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Search Results: 1 - 10 of 5944 matches for " Friedrich-Karl Holtmeier "
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Subalpine Forest and Treeline Ecotone under the Influence of Disturbances: A Review  [PDF]
Friedrich-Karl Holtmeier, Gabriele Broll
Journal of Environmental Protection (JEP) , 2018, DOI: 10.4236/jep.2018.97051
Abstract: This article considers disturbances caused by abiotic and biotic factors and human impact in the ecological region extending from subalpine forest to the upper tree limit. Both abiotic and biotic factors may cause reversible or irreversible disturbances. Disturbances by mass movement and avalanches give the subalpine forest and the treeline ecotone a distinct spatial pattern characterized by forest on safe topography and sites that preclude forest. Removal of the upper subalpine forests by humans has enlarged the snow-catchment area of avalanches and elongated the avalanche pathways. Consequently, avalanche destructive potential has increased. Hazards will probably increase due to climate change. External factors, like cyclonic storms, may cause fundamental disturbances. Fires have played a major role in the removal of high-elevation forests. Forest destruction by fire is often followed by soil erosion. Wild fires are likely to increase as a result of warming climate and would possibly prevent climatically-driven treeline advance. Cyclic or episodic mass outbreaks of defoliating insects and bark beetles, and pathogens also cause severe disturbances. Oversized populations of wild ungulates impede tree regeneration and can cause local soil erosion. Inadequate game management is the primary cause of intolerable ungulate numbers. Due to man-caused habitat fragmentation, the animals’ impact on the remained habitats has increased. Subalpine forest may recover from disturbance or become replaced by a substitute formation (e.g. krummholz). A subsequent absence of natural disturbances may also be considered a disturbance initiating a new development. Both natural and anthropogenic disturbances may counteract positive influences of climatic warming on subalpine forests and treeline. Effective measures to reduce or prevent abiotic and biotic disturbances of high-elevation forest may contribute to greater safety for people living in the endangered areas of the mountain valleys and also improve other ecosystem services of the subalpine forest.
Silicon Burning II: Quasi-Equilibrium and Explosive Burning
W. Raphael Hix,Friedrich-Karl Thielemann
Physics , 1998, DOI: 10.1086/306692
Abstract: Having examined the application of quasi-equilibrium to hydrostatic silicon burning in Paper I of this series, Hix & Thielemann (1996), we now turn our attention to explosive silicon burning. Previous authors have shown that for material which is heated to high temperature by a passing shock and then cooled by adiabatic expansion, the results can be divided into three broad categories; \emph{incomplete burning}, \emph{normal freezeout} and \emph{$\alpha$-rich freezeout}, with the outcome depending on the temperature, density and cooling timescale. In all three cases, we find that the important abundances obey quasi-equilibrium for temperatures greater than approximately 3 GK, with relatively little nucleosynthesis occurring following the breakdown of quasi-equilibrium. We will show that quasi-equilibrium provides better abundance estimates than global nuclear statistical equilibrium, even for normal freezeout and particularly for $\alpha$-rich freezeout. We will also examine the accuracy with which the final nuclear abundances can be estimated from quasi-equilibrium.
Numerical Investigation on Pressure Fluctuations for Different Configurations of Vaned Diffuser Pumps
Jianjun Feng,Friedrich-Karl Benra,Hans Josef Dohmen
International Journal of Rotating Machinery , 2007, DOI: 10.1155/2007/34752
Abstract: Numerical simulations on impeller-diffuser interactions in radial diffuser pumps are conducted to investigate the unsteady flow, and more attention is paid to pressure fluctuations on the blade and vane surfaces. Calculations are performed at different operating points, different blade number configurations, and different radial gaps between the impeller and diffuser to examine their effects on the unsteady flow. Computational results show that a jet-wake flow structure is observed at the impeller outlet. The biggest pressure fluctuation on the blade is found to occur at the impeller trailing edge, on the pressure side near the impeller trailing edge, and at the diffuser vane leading edge, independent of the flow rate, radial gap, and blade number configuration. All of the flow rate, blade number configuration, and radial gap influence significantly the pressure fluctuation and associated unsteady effects in the diffuser pumps.
