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The injection and evolution of turbulence in the intergalactic medium is studied by means of mesh-based hydrodynamical simulations, including a subgrid scale (SGS) model for small-scale unresolved turbulence. The simulations show that the production of turbulence has a different redshift dependence in the intracluster medium (ICM) and the warm-hot intergalactic medium (WHIM). We show that turbulence in the ICM is produced chiefly by merger-induced shear flows, whereas the production in the WHIM is dominated by shock interactions. Secondly, the effect of dynamical pressure support on the gravitational contraction has been studied. This turbulent support is stronger in the WHIM gas at baryon overdensities 1 < delta < 100, and less relevant for the ICM. Although the relative mass fraction of the gas with large vorticity is considerable (52% in the ICM), we find that for only about 10% in mass this is dynamically relevant, namely not associated to an equally large thermal pressure support. According to this result, a significant non-thermal pressure support counteracting the gravitational contraction is a localised characteristic in the cosmic flow, rather than a widespread feature., 14 pages, 11 figure. MNRAS, accepted

FEARLESS (Fluid mEchanics with Adaptively Refined Large Eddy SimulationS) is a numerical scheme for modelling subgrid-scale turbulence in cosmological adaptive mesh refinement simulations. In this contribution, the main features of this tool will be outlined. We discuss the application of this method to cosmological simulations of the large-scale structure. The simulations show that the production of turbulence has a different redshift dependence in the intra-cluster medium and the warm-hot intergalactic medium, caused by the distinct stirring mechanisms (mergers and shock interactions) acting in them. Some properties of the non-thermal pressure support in the two baryon phases are also described., Comment: 4 pages, 1 figure, proceedings of "Advances in Computational Astrophysics: methods, tools and outcomes", Cefal\`u (Italy), June 2011, to appear in ASP Conf. Series

The delayed detonation model describes the observational properties of the majority of type Ia supernovae very well. Using numerical data from a three-dimensional deflagration model for type Ia supernovae, the intermittency of the turbulent velocity field and its implications on the probability of a deflagration-to-detonation (DDT) transition are investigated. From structure functions of the turbulent velocity fluctuations, we determine intermittency parameters based on the log-normal and the log-Poisson models. On the other hand, the analysis of the turbulent velocity fluctuations in the vicinity of the flame front by Roepke suggests a much higher probability of large velocity fluctuations on the grid scale in comparison to the log-normal intermittency model. Following Pan et al., we computed probability density functions for a DDT for the different distributions. Assuming that a DDT can occur in the stirred flame regime, as proposed by Woosley et al., the log-normal model would imply a delayed detonation between 0.7 and 0.8 seconds after the beginning of the deflagration phase for the multi-spot ignition scenario used in the simulation. However, the probability drops to virtually zero if a DDT is further constrained by the requirement that the turbulent velocity fluctuations reach about 500 km/s. Under this condition, delayed detonations are only possible if the distribution of the velocity fluctuations is not log-normal. From our calculations follows that the distribution obtained by Roepke allow for multiple DDTs around 0.8 seconds after ignition at a transition density close to 1x10^7 g/cm^3., Comment: 33 pages, 10 figures, submitted to ApJ

We present a numerical scheme for modelling unresolved turbulence in cosmological adaptive mesh refinement codes. As a first application, we study the evolution of turbulence in the intra-cluster medium and in the core of a galaxy cluster. Simulations with and without subgrid scale model are compared in detail. Since the flow in the ICM is subsonic, the global turbulent energy contribution at the unresolved length scales is smaller than 1% of the internal energy. We find that the production of turbulence is closely correlated with merger events occurring in the cluster environment, and its dissipation locally affects the cluster energy budget. Because of this additional source of dissipation, the core temperature is larger and the density is smaller in the presence of subgrid scale turbulence than in the standard adiabatic run, resulting in a higher entropy core value., Comment: Submitted to ApJ, 14 pages, 14 figures, 3 tables

The onset of the thermonuclear runaway in a Chandrasekhar-mass white dwarf, leading to the explosion as a type Ia supernova, is studied with hydrodynamical simulations. We investigate the evolution of temperature fluctuations (``bubbles'') in the WD's convective core by means of 2D numerical simulations. We show how the occurrence of the thermonuclear runaway depends on various bubble parameters. The relevance of the progenitor's composition for the ignition process is also discussed., 4 pages, 2 figures, proceedings of the Catania Workshop on Nuclear and Neutrino Astrophysics (WNNA 2007)

