An analytic interface dynamo over a shear layer of finite depth

Parker's analytic Cartesian interface dynamo is generalized to the case of a shear layer of finite thickness and low resistivity ("tachocline"), bounded by a perfect conductor ("radiative zone") on the one side, and by a highly diffusive medium ("convective zone") supporting an $\alpha$-effect on the other side. In the limit of high diffusivity contrast between the shear layer and the diffusive medium, thought to be relevant for the Sun, a pair of exact dispersion relations for the growth rate and frequency of dynamo modes is analytically derived. Graphic solution of the dispersion relations displays a somewhat unexpected, non-monotonic behaviour, the mathematical origin of which is elucidated. The dependence of the results on the parameter values (dynamo number and shear layer thickness) is investigated. The implications of this result for the solar dynamo problem are discussed.Comment: 11 pages, 4 figures Geophys. Astrophys. Fluid Dyn., in pres

Evaluation of the Thermal NO formation mechanism under low-temperature diesel combustion conditions

Over the past two decades, the amount of exhaust gas pollutants emissions has been significantly reduced due to the severe emission legislation imposed in most countries worldwide. Initial strategies simply required the employment of simple after-treatment and engine control devices; however, as the restrictions become more stringent, these strategies are evolving in the development of different combustion modes, specially characterized by having low-temperature combustion characteristics. These new working conditions demand the need to check the suitability of the current NO predictive models that coexist nowadays under standard diesel combustion characteristics, paying closer attention to the Thermal mechanism. In order to do so, a common chemical-kinetic software was employed to simulate, for n-heptane and methane fuels, fixed local conditions (standard diesel and low-temperature combustion) described by constant pressure, relative mixture fraction, oxygen mass fraction and initial and final reaction temperature. The study reflects a common trend between all the studied cases, independently of the considered local conditions, making it applicable to more complex situations such as real NO formation processes in diesel sprays. This relationship was characterized by a fourth-degree polynomial equation capable of substantially improving the NO prediction by just using the Thermal NO predictive model.The authors thank the Ministerio de Ciencia e Innovacion of the Spanish government for contributing to this work with the grant BES-2009-021897.Desantes Fernández, JM.; López, JJ.; Redón Lurbe, P.; Arregle, JJP. (2012). Evaluation of the Thermal NO formation mechanism under low-temperature diesel combustion conditions. International Journal of Engine Research. 13(6):531-539. https://doi.org/10.1177/1468087411429638S53153913

Influence of ambient temperature on diesel engine raw pollutants and fuel consumption in different driving cycles

