Analysis of thermal environment in the thrustor cavity of space vehicles

Wind tunnel tests to determine the mechanisms involved in producing the high local heating rates in and around the thrustor nozzle in a non-firing condition are reported. The geometry of a typical nozzle installation is described. Procedures for reducing the effects of hypersonic flow interactions in order to prevent excess local heating are explained

Static and non-static quantum effects in two-dimensional dilaton gravity

We study backreaction effects in two-dimensional dilaton gravity. The backreaction comes from an $R^2$ term which is a part of the one-loop effective action arising from massive scalar field quantization in a certain approximation. The peculiarity of this term is that it does not contribute to the Hawking radiation of the classical black hole solution of the field equations. In the static case we examine the horizon and the physical singularity of the new black hole solutions. Studying the possibility of time dependence we see the generation of a new singularity. The particular solution found still has the structure of a black hole, indicating that non-thermal effects cannot lead, at least in this approximation, to black hole evaporation.Comment: 10 pages, no figure

A study of forced flow separation in rocket nozzle

The characteristics of flow profile in a rocket nozzle during the start transient were investigated as well as possibilities of reducing the side-load thrust by sticking trip wires in the nozzle. To simplify the geometry of flow configuration around the trip wire, it was assumed that the flow is passing through square steps instead of round wires. Since a purely analytic solution is not available, a series of semiempirical solutions was proposed

On the unitarity of higher-dervative and nonlocal theories

We consider two simple models of higher-derivative and nonlocal quantu systems.It is shown that, contrary to some claims found in literature, they can be made unitary.Comment: 8 pages, no figure

A Deformation of Twistor Space and a Chiral Mass Term in N=4 Super Yang-Mills Theory

Super twistor space admits a certain (super) complex structure deformation that preserves the Poincare subgroup of the symmetry group PSL(4|4) and depends on 10 parameters. In a previous paper [hep-th/0502076], it was proposed that in twistor string theory this deformation corresponds to augmenting N=4 super Yang-Mills theory by a mass term for the left-chirality spinors. In this paper we analyze this proposal in more detail. We calculate 4-particle scattering amplitudes of fermions, gluons and scalars and show that they are supported on holomorphic curves in the deformed twistor space.Comment: 52 pages, 15 figure

X-ray absorption spectroscopy studies of Ba1-xCaxTiO3

[[abstract]]We report x-ray absorption near edge spectroscopy (XANES) of Ca and O K-edges of Ba1-xCaxTiO3 (x = 0, 0.01, 0.08, 1) and understand the spectral features related to the electronic structure of these perovskites. The XANES spectra of Ca K-edge possess a pre-edge peak similar to other 3d transition metals like Ti, Ni when present in perovskite structure and provides information about p-type or hole doping. Presence of considerable amount of 3d states justifies the reason to consider it as a light 3d transition metal. The O K-edge spectra display characteristic spectral features assigned as eg and t2g and show strong dependence on concentration.[[notice]]補正完畢[[journaltype]]國外[[booktype]]紙本[[booktype]]電子版[[countrycodes]]US

Noncommutative Dipole Field Theories And Unitarity

We extend the argument of Gomis and Mehen for violation of unitarity in field theories with space-time noncommutativity to dipole field theories. In dipole field theories with a timelike dipole vector, we present 1-loop amplitudes that violate the optical theorem. A quantum mechanical system with nonlocal potential of finite extent in time also shows violation of unitarity.Comment: typos corrected, more details added in Sec 5, version to appear in JHE

Gauge symmetries of strings in supertwistor space

Recently we have considered supertwistor reformulation of the D=4 N=1,2 superstring action that comprises Newman-Penrose dyad components and is classically equivalent to the Green-Schwarz one. It was shown that in the covariant kappa-symmetry gauge the supertwistor representation of the string action simplifies. Here we analyze its Hamiltonian formulation, classify the constraints on the phase-space variables, and find the covariant set of generators of the gauge symmetries. Quantum symmetries of the supertwistor representation of the string action are examined applying the world-sheet CFT technique. Considered are various generalizations of the model from the perspective of their possible relation to known twistor superstring models.Comment: 17 pages, LaTeX; v.2 minor changes in the text, references added, misprints correcte

Cosmological Evolution of a Brane Universe in a Type 0 String Background

We study the cosmological evolution of a D3-brane Universe in a type 0 string background. We follow the brane-universe along the radial coordinate of the background and we calculate the energy density which is induced on the brane because of its motion in the bulk. We find that for some typical values of the parameters and for a particular range of values of the scale factor of the brane-universe, the effective energy density is dominated by a term proportional to $\frac{1}{(loga)^{4}}$ indicating a slow varying inflationary phase. For larger values of the scale factor the effective energy density takes a constant value and the brane-universe enters its usual inflationary period.Comment: 25 pages,1 figure,LaTex file,final version to appear in Phys. Rev.