Multi-Level quasi-Newton methods for the partitioned simulation of fluid-structure interaction

In previous work of the authors, Fourier stability analyses have been performed of Gauss-Seidel iterations between the flow solver and the structural solver in a partitioned fluid-structure interaction simulation. These analyses of the flow in an elastic tube demonstrated that only a number of Fourier modes in the error on the interface displacement are unstable. Moreover, the modes with a low wave number are most unstable and these modes can be resolved on a coarser grid. Therefore, a new class of quasi-Newton methods with more than one grid level is introduced. Numerical experiments show a significant reduction in run time

Asteroseismology of eclipsing binary stars using Kepler and the HERMES spectrograph

We introduce our PhD project in which we focus on pulsating stars in eclipsing binaries. The combination of high-precision Kepler photometry with high-resolution HERMES spectroscopy allows for detailed descriptions of our sample of target stars. We report here the detection of three false positives by radial velocity measurements.Comment: Proceedings paper, 2 pages, 2 figures, to appear in "Setting a New Standard in the Analysis of Binary Stars", Eds K. Pavlovski, A. Tkachenko, and G. Torres, EAS Publications Serie

On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars

The Fourier Transform method is a popular tool to derive the rotational velocities of stars from their spectral line profiles. However, its domain of validity does not include line-profile variables with time-dependent profiles. We investigate the performance of the method for such cases, by interpreting the line-profile variations of spotted B stars, and of pulsating B tars, as if their spectral lines were caused by uniform surface rotation along with macroturbulence. We perform time-series analysis and harmonic least-squares fitting of various line diagnostics and of the outcome of several implementations of the Fourier Transform method. We find that the projected rotational velocities derived from the Fourier Transform vary appreciably during the pulsation cycle whenever the pulsational and rotational velocity fields are of similar magnitude. The macroturbulent velocities derived while ignoring the pulsations can vary with tens of km/s during the pulsation cycle. The temporal behaviour of the deduced rotational and macroturbulent velocities are in antiphase with each other. The rotational velocity is in phase with the second moment of the line profiles. The application of the Fourier method to stars with considerable pulsational line broadening may lead to an appreciable spread in the values of the rotation velocity, and, by implication, of the deduced value of the macroturbulence. These two quantities should therefore not be derived from single snapshot spectra if the aim is to use them as a solid diagnostic for the evaluation of stellar evolution models of slow to moderate rotators.Comment: 13 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

Wind-structure interaction simulations for the prediction of ovalling vibrations in silo groups

Wind-induced ovalling vibrations were observed during a storm in October 2002 on several empty silos of a closely spaced group consisting of 8 by 5 thin-walled silos in the port of Antwerp (Belgium). The purpose of the present research is to investigate if such ovalling vibrations can be predicted by means of numerical simulations. More specifically, the necessity of performing computationally demanding wind-structure interaction (WSI) simulations is assessed. For this purpose, both one-way and two-way coupled simulations are performed. Before considering the entire silo group, a single silo in crosswind is simulated. The simulation results are in reasonably good agreement with observations and WSI simulations seem to be required for a correct prediction of the observed ovalling vibrations

An examination of some characteristics of Kepler Short and Long Cadence Data

A close comparison of Kepler short- and long-cadence data released prior to 2011 Nov 1 has shown some subtle differences that make the short-cadence data superior to their long-cadence counterparts. The inevitable results of a faster sampling rate are present: the short-cadence data provide greater time resolution for short-lived events like flares, and have a much higher Nyquist frequency than the long-cadence data; however, they also contain fewer high-amplitude peaks at low frequency and allow a more precise determination of pulsation frequencies, amplitudes and phases. The latter observation indicates that Kepler data are not normally distributed. Moreover, a close inspection of the Pre-search Data Conditioned (PDC) long-cadence data show residuals that have increased noise on time-scales important to asteroseismology, but unimportant to planet searches.Comment: Accepted for publication in MNRAS. 7 pages, 5 figure

Nitrogen Risk Assessment Model for Scotland: II. Hydrological transport and model testing

