Simulation of adaptive optics for the Vacuum Tower Telescope

A simulation model of the adaptive optics of the German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, is presented. The model uses modules from the integrated model of the Euro50 extremely large telescope, and includes submodels of a Shack-Hartmann wavefront sensor, a de-formable mirror, a tip-tilt mirror, high-voltage amplifier low-pass filters, a reconstructor and a controller. We investigate the impact on the closed loop bandwidth of changes in controller configuration and certain system parameters, such as low pass filter bandwidth and camera integration and readout time. Control strategies were tested on simple models before implementation on the full VTT model. Using the models, different control strategies are compared.     

Chromospheric evolution and the flare activity of super-active region NOAA 6555

Super-active region NOAA 6555 was highly flare productive during the period March 21st–27th, 1991 of its disk passage. We have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed surges are also discussed.     

Observed foundation behaviour of concrete gravity platforms installed in the North Sea 1973–1978

Available measurements of foundation behaviour of the 13 concrete gravity platforms installed in the North Sea in the years 1973–1978 are presented. The measurements cover both the installation phase and the early part of the operation period. For the installation phase, measurements of dowel and skirt penetration resistance, base contact stresses, tilt, piping and erosion, are presented. For the operation period, the presentation includes measurements of settlements, pore water pressures, base contact stresses, permanent lateral displacements, cyclic displacements, dynamic behaviour and erosion. The measurements are used to back calculate soil properties and to evaluate the adequacy of the design procedures which have been used. The observations give valuable experience with respect to the foundation performance of offshore gravity platforms.     

Reduction of ground vibration by means of barriers or soil improvement along a railway track

Trains running in built-up areas are a source to ground-borne noise. A careful design of the track may be one way of minimizing the vibrations in the surroundings. For example, open or infilled trenches may be constructed along the track, or the soil underneath the track may be improved. In this work, the influence of the track design and properties on the level of ground vibration due to a vehicle moving with subsonic speed is examined. A coupled finite element-boundary element model of the track and subsoil is employed, adopting a formulation in the moving frame of reference following the vehicle. The computations are carried out in the frequency domain for various combinations of the vehicle speed and the excitation frequency. The analyses indicate that open trenches are more efficient than infilled trenches or soil stiffening–even at low frequencies. However, the direction of the load is of paramount importance. For example, the response outside a shallow open trench may change dramatically when horizontal load is applied instead of vertical load.     

An Overview of Electrokinetic Consolidation of Soils

Electrokinetic stabilization is one of the techniques that improve the geotechnical properties of the soils. It was pioneered by Casagrande in late 1940s and has not seen much development since then, especially in large-scale field applications. Some bench scale studies have been carried out during the past two decades and there have been some small scale field studies and limited field applications, mostly on soft soils. Due to lack of understanding of the physiochemical and electrochemical changes in the soil during electrokinetic stabilization, uncertain energy costs, loss of efficiency with time and the corrosion of electrodes, this method is usually considered as a last resort for large-scale practical applications. The objective of this paper is to highlight the critical parameters affecting electrokinetic consolidation, and to discuss their effects on the efficiency of the process. A better understanding of these critical parameters and their effects will enable geotechnical engineers to design the electrokinetic consolidation operation more effectively and make it an economically viable process for many situations.