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- Aalto, Susanne, 1964
(författare)
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Molecular Clouds in Starburst Galaxies
- 1994
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Starbursts in the centres of galaxies are believed to be caused by funneling of gas towards the inner regions. Gravitational interactions between galaxies are known to cause vast rearrangements of their structure as well as induce bursts of star formation in their nuclear regions. However, radial gas-transport may also be the result of a bar- instability intrinsic to the system. This report deals with the effect of a starburst on the molecular medium of galaxies. The mm-wave, rotational transitions of interstellar molecules serve as probes of the physical conditions, kinematics and total mass of the molecular gas. Observations of the J=1-0, J=2-1 and J=3-2 transitions of 12Co, the J=1-0 and J=2-1 transitions of 13Co, the J=1-0 transitions of C180 and HCN and the J=2-1 transition of CS are used to determine the properties of the molecular gas in starbursting and normal galaxies. The mean escape probability approximation is used to solve the radiative transfer equations. The inferred properties of the molecular clouds in the centres of starburst galaxies differ from those of Galactic disk clouds. Such a deviation is likely to be induced by a central gas concentration and/or the nuclear starburst disrupting the clouds. Large 12CO/13CO J=1-0 intensity ratios (R>20) are measured towards luminous mergers implying unusual, high-pressure, molecular cloud ensembles. Model cloud ensembles for the merging system, NGC 3256, and the starburst spiral NGC 1808 suggest that the 12CO-emission is dominated by warm (Tk = 100 - 300 K), small (0.5-1 pc) and moderately dense (n = 2 - 7 x 103 cm-3) molecular clouds. Such temperatures and densities are typical of Galactic photon dominated regions (PDRs). Statistical studies of molecular line ratios towards galaxy centres indicate that the optical depth of the 12CO 1-0 emission is moderate, .tau.Ã? 1. Many starburst galaxies have large-scale excitation gradients indicating warm, dense gas in the centre, and normal cool molecular gas in their extended disks. NGC 3256, in particular, has a large R = 35 in the centre, while it is found to be only R = 10 - 15 at positions 43" off centre. The model cloud ensemble of NGC 1808 indicate that the 12CO-emitting gas may be diffuse rather than self-gravitating, this could be a common phenomenon in starburst galaxies.
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| 2. |
- Aasa, Annika, 1963
(författare)
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User Defined Syntax
- 1992
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes two examples of user defined syntax. The first, and most thoroughly investigated, is a new datatype construction, the conctype, the elements of which have a very flexible syntax. An embedded language can easily be introduced into a programming language using conctypes and computations are easily expressed using the concrete syntax and a special pattern matching form. The second example is user defined distfix operators which give a user possibility to extend the syntax for expressions in a programming language. We describe both a user's view and the implementation of these two examples. In both cases, context-free grammars serve as a basis for the definition of the new syntax. A problem that is investigated is how to disambiguate grammars with precedences. To see how this should be done we investigate which language a grammar together with precedence rules defines. For a sub-class of context-free grammars we give a predicate that defines the precedence correct syntax trees according to some precedence rules. We also give an algorithm that transforms such a grammar to an ordinary unambiguous context-free grammar and prove the correctness of the algorithm. We use the algorithm in our implementation of distfix operators. For more general grammars, we isolate one kind of ambiguity which is suitable to resolve with precedence rules. We define the generated language for such a grammar by an attribute grammar. This approach of resolving ambiguity is used in the implementation of conctypes.
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| 3. |
- Abarkan, Abdellah, 1960
(författare)
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Bebyggelsemönster i medelhavsområdet: en studie av medinan i Fes ställd i relief mot Siena|
- 1995
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is an analysis and comparative study of medieval Mediterranean urban patterns. With urban patterns is meant the arrangement of houses and public spaces with special regard to the relations between them. The thesis contains four parts. Previous research on Muslim cities is described and commented in the first part. A difference between research before and after 1960 is noted. The Medina of Fes is discussed in the second part. In an historical background sketch the foundation of the Medina and its development are presented. The spatial analysis shows that the physical structure of the Medina is based on a primary pattern unit of a square courtyard house with a closed, inward-turned physical structure. A number of these primary units are grouped around a cul de sac to form a second-level community unit; a number of these latter form a quarter. The quarters are adjacent and close to each other which means that the Medina as a whole can be regarded as a complex physical and social unit surrounded with the city walls. In a comparable approach the physical and social structure of Siena is studied. The primary units here are the casa corte, which is similar to the courtyard of the Medina, and the casa a schiera which is a filehouse without any comparative physical form in the medina. These two different primary units engendered two types of building patterns, one in the city centre where courtyard units are grouped around a central square, and the other one in the periphery of the city formed by the alignment of the filehouses along the street. An attempt to compare the physical and social units in the Medina and Siena is made in the fourth part of this thesis. The primary units, the community units, the quarters and the cities are compared. This comparative study shows that the similarities between the Medina and Siena are much more apparent on the social level and on the level of the smaller physical units, i.e. the habitat, than between the larger social and physical composition of the quarters.