We present numerical calculations of the parameters $C_{\nu}$, $C_{\epsilon}$ and $C_{\kappa}$ associated with the common closures for turbulence production, dissipation and diffusion. In the case of homogeneous and isotropic turbulence, these parameters are expected to be statistically scale-invariant within the inertial subrange. In order to scrutinise this conjecture, we utilised a generalisation of the Germano filtering formalism, which is applicable to compressible flows as well. The filtering of data obtained from three-dimensional direct numerical simulations of forced isotropic turbulence with Mach numbers in the range $\sim 0.1...1$ then yielded values of the closure parameters associated with different length scales. The results indicate that the closure parameters are nearly universal for subsonic or moderately transonic flows, although the resolution of $432^{3}$ grid cells in our simulations is not quite sufficient to clearly establish scale invariance. In addition, it was found that the customary assumption of a kinetic Prandtl number of about unity for the gradient-diffusion closure is flawed due to the misalignment between turbulent flux and the gradient of the turbulence energy. Nevertheless, sound correlation can be achieved if the flux magnitude rather than the flux vector is locally matched. This conclusion is particularly useful for the family of subgrid scale models based on the turbulence energy equation. Furthermore, the parameter of production $C_{\nu}$ was computed in the fashion of dynamical procedures. Thereby, superior agreement between modelled and explicitly evaluated turbulence stresses in comparison to the eddy-viscosity closure with constant $C_{\nu}$ was verified., Comment: 17 pages, 4 figures; submitted to J. Fluid Mech. PDF file with full resolution figures available at http://www.astro.uni-wuerzburg.de/~schmidt/SimlTurb_JFM.pdf

Type Ia supernovae have recently received considerable attention because it appears that they can be used as "standard candles" to measure cosmic distances out to billions of light years away from us. Observations of type Ia supernovae seem to indicate that we are living in a universe that started to accelerate its expansion when it was about half its present age. These conclusions rest primarily on phenomenological models which, however, lack proper theoretical understanding, mainly because the explosion process, initiated by thermonuclear fusion of carbon and oxygen into heavier elements, is difficult to simulate even on supercomputers. Here, we investigate a new way of modeling turbulent thermonuclear deflagration fronts in white dwarfs undergoing a type Ia supernova explosion. Our approach is based on a level set method which treats the front as a mathematical discontinuity and allows for full coupling between the front geometry and the flow field. New results of the method applied to the problem of type Ia supernovae are obtained. It is shown that in 2-D with high spatial resolution and a physically motivated subgrid scale model for the nuclear flames numerically "converged" results can be obtained, but for most initial conditions the stars do not explode. In contrast, simulations in 3-D, do give the desired explosions and many of their properties, such as the explosion energies, lightcurves and nucleosynthesis products, are in very good agreement with observed type Ia supernovae., Comment: 17 pages, 13 figures, some figures are reduced in resolution, full-resolution version of this text available at http://www.mpa-garching.mpg.de/mpa/publications/preprints/pp2004/MPA1621.pdf -- to appear in "Analysis and Numerics of Conservation Laws", ed. G. Warnecke (Springer-Verlag)

We demonstrate that shunting of Superconductor-Insulator-Superconductor Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced non-linear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlO${_x}$/Nb junction shunted by Nb/AlO${_x}$/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited non-hysteretic I-V characteristics at temperatures down to at least 1.4 K., Comment: 4 pages incl. 3 figures

We investigate the interaction of thermonuclear flames in Type Ia supernova explosions with vortical flows by means of numerical simulations. In our study, we focus on small scales, where the flame propagation is no longer dominated by the turbulent cascade originating from large-scale effects. Here, the flame propagation proceeds in the cellular burning regime, resulting from a balance between the Landau-Darrieus instability and its nonlinear stabilization. The interaction of a cellularly stabilized flame front with a vortical fuel flow is explored applying a variety of fuel densities and strengths of the velocity fluctuations. We find that the vortical flow can break up the cellular flame structure if it is sufficiently strong. In this case the flame structure adapts to the imprinted flow field. The transition from the cellularly stabilized front to the flame structure dominated by vortices of the flow proceeds in a smooth way. The implications of the results of our simulations for Type Ia Supernova explosion models are discussed., 13 pages, 2 tables, 10 figures, resolution of Figs. 4 and 5 degraded due to archive file size restrictions, submitted to A&A

