Numerical investigation of the wall pressure fluctuations in channel flows

Numerically simulated turbulent pressure fields in a plane channel and an annulus with a 5:1 ratio of radii are used to evaluate the spatial structure of pressure fluctuations at the walls. Wall-pressure correlation functions in the axial and azimuthal directions, spectra, correlation functions and angular correlations of the forces acting on the cylinders of the annulus are considered. It is found that no correlation exists between the pressure values at two points which are diametrically opposed to each other in the annulus. The spectrum of the forces starts with a zero value for small wave numbers, reaches its maximum at about unity wave number and then decreases, as a −7/3rd power of the wave number

The global lightning-induced nitrogen oxides source

The knowledge of the lightning-induced nitrogen oxides (LNO<sub>x</sub>) source is important for understanding and predicting the nitrogen oxides and ozone distributions in the troposphere and their trends, the oxidising capacity of the atmosphere, and the lifetime of trace gases destroyed by reactions with OH. This knowledge is further required for the assessment of other important NO<sub>x</sub> sources, in particular from aviation emissions, the stratosphere, and from surface sources, and for understanding the possible feedback between climate changes and lightning. This paper reviews more than 3 decades of research. The review includes laboratory studies as well as surface, airborne and satellite-based observations of lightning and of NO<sub>x</sub> and related species in the atmosphere. Relevant data available from measurements in regions with strong LNO<sub>x</sub> influence are identified, including recent observations at midlatitudes and over tropical continents where most lightning occurs. Various methods to model LNO<sub>x</sub> at cloud scales or globally are described. Previous estimates are re-evaluated using the global annual mean flash frequency of 44±5 s<sup>−1</sup> reported from OTD satellite data. From the review, mainly of airborne measurements near thunderstorms and cloud-resolving models, we conclude that a "typical" thunderstorm flash produces 15 (2–40)×10<sup>25</sup> NO molecules per flash, equivalent to 250 mol NO<sub>x</sub> or 3.5 kg of N mass per flash with uncertainty factor from 0.13 to 2.7. Mainly as a result of global model studies for various LNO<sub>x</sub> parameterisations tested with related observations, the best estimate of the annual global LNO<sub>x</sub> nitrogen mass source and its uncertainty range is (5±3) Tg a<sup>−1</sup> in this study. In spite of a smaller global flash rate, the best estimate is essentially the same as in some earlier reviews, implying larger flash-specific NO<sub>x</sub> emissions. The paper estimates the LNO<sub>x</sub> accuracy required for various applications and lays out strategies for improving estimates in the future. An accuracy of about 1 Tg a<sup>−1</sup> or 20%, as necessary in particular for understanding tropical tropospheric chemistry, is still a challenging goal

The genus Metastelma (Asclepiadoideae-Asclepiadeae-Metastelmatinae) in South America

Molecular and morphological analyses are used to circumscribe the genus Metastelma in South America. In northem South America, fourteen species are identified, nine o f them new; and two species are found in Southern South America, one o f them new. The northem South American lineage is retrieved as sister to a clade comprising the type species of Metastelma, M. parviflorum, one o f several Caribbean lineages. The Southern South American lineage remains unresolved. The Central American lineage remains poorly supported monophyletic, but ineludes a species from Jamaica, M. priorii. Several specimens from the Andes o f Argentina and Bolivia are retrieved in a well-supported clade outside Metastelma s.s. For these specimens, a new genus is described comprising presently seven species. Four o f these species are transferred from Metastelma, one from Ditassa, and two species are described as new Stelmation E.Foum. is resurrected, comprising presently a single species, S. myrtifolium from Brazil. Metastelma giuliettianum and M. harleyi, also from Brazil, are excluded from Metastelma, but cannot yet be transferred to another genus, because the taxonomy o f Metastelmatinae is still in flux. Thus, Metastelma s.s. is absent from Brazil

Cool contrails

Contrails are cirrus clouds which warm or cool the Earth depending on flight route and weather. Hence, the climate impact of aviation can be minimised by avoiding warming contrails and allowing for cooling contrails by proper weather dependent route selection. This article summarises recent research results on this topic

Realizability of Reynolds-Stress Turbulence Models

It is shown that certain existing phenomenological models of turbulence in terms of differential equations for the Reynolds stresses Rαβ=(ulα ulβ) do not guarantee realizable solutions. The known realizability conditions are Rαβ ≥ 0 for α=β and R2αβ ≥ RααRββ for α≠β. A stronger requirement is that the matrix Rαβ be positive semi-definite. This implies three conditions like non-negative eigenvalues or non-negative principal invariants. Conditions are given which must be satisfied by the model itself in order to guarantee realizable solutions for any realizable initial and boundary conditions. Some means are proposed which can be used to change the existing models into realizable ones