Sigfried Vanaverbeke

Two-dimensional modeling of wave propagation in materials with hysteretic nonlinearity.

A multiscale model for the two-dimensional nonlinear wave propagation in a locally microdamaged medium is presented, and numerical simulations are analyzed in view of nondestructive testing applications. The multiscale model uses a statistical distribution of hysterons and upscales their microscopic stress-strain relations to a mesoscopic level. Macroscopic observations are then predicted by finite integration techniques. The influence of a small region with hysteretic nonlinearity on the generation of harmonics is investigated, and numerical results for different amplitudes of the input signal and different analysis techniques of the response signal are presented. Second, a study is conducted on the interaction of a Rayleigh wave with a microdamaged zone with hysteretic nonlinearity at the surface of an otherwise linear body, and the influence of the microdamaged zone on the surface wave velocity and on the generation of harmonics is examined. It is found that the effect of hysteresis on the Rayleigh wave propagation can be barely seen in the surface wave velocity measurement, but shows up nicely in the wave spectrum. The potential of a nonlinearity based depth profiling technique is explored by evaluating the nonlinear responses at different frequencies for a vertically stratified medium with spatially varying hysteresis properties.

Three-Dimensional–Two-Dimensional Coupled Model for Eddy Currents in Laminated Iron Cores

Eddy currents due to magnetic flux perpendicular to the sheets of a lamination iron core are represented on a number of 2D slice models, which are embedded in a 3D model of the entire device using a multi-scale technique. The choice of a different spatial resolution enables to attain an advantageous convergence of the discretization error for the eddy-current power losses, compared to a standard modelling technique using an anisotropic surrogate material.

On the thermal sensitivity of binary formation in collapsing molecular clouds

We report the results of a numerical study on the initial formation stages of low-mass protostellar binary systems. We determine the separation of protostellar binaries formed as a function of the initial thermal state by varying the initial temperature in a slightly modified version of the Burkert and Bodenheimer collapse test. We find that the outcome is highly sensitive to both the initial temperature of the cloud and the initial amplitude of azimuthal density perturbation A. For A=10 %, variations of only 1 unit Kelvin below 10 K lead to changes of up to 100 AU ( i.e. of order 30 %) in the instantaneous separation, whereas for this small A the initial temperatures above 10 K yield, instead of a binary, a single low-mass fragment that never reaches protostellar densities. Protostellar binaries, however, do emerge when the perturbation amplitude is increased from 10 % to 25 %. We also investigate the impact of the critical density which governs the transition from isothermal to adiabatic thermodynamic behaviour of the collapsing gas. We find that the critical density not only affects the overall structural evolution of the gas envelope, but also the size of the rotating disk structures formed during collapse as well as the number of protostellar fragments resulting from the final fragmentation of the disks. This mechanism can give rise to young protostellar objects constituting bound multiple stellar systems.

Extinction and the Dimming of KIC 8462852

To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star over a wide wavelength range from the UV to the mid-infrared from October 2015 through December 2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the long-term fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period reported is 22.1 +\- 9.7 milli-mag/yr in the Swift wavebands, with amounts of 21.0 +\- 4.5 mmag in the groundbased B measurements, 14.0 +\- 4.5 mmag in V, and 13.0 +\- 4.5 in R, and a rate of 5.0 +\- 1.2 mmag/yr averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at >= 3 sigma by three different observatories operating from the UV to the IR. The presence of long-term secular dimming means that previous SED models of the star based on photometric measurements taken years apart may not be accurate. We find that stellar models with T_{eff} = 7000 - 7100 K and A_V ~ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the near-simultaneous Spitzer photometry at 3.6 and 4.5 microns, although a longer wavelength excess from a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of the fading favors a relatively neutral color (i.e., R_V >= 5, but not flat across all the bands) compared with the extinction law for the general ISM (R_V = 3.1), suggesting that the dimming arises from circumstellar material.

GRADSPMHD: A parallel MHD code based on the SPH formalism

We present GRADSPMHD, a completely Lagrangian parallel magnetohydrodynamics code based on the SPH formalism. The implementation of the equations of SPMHD in the “GRAD-h” formalism assembles known results, including the derivation of the discretized MHD equations from a variational principle, the inclusion of time-dependent artificial viscosity, resistivity and conductivity terms, as well as the inclusion of a mixed hyperbolic/parabolic correction scheme for satisfying the ∇⋅B→ constraint on the magnetic field. The code uses a tree-based formalism for neighbor finding and can optionally use the tree code for computing the self-gravity of the plasma. The structure of the code closely follows the framework of our parallel GRADSPH FORTRAN 90 code which we added previously to the CPC program library. We demonstrate the capabilities of GRADSPMHD by running 1, 2, and 3 dimensional standard benchmark tests and we find good agreement with previous work done by other researchers. The code is also applied to the problem of simulating the magnetorotational instability in 2.5D shearing box tests as well as in global simulations of magnetized accretion disks. We find good agreement with available results on this subject in the literature. Finally, we discuss the performance of the code on a parallel supercomputer with distributed memory architecture.