We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In our samples with relatively large charging energy values Ec > EJ, where EJ is the energy of the Josephson coupling, two transport mechanisms were found to be dominating, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island (in the open state of the transistor around the points Qg = CgVg = e mod(2e), where Cg is the gate capacitance) or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the open states of our transistors had been found to be unstable with respect to quasiparticle poisoning, high-frequency gate cycling (at f ~ 1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data., 8 pages, 6 figures

We present a numerical model to study the behavior of thermonuclear flames in the discontinuity approximation. This model is applied to investigate the Landau-Darrieus instability under conditions found in Type Ia supernova explosions of Chandrasekhar mass white dwarfs. This is a first step to explore the flame microstructure in these events. The model reproduces Landau's linearized stability analysis in early stages of the flame evolution and the stabilization in a cellular flame structure in the nonlinear stage., 6 pages, 3 figures, to appear in Proc. 11th Workshop on Nuclear Astrophysics, Ringberg Castle, Germany, 2002

Type Ia supernovae have become an indispensable tool for studying the expansion history of the universe, yet our understanding of the explosion mechanism is still incomplete. We describe the variety of discussed scenarios, sketch the most relevant physics, and report recent advances in multidimensional simulations of Chandrasekhar mass white dwarf explosions., Based on an invited review at the XIII Rencontres de Blois - Frontiers of the Universe, June 17-23,2001

We report on the effect of the frequency-locked transfer of single Cooper pairs in a superconducting single-electron Al transistor embedded in a dissipative environment (on-chip Cr resistor of R = 40 kOhm). The transistor was dc voltage biased, and the harmonic signal of frequency f of several MHz was applied to the gate. Due to the substantial rate of relaxation, the unidirectional transfer of single pairs occurred in each junction once per clock cycle and the current plateaus at I = 2ef were developed in the transistor's I-V curves. The mechanisms (supercurrent, Landau-Zener tunneling, quasiparticle tunneling, etc.) deteriorating the phase-locking regime are discussed., 5 pages incl. 3 figures

We report on a new type of single-electron transistor (SET) comprising two highly resistive Cr thin-film strips (~ 1um long) connecting a 1 um-long Al island to two Al outer electrodes. These resistors replace small-area oxide tunnel junctions of traditional SETs. Our transistor with a total asymptotic resistance of 110 kOhm showed a very sharp Coulomb blockade and reproducible, deep and strictly e-periodic gate modulation in wide ranges of bias currents I and gate voltages V_g. In the Coulomb blockade region (|V| < 0.5 mV), we observed a strong suppression of the cotunneling current allowing appreciable modulation curves V-V_g to be measured at currents I as low as 100 fA. The noise figure of our SET was found to be similar to that of typical Al/AlOx/Al single-electron transistors., 5 pages incl. 4 figs

We present a new way of modeling turbulent thermonuclear deflagration fronts in Chandrasekhar-mass white dwarfs, consisting of carbon and oxygen, undergoing a type Ia supernova explosion. Our approach is a front capturing/tracking hybrid scheme, based on a level set method, which treats the front as a mathematical discontinuity and allows for full coupling between the front geometry and the flow field. First results of the method applied to the problem of type Ia supernovae are discussed. It will be shown that even in 2-D and even with a physically motivated sub-grid model numerically ``converged'' results are difficult to obtain., 13 pages, 5 figures. To appear in Proc. ``Two decades of numerical astrophysics'', Port Douglas, Australia 1999

Type Ia supernovae (SN Ia) are generally believed to be the result of the thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs, mainly because such thermonuclear explosions can account for the right amount of nickel, which is needed to explain the light curves and the late-time spectra, and the abundances of intermediate-mass nuclei which dominate the spectra near maximum light. Because of their enormous brightness and apparent homogeneity SN Ia have become an important tool to measure cosmological parameters. In this article the present understanding of the physics of thermonuclear explosions is reviewed. In particular, we focus our attention on subsonic ("deflagration") fronts, i.e. we investigate fronts propagating by heat diffusion and convection rather than by compression. Models based upon this mode of nuclear burning have been applied very successfully to the SN Ia problem, and are able to reproduce many of their observed features remarkably well. However, the models also indicate that SN Ia may differ considerably from each other, which is of importance if they are to be used as standard candles., Comment: 9 pages, 3 figures, conference proceedings

