Emergence of spatio-temporal dynamics from exact coherent solutions in pipe flow

Turbulent-laminar patterns are ubiquitous near transition in wall-bounded shear flows. Despite recent progress in describing their dynamics in analogy to non-equilibrium phase transitions, there is no theory explaining their emergence. Dynamical-system approaches suggest that invariant solutions to the Navier–Stokes equations, such as traveling waves and relative periodic orbits in pipe flow, act as building blocks of the disordered dynamics. While recent studies have shown how transient chaos arises from such solutions, the ensuing dynamics lacks the strong fluctuations in size, shape and speed of the turbulent spots observed in experiments. We here show that chaotic spots with distinct dynamical and kinematic properties merge in phase space and give rise to the enhanced spatio-temporal patterns observed in pipe flow. This paves the way for a dynamical-system foundation to the phenomenology of turbulent-laminar patterns in wall-bounded extended shear flows.Peer ReviewedPostprint (published version

Short Interest and Stock Returns

Using a longer time period and both NYSE-Amex and Nasdaq stocks, this paper examines short interest and stock returns in more detail than any previous study and finds that many documented patterns are not robust. While equally weighted high short interest portfolios generally underperform, value weighted portfolios do not. In addition, there is a negative correlation between market returns and short interest over our whole period. Finally, inferences from short time periods, such as 1988-1994 when the underperformance of high short interest stocks was exceptional or 1995-2002, when high short interest Nasdaq stocks did not underperform, are misleading.

Biodiversity and phytogeography of Bolivia\u27s wetland flora

The composition, diversity, and phytogeographic affinities of the vascular flora of Bolivia\u27s wetlands were examined and compared with wetland floras from the other Neotropical countries and the New World Temperate region. Forty-six wetlands distributed throughout Bolivia and ranging in elevation from 90 in to \u3e4400 m were established as study sites. Regional and national wetland floras were compiled from field research, the literature, herbarium specimens, and available databases. Two thousand and sixty species in 149 families and 666 genera were identified as associated with wetlands in Mesoamerica and tropical and subtropical South America. Of these, 1026 species in 126 families and 450 genera were noted for Bolivian wetlands. When considered both in terms of number of species per system and in comparisons of species-area curves, Bolivia\u27s wetlands were found to be less diverse than wetlands of the New World Temperate region. At the macroregional level, both South and Central America were less diverse than the New World Temperate region. Thus, at both the system and regional scales, it appeared that phytodiversity in New World wetlands constituted an exception to the widely recognized latitudinal gradient in species richness. Additionally, phytodiversity in Neotropical wetlands was generally not well-correlated with diversity in terrestrial habitats. A large portion of the Neotropical wetland species possessed very broad ranges, and few endemic species were noted relative to the Neotropical terrestrial habitats. Similarity Indices were generally unsatisfactory for analyzing floristic similarities at the system level. Both Detrended Correspondence Analysis (DCA) and Frequency Analysis produced more satisfactory results. Species that typically tended towards dominance in Bolivian wetlands most often were extremely widely distributed, precluding the identification of regional wetland associations based on dominant species. Nevertheless, ordination of the study sites by DCA generally grouped wetlands from within the same Bolivian region. At the macroregional level, an ordination by DCA ordered the countries of the Neotropics into three groups: (1) Bolivia, Brazil and Peru; (2) Colombia, The Guianas, and Venezuela; and, (3) all Central American countries. Two countries, Mexico and Ecuador, were not clearly associated with any group

