4,094 research outputs found
Numerical investigation of transitional supersonic axisymmetric wakes
No full textTransitional supersonic axisymmetric wakes are investigated by
conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M=2.46 for several Reynolds numbers, and relating these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. Three high-order accurate compressible codes were developed in cylindrical coordinates for this research: a spatial Navier-Stokes (N-S) code to conduct Direct Numerical Simulations (DNS), a linearized N-S code for linear stability investigations using axisymmetric basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to deliberately exclude physical effects is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain-sizes. With this approach, the impact of structures associated with certain modes on the global wake-behavior can be scrutinized. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region co-exist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k>0 for ReD>5,000 and with respect to the axisymmetric mode k=0 for ReD>100,000. It is concluded that azimuthal modes k=2 and k=4 are the dominant modes in the trailing wake, producing a four-lobe wake pattern. Two possible mechanisms responsible for the generation of longitudinal structures within the recirculation region are suggested
Hadron Production in Proton-Proton Collisions
No full textOnTEAM metadata: GDSID: DOC-2008-Sep-154; Attribute ID: LIBRARY-thesis_ma-2008-001; Title: [GSI Master 2008-01] Hadron Production in Proton-Proton Collisions [30.7.2008]; Author(s): Fasel, Markus; Corporate author(s): ; Publication date: 20080929; Creator: manton; Creation date: 29.09.2008 15:09:47; Change date: 30.09.2010 16:05:30; Access: Welt; Attribute type: Text.Thesis.MA; Directory path: ['GSI Publications', 'GSI as Publisher']; Attribute path: ['Infrastructure', 'Library and Documentation', 'thesis_ma', 'Added in 2008']; File name(s): ['DOC-2008-Sep-154-1.pdf']; File title(s): ['']; File access: ['GSI-intern'
Motivic Approach to Enumerating Vector Bundles
No full textDans ce travail, nous utilisons la théorie de l’obstruction en théorie homotopique des schémas pour obtenir certains résultats d’énumération de fibrés vectoriels sur des algèbres lisses de dimension d sur un corps k fixé. Dans un premier temps, nous énumérons les fibrés vectoriels de rang d sur ces algèbres, obtenant au passage de nouvelles preuves de certains théorèmes de Suslin et Bhatwadekar. Nous étudions ensuite les fibrés de rang d-1, prouvant au passage une conjecture de Suslin en admettant une conjecture de Asok et Fasel. Finalement, nous utilisons des méthodes similaires pour prouver des résultats de simplification pour des fibrés symplectiques de rang critique.In this thesis, we establish, via obstruction theory in motivic homotopy theory, some enumeration results on vector bundles of rank dover a smooth affine k-algebra A of dimension d for a base field k, in analogy with some results of James-Thomas. In the rank d case, we recover in particular results of Suslin and Bhatwadekar on cancellation of such vector bundles. Admitting a conjecture of Asok and Fasel, we prove cancellation of such modules of rank d-1 if the base field k is algebraically closed. Using similar methods, we also obtain cancellation results for symplectic vector bundles of critical rank
High-accuracy DNS of supersonic base flows and control of the near wake
No full textLarge-scale numerical simulations of axisymmetric, supersonicbase flows were conducted at various Reynolds numbers. Direct Numerical Simulations (DNS) were employed to investigate the hydrodynamic stability behaviorof the near-wake region. As a consequence of physical flow instabilities, large coherent structures evolve that have a significant impact on the mean flow and are responsible for a considerable amount of base-drag. It is demonstrated that the deliberate exclusion or reinforcement of certain helical modes can lead to a rise in base-pressure and thus decreasing the drag of a blunt body at supersonic speed. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates with high parallel efficiency was developed and employed on the SGI Origin3900 shared memory complex at the ERDC MSRC. In addition to providing vital insight into the physical mechanisms in supersonic base flows, the DNS results are intended for use as benchmark data for the development of a Flow Simulation Methodology (FSM) for high Reynolds number turbulent flow
Direct numerical simulations of transitional supersonic base flows
No full textTransitional supersonic base flows at M=2.46 are investigated
using Direct Numerical Simulations. Results are presented for Reynolds numbers based on the cylinder diameter ReD=30,000-100,000. As a consequence of flow instabilities, coherent structures develop that have a profound impact on the global flow behavior. Simulations with various circumferential domain sizes are conducted to investigate the effect of coherent structures associated with different azimuthal modes on the mean flow, in particular on the base pressure which determines the base drag.Temporal spectra reveal that frequencies found in the axisymmetric mode can be related to dominant higher modes present in the flow. It is shown that azimuthal modes with low wavenumbers cause a flat base pressure distribution and that the mean base pressure value increases when the most dominant modes are deliberately eliminated. Visualizations of instantaneous flow quantities and turbulence statistics at ReD=100,000 show good agreement with experiments at a significantly higher Reynolds number. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates developed specifically for this research was used
Damage detection using frequency domain ARX models and extreme value statistics
No full textThe author acknowledges Tim Johnson and Seth Gregg
and the Los Alamos Dynamic Summer School for providing
the test structure as well as helping with the set-up,
instrumentation and acquisition of data from the test
structure. Funding for the summer school was provided by
the Engineering Sciences and Application Division at Los
Alamos National Laboratory and the Department of
Energy’s Education Program Office
Une approche motivique de l'énumération de fibrés vectoriels
No full textIn this thesis, we establish, via obstruction theory in motivic homotopy theory, some enumeration results on vector bundles of rank dover a smooth affine k-algebra A of dimension d for a base field k, in analogy with some results of James-Thomas. In the rank d case, we recover in particular results of Suslin and Bhatwadekar on cancellation of such vector bundles. Admitting a conjecture of Asok and Fasel, we prove cancellation of such modules of rank d-1 if the base field k is algebraically closed. Using similar methods, we also obtain cancellation results for symplectic vector bundles of critical rank.Dans ce travail, nous utilisons la théorie de l’obstruction en théorie homotopique des schémas pour obtenir certains résultats d’énumération de fibrés vectoriels sur des algèbres lisses de dimension d sur un corps k fixé. Dans un premier temps, nous énumérons les fibrés vectoriels de rang d sur ces algèbres, obtenant au passage de nouvelles preuves de certains théorèmes de Suslin et Bhatwadekar. Nous étudions ensuite les fibrés de rang d-1, prouvant au passage une conjecture de Suslin en admettant une conjecture de Asok et Fasel. Finalement, nous utilisons des méthodes similaires pour prouver des résultats de simplification pour des fibrés symplectiques de rang critique
Application of frequency domain ARX models and extreme value statistics to impedance-based damage detection
No full textFunding for this project was provided by the Department of Energy through the internal funding program at Los Alamos National Laboratory known as Laboratory Directed Research and Development. The author acknowledges Tim Johnson and Seth Gregg and the Los Alamos Dynamic Summer School for providing the test structure as well as helping with the set-up, instrumentation and acquisition of data from the test structure. Funding for the summer school was provided by the Engineering Sciences and Application Division at Los Alamos National Laboratory and the Department of Energy’s Education Program Office
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