Showing papers by "Swedish Defence Research Agency published in 2000"

Sensitivity analysis using adjoint parabolized stability equations for compressible flows

Abstract: An input/output framework is used to analyze the sensitivity of two- and three-dimensional disturbances in a compressible boundary layer for changes in wall and momentum forcing The sensitivity is defined as the gradient of the kinetic disturbance energy at a given downstream position with respect to the forcing The gradients are derived using the parabolized stability equations (PSE) and their adjoint (APSE) The adjoint equations are derived in a consistent way for a quasi-two-dimensional compressible flow in an orthogonal curvilinear coordinate system The input/output framework provides a basis for optimal control studies Analysis of two-dimensional boundary layers for Mach numbers between 0 and 12 show that wall and momentum forcing close to branch I of the neutral stability curve give the maximum magnitude of the gradient Forcing at the wall gives the largest magnitude using the wall normal velocity component In case of incompressible flow, the two-dimensional disturbances are the most sensitive ones to wall inhomogeneity For compressible flow, the three-dimensional disturbances are the most sensitive ones Further, it is shown that momentum forcing is most effectively done in the vicinity of the critical layer

Sequencing of the Francisella tularensis strain Schu 4 genome reveals the shikimate and purine metabolic pathways, targets for the construction of a rationally attenuated auxotrophic vaccine.

Abstract: Francisella tularensis is the etiological agent of tularemia, a serious disease in several Northern hemisphere countries. The organism has fastidious growth requirements and is very poorly understood at the genetic and molecular levels. Given the lack of data on this organism, we undertook the sample sequencing of its genome. A random library of DNA fragments from a highly virulent strain (Schu 4) of F. tularensis was constructed and the nucleotide sequences of 13,904 cloned fragments were determined and assembled into 353 contigs. A total of 1.83 Mb of nucleotide sequence was obtained that had a G+C content of 33.2%. Genes located on plasmids pOM1 and pNFL10, which had been previously isolated from low virulence strains of F. tularensis, were absent but all of the other known F. tularensis genes were represented in the assembled data. F. tularensis Schu4 was able to grow in the absence of aromatic amino acids and orthologues of genes which could encode enzymes in the shikimate pathway in other bacteria were identified in the assembled data. Genes that could encode all of the enzymes in the purine biosynthetic and most of the en- zymes in the purine salvage pathways were also identified. This data will be used to develop defined rationally attenuated mutants of F. tularensis, which could be used as replacements for the existing genetically undefined live vaccine strain.

Physical Simulations on the Operation of 4H-SiC Microwave Power Transistors.

Abstract: Ibis work deals with silicon carbide (SiC) metal semieonduetor field effect transistors (MESFETs) and microwave amplifiers using them. The wide bandgap (WBG) semiconductors silicon carbide and gallium nitride have a large potential for microwave power generation. The high power density combined with the comparably high impedance attainable by devices in these materials also offers new possibilities for wideband high power microwave systems. To realise these possibilities we need transistors that are well understood and optimised and amplifier designs that take advantage of the broadband possibilities offered by the transistors. We have developed and used physical drift-diffusion simulation models for SiC MESFETs. The simulation results showed a large influence of the doping and thickness of the channel and buffer layers, and the properties of the semi-insulating substrate on the DC and small signal device performance. A comparison between the physical simulations and measured device characteristics has also been carried out. A novel and efficient way to extend the physical simulations to the large signal high frequency domain has been developed. The method was used to investigate experimentally detected problems in the dynamies of the transistors during large signal operation and to further optimise the device structure. In this work a number of broadband SiC MESFET amplifiers were designed, fabricated and characterized. The packaging and charaeterisation of transistors is described and the design and characterisation of the amplifiers are presented. A 100-500 MHz amplifier showed a measured output power above 20 W and gain >15 dB across the band and a peak power of 26 W at 400 MHz, corresponding to a power density of 5.2 W/mm, with an associated power added efficiency (PAE) of 46 %. A 0.8-2 GHz feedback amplifier showed a measured output power above 5 W, gain >7 dB and PAE above 15 % across the band. A two stage 2.8-3.3 GHz amplifier had a maximum measured output power of 12.6 W with 22 dB gain and 9% PAE measured at 2.8 GHz. These results clearly show that SiC devices have a large potential for microwave power amplifiers in general and broadband amplifiers in particular.

Assessing Camouflage Methods Using Textural Features

Abstract: Developments in the area of signature suppression make it progressively more difficult to recognize targets. In order to obtain a sufficient low degree of false alarms it is necessary to observe spatial and spectral properties. There is a genuine need to use spatial properties when analyzing the difference between a target area and a background area. This is more relevant today since modern signature suppression techniques have focused on the reduction of distinct features, like hot spots in the infrared band. The approach is to apply texture descriptors to characterize the background and also more or less camouflaged targets. In addition, other descriptors are used to characterize man made objects. It is necessary to focus on features which discriminate targets from the background, and this demands a more precise description of the background and the targets than usual. The underlying assumption is that an area with more or less observable targets has different statistical properties from other areas. Statistical properties together with detected target specific features like straight lines. edges, corners or perhaps reflections from a window have to be combined with methods used in data fusion. Experiments with a computer program that estimates the statistical differences between targets and background are described. These differences are computed using a number of different distance measures.

A novel algorithm for passive demodulation of optical fiber interferometric signals using a 3x3 coupler

Abstract: Digital processing is used to linearly reproduce dynamic phase signals in fiber interferometers passively demodulated using a 3x3 coupler. The phase is retrieved as a weighted sum of the three received signals. High resolution down to the 1 μrad level is achieved.