We propose a circuit comprising a linear array of three small-capacitance superconducting tunnel junctions with miniature resistors (R > R_Q = h/4e^2 = 6.5 kOhm) attached to the ends of this array. Owing to the Coulomb blockade effect and the effect of dissipative environment on the supercurrent, this circuit enables the gate-controlled transfer of individual Cooper pairs. The first experiment on operating the Al-Cr Cooper pair R-pump is described., Reported at International Workshop on "Macroscopic Quantum Coherence and Computing - MQC2" (14-17 June, 2000, Naples, Italy)

The mechanism for deflagration-detonation-transition (DDT) by turbulent preconditioning, suggested to explain the possible occurrence of delayed detonations in Type Ia supernova explosions, is argued to be conceptually inconsistent. It relies crucially on diffusive heat losses of the burned material on macroscopic scales. Regardless of the amplitude of turbulent velocity fluctuations, the typical gradient scale for temperature fluctuations is shown to be the laminar flame width or smaller, rather than the factor of thousand more required for a DDT. Furthermore, thermonuclear flames cannot be fully quenched in regions much larger than the laminar flame width as a consequence of their simple ``chemistry''. Possible alternative explosion scenarios are briefly discussed., 8 pages, uses aastex; added references. Accepted by ApJ Letters

We report on the operation of a single electron trap comprising a chain of four Al/AlOx/Al tunnel junctions attached, at one side, to a memory island and, at the other side, to a miniature on-chip Cr resistor R=50 kOhm which served to suppress cotunneling. At appropriate voltage bias the bi-stable states of the trap, with the charges differing by the elementary charge e, were realized. At low temperature, spontaneous switching between these states was found to be infrequent. For instance, at T=70 mK the system was capable of holding an electron for more than 2 hours, this time being limited by the time of the measurement., 3 pages of text and 2 figures

The low-frequency noise figures of single-electron transistors (electrometers) of traditional planar and new stacked geometry were compared. We observed a correlation between the charge noise and the contact area of the transistor island with a dielectric substrate in the set of Al transistors located on the same chip and having almost similar electric parameters. We have found that the smaller the contact area the lower the noise level of the transistor. The lowest noise value 8*10E-6 e/sqrt(Hz) at f = 10 Hz. has been measured in a stacked transistor with an island which was completely isolated from a substrate. Our measurements have unambiguously indicated that the dominant source of the background charge fluctuations is associated with a dielectric substrate, Comment: Review paper, latex, 10 pages, 7 figures, to be publ. in JLTP, 2000; Proceeding of "Electron Transport in Mesoscopic Systems", August 12-15, 1999 Geteborg, Sweden, http://fy.chalmers.se/meso_satellite/index.html See also LT22 manuscript: http://lt22.hut.fi/cgi/view?id=S11138

First results of a numerical investigation of primordial black hole formation in the radiation dominated phase of the Early Universe are presented. The simulations follow the gravitational collapse of three different families of high-amplitude density fluctuations imposed at the time of horizon crossing. The threshold for black hole formation, \delta_{c} \approx 0.7, is found to be nearly identical for all perturbation families if the control parameter, \delta, is chosen as the total excess mass within the initial horizon volume. Furthermore, we demonstrate that the scaling of black hole mass with distance from the formation threshold, known to occur in near-critical gravitational collapse, applies to primordial black hole formation., Comment: 4 pages, 1 figure included, uses prcrc.sty. To appear in the proceedings of Dark Matter '98, Los Angeles (ed. D. Cline). Also available at http://bigwhirl.uchicago.edu/jcn/pub_pbh.html

