Identification of Nonlinear Normal Modes of Engineering Structures under Broadband Forcing

The objective of the present paper is to develop a two-step methodology integrating system identification and numerical continuation for the experimental extraction of nonlinear normal modes (NNMs) under broadband forcing. The first step processes acquired input and output data to derive an experimental state-space model of the structure. The second step converts this state-space model into a model in modal space from which NNMs are computed using shooting and pseudo-arclength continuation. The method is demonstrated using noisy synthetic data simulated on a cantilever beam with a hardening-softening nonlinearity at its free end.Comment: Journal pape

Efficient numerical calculation of drift and diffusion coefficients in the diffusion approximation of kinetic equations

In this paper we study the diffusion approximation of a swarming model given by a system of interacting Langevin equations with nonlinear friction. The diffusion approximation requires the calculation of the drift and diffusion coefficients that are given as averages of solutions to appropriate Poisson equations. We present a new numerical method for computing these coefficients that is based on the calculation of the eigenvalues and eigenfunctions of a Schr\"odinger operator. These theoretical results are supported by numerical simulations showcasing the efficiency of the method

Risks associated with endotoxins in feed additives produced by fermentation

Acknowledgements We thank Jordi Tarrés Call, who recorded the discussions and offered advice about procedures, and Nicole Reisinger and Gerd Schatzmayr, who provided valuable information about endotoxins in animal feeds. The Rowett Institute of Nutrition and Health is funded by the Rural and Environment Science and Analytical Services Division (RESAS) of the Scottish Government.Peer reviewedPublisher PD

Stellar populations in the surrounding field of the LMC clusters NGC 2154 and NGC 1898

In this paper we present a study and comparison of the star formation rates (SFR) in the fields around NGC 1898 and NGC 2154, two intermediate-age star clusters located in very different regions of the Large Magellanic Cloud. We also present a photometric study of NGC 1898, and of seven minor clusters which happen to fall in the field of NGC 1898, for which basic parameters were so far unknown. We do not focus on NGC 2154, because this cluster was already investigated in Baume et al. 2007, using the same theoretical tools. The ages of the clusters were derived by means of the isochrone fitting method on their $clean$ color-magnitude diagrams. Two distinct populations of clusters were found: one cluster (NGC 2154) has a mean age of 1.7 Gyr, with indication of extended star formation over roughly a 1 Gyr period, while all the others have ages between 100 and 200 Myr. The SFRs of the adjacent fields were inferred using the downhill-simplex algorithm. Both SFRs show enhancements at 200, 400, 800 Myr, and at 1, 6, and 8 Gyr. These bursts in the SFR are probably the result of dynamical interactions between the Magellanic Clouds (MCs), and of the MCs with the Milky Way.Comment: 10 pages, 11 eps figures, in press in MNRAS. For a version including references contact the author

Propagation dynamics on networks featuring complex topologies

Analytical description of propagation phenomena on random networks has flourished in recent years, yet more complex systems have mainly been studied through numerical means. In this paper, a mean-field description is used to coherently couple the dynamics of the network elements (nodes, vertices, individuals...) on the one hand and their recurrent topological patterns (subgraphs, groups...) on the other hand. In a SIS model of epidemic spread on social networks with community structure, this approach yields a set of ODEs for the time evolution of the system, as well as analytical solutions for the epidemic threshold and equilibria. The results obtained are in good agreement with numerical simulations and reproduce random networks behavior in the appropriate limits which highlights the influence of topology on the processes. Finally, it is demonstrated that our model predicts higher epidemic thresholds for clustered structures than for equivalent random topologies in the case of networks with zero degree correlation.Comment: 10 pages, 5 figures, 1 Appendix. Published in Phys. Rev. E (mistakes in the PRE version are corrected here

A Summary of Main Experimental Results Concerning the Secondary Electron Emission of Copper

The secondary electron emission of surfaces exposed to the impact of energetic electrons contributes significantly to the electron cloud build-up. For the prediction of the consequences of this effect the measurements of the secondary electron yield carried out at CERN are an important source of information. New experimental results concerning the total secondary electron yield for very low primary electron energy (between 5 eV and 50 eV) will be also given in the case of as received copper. Furthermore the energy distribution of the re-emitted electrons is drastically influenced by the primary electron energy. The ratio of the number of reflected electrons to the total number of re-emitted electrons has been measured and its variation with the primary electron energy will be shown. As a consequence of these new experimental data, a numerical approximation to express the secondary electron yield as a function of the primary electron energy will be given for the low incident electron energy region (E < 50 eV). It has been shown that the decrease of the secondary electron yield due to the electron bombardment could reduce sufficiently the consequences electron cloud effect. To understand further the origin of this decrease, the results of experiments showing the variation of the electron induced desorption yield with the incident electron dose will be compared to the concomitant reduction of the secondary electron yield

The variation of the secondary electron yield and of the desorption yield of copper under electron bombardment: Origin and impact on the conditioning of the LHC

The operation of most radio-frequency components in accelerators rely on the conditioning obtained usually by gradually increasing the power fed into these devices. This effect is related to the reduction of the desorption yields and of the secondary electron yields of surfaces exposed to electron bombardment. In LHC, similar decreases will also limit the detrimental consequences of the electron cloud phenomenon on the beam stability, the power deposited in the cryogenic system and the gas density. The parallel evolution of the desorption yield and of the secondary electron yield will be discussed and compared to the changes to the surface composition as observed by surface analytical techniques. The possible origin of the secondary electron yield decrease will also be discussed at the light of these new results