Characterization of Mean and Turbulent Flow over Complex Topography under various Inflow and Geometric Configurations

An experimental investigation of flow over a complex topography was undertaken to study the influence of inflow conditions including Reynolds number, upstream roughness, and inflow shear profile on mean and turbulent flow behaviour. Large-scale physical testing was employed over the escarpment of a hill, covering a Reynolds number range of 3.6×104 to 5.2×105. Measurements taken using Particle Image Velocimetry (PIV) and Cobra Probes were analyzed in terms of mean and turbulent statistics. Coherent structures were characterized through Proper Orthogonal Decomposition (POD). The results show that the Reynolds number had little effect on the flow behaviour, while the effect of changing upstream roughness was low to moderate. The modified inflow shear profile had a significant impact, greatly increasing turbulent kinetic energy. A sharper escarpment leading edge had the largest impact by far, significantly altering the flow dynamics. The turbulent flow behavior over the sharper escarpment was found to be similar to the classical case of forward-facing step

A Neutron Star Binary Merger Model for GW170817/GRB170817a/SSS17a

The merging neutron star gravitational wave event GW170817 has been observed throughout the entire electromagnetic spectrum from radio waves to $\gamma$-rays. The resulting energetics, variability, and light curves are shown to be consistent with GW170817 originating from the merger of two neutron stars, in all likelihood followed by the prompt gravitational collapse of the massive remnant. The available $\gamma$-ray, X-ray and radio data provide a clear probe for the nature of the relativistic ejecta and the non-thermal processes occurring within, while the ultraviolet, optical and infrared emission are shown to probe material torn during the merger and subsequently heated by the decay of freshly synthesized $r$-process material. The simplest hypothesis that the non-thermal emission is due to a low-luminosity short $\gamma$-ray burst (sGRB) seems to agree with the present data. While low luminosity sGRBs might be common, we show here that the collective prompt and multi-wavelength observations are also consistent with a typical, powerful sGRB seen off-axis. Detailed follow-up observations are thus essential before we can place stringent constraints on the nature of the relativistic ejecta in GW170817.Comment: 9 pages, 5 figures, accepted to ApJ Letter

Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger

11 hours after the detection of gravitational wave source GW170817 by the Laser Interferometer Gravitational-Wave Observatory and Virgo Interferometers, an associated optical transient SSS17a was discovered in the galaxy NGC 4993. While the gravitational wave data indicate GW170817 is consistent with the merger of two compact objects, the electromagnetic observations provide independent constraints of the nature of that system. Here we synthesize all optical and near-infrared photometry and spectroscopy of SSS17a collected by the One-Meter Two-Hemisphere collaboration. We find that SSS17a is unlike other known transients. The source is best described by theoretical models of a kilonova consisting of radioactive elements produced by rapid neutron capture (the r-process). We find that SSS17a was the result of a binary neutron star merger, reinforcing the gravitational wave result.Comment: 21 pages, 4 figures, accepted to Scienc