Finite-size effects in the dynamics of neutrally buoyant particles in turbulent flow

The dynamics of neutrally buoyant particles transported by a turbulent flow is investigated for spherical particles with radii of the order of the Kolmogorov dissipative scale or larger. The pseudo-penalisation spectral method that has been proposed by Pasquetti et al. (2008) is adapted to integrate numerically the simultaneous dynamics of the particle and of the fluid. Such a method gives a unique handle on the limit of validity of point-particle approximations, which are generally used in applicative situations. Analytical predictions based on such models are compared to result of very well resolved direct numerical simulations. Evidence is obtained that Faxen corrections give dominant finite-size corrections to velocity and acceleration fluctuations for particle diameters up to four times the Kolmogorov scale. The dynamics of particles with larger diameters is dominated by inertial-range physics, and is consistent with predictions obtained from dimensional analysis.Comment: 10 pages, 5 figure

Work Organisation and Innovation - Case Study: LHT, Germany

[Excerpt] Lufthansa Technik AG (LHT) provides aircraft-related technical services to a worldwide customer base comprising airlines, aircraft leasing companies, maintenance organisations, and operators of business and VIP aircrafts. Besides the maintenance, repair, and overhaul (MRO) services that form the organisation’s core business, activities also include development and production activities, as well as logistics

Photon Stars

We discuss numerical solutions of Einstein's field equation describing static, spherically symmetric conglomerations of a photon gas. These equations imply a back reaction of the metric on the energy density of the photon gas according to Tolman's equation. The 3-fold of solutions corresponds to a class of physically different solutions which is parameterized by only two quantities, e.g. mass and surface temperature. The energy density is typically concentrated on a shell because the center contains a repelling singularity, which can, however, not be reached by timelike or null geodesics. The physical relevance of these solutions is completely open, although their existence may raise some doubts w.r. to the stability of black holes.Comment: 10 pages, 5 figures, talk at the DPG Spring Meeting 199

A Lagrangian model for the evolution of turbulent magnetic and passive scalar fields

In this paper we present an extension of the \emph{Recent Fluid Deformation (RFD)} closure introduced by Chevillard and Meneveau (2006) which was developed for modeling the time evolution of Lagrangian fluctuations in incompressible Navier-Stokes turbulence. We apply the RFD closure to study the evolution of magnetic and passive scalar fluctuations. This comparison is especially interesting since the stretching term for the magnetic field and for the gradient of the passive scalar are similar but differ by a sign such that the effect of stretching and compression by the turbulent velocity field is reversed. Probability density functions (PDFs) of magnetic fluctuations and fluctuations of the gradient of the passive scalar obtained from the RFD closure are compared against PDFs obtained from direct numerical simulations

Slipping motion of large neutrally-buoyant particles in turbulence

Direct numerical simulations are used to investigate the individual dynamics of large spherical particles suspended in a developed homogeneous turbulent flow. A definition of the direction of the particle motion relative to the surrounding flow is introduced and used to construct the mean fluid velocity profile around the particle. This leads to an estimate of the particle slipping velocity and its associated Reynolds number. The flow modifications due to the particle are then studied. The particle is responsible for a shadowing effect that occurs in the wake up to distances of the order of its diameter: the particle pacifies turbulent fluctuations and reduces the energy dissipation rate compared to its average value in the bulk. Dimensional arguments are presented to draw an analogy between particle effects on turbulence and wall flows. Evidence is obtained on the presence of a logarithmic sublayer at distances between the thickness of the viscous boundary layer and the particle diameter \Dp. Finally, asymptotic arguments are used to relate the viscous sublayer quantities to the particle size and the properties of the outer turbulence. It is shown in particular that the skin-friction Reynolds number behaves as Re_\tau \propto (\Dp/\eta)^{4/3}.Comment: 10 pages, 6 figure

Numerical Simulations and Analysis of Thermally Excited Waves in Plasma Crystals

A numerical model for a 2D-monolayer plasma crystal was established using the Box_tree code. Box_tree is a Barnes_Hut tree code which has proven effective in modeling systems composed of large numbers of particles. Thermally excited waves in this plasma crystal were numerically simulated and dispersion relations for both the longitudinal and transverse wave modes were found. These were compared with the dispersion relations extrapolated from experiment as well as a theory based on harmonic approximation. The results were found to agree with theoretical dispersion relations under different wave propagation directions with different particle charges and over a range of 0.9<k<5.Comment: 7 pages, Presented at COSPAR '0

Unexploited Agricultural Growth: The Case of Crop–Livestock Production Systems in Zimbabwe

Livestock is the most important source of cash for small-scale farmers in the semi-arid tropics of southern Africa. However, with limited access to markets, farmers do not have the incentive to invest in improved livestock management. Livestock production and off-takes remain low and farmers are unable to realize the full potential of their herds. We believe that improved market access will be the driver to increase technology adoption for income growth and poverty reduction. In Zimbabwe, a recent baseline diagnosis by ICRISAT and partners found that cash income from goats is crucial to cover day-to-day expenditures for food, education and human health. Cattle are more important for draft power and milk, and support subsistence cropping activities. Major production constraints include high mortality rates attributed to dry season feed shortages, particularly affecting farmers with small herds. An increasing demand for livestock products in rural and urban areas offers small-scale farmers opportunities for market participation. However, the existing markets are underdeveloped, with high transaction costs implying low producer prices and poor access to information for farmers. The challenge is to sustain livestock production, develop more effective market facilities, and thereby increase off-take. The potential of market-led technology development in crop–livestock systems has not been sufficiently exploited by research and development. To have an impact on incomes and poverty, we develop an innovative approach that would first evaluate local constraints in production and marketing, and then test alternative livestock markets and management strategies, with a strong linkage between private and public sectorsAgricultural and Food Policy, Consumer/Household Economics, Environmental Economics and Policy, Farm Management, Food Consumption/Nutrition/Food Safety, Food Security and Poverty, Livestock Production/Industries, Marketing, Productivity Analysis, Research and Development/Tech Change/Emerging Technologies,

Bibliography on inactivation of viruses and rickettsiae by heat

Inactivation of viruses and rickettsiae by heat - bibliograph