide analysis of immunity in a social insect, the honey bee Apis mellifera. We present honey bee models for each of four signalling pathways associated with immunity, identifying plausible orthologues for nearly all predicted pathway members. When compared to the sequenced Drosophila and Anopheles genomes, honey bees possess roughly one-third as many genes in 17 gene families implicated in insect immunity. We suggest that an implied reduction in immune flexibility in bees reflects either the strength of social barriers to disease, or a tendency for bees to be attacked by a limited set of highly coevolved pathogens

Carbohydrate metabolism genes and pathways in insects: insights from the honey bee genome

Carbohydrate-metabolizing enzymes may have particularly interesting roles in the honey bee, Apis mellifera, because this social insect has an extremely carbohydrate-rich diet, and nutrition plays important roles in caste determination and socially mediated behavioural plasticity. We annotated a total of 174 genes encoding carbohydrate-metabolizing enzymes and 28 genes encoding lipid-metabolizing enzymes, based on orthology to their counterparts in the fly, Drosophila melanogaster, and the mosquito, Anopheles gambiae. We found that the number of genes for carbohydrate metabolism appears to be more evolutionarily labile than for lipid metabolism. In particular, we identified striking changes in gene number or genomic organization for genes encoding glycolytic enzymes, cellulase, glucose oxidase and glucose dehydrogenases, glucose-methanol-choline (GMC) oxidoreductases, fucosyltransferases, and lysozymes