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| 5. |
- Abdulwahab, Fethi, 1964
(författare)
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Postprocessing Techniques and Adaptivity in the Finite Element Method
- 1995
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Finite Element (FE) method is an approximate method and as such the accuracy of its solutions must be controlled to allow for a reliable application. This thesis deals with error estimation and improvements of the FE solutions based on superconvergent properties of the FE solution in certain points. Two new postprocessing procedures are developed for elliptic problems. The procedures are based on local patches of elements and developed in such a way that the methods function at local level. Being local methods, the cost involved in implementing the procedures is small. The derivatives of the FE solutions are discontinuous across element boundaries. The primary purpose of the proposed postprocessing procedures is to smooth the discontinuous derivatives of finite element solutions like stresses and fluxes. The smoothed solution is used both as a new improved solution and in an error estimation. The presented postprocessing methods do not only smooth the stress discontinuities but also improve the satisfaction of the governing differential equation. This is in contrast to the fact that the FE solutions fulfill equilibrium conditions only in a weak and global sense. Similarly, the FE solutions do not generally satisfy the prescribed natural boundary conditions satisfactorily. On the other hand, the solutions obtained by the proposed postprocessing procedure satisfy the prescribed boundary conditions. Satisfaction of both the equilibrium and boundary conditions is achieved in the sense of the least squares. The proposed methods yield superconvergent solutions meaning that the accuracy of the postprocessed solutions are at least one order higher than the finite element solutions. This in its turn means that use of these postprocessed solutions in error estimations gurantees that the error estimate is asymptotically exact or that the estimated discretization error converges towards the true error in the limit. Several numerical test examples are given to validate the accuracy and effectiveness of these new methods. The presented methods are also implemented in an adaptive finite element procedure of the h-version or mesh enrichment. The postprocessing procedures proposed in this thesis can be regarded as a step forward towards an accurate and automated adaptive analysis in which the ultimate goal is to provide finite element solutions of high quality in an economical manner. Starting from some arbitrary mesh, such an automated adaptive procedure should provide an improved finite element model that meets a user specified level of accuracy. To accomplish this goal, every FE program package should be equipped with efficient postprocessing procedures.
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| 8. |
- Abrahamsson, Hans, 1966
(författare)
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On Turbulent Wall Jets
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An experimental investigation of the two- and three-dimensional turbulent wall jets has been conducted. The measurements have been performed using different hot-wire techniques in large scale wall jet facilities, and the measurements were focused on the fully-developed region of the flow. The two-dimensional wall jet is found to be self-preserving, and the streamwise development of the maximum mean velocity and the half-width is independent of the inlet Reynolds number when the viscosity and inlet momentum flux are used as scaling parameters. The near wall region is characterised by a short universal mean velocity profile and a high turbulence level caused by the influence of the outer region. The lateral spreading rate of the three-dimensional wall jet is found to be five times larger than the normal spreading rate, and this is connected to a secondary mean motion in the lateral plane which shows a negative normal mean velocity and a sharp twist of the velocity vector away from the symmetry plane near the wall. The turbulent kinetic energy budget indicates a transport of turbulent energy by advection and turbulent diffusion towards the wall region, where the turbulence level is enhanced and the near wall peak in the streamwise turbulence intensity is attenuated. A similarity analysis of the equations governing the 2DWJ show that the shear stress in the outer region scales with the friction velocity instead of the commonly used maximum mean velocity, and thus the outer region is governed by two velocity scales. The similarity analysis also shows that the growth of the wall jet (i.e. the half-width) is not linear but depends on the local Reynolds number. The velocity profiles in the overlap region are not logarithmic (as commonly believed) but show a power law behaviour. The similarity theory is in good agreement with experimental observations. Pulsed hot-wire anemometry measurements (PHWA) show good agreement with LDA data in the outer region of the 2DWJ, and these techniques give a significantly higher turbulence level (e.g. the shear stress) than the HWA. This difference can be explained by the high local turbulence levels, which are so high that local flow reversals occur. Even larger differences between PHWA and HWA are found in the 3DWJ because the local turbulence levels are higher and negative velocity samples are more frequent in the 3DWJ than the 2DWJ.
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