Exploring strong-field deviations from general relativity via gravitational waves

Two new observational windows have been opened to strong gravitational physics: gravitational waves, and very long baseline interferometry. This suggests observational searches for new phenomena in this regime, and in particular for those necessary to make black hole evolution consistent with quantum mechanics. We describe possible features of "compact quantum objects" that replace classical black holes in a consistent quantum theory, and approaches to observational tests for these using gravitational waves. This is an example of a more general problem of finding consistent descriptions of deviations from general relativity, which can be tested via gravitational wave detection. Simple models for compact modifications to classical black holes are described via an effective stress tensor, possibly with an effective equation of state. A general discussion is given of possible observational signatures, and of their dependence on properties of the colliding objects. The possibility that departures from classical behavior are restricted to the near-horizon regime raises the question of whether these will be obscured in gravitational wave signals, due to their mutual interaction in a binary coalescence being deep in the mutual gravitational well. Numerical simulation with such simple models will be useful to clarify the sensitivity of gravitational wave observation to such highly compact departures from classical black holes.Comment: 20 pages, 9 figures. v2: references and CERN preprint number adde

Euler flow solutions for transonic wind tunnel section

Steady, 2D Euler flow computations have been performed for a wind tunnel section, designed for research on transonic shock wave - boundary layer interaction. For the discretization of the steady Euler equations, an upwind finite volume technique has been applied. The solution method used is collective, symmetric point Gauss-Seidel relaxation, accelerated by nonlinear multigrid. Initial finest grid solutions have been obtained by nested iteration. Automatic grid adaptation has been applied for obtaining sharp shocks. An indication is given of the mathematical quality of four different boundary conditions for the outlet flow. Two transonic flow solutions with shock are presented; a choked and a non-choked flow. Both flow solutions show a good shock capturing. A comparison is made with results obtained by holographic interferometry. Keywords: steady Euler equations - transonic flows - mutigrid methods - grid generation and adaptation - boundary condition

Upwind schemes for the Navier-Stokes equations

A discretization method is presented for the full, steady, compressible Navier-Stokes equations. The method makes use of quadrilateral finite volumes and consists of an upwind discretization of the convective part and a central discnetization of the diffusive part. In the present paper, the emphasis lies on the discretization of the convective part. The applied solution method directly solves the steady equations by means of a Newton method, which requires the discretization to be continuously differentiable. For two upwind schemes which satisfy this requirement (Osher's and van Leer's scheme}, results of a quantitative error analysis are presented. Osher's scheme appaars to be more and more accurate than van Leer's scheme with increasing Reynolds number. A suitable higher-order accurate discretization of convection is chosen. Based on this higher-order scheme, a new limner is constructed. Further, for van Leer's scheme, a solid wall - boundary condition treatment is proposed, which ensures a continuous transition from the Navier-Stokes flow regime to the Euler flow regime. Numerical results are presented for a subsonic flat plate flow and a supersonic flat plate flow with oblique shock wave - boundary layer interaction. The resu Its obtained agree with the predictions made. Useful properties of the discretization method are that it allows an easy check of false diffusion and that it needs no tuning of parameters

Cascaded wavelength conversions using four-wave mixing in semiconductor optical amplifiers

Wavelength conversion in wavelength-division multiplexed (WDM) communication systems would provide significant network performance improvement. Optoelectronic, cross-gain saturation, and cross phase saturation wavelength converters are candidate technologies that have been well characterized, however, they are not “transparent” to either bit-rate or modulation format. Complete transparency is offered only by ultrafast wave mixing techniques-in the present case four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs)