Phase Field Model for Dynamics of Sweeping Interface

Motivated by the drying pattern experiment by Yamazaki and Mizuguchi[J. Phys. Soc. Jpn. {\bf 69} (2000) 2387], we propose the dynamics of sweeping interface, in which material distributed over a region is swept by a moving interface. A model based on a phase field is constructed and results of numerical simulations are presented for one and two dimensions. Relevance of the present model to the drying experiment is discussed.Comment: 4 pages, 7 figure

Winds from Nuclear Starbursts: Old Truths and Recent Progress on Superwinds

I will discuss a few select aspects of the most common and best understood galactic-scale outflow -- starburst-driven superwinds, focusing on winds from nuclear starburst galaxies. I will show that modern observations, in particular in the soft and hard X-ray bands, complement and reinforce the existing paradigm of superwinds as flows collectively driven by multiple SNe. The properties of the diffuse X-ray emission from dwarf starburst galaxies, L_BOL ~ L_* starbursts in spiral galaxies, and ULIRGS, are all consistent with superwind activity. Where appropriate, I contrast the physics of starburst-driven winds with poorly collimated winds from AGN, and discuss what we know of the role of LLAGN and Seyfert nuclei in starburst superwind galaxies.Comment: To appear in the proceedings of IAU symposium 222: The Interplay among Black Holes, Stars and ISM in Galactic Nuclei, held in Gramado, Brazil, March 1-5 2004. Eds. Th. Storchi Bergmann, L.C. Ho & H.R. Schmitt. 6 pages, 1 figur

Black hole foraging: feedback drives feeding

We suggest a new picture of supermassive black hole (SMBH) growth in galaxy centers. Momentum-driven feedback from an accreting hole gives significant orbital energy but little angular momentum to the surrounding gas. Once central accretion drops, the feedback weakens and swept-up gas falls back towards the SMBH on near-parabolic orbits. These intersect near the black hole with partially opposed specific angular momenta, causing further infall and ultimately the formation of a small-scale accretion disk. The feeding rates into the disk typically exceed Eddington by factors of a few, growing the hole on the Salpeter timescale and stimulating further feedback. Natural consequences of this picture include (i) the formation and maintenance of a roughly toroidal distribution of obscuring matter near the hole; (ii) random orientations of successive accretion disk episodes; (iii) the possibility of rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed from infalling gas, resulting in visible flares and ejection of hypervelocity stars; (v) super-solar abundances of the matter accreting on to the SMBH; and (vi) a lower central dark-matter density, and hence annihilation signal, than adiabatic SMBH growth implies. We also suggest a simple sub-grid recipe for implementing this process in numerical simulations.Comment: accepted for publication in ApJ Letters, 5 pages, 1 figur

A review of recent analytical and experimental studies applicable to LMFBR fuel and blanket assembly design / by E. Khan and N. Todreas

"September, 1973."Includes bibliographical references (pages [40]-[44])U.S. Atomic Energy Commission contract AT(11-1)-224

Development and Application of a Model for the Cross-Flow Induced by Mixing Vane Spacers in Fuel Assemblies

CFD investigations of the flow in a 5x5 rod bundle with mixing vane spacer grid were performed with single-phase and two-phase flow conditions. A non-linear k-epsilon model was used to simulate secondary flow and non-isotropic turbulence. A detailed analysis of the results showed that mixing vane induced swirling flow strongly affects the cross-flow. A model was developed to predict the forced cross-flow between the sub-channels and implemented into the sub-channel analysis code COBRA-FLX

Opportunities for use of exact statistical equations

Exact structure function equations are an efficient means of obtaining asymptotic laws such as inertial range laws, as well as all measurable effects of inhomogeneity and anisotropy that cause deviations from such laws. "Exact" means that the equations are obtained from the Navier-Stokes equation or other hydrodynamic equations without any approximation. A pragmatic definition of local homogeneity lies within the exact equations because terms that explicitly depend on the rate of change of measurement location appear within the exact equations; an analogous statement is true for local stationarity. An exact definition of averaging operations is required for the exact equations. Careful derivations of several inertial range laws have appeared in the literature recently in the form of theorems. These theorems give the relationships of the energy dissipation rate to the structure function of acceleration increment multiplied by velocity increment and to both the trace of and the components of the third-order velocity structure functions. These laws are efficiently derived from the exact velocity structure function equations. In some respects, the results obtained herein differ from the previous theorems. The acceleration-velocity structure function is useful for obtaining the energy dissipation rate in particle tracking experiments provided that the effects of inhomogeneity are estimated by means of displacing the measurement location.Comment: accepted by Journal of Turbulenc

Early Enrichment of the Intergalactic Medium and its Feedback on Galaxy Formation

Supernova-driven outflows from early galaxies may have had a large impact on the kinetic and chemical structure of the intergalactic medium (IGM). We use three-dimensional Monte Carlo cosmological realizations of a simple linear peaks model to track the time evolution of such metal-enriched outflows and their feedback on galaxy formation. We find that at most 30% of the IGM by volume is enriched to values above 10^-3 solar in models that only include objects that cool by atomic transitions. The majority of enrichment occurs relatively early (5 < z < 12) and resulting in a mass-averaged cosmological metallicity between 10^-3 and 10^-1.5 solar. The inclusion of Population III objects that cool through H2 line emission has only a minor impact on these results: increasing the mean metallicity and filling factor by at most a factor of 1.4, and moving the dawn of the enrichment epoch to a redshift of approximately 14 at the earliest. Thus enrichment by outflowing galaxies is likely to have been incomplete and inhomogeneous, biased to the areas near the starbursting galaxies themselves. Models with a 10% star formation efficiency can satisfactorily reproduce the nearly constant (2 < z < 5, Z approximately 3.5 x 10^-4 solar) metallicity of the low column density Ly-alpha forest derived by Songaila (2001), an effect of the decreasing efficiency of metal loss from larger galaxies. Finally, we show that IGM enrichment is intimately tied to the ram-pressure stripping of baryons from neighboring perturbations. This results in the suppression of at least 20% of the dwarf galaxies in the mass range 10^8.5 to 10^9.5 solar, in all models with filling factors greater than 2%, and an overall suppression of approximately 50% of dwarf galaxies in the most observationally-favored model.Comment: 8 pages, 5 figures, accepted to Ap