Effect of blade geometry on the aerodynamic loads produced by vertical-axis wind turbines

Accurate aerodynamic modelling of vertical-axis wind turbines poses a significant challenge. The rotation of the turbine induces large variations in the angle of attack of its blades that can manifest as dynamic stall. In addition, interactions between the blades of the turbine and the wake that they produce can result in impulsive changes to the aerodynamic loading. The Vorticity Transport Model has been used to simulate the aerodynamic performance and wake dynamics of three different vertical-axis wind turbine configurations. It is known that vertical-axis turbines with either straight or curved blades deliver torque to their shaft that fluctuates at the blade passage frequency of the rotor. In contrast, a turbine with helically twisted blades delivers a relatively steady torque to the shaft. In this article, the interactions between helically twisted blades and the vortices within their wake are shown to result in localized perturbations to the aerodynamic loading on the rotor that can disrupt the otherwise relatively smooth power output that is predicted by simplistic aerodynamic tools that do not model the wake to sufficient fidelity. Furthermore, vertical-axis wind turbines with curved blades are shown to be somewhat more susceptible to local dynamic stall than turbines with straight blades

Quality Premiums and the Post-Harvest Spot Market Thinness: The Case of U.S. Peanuts

In the paper, we analyze two issues that are important in the U.S. peanut markets: the absence of explicit quality premiums in the crop contracts, and the thinness of the postharvest cash/spot markets, which obstructs the operation of current production support policies. We argue that introduction of quality differentials in the form of either fixed premium or rank-order tournament contracts may kill two birds with one stone by increasing the spot market turnover and providing incentives for increasing crop quality. In addition, this arrangement is likely to reduce the costs of the federal support programs.contracts, tournaments, efficiency, cash markets, peanuts, Marketing,

Strategic Behavior and Trade in Agricultural Commodities - Competition in World Peanut Markets

In this paper, we make an attempt to rationalize the strategic behavior of major peanut exporting and importing countries in the framework of imperfectly competitive markets with the focus on the global and inter-American peanut trade. This study is motivated by the fact that liberalizing imperfectly competitive and often distorted markets can have unorthodox effects, in particular increase the incentives to overuse certain trade policies. The results suggest that the South American peanut producers stand to benefit from the reductions in the U.S. peanut production supports but, paradoxically, preservation of a tariff may still be mutually welfare enhancing. In the broader context of global peanut trade, multi-lateral tariff reduction increases the low-cost exporters' incentives to subsidize export production, which benefits the consumers but hurts the higher-cost producers.trade, peanuts, strategic behavior, tariffs, TRQs, subsidies, Crop Production/Industries, International Relations/Trade, F12, F13, Q17,

Dynamically dominant magnetic fields in the diffuse interstellar medium

Observations show that magnetic fields in the interstellar medium (ISM) often do not respond to increases in gas density as would be naively expected for a frozen-in field. This may suggest that the magnetic field in the diffuse gas becomes detached from dense clouds as they form. We have investigated this possibility using theoretical estimates, a simple magneto-hydrodynamic model of a flow without mass conservation and numerical simulations of a thermally unstable flow. Our results show that significant magnetic flux can be shed from dense clouds as they form in the diffuse ISM, leaving behind a magnetically dominated diffuse gas.Comment: 2 pages, 1 figure. In proceedings of IAU Symposium 259: "Cosmic magnetic fields: from planets to stars and galaxies", eds. K.G. Strassmeier, A.G. Kosovichev & J.E. Beckman in pres

Magnetic fields and spiral arms in the galaxy M51

(Abridged) We use new multi-wavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from 200\pc to several \kpc. Interferometric and single dish data are combined to obtain new maps at \wwav{3}{6} in total and polarized emission, and earlier \wav{20} data are re-reduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the \wav{20} emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at \wav{6} must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy: the disc is dominated by a combination of azimuthal modes, $m=0+2$, but in the halo only an $m=1$ mode is required to fit the observations. We disuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the spiral shocks. The average arm--interam contrast, representative of the radii r>2\kpc where the spiral arms are broader, is not compatible with straightforward compression: lower arm--interarm contrasts than expected may be due to resolution effects and \emph{decompression} of the magnetic field as it leaves the arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of 50\pc for the size of the turbulent eddies.Comment: 21 pages, 18 figures (some at lower resolution than submitted version), accepted for publication in MNRA

The supernova-regulated ISM. I. The multi-phase structure

We simulate the multi-phase interstellar medium randomly heated and stirred by supernovae, with gravity, differential rotation and other parameters of the solar neighbourhood. Here we describe in detail both numerical and physical aspects of the model, including injection of thermal and kinetic energy by SN explosions, radiative cooling, photoelectric heating and various transport processes. With 3D domain extending 1 kpc^2 horizontally and 2 kpc vertically, the model routinely spans gas number densities 10^-5 - 10^2 cm^-3, temperatures 10-10^8 K, local velocities up to 10^3 km s^-1 (with Mach number up to 25). The thermal structure of the modelled ISM is classified by inspection of the joint probability density of the gas number density and temperature. We confirm that most of the complexity can be captured in terms of just three phases, separated by temperature borderlines at about 10^3 K and 5x10^5 K. The probability distribution of gas density within each phase is approximately lognormal. We clarify the connection between the fractional volume of a phase and its various proxies, and derive an exact relation between the fractional volume and the filling factors defined in terms of the volume and probabilistic averages. These results are discussed in both observational and computational contexts. The correlation scale of the random flows is calculated from the velocity autocorrelation function; it is of order 100 pc and tends to grow with distance from the mid-plane. We use two distinct parameterizations of radiative cooling to show that the multi-phase structure of the gas is robust, as it does not depend significantly on this choice.Comment: 28 pages, 22 figures and 8 table