The r-process nucleosynthesis in the various jet-like explosions of magnetorotational core-collapse supernovae
Nobuya Nishimura,Tomoya Takiwaki,Friedrich-Karl Thielemann
Physics , 2015, DOI: 10.1088/0004-637X/810/2/109
Abstract: The r-process nucleosynthesis in core-collapse supernovae (CC-SNe) is studied, with a focus on the explosion scenario induced by rotation and strong magnetic fields. Nucleosynthesis calculations are conducted based on magneto-hydrodynamical explosion models with a wide range of parameters for initial rotation and magnetic fields. The explosion models are classified in two different types: i.e., prompt-magnetic-jet and delayed-magnetic-jet, for which the magnetic fields of proto-neutron stars (PNSs) during collapse and the core-bounce are strong and comparatively moderate, respectively. Following the hydrodynamical trajectories of each explosion model, we confirmed that r-processes successfully occur in the prompt-magnetic-jets, which produce heavy nuclei including actinides. On the other hand, the r-process in the delayed-magnetic-jet is suppressed, which synthesizes only nuclei up to the second peak ($A \sim 130$). Thus, the r-process in the delayed-magnetic-jets could explain only "weak r-process" patterns observed in metal-poor stars rather than the "main r-process", represented by the solar abundances. Our results imply that core-collapse supernovae are possible astronomical sources of heavy r-process elements if their magnetic fields are strong enough, while weaker magnetic explosions may produce "weak r-process" patterns ($A \lesssim 130$). We show the potential importance and necessity of magneto-rotational supernovae for explaining the galactic chemical evolution, as well as abundances of r-process enhanced metal-poor stars. We also examine the effects of the remaining uncertainties in the nature of PNSs due to weak interactions that determine the final neutron-richness of ejecta. Additionally, we briefly discuss radioactive isotope yields in primary jets (e.g., $^{56}$Ni), with relation to several optical observation of SNe and relevant high-energy astronomical phenomena.
An Adaptive Grid, Implicit Code for Spherically Symmetric, General Relativistic Hydrodynamics in Comoving Coordinates
Matthias Liebendoerfer,Stephan Rosswog,Friedrich-Karl Thielemann
Physics , 2001, DOI: 10.1086/339872
Abstract: We describe an implicit general relativistic hydrodynamics code. The evolution equations are formulated in comoving coordinates. A conservative finite differencing of the Einstein equations is outlined, and artificial viscosity and numerical diffusion are discussed. The time integration is performed with AGILE, an implicit solver for stiff algebrodifferential equations on a dynamical adaptive grid. We extend the adaptive grid technique, known from nonrelativistic hydrodynamics, to the general relativistic application and identify it with the concept of shift vectors in a 3+1 decomposition. The adaptive grid minimizes the number of required computational zones without compromising the resolution in physically important regions. Thus, the computational effort is greatly reduced when the zones are subject to computationally expensive additional processes, such as Boltzmann radiation transport or a nuclear reaction network. We present accurate results in the standard tests for supernova simulations: Sedov's point-blast explosion, the nonrelativistic and relativistic shock tube, the Oppenheimer-Snyder dust collapse, and homologous collapse.
Conservative General Relativistic Radiation Hydrodynamics in Spherical Symmetry and Comoving Coordinates
Matthias Liebendoerfer,Anthony Mezzacappa,Friedrich-Karl Thielemann
Physics , 2000, DOI: 10.1103/PhysRevD.63.104003
Abstract: The description of general relativistic radiation hydrodynamics in spherical symmetry is presented in natural coordinate choices. For hydrodynamics, comoving coordinates are chosen, and the momentum phase space for the radiation particles is described in comoving frame four momenta. We also investigate a description of the momentum phase space in terms of particle impact parameter and energy at infinity and derive a simple approximation to the general relativistic Boltzmann equation. Further developed are, however, the exact equations in comoving coordinates, because the description of the interaction between matter and radiation particles is best described in the closely related orthonormal basis comoving with the fluid elements. We achieve a conservative and concise formulation of radiation hydrodynamics that is well suited for numerical implementation by a variety of methods. The contribution of radiation to the general relativistic jump conditions at shock fronts is discussed, and artificial viscosity is consistently included in the derivations in order to support approaches relying on this option.