Optimization and Design for Heavy Lift Launch Vehicles

The simulation and evaluation of an orbital launch vehicle requires consideration of numerous factors. These factors include, but are not limited to the propulsion system, aerodynamic effects, rotation of the earth, oblateness, and gravity. A trajectory simulation that considers these different factors is generated by a code developed for this thesis titled Trajectories for Heavy-lift Evaluation and Optimization (THEO). THEO is a validated trajectory simulation code with the ability to model numerous launch configurations. THEO also has the capability to provide the means for an optimization objective. Optimization of a launch vehicle can be specified in terms of many different variables. For a heavy lift launch vehicle in this thesis, the goal of optimization is to minimize Gross Lift Off Weight (GLOW). THEO provides the capability to optimize by simulating hundreds of thousands of trajectories for a single configuration through the variation of preset independent variables. The sheer volume of these trajectories provides the means to locate configurations that minimize GLOW. Optimization can also be performed by determining the minimum amount of energy necessary to reach target burnout conditions. The energy requirements are then correlated to the propellant mass which can be used to estimate GLOW. This thesis first discusses the validation of THEO as a simulation program and the properties associated with accurately modeling a trajectory. It then relates how THEO and other developed tools can be utilized to determine a configuration that is optimized to minimize GLOW to orbit for adaptable payload sizes

On the Lie-algebraic origin of metric 3-algebras

Since the pioneering work of Bagger-Lambert and Gustavsson, there has been a proliferation of three-dimensional superconformal Chern-Simons theories whose main ingredient is a metric 3-algebra. On the other hand, many of these theories have been shown to allow for a reformulation in terms of standard gauge theory coupled to matter, where the 3-algebra does not appear explicitly. In this paper we reconcile these two sets of results by pointing out the Lie-algebraic origin of some metric 3-algebras, including those which have already appeared in three-dimensional superconformal Chern-Simons theories. More precisely, we show that the real 3-algebras of Cherkis-Saemann, which include the metric Lie 3-algebras as a special case, and the hermitian 3-algebras of Bagger-Lambert can be constructed from pairs consisting of a metric real Lie algebra and a faithful (real or complex, respectively) unitary representation. This construction generalises and we will see how to construct many kinds of metric 3-algebras from pairs consisting of a real metric Lie algebra and a faithful (real, complex or quaternionic) unitary representation. In the real case, these 3-algebras are precisely the Cherkis-Saemann algebras, which are then completely characterised in terms of this data. In the complex and quaternionic cases, they constitute generalisations of the Bagger-Lambert hermitian 3-algebras and anti-Lie triple systems, respectively, which underlie N=6 and N=5 superconformal Chern-Simons theories, respectively. In the process we rederive the relation between certain types of complex 3-algebras and metric Lie superalgebras.Comment: 29 pages (v4: really final version to appear in CMP. Example 7 has been improved.

An analytical model for bore-driven run-up

We use a hodograph transformation and a boundary integral method to derive a new analytical solution to the shallow-water equations describing bore-generated run-up on a plane beach. This analytical solution differs from the classical Shen-Meyer runup solution in giving significantly deeper and less asymmetric swash flows, and also by predicting the inception of a secondary bore in both the backwash and the uprush in long surf. We suggest that this solution provides a significantly improved model for flows including swash events and the run-up following breaking tsunamis

How patterns of injecting drug use evolve in a cohort of people who inject drugs

This research found an overall movement away from street based drug purchasing and drug use, towards more activity in private settings, which has important implications for the harms experienced by people who inject drugs. Foreword This paper investigates the frequency of intravenous drug use in a cohort of people who inject drugs, and the decline in use over time. It provides an important indication of the effectiveness of current interventions at reducing the consumption of illicit drugs. Comparisons are made between the injection frequency of participants on or off Opioids Substitution Therapy (OST), and according to the settings in which drugs are most frequently purchased and used (eg street, house). This research found an overall movement away from street based drug purchasing and drug use, towards more activity in private settings. This has important implications for the harms experienced by people who inject drugs. Intravenous drug use was persistent, with only slow declines observed in the frequency of the cohort’s overall use. Lower injection frequency was associated with use in private rather than public locations as well as the uptake of OST. Additional work is needed to understand how this change in setting is affected by and also affects current interventions, and whether it can be used to help further reduce injecting drug use