A greedy reduced basis scheme for multifrequency solution of structural acoustic systems

The solution of frequency dependent linear systems arising from the discretization of vibro-acoustic problems requires a significant computational effort in the case of rapidly varying responses. In this paper, we review the use of a greedy reduced basis scheme for the efficient solution in a frequency range. The reduced basis is spanned by responses of the system at certain frequencies that are chosen iteratively based on the response that is currently worst approximated in each step. The approximations at intermediate frequencies as well as the a posteriori estimations of associated errors are computed using a least squares solver. The proposed scheme is applied to the solution of an interior acoustic problem with boundary element method (BEM) and to the solution of coupled structural acoustic problems with finite element method and BEM. The computational times are compared to those of a conventional frequencywise strategy. The results illustrate the efficiency of the method.     

Growth condition dependence of unintentional oxygen incorporation in epitaxial GaN

Growth conditions have a tremendous impact on the unintentional background impurity concentration in gallium nitride (GaN) synthesized by molecular beam epitaxy and its resulting chemical and physical properties. In particular for oxygen identified as the dominant background impurity we demonstrate that under optimized growth stoichiometry the growth temperature is the key parameter to control its incorporation and that an increase by 55?°C leads to an oxygen reduction by one order of magnitude. Quantitatively this reduction and the resulting optical and electrical properties are analyzed by secondary ion mass spectroscopy, photoluminescence, capacitance versus voltage measurements, low temperature magneto-transport and parasitic current paths in lateral transistor test structures based on two-dimensional electron gases. At a growth temperature of 665?°C the residual charge carrier concentration is decreased to below 10¹⁵ cm^?3, resulting in insulating behavior and thus making the material suitable for beyond state-of-the-art device applications.     

Performance of the radphi detector and trigger in a high rate tagged photon beam

We describe the design and operation of a detector system for measuring all-photon decays of mesons photoproduced in a tagged photon beam with energies between 4.3 and 5.4 GeV and a flux of 5×10⁷ tagged photons per second. Photons from meson decays were detected with a lead-glass calorimeter with an energy resolution of 11% at 1 GeV. Various veto and trigger components were also present. Final states with as many as six photons were successfully detected and reconstructed.     

Measuring of the quenching effect of liquid hardening agents on the basis of synthetics

To judge liquids for steel hardening, normally special test specimens are quenched by means of immersion. Characteristics are taken from temperature-time curves, which are picked up in the sample center. These characteristics (i.e. temperature of transition from filmboiling to nucleate boiling – LEIDENFROST-Temperature – and cooling rates) serve as criterions for the judgement of quenching effect. The effect of sample characteristics (geometry and material) upon characteristic values of the cooling curve was determined. Hence a proposal to design a test specimen was derived. This specimen (Chromenickel-steel-cylinder, diameter 15 mm, length 45 mm, rounded edges) presents of all examined configurations a very good possibility to a precise judgement of quenching media as well as chances to estimate the quenching process in case of steel hardening. As common quenching tests (with a single – central – hot junction) do not allow to draw conclusions referring to steel hardening, an additional method was developed to calculate the surface-temperature dependent heatflux density (respectively heattransfer coefficient) from three temperature-time-curves, measured on different radii of the sample cylinder. The reliability of this method was proved (differential method according to Binder-Schmidt).