On the Spatial Distribution of Hard X-Rays from Solar Flare Loops

The aim of this paper is to investigate the spatial structure of the impulsive phase hard X-ray emission from solar flares. This work is motivated by the YOHKOH and the forthcoming HESSI observations. Summarizing past results, it is shown that the transport effects can account for the observations by inhomogeneous loops where there is a strong field convergence and/or density enhancement at the top of the flaring loop. Scattering by plasma turbulence at the acceleration site or pancake type pitch angle distribution of the accelerated electrons can also give rise to enhanced emission at the loop tops. These could be a natural consequence of acceleration by plasma waves. This paper considers a general case of stochastic scattering and acceleration that leads to an isotropic pitch angle distribution and an enhanced emission from the loop tops or the acceleration site. Following the formalism developed in earlier papers the strength and the spectrum of the radiation expected from the acceleration site and the foot points are evaluated and their dependence on the parameters describing the acceleration process and the flare plasma are determined. The theoretical ratio of these two intensities and relative values of their spectral indices are compared with the YOHKOH observations, demonstrating that the above mentioned parameters can be constrained with such observations. It is shown that future high spatial and spectral resolution observations, for example those expected from HESSI, can begin to distinguish between different models and constrain their parameters.Comment: 37 pages with 20 figures. Accepted for publication in ApJ http://www.astronomy.stanford.ed

The Magnetized Disk-Halo Transition Region of M51

An excellent laboratory for studying large scale magnetic fields is the grand de- sign face-on spiral galaxy M51. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron emission at different radio frequencies gives a picture of the galaxy at different depths: Observations at L-band (1-2 GHz) probe the halo region while at C- and X- band (4-8 GHz) the linearly polarized emission probe the disk region of M51. We present new observations of M51 using the Karl G. Jansky Very Large Array (VLA) at S-band (2-4 GHz), where previously no polarization observations existed, to shed new light on the transition region between the disk and the halo. We discuss a model of the depolarization of synchrotron radiation in a multilayer magneto-ionic medium and compare the model predictions to the multi-frequency polarization data of M51 between 1-8GHz. The new S-band data are essential to distinguish between different models. Our study shows that the initial model parameters, i.e. the total reg- ular and turbulent magnetic field strengths in the disk and halo of M51, need to be adjusted to successfully fit the models to the data.Comment: 4 Pages, 3 Figures, Conference Proceeding to IAU Focus Meeting 8: New Insights in Extragalactic Magnetic Field

Living Dead in the United States: Felon Disenfranchisement and White Privilege

The purpose of this study is to re-examine the policy of felon disenfranchisement through an analysis of its historical lineage from the Jim Crow Era to the contemporary era of Black Lives Matter and identify the influence of White Privilege in its development. Review of previous research indicates a racial bias in the early implementation of felon disenfranchisement intended to prevent Blacks from exercising the right to vote as well as identifies racial motivations behind the use of the policy until present day. The United States has a history of trying to bar Black people from voting.1 Disenfranchisement prevents the exercise of full citizenship for felons and ex-felons in the United States. Primary and secondary sources that address the history of felon disenfranchisement will be interpreted through the lens of critical race theory to identify White Privilege in the development of felon disenfranchisement. This study provides a revised way of thinking on historical race relations in the United States and of the racially disproportionate disenfranchisement of Black United States citizens. This research indicates explicit and passive racial bias in the policy of felon disenfranchisement throughout its historical lineage. It further defines the impact of White Privilege in the policy of felon disenfranchisement. This research proves that policies with racially disproportionate outcomes, like felon disenfranchisement, are perpetuated and left unaddressed because of the absence of White voices and White involvement in the conversations regarding these policies

Contract Marketing after the 2002 Farm Act: The Case of Peanuts

The elimination of the marketing quota system that regulated the peanut market since the 1930s has been accompanied by the emergence of marketing contracts between farmers and peanut buyers (mainly peanut shellers). Two types of contracts have been observed, forward contracts for delivery at harvest or at a later date and "option to purchase" contracts. We analyze the clauses of contracts used by major shellers in order to infer the motivation behind these contracts (i.e., risk sharing, reduction of transaction costs, improve coordination, exercise of market power, etc.). The analysis points out that the main role of the contracts is to replace the marketing structure existing prior the 2002 Farm Act, where peanut marketing was quite regulated. In this sense, the reduction of transaction costs associated to the need for coordinating a continuous supply of homogeneous quality seems to be the most plausible explanation.Marketing,