International audienceThe amount and concentration of N in catchment runoff is strongly controlled by a number of hydrological influences, such as leaching rates and the rate of transport of N from the land to surface water bodies. This paper describes how the principal hydrological controls at a catchment scale have been represented within the Nitrogen Risk Assessment Model for Scotland (NIRAMS); it demonstrates their influence through application of the model to eight Scottish catchments, contrasting in terms of their land use, climate and topography. Calculation of N leaching rates, described in the preceding paper (Dunn et al., 2004), is based on soil water content determined by application of a weekly water balance model. This model uses national scale datasets and has been developed and applied to the whole of Scotland using five years of historical meteorological data. A catchment scale transport model, constructed from a 50m digital elevation model, routes flows of N through the sub-surface and groundwater to the stream system. The results of the simulations carried out for eight different catchments demonstrate that the NIRAMS model is capable of predicting time-series of weekly stream flows and N concentrations, to an acceptable degree of accuracy. The model provides an appropriate framework for risk assessment applications requiring predictions in ungauged catchments and at a national scale. Analysis of the model behaviour shows that streamwater N concentrations are controlled both by the rate of supply of N from leaching as well as the rate of transport of N from the land to the water. Keywords: nitrogen, diffuse pollution, hydrology, model, transport, catchmen

Transfer Matrices and Excitations with Matrix Product States

We investigate the relation between static correlation functions in the ground state of local quantum many-body Hamiltonians and the dispersion relations of the corresponding low energy excitations using the formalism of tensor network states. In particular, we show that the Matrix Product State Transfer Matrix (MPS-TM) - a central object in the computation of static correlation functions - provides important information about the location and magnitude of the minima of the low energy dispersion relation(s) and present supporting numerical data for one-dimensional lattice and continuum models as well as two-dimensional lattice models on a cylinder. We elaborate on the peculiar structure of the MPS-TM's eigenspectrum and give several arguments for the close relation between the structure of the low energy spectrum of the system and the form of static correlation functions. Finally, we discuss how the MPS-TM connects to the exact Quantum Transfer Matrix (QTM) of the model at zero temperature. We present a renormalization group argument for obtaining finite bond dimension approximations of MPS, which allows to reinterpret variational MPS techniques (such as the Density Matrix Renormalization Group) as an application of Wilson's Numerical Renormalization Group along the virtual (imaginary time) dimension of the system.Comment: 39 pages (+8 pages appendix), 14 figure

Transient modelling of the rotor-tower interaction of wind turbines using fluid-structure interaction simulations

In this work, we focus on the effect of supporting structures on the loads acting on a large horizontal axis wind turbine. The transient fluid-structure interaction (FSI) is simulated by an in-house code which couples two solvers, one for the computational fluid dynamics (CFD) and one for the computational structure mechanics (CSM). Strong coupling is applied as the force and displacement equilibriums are always enforced on the fluid- structure interface. The flexibility of the three blades of the considered machine is taken into account. The accurate CSM model reproduces in details the composite layups, foam, adhesive and internal stiffeners of the blades. On the other hand, the supporting structures (tower and nacelle) are considered to be rigid. On the fluid side, a fully hexahedral mesh is generated by a multi-block strategy. The same mesh is continuously deformed and adapted according to the displacement of the fluid- structure interface. The atmospheric boundary layer (ABL) under neutral conditions is included and consistently preserved along the computational domain. Using the outlined model, the blade deflections with and without supporting structure are compared. The effects of this transient interaction are highlighted throughout the rotation of the rotor, in terms of both wind energy conversion performance of the machine and structural response of each component. The maximal stress in the blade material as a function of time is compared with and without the presence of the tower in the wake of the rotor. Only a few similar works are reported to appear in literature [1, 2], whereas none of them currently includes the ABL or show detailed information about the internal stresses in the composite blades

Modal characteristics of a flexible tube in turbulent axial flow: a numerical approach and validation with experimental data

Flow-induced vibration is an important concern in the design of tube bundles. Due to the coupling of fluid motion and structural motion, instabilities such as flutter and divergence can arise. Next to the instabilities caused by the coupling of fluid motion and structural motion, turbulence could cause small amplitude vibrations, which in turn could give rise to long-term damage. Currently, the dynamical behavior of a tube in axial flow is studied by splitting the flow forces into inviscid and viscous components. The inviscid flow forces are determined from potential flow theory while the viscous flow forces come from empirical formulations. In this paper, a computational methodology is proposed to improve the accuracy of the predicted dynamical behaviour. In this methodology partitioned fluid-structure interaction simulations are performed to calculate the free vibration decay of a tube in axial flow. The tube is initially deformed according to an eigenmode in vacuum. Modal characteristics are then derived from the free vibration decay of the tube surrounded by the turbulent water flow. To validate this computational methodology a series of experiments is reproduced. In these experiments the frequency and damping of the fundamental mode of a solid brass cylinder were measured