This is the author s version of a work that was accepted for publication in International Journal of Engine Research. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published as https://doi.org/10.1177/1468087418792353[EN] The effect of low ambient temperature on diesel raw pollutant emissions is analysed in two different driving cycles: NEDC and WLTC. The study is focused on hydrocarbons, carbon monoxide, nitrogen oxides and fuel consumption. Tests are conducted at cold start in a HSDI light-duty diesel engine with two levels of ambient temperature: 20 degrees C and -7 degrees C. Results showed a general detriment of pollutant emissions and break thermal efficiency at low ambient temperatures. NOx is increased around 250% in both cycles when running at low temperatures. Effect on hydrocarbons is more noticeable in the NEDC, where it rises in 270%, compared with the 150% of increase in the WLTC. In the case of carbon monoxide, uncorrelated tendencies are observed between both driving cycles. Concerning the NEDC, carbon monoxide emissions increase up to 125%, while at the WLTC, they are reduced up to 20%. Finally, from the point of view of the thermal efficiency, a reduction of nearly 10% in the NEDC is observed. However, no fuel penalty is spotted regarding the WLTC.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge the 'Apoyo para la investigacion y Desarrollo (PAID)', grant for doctoral studies (FPI S1 2015 2512), of Universitat Politecnica de Valencia.Luján, JM.; Climent, H.; Ruiz-Rosales, S.; Moratal, A. (2019). 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International Journal of Engine Research, 20(2), 247-260. doi:10.1177/1468087417749391Ludykar, D., Westerholm, R., & Almén, J. (1999). Cold start emissions at +22, −7 and −20°C ambient temperatures from a three-way catalyst (TWC) car: regulated and unregulated exhaust components. Science of The Total Environment, 235(1-3), 65-69. doi:10.1016/s0048-9697(99)00190-4Weilenmann, M., Soltic, P., Saxer, C., Forss, A.-M., & Heeb, N. (2005). Regulated and nonregulated diesel and gasoline cold start emissions at different temperatures. Atmospheric Environment, 39(13), 2433-2441. doi:10.1016/j.atmosenv.2004.03.081Dardiotis, C., Martini, G., Marotta, A., & Manfredi, U. (2013). Low-temperature cold-start gaseous emissions of late technology passenger cars. Applied Energy, 111, 468-478. doi:10.1016/j.apenergy.2013.04.093Pavlovic, J., Marotta, A., & Ciuffo, B. (2016). CO2 emissions and energy demands of vehicles tested under the NEDC and the new WLTP type approval test procedures. Applied Energy, 177, 661-670. doi:10.1016/j.apenergy.2016.05.110Tsokolis, D., Tsiakmakis, S., Dimaratos, A., Fontaras, G., Pistikopoulos, P., Ciuffo, B., & Samaras, Z. (2016). Fuel consumption and CO2 emissions of passenger cars over the New Worldwide Harmonized Test Protocol. Applied Energy, 179, 1152-1165. doi:10.1016/j.apenergy.2016.07.091Giakoumis, E., & Zachiotis, A. (2017). Investigation of a Diesel-Engined Vehicle’s Performance and Emissions during the WLTC Driving Cycle—Comparison with the NEDC. Energies, 10(2), 240. doi:10.3390/en10020240Myung, C.-L., Jang, W., Kwon, S., Ko, J., Jin, D., & Park, S. (2017). Evaluation of the real-time de-NO x performance characteristics of a LNT-equipped Euro-6 diesel passenger car with various vehicle emissions certification cycles. Energy, 132, 356-369. doi:10.1016/j.energy.2017.05.089Marotta, A., Pavlovic, J., Ciuffo, B., Serra, S., & Fontaras, G. (2015). Gaseous Emissions from Light-Duty Vehicles: Moving from NEDC to the New WLTP Test Procedure. Environmental Science & Technology, 49(14), 8315-8322. doi:10.1021/acs.est.5b01364Luján, J. M., Climent, H., García-Cuevas, L. M., & Moratal, A. (2018). Pollutant emissions and diesel oxidation catalyst performance at low ambient temperatures in transient load conditions. Applied Thermal Engineering, 129, 1527-1537. doi:10.1016/j.applthermaleng.2017.10.138Ko, J., Jin, D., Jang, W., Myung, C.-L., Kwon, S., & Park, S. (2017). Comparative investigation of NOx emission characteristics from a Euro 6-compliant diesel passenger car over the NEDC and WLTC at various ambient temperatures. Applied Energy, 187, 652-662. doi:10.1016/j.apenergy.2016.11.105Armas, O., García-Contreras, R., & Ramos, A. (2016). On-line thermodynamic diagnosis of diesel combustion process with paraffinic fuels in a vehicle tested under NEDC. 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A four-dimensional {\Lambda}CDM-type cosmological model induced from higher dimensions using a kinematical constraint

A class of cosmological solutions of higher dimensional Einstein field equations with the energy-momentum tensor of a homogeneous, isotropic fluid as the source are considered with an anisotropic metric that includes the direct sum of a 3-dimensional (physical, flat) external space metric and an n-dimensional (compact, flat) internal space metric. A simple kinematical constraint is postulated that correlates the expansion rates of the external and internal spaces in terms of a real parameter {\lambda}. A specific solution for which both the external and internal spaces expand at different rates is given analytically for n=3. Assuming that the internal dimensions were at Planck length scales when the external space starts with a Big Bang (t=0), they expand only 1.49 times and stay at Planck length scales even in the present age of the universe (13.7 Gyr). The effective four dimensional universe would exhibit a behavior consistent with our current understanding of the observed universe. It would start in a stiff fluid dominated phase and evolve through radiation dominated and pressureless matter dominated phases, eventually going into a de Sitter phase at late times.Comment: 12 pages, 8 figures; matches the version published in General Relativity and Gravitatio