Optical Dimming of RW Aur Associated with an Iron-rich Corona and Exceptionally High Absorbing Column Density

RW Aur is a binary system composed of two young, low-mass stars. The primary, RW Aur A, has undergone visual dimming events (Delta V = 2-3 mag) in 2011, 2014-16, and 2017-2018. Visual and IR observations indicate a gray absorber that moved into the line of sight. This dimming is also associated with changes in the outflow. In 2017, when the optical brightness was almost 2 mag below the long-term average, we triggered a Chandra observation to measure the absorbing column density N H and to constrain dust properties and the gas-to-dust ratio of the absorber. In 2017, the X-ray spectrum is more absorbed than it was in the optically bright state (N-H = ( 4 +/- 1) x 10(23) cm(-2)) and shows significantly more hot plasma than in X-ray observations taken before. Furthermore, a new emission feature at 6.63 +/- 0.02 keV (statistic) +/- 0.02 keV (systematic) appeared, indicating an Fe abundance an order of magnitude above solar, in contrast with previous sub-solar Fe abundance measurements. Comparing X-ray absorbing column density N-H and optical extinction A(v), we find that either the gas-to-dust ratio in the absorber is orders of magnitude higher than in the ISM, or the absorber has undergone significant dust evolution. Given the high column density coupled with changes in the X-ray spectral shape, this absorber is probably located in the inner disk. We speculate that a breakup of planetesimals or a terrestrial planet could supply large grains, causing gray absorption;some of these grains would be accreted and enrich the stellar corona with iron, which could explain the inferred high abundance.

Separation of multiple scatterers in NEWS‐TR experiments

Nonlinear elastic wave spectroscopy combined with acoustic time reversal (NEWS‐TR) is a promising new methodology for detecting microdamage at an early stage. When dealing with structures which could potentially contain many pointlike nonlinear scatterers, there is a need to develop techniques for separately imaging the defects using a distributed sensor network which acts as a time‐reversal mirror. In this contribution, we present numerical simulations of a newly developed version of the DORT method for nonlinear imaging and also discuss the possibility of applying PARAFAC (parallel factor analysis) and ICA (independent component analysis) methods to solve the problem of separating multiple nonlinear scatterers in the time‐frequency domain.

Multiscale Approach For Simulating Nonlinear Wave Propagation In Materials with Localized Microdamage

A multiscale model for the simulation of two‐dimensional nonlinear wave propagation in microcracked materials exhibiting hysteretic nonlinearity is presented. We use trigger‐like elements with a two state nonlinear stress‐strain relation to simulate microcracks at the microlevel. A generalized Preisach space approach, based on the eigenstress‐eigenstrain formulation, upscales the microscopic state relation to the mesoscopic level. The macroscopic response of the sample to an arbitrary excitation signal is then predicted using a staggered grid Elastodynamic Finite Integration Technique (EFIT) formalism. We apply the model to investigate spectral changes of a pulsed signal traversing a localized microdamaged region with hysteretic nonlinearity in a plate, and to study the influence of a superficial region with hysteretic nonlinearity on the nonlinear Rayleigh wave propagation.

Simulation of Microdamage Depth Profiling Using Nonlinear Rayleigh Wave Propagation in Media with Stratified Hysteretic Nonlinearity

The examination of near‐surface deterioration is of high importance in quality assessment applications such as the in situ characterization of environmental degradation of natural building stones in historical buildings, and the detection of surface breaking cracks in metal and composite constructions used in aeronautics. In this paper, we report on numerical experiments of Rayleigh wave propagation along the surface of a microcracked solid. Using a multiscale model, we investigate the influence of a microdamaged zone on the surface wave velocity, and on the generation of harmonics. We found that the nonlinear measures are far more sensitive in assessing microdamage than the linear characteristics. We illustrate this with a comparison of the results for linear and nonlinear depth profiling techniques by evaluating the linear and nonlinear response for excitation signals at different frequencies in the case of a stratified medium with spatially varying hysteresis properties.