A Numerical Scheme for the Compressible Low-Mach Number Regime of Ideal Fluid Dynamics

Based on the Roe solver a new technique that allows to correctly represent low Mach number flows with a discretization of the compressible Euler equations was proposed in Miczek et al. (Astron Astrophys 576:A50, 2015). We analyze properties of this scheme and demonstrate that its limit yields a discretization of the continuous limit system. Furthermore we perform a linear stability analysis for the case of explicit time integration and study the performance of the scheme under implicit time integration via the evolution of its condition number. A numerical implementation demonstrates the capabilities of the scheme on the example of the Gresho vortex which can be accurately followed down to Mach numbers of \({\sim }10^{-10}\).     

An interfacial polarization model for activated electrorheological suspensions

Surfactant influences the ER response in two different ways. At low surfactant concentrations, it enhances the ER response by enhancing the particle polarizability. While at large concentrations, the response degrades due to the non-linear conductivity in the continuous phase. The yield stress is proportional to the electric field strength squared at small surfactant concentrations, but increases more slowly with field strength at large concentrations. In this paper, an interfacial polarization model is introduced to predict the ER behavior of surfactant-activated ER suspensions. Maxwell-Wagner model was modified by incorporating the effects of surfactant adsorption and field-induced alteration of the surfactant structure. The modified interfacial polarization model predicts well the qualitative behavior of the surfactant activated ER suspensions over all surfactant concentration ranges.     

Shear-controlled electrical conductivity of carbon nanotubes networks suspended in low and high molecular weight liquids

Controlling the electrical conductivity is a critical issue when processing material systems consisting of an insulating matrix filled with conductive particles. We provide experimental evidence that given shear rates result in specific conductivity levels in such different systems as high-viscosity carbon nanotube/polymer melt or