In the framework of the Chandrasekhar mass white dwarf model for Type Ia supernovae, various stages of the explosion are described in terms of the burning regimes of the thermonuclear flame front. In the early flamelet regime following the ``smoldering'' phase prior to the explosion, the flame is sufficiently thin and fast to remain laminar on small scales. As the white dwarf density declines, the thermal flame structure becomes subject to penetration by turbulent eddies, and it enters the ``distributed burning'' regime. A specific control parameter for this transition is proposed. Furthermore, we outline an argument for the coincidence of the transition between burning regimes with the onset of a deflagration-detonation-transition (DDT) in the late phase of the explosion., Comment: 5 pages, to appear in the proceedings of the Second Oak Ridge Symposium on Atomic and Nuclear Astrophysics. See also http://bigwhirl.uchicago.edu/jcn/pub_snIa.html

Gibson scaling and related properties of flame-surface geometry in turbulent premixed combustion are demonstrated using a novel computational model, Deterministic Turbulent Mixing (DTM). In DTM, turbulent advection is represented by a sequence of maps applied to the computational domain. The structure of the mapping sequence incorporates pertinent scaling properties of the turbulent cascade. Here, combustion in Kolmogorov turbulence (kinetic-energy cascade) and in Bolgiano-Obukhov convective turbulence (potential-energy cascade) is simulated. Implications with regard to chemical flames and astrophysical (thermonuclear) flames are noted., 16 pages, 5 ps figures included, accepted by Combust. Sci. Tech., also available at http://www.mpa-garching.mpg.de/~jcn/pub_flame.html

A series of two dimensional numerical simulations of explosive nuclear burning is presented for white dwarfs near the Chandraskhar mass. We assume that the burning begins as a slow deflagration front at or near the center of the star, and continues until the density in the burning regions has declined to about 10$^7$ g cm$^{-3}$, where the flame is essentially extinguished. We employ a novel numerical representation of the turbulent flame brush based upon ideas previously developed for modelling laboratory combustion and explore in some detail the sensitivity of the outcome to the manner in which burning is initiated. In particular, we simulate 1) a centrally ignited deflagration, 2) off-center ignition at a single ``point", and 3) simultaneous off-center ignition at five ``points". We find that the amount of $^{56}$Ni that is produced and other observable properties depend sensitively upon how the fuel is ignited., 14 pages, 9 figures included (200kB), uses aas2pp4.sty. Accepted by ApJ

We present a numerical investigation of the cellular burning regime in Type Ia supernova explosions. This regime holds at small scales (i.e. below the Gibson scale), which are unresolved in large-scale Type Ia supernova simulations. The fundamental effects that dominate the flame evolution here are the Landau-Darrieus instability and its nonlinear stabilization, leading to a stabilization of the flame in a cellular shape. The flame propagation into quiescent fuel is investigated addressing the dependence of the simulation results on the specific parameters of the numerical setup. Furthermore, we investigate the flame stability at a range of fuel densities. This is directly connected to the questions of active turbulent combustion (a mechanism of flame destabilization and subsequent self-turbulization) and a deflagration-to-detonation transition of the flame. In our simulations we find no substantial destabilization of the flame when propagating into quiescent fuels of densities down to ~10^7 g/cm^3, corroborating fundamental assumptions of large-scale SN Ia explosion models. For these models, however, we suggest an increased lower cutoff for the flame propagation velocity to take the cellular burning regime into account., 12 pages, 2 tables, 10 figures, resolution of figures degraded due to archive file size restrictions, submitted to A&A

The Square Kilometre Array (SKA) is the most ambitious radio telescope ever planned. With a collecting area of about a square kilometre, the SKA will be far superior in sensitivity and observing speed to all current radio facilities. The scientific capability promised by the SKA and its technological challenges provide an ideal base for interdisciplinary research, technology transfer, and collaboration between universities, research centres and industry. The SKA in the radio regime and the European Extreme Large Telescope (E-ELT) in the optical band are on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI) and have been recognised as the essential facilities for European research in astronomy. This "White Paper" outlines the German science and R&D interests in the SKA project and will provide the basis for future funding applications to secure German involvement in the Square Kilometre Array., Editors: H. R. Kl\"ockner, M. Kramer, H. Falcke, D.J. Schwarz, A. Eckart, G. Kauffmann, A. Zensus; 150 pages (low resolution- and colour-scale images), published in July 2012, language English (including a foreword and an executive summary in German), the original file is available via the MPIfR homepage