Post-Newtonian Freely Specifiable Initial Data for Binary Black Holes in Numerical Relativity

Construction of astrophysically realistic initial data remains a central problem when modelling the merger and eventual coalescence of binary black holes in numerical relativity. The objective of this paper is to provide astrophysically realistic freely specifiable initial data for binary black hole systems in numerical relativity, which are in agreement with post-Newtonian results. Following the approach taken by Blanchet, we propose a particular solution to the time-asymmetric constraint equations, which represent a system of two moving black holes, in the form of the standard conformal decomposition of the spatial metric and the extrinsic curvature. The solution for the spatial metric is given in symmetric tracefree form, as well as in Dirac coordinates. We show that the solution differs from the usual post-Newtonian metric up to the 2PN order by a coordinate transformation. In addition, the solutions, defined at every point of space, differ at second post-Newtonian order from the exact, conformally flat, Bowen-York solution of the constraints.Comment: 41 pages, no figures, accepted for publication in Phys. Rev. D, significant revision in presentation (including added references and corrected typos

A large strain anisotropic elastoplastic continuum theory for nonlinear kinematic hardening and texture evolution

In this paper we present a continuum theory for large strain anisotropic elastoplasticity based on a decomposition of the modified plastic velocity gradient into energetic and dissipative parts. The theory includes the Armstrong and Frederick hardening rule as well as multilayer models as special cases even for large strain anisotropic elastoplasticity. Texture evolution may also be modelled by the formulation, which allows for a meaningful interpretation of the terms of the dissipation equatio

NASA space station automation: AI-based technology review

Research and Development projects in automation for the Space Station are discussed. Artificial Intelligence (AI) based automation technologies are planned to enhance crew safety through reduced need for EVA, increase crew productivity through the reduction of routine operations, increase space station autonomy, and augment space station capability through the use of teleoperation and robotics. AI technology will also be developed for the servicing of satellites at the Space Station, system monitoring and diagnosis, space manufacturing, and the assembly of large space structures

The structure of a statistically steady turbulent boundary layer near a free-slip surface

The interaction between a free-slip surface with unsheared but sustained turbulence is investigated in a series of direct numerical simulations. By changing (i) the distance between the (plane) source of turbulence and the surface, and (ii) the value of the viscosity, a set of five different data sets has been obtained in which the value of the Reynolds-number varies by a factor of 4. The observed structure of the interaction layer is in agreement with current knowledge, being made of three embedded sublayers: a blockage layer, a slip layer, and a Kolmogorov layer. Practical measures of the different thicknesses are proposed that lead to a new Reynolds-number scaling based on easy-to-evaluate surface quantities. This scaling is consistent with previous proposals but makes easier the comparison between free-surface flows when they differ by the characteristics of the distant turbulent field. Its use will be straightforward in a turbulence-modeling framework