On the lithium content of the globular cluster M92

I use literature data and a new temperature calibration to determine the Li abundances in the globular cluster M 92. Based on the same data, Boesgaard et al. have claimed that there is a dispersion in Li abundances in excess of observational errors. This result has been brought as evidence for Li depletion in metal-poor dwarfs. In the present note I argue that there is no strong evidence for intrinsic dispersion in Li abundances, although a dispersion as large as 0.18 dex is possible. The mean Li abundance, A(Li)=2.36, is in good agreement with recent results for field stars and TO stars in the metal-poor globular cluster NGC 6397. The simplest interpretation is that this constant value represents the primordial Li abundance.Comment: A&A accepte

Propeller blade stress estimates using lifting line theory

OpenProp, an open-source computational tool for the design and analysis of propellers and horizontal-axis turbines, is extended to provide estimates of normal stresses in the blades for both on- and off-design operating conditions. The numerical model is based on propeller lifting theory, and the present implementation of the code includes an analysis capability to estimate the off-design performance of the propeller or turbine and to make blade stress predictions. As an example, we present the design and performance of a two-bladed propeller. Experimental measurements of the propeller performance over a wide range of off-design operating conditions agree with performance predictions. Estimates of the blade stress are given for on-design and off-design operating states of the propeller.United States. Office of Naval Research (N000140810080)United States. National Oceanic and Atmospheric Administration (NOAA NSG NA060AR4170019)Project Ocean (Robert Damus

An impulse framework for hydrodynamic force analysis : fish propulsion, water entry of spheres, and marine propellers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.This thesis presents an impulse framework for analyzing the hydrodynamic forces on bodies in flow. This general theoretical framework is widely applicable, and it is used to address the hydrodynamics of fish propulsion, water entry of spheres, and the offdesign performance of marine propellers. These seemingly-unrelated physics problems share a key common thread: The forces on these fish, spheres, and propellers can be modeled as the sum of the reaction to the rate of change of (1) the pressure impulse required to set up the potential flow about the body, and (2) the vortex impulse required to create the vortical structures in the wake of the body. Fish generate propulsive forces by creating and manipulating large-scale vortical structures using their body and tail. High-speed particle image velocimetry experiments show that a fish generates two vortex rings during a C-turn maneuver and that the change in momentum of the fish balances the change in pressure impulse plus the vortex impulse of these rings. When a sphere plunges into a basin of water and creates a sub-surface air cavity in place of a vortical wake, the vortex impulse is zero, and the force on the sphere is given by the pressure impulse component. Using data from high-speed imaging experiments, a semi-empirical numerical simulation is developed herein; this numerical model shows how the presence of the cavity alters the unsteady pressure force on the sphere and modulates the dynamics of the impact event. During steady propeller operation, the pressure impulse is constant, and the loads on the propeller are given by the vortex impulse component. To analyze these loads, a computational design and analysis tool is presented; this code suite is based on propeller lifting line theory, which is shown to be a special case of the general impulse framework of this thesis. A marine propeller is designed, built, and tested over a range of off-design operating conditions. Experimental results match the predicted performance curve for this propeller, which provides important validation data for the numerical method presented herein. 3 Bringing this thesis full circle, the unsteady startup of the propellor is addressed, which is analogous to the impulsive maneuvering of the swimming fish. As in the fish maneuvering problem, the propellor generates a ring-like vortical wake, and it is shown herein how the vortex impulse of these rings provides thrust for the propellor. With the perspective of the impulse framework developed in this thesis, the results of these tandem experimental investigations and numerical simulations provide deeper insight into classical fluid-dynamics theory and modern experimental hydrodynamics.by Brenden P. Epps.Ph.D

Design of spacecraft for exploration of the Moon and Mars

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 123-125).In this thesis, I develop the conceptual design of the spacecraft required for human-Lunar and human-Mars exploration. The requirements for these vehicles are derived in the context of the NASA Concept Exploration & Refinement project. Similarly, the concepts generated are intended to operate within the transportation architecture developed during this project. Therefore, this thesis serves as a vehicle-level design exercise. Four vehicle architecture options are synthesized by combining system concepts in a logical fashion. These four options are evaluated on several performance criteria, and one vehicle design concept is selected for detailed modeling. In addition, I investigate the conceptual design of the airlock system, as a system-level design exercise. This research project culminated in a set of vehicle concept designs and design recommendations for NASA.by Brenden P. Epps.S.M

Emergence of time-horizon invariant correlation structure in financial returns by subtraction of the market mode

We investigate the emergence of a structure in the correlation matrix of assets' returns as the time-horizon over which returns are computed increases from the minutes to the daily scale. We analyze data from different stock markets (New York, Paris, London, Milano) and with different methods. Result crucially depends on whether the data is restricted to the ``internal'' dynamics of the market, where the ``center of mass'' motion (the market mode) is removed or not. If the market mode is not removed, we find that the structure emerges, as the time-horizon increases, from splitting a single large cluster. In NYSE we find that when the market mode is removed, the structure of correlation at the daily scale is already well defined at the 5 minutes time-horizon, and this structure accounts for 80 % of the classification of stocks in economic sectors. Similar results, though less sharp, are found for the other markets. We also find that the structure of correlations in the overnight returns is markedly different from that of intraday activity.Comment: 12 pages, 17 figure

Multiscaled Cross-Correlation Dynamics in Financial Time-Series

The cross correlation matrix between equities comprises multiple interactions between traders with varying strategies and time horizons. In this paper, we use the Maximum Overlap Discrete Wavelet Transform to calculate correlation matrices over different timescales and then explore the eigenvalue spectrum over sliding time windows. The dynamics of the eigenvalue spectrum at different times and scales provides insight into the interactions between the numerous constituents involved. Eigenvalue dynamics are examined for both medium and high-frequency equity returns, with the associated correlation structure shown to be dependent on both time and scale. Additionally, the Epps effect is established using this multivariate method and analyzed at longer scales than previously studied. A partition of the eigenvalue time-series demonstrates, at very short scales, the emergence of negative returns when the largest eigenvalue is greatest. Finally, a portfolio optimization shows the importance of timescale information in the context of risk management

On the inter-foil spacing and phase lag of tandem flapping foil propulsors

The aim of this article is to provide a theoretical basis upon which to advance and deploy novel tandem flapping foil systems for efficient marine propulsion. We put forth three key insights into tandem flapping foil hydrodynamics related to their choreography, propulsive efficiency, and unsteady loading. In particular, we propose that the performance of the aft foil depends on a new nondimensional number, s/Utau, which is the inter-foil separation s normalized by the distance that the freestream U advects in one flapping period tau. Additionally, we show how unsteady loading can be mitigated through choice of phase lag

“Digital James Dickey” Developing an App for the iPad based on a poem by James Dickey (grade level 11 & 12)

The purpose of our project was to create an app for the ipad based on a nature poem by James Dickey that could be used by a high school teacher in a classroom. The app includes a poem, video readings by the poet, perspectives (comment and literary criticisms) by a number of experts, biography, a gallery of photos, and a resources page. In fall of 2013 we will test the app in the high school classroo