A Comparison of One-Way and Two-Way Coupling Methods for Numerical Analysis of Fluid-Structure Interactions
Friedrich-Karl Benra,Hans Josef Dohmen,Ji Pei,Sebastian Schuster,Bo Wan
Journal of Applied Mathematics , 2011, DOI: 10.1155/2011/853560
Abstract: The interaction between fluid and structure occurs in a wide range of engineering problems. The solution for such problems is based on the relations of continuum mechanics and is mostly solved with numerical methods. It is a computational challenge to solve such problems because of the complex geometries, intricate physics of fluids, and complicated fluid-structure interactions. The way in which the interaction between fluid and solid is described gives the largest opportunity for reducing the computational effort. One possibility for reducing the computational effort of fluid-structure simulations is the use of one-way coupled simulations. In this paper, different problems are investigated with one-way and two-way coupled methods. After an explanation of the solution strategy for both models, a closer look at the differences between these methods will be provided, and it will be shown under what conditions a one-way coupling solution gives plausible results. 1. Introduction The interaction between fluids and solids is a phenomenon that can often be observed in nature, for example, the deformation of trees or the movement of sand dunes caused by wind. In almost the same manner, wind can interact with buildings, sometimes with dramatic consequences, such as the collapse of the Tacoma-Narrows Bridge in November 1940. These processes can only be calculated using laws and equations from different physical disciplines. Examples like this, where the arising subproblems cannot be solved independently, are called multiphysics applications. A very important class of these multiphysics problems are fluid-structure interactions (FSIs), which are characterized by the fact that the flow around a body has a strong impact on the structure, and vice versa; the modification of the structure has a nonnegligible influence on the flow. Two disciplines involved in these kinds of multiphysics problems are fluid dynamics and structural dynamics, which can both be described by the relations of continuum mechanics. On this note, FSI is a subset of multiphysics applications and is defined well by Zienkiewicz and Taylor [1]: Coupled systems and formulations are those applicable to multiple domains and dependent variables which usually describe different physical phenomena and in which neither domain can be solved while separated from the other and neither set of dependent variables can be explicitly eliminated at the differential equation level. Solution strategies for FSI simulations are mainly divided into monolithic and partitioned methods; this paper will focus only on
Complete inclusion of parity-dependent level densities in the statistical description of astrophysical reaction rates
Hans Peter Loens,Karlheinz Langanke,Gabriel Martínez-Pinedo,Thomas Rauscher,Friedrich-Karl Thielemann
Physics , 2008, DOI: 10.1016/j.physletb.2008.07.073
Abstract: Microscopic calculations show a strong parity dependence of the nuclear level density at low excitation energy of a nucleus. Previously, this dependence has either been neglected or only implemented in the initial and final channels of Hauser-Feshbach calculations. We present an indirect way to account for a full parity dependence in all steps of a reaction, including the one of the compound nucleus formed in a reaction. To illustrate the impact on astrophysical reaction rates, we present rates for neutron captures in isotopic chains of Ni and Sn. Comparing with the standard assumption of equipartition of both parities, we find noticeable differences in the energy regime of astrophysical interest caused by the parity dependence of the nuclear level density found in the compound nucleus even at sizeable excitation energies.
Massive Stars and their Supernovae
Friedrich-Karl Thielemann,Raphael Hirschi,Matthias Liebend?rfer,Roland Diehl
Physics , 2010, DOI: 10.1007/978-3-642-12698-7_4
Abstract: Massive stars and their supernovae are prominent sources of radioactive isotopes, the observations of which thus can help to improve our astrophysical models of those. Our understanding of stellar evolution and the final explosive endpoints such as supernovae or hypernovae or gamma-ray bursts relies on the combination of magneto-hydrodynamics, energy generation due to nuclear reactions accompanying composition changes, radiation transport, and thermodynamic properties (such as the equation of state of stellar matter). Nuclear energy production includes all nuclear reactions triggered during stellar evolution and explosive end stages, also among unstable isotopes produced on the way. Radiation transport covers atomic physics (e.g. opacities) for photon transport, but also nuclear physics and neutrino nucleon/nucleus interactions in late phases and core collapse. Here we want to focus on the astrophysical aspects, i.e. a description of the evolution of massive stars and their endpoints, with a special emphasis on the composition of their ejecta (in form of stellar winds during the evolution or of explosive ejecta). Low and intermediate mass stars end their evolution as a white dwarf with an unburned C and O composition. Massive stars evolve beyond this point and experience all stellar burning stages from H over He, C, Ne, O and Si-burning up to core collapse and explosive endstages. In this chapter we discuss the nucleosynthesis processes involved and the production of radioactive nuclei in more detail.
A New Approach to Determine the Initial Mass Function in the Solar Neighborhood
Takuji Tsujimoto,Yuzuru Yoshii,Ken'ichi Nomoto,Francesca Matteucci,Friedrich-Karl Thielemann,Masaaki Hashimoto
Physics , 1997, DOI: 10.1086/304215
Abstract: Oxygen to iron abundance ratios of metal-poor stars provide information on nucleosynthesis yields from massive stars which end in Type II supernova explosions. Using a standard model of chemical evolution of the Galaxy we have reproduced the solar neighborhood abundance data and estimated the oxygen and iron yields of genuine SN II origin. The estimated yields are compared with the theoretical yields to derive the relation between the lower and upper mass limits in each generation of stars and the IMF slope. Independently of this relation, we furthermore derive the relation between the lower mass limit and the IMF slope from the stellar mass to light ratio in the solar neighborhood. These independent relations unambiguously determine the upper mass limit of $m_u=50 \pm 10 M_sun$ and the IMF slope index of 1.3 - 1.6 above 1 M_sun. This upper mass limit corresponds to the mass beyond which stars end as black holes without ejecting processed matter into the interstellar medium. We also find that the IMF slope index below 0.5 M_sun cannot be much shallower than 0.8.
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