832 research outputs found
Standard 1D solar atmosphere as initial condition for MHD simulations and switch-on effects
Many applications in Solar physics need a 1D atmospheric model as initial
condition or as reference for inversions of observational data. The VAL
atmospheric models are based on observations and are widely used since decades.
Complementary to that, the FAL models implement radiative hydrodynamics and
showed the shortcomings of the VAL models since almost equally long time. In
this work, we present a new 1D layered atmosphere that spans not only from the
photosphere to the transition region, but from the solar interior up to far in
the corona. We also discuss typical mistakes that are done when switching on
simulations based on such an initial condition and show how the initial
condition can be equilibrated so that a simulation can start smoothly. The 1D
atmosphere we present here served well as initial condition for HD and MHD
simulations and should also be considered as reference data for solving inverse
problems.Comment: 10 pages, 3 figures, published versio
Application of the electromotive force as a shock front indicator in the inner heliosphere
The electromotive force (EMF) describes how the evolution and generation of a
large-scale magnetic field is influenced by small-scale turbulence. Recent
studies of in-situ measurements have shown a significant peak in the EMF while
a coronal mass ejection (CME) shock front passes by the spacecraft. The goal of
this study is to use the EMF as an indicator for the arrival of CME shock
fronts. With Helios spacecraft measurements we carry out a statistical study on
the EMF during CMEs in the inner heliosphere. We develop an automated shock
front detection algorithm using the EMF as the main detection criterion and
compare the results to an existing CME database. The properties of the EMF
during the recorded events are discussed as a function of the heliocentric
distance. Our algorithm reproduces most of the the events from Kilpua et al.
(2015) and finds many additional CME-like events which proves the EMF as a good
shock front indicator. The largest peaks in the EMF are found from 0 to 50
minutes after the initial shock. We find a power law of -1.54 and -2.18 for two
different formulations of the EMF with the heliocentric distance.Comment: 6 pages, 5 figures, publishe
A multiscale regularized restoration algorithm for XMM-Newton data
We introduce a new multiscale restoration algorithm for images with few
photons counts and its use for denoising XMM data. We use a thresholding of the
wavelet space so as to remove the noise contribution at each scale while
preserving the multiscale information of the signal. Contrary to other
algorithms the signal restoration process is the same whatever the signal to
noise ratio is. Thresholds according to a Poisson noise process are indeed
computed analytically at each scale thanks to the use of the unnormalized Haar
wavelet transform. Promising preliminary results are obtained on X-ray data for
Abell 2163 with the computation of a temperature map.Comment: To appear in the Proceedings of `Galaxy Clusters and the High
Redshift Universe Observed in X-rays', XXIth Moriond Astrophysics Meeting
(March 2001), Eds. Doris Neumann et a
Fracture
These notes begin with a review of the mainstream theory of brittle fracture, as it has emerged from the works of Griffi th and Irwin. We propose a re-formulation of that theory within the confi nes of the calculus of variations, focussing on crack path prediction. We then illustrate the various possible minimality criteria in a simple 1d-case as well as in a tearing experiment and discuss in some details the only complete mathematical formulation so far, that is that where global minimality for the total energy holds at each time. Next we focus on the numerical treatment of crack evolution and detail crack regularization which turns out to be a good approximation from the standpoint of crack propagation. This leads to a discussion of the computation of minimizing states for a non-convex functional. We illustrate the computational issues with a detailed investigation of the tearing experiment
Aircraft control via variable cant-angle winglets
Copyright @ 2008 American Institute of Aeronautics and AstronauticsThis paper investigates a novel method for the control of "morphing" aircraft. The concept consists of a pair of winglets; with adjustable cant angle, independently actuated and mounted at the tips of a baseline flying wing. The general philosophy behind the concept was that for specific flight conditions such as a coordinated turn, the use of two control devices would be sufficient for adequate control. Computations with a vortex lattice model and subsequent wind-tunnel tests demonstrate the viability of the concept, with individual and/or dual winglet deflection producing multi-axis coupled control moments. Comparisons between the experimental and computational results showed reasonable to good agreement, with the major discrepancies thought to be due to wind-tunnel model aeroelastic effects.This work has been supported by a Marie Curie excellence research grant funded by the European Commission
- …