Writhe in the Stretch-Twist-Fold Dynamo

This is an Author's Original Manuscript of an article whose final and definitive form, the Version of Record, has been published in Geophysical and Astrophysical Fluid Dynamics (2008) Copyright © 2008 Taylor & Francis, available online at: http://www.tandfonline.com/10.1080/03091920802531791This article looks at the influence of writhe in the stretch-twist-fold dynamo. We consider a thin flux tube distorted by simple stretch, twist, and fold motions and calculate the helicity and energy spectra. The writhe number assists in the calculations, as it tells us how much the internal twist changes as the tube is distorted. In addition it provides a valuable diagnostic for the degree of distortion. Non mirror-symmetric dynamos typically generate magnetic helicity of one sign on large-scales and the opposite sign on small scales. The calculations presented here confirm the hypothesis that the large-scale helicity corresponds to writhe and the small scale corresponds to twist. In addition, the writhe helicity spectrum exhibits an interesting oscillatory behavior. The technique of calculating Fourier spectra for the writhe helicity may be useful in other areas of research, for example, the study of highly coiled molecules

Slightly Non-Minimal Dark Matter in PAMELA and ATIC

We present a simple model in which dark matter couples to the standard model through a light scalar intermediary that is itself unstable. We find this model has several notable features, and allows a natural explanation for a surplus of positrons, but no surplus of anti-protons, as has been suggested by early data from PAMELA and ATIC. Moreover, this model yields a very small nucleon coupling, well below the direct detection limits. In this paper we explore the effect of this model in both the early universe and in the galaxy.Comment: 7 pages, 6 figures, v3: updated for new data, added discussion of Ferm

The Sunyaev-Zeldovich Effect and Its Cosmological Significance

Comptonization of the cosmic microwave background (CMB) radiation by hot gas in clusters of galaxies - the Sunyaev-Zeldovich (S-Z) effect - is of great astrophysical and cosmological significance. In recent years observations of the effect have improved tremendously; high signal-to-noise images of the effect (at low microwave frequencies) can now be obtained by ground-based interferometric arrays. In the near future, high frequency measurements of the effect will be made with bolomateric arrays during long duration balloon flights. Towards the end of the decade the PLANCK satellite will extensive S-Z surveys over a wide frequency range. Along with the improved observational capabilities, the theoretical description of the effect and its more precise use as a probe have been considerably advanced. I review the current status of theoretical and observational work on the effect, and the main results from its use as a cosmological probe.Comment: Invited review; in proceedings of the Erice NATO/ASI `Astrophysical Sources of High Energy Particles and Radiation'; 11 pages, 3 figure

Viscosity Sum Rules at Large Scattering Lengths

We use the operator product expansion (OPE) and dispersion relations to obtain new model-independent "Borel-resummed" sum rules for both shear and bulk viscosity of many-body systems of spin-1/2 fermions with predominantly short range S-wave interactions. These sum rules relate Gaussian weights of the frequency-dependent viscosities to the Tan contact parameter C(a). Our results are valid for arbitrary values of the scattering length a, but receive small corrections from operators of dimension larger than 5 in the OPE, and can be used to study transport properties in the vicinity of the infinite scattering length fixed point. In particular, we find that the exact dependence of the shear viscosity sum rule on scattering length is controlled by the function C(a). The sum rules that we obtain depend on a frequency scale w that can be optimized to maximize their overlap with low-energy data

Type Ia Supernova Explosion Models

Because calibrated light curves of Type Ia supernovae have become a major tool to determine the local expansion rate of the Universe and also its geometrical structure, considerable attention has been given to models of these events over the past couple of years. There are good reasons to believe that perhaps most Type Ia supernovae are the explosions of white dwarfs that have approached the Chandrasekhar mass, M_ch ~ 1.39 M_sun, and are disrupted by thermonuclear fusion of carbon and oxygen. However, the mechanism whereby such accreting carbon-oxygen white dwarfs explode continues to be uncertain. Recent progress in modeling Type Ia supernovae as well as several of the still open questions are addressed in this review. Although the main emphasis will be on studies of the explosion mechanism itself and on the related physical processes, including the physics of turbulent nuclear combustion in degenerate stars, we also discuss observational constraints.Comment: 38 pages, 4 figures, Annual Review of Astronomy and Astrophysics, in pres

Primordial Black Holes: sirens of the early Universe

Primordial Black Holes (PBHs) are, typically light, black holes which can form in the early Universe. There are a number of formation mechanisms, including the collapse of large density perturbations, cosmic string loops and bubble collisions. The number of PBHs formed is tightly constrained by the consequences of their evaporation and their lensing and dynamical effects. Therefore PBHs are a powerful probe of the physics of the early Universe, in particular models of inflation. They are also a potential cold dark matter candidate.Comment: 21 pages. To be published in "Quantum Aspects of Black Holes", ed. X. Calmet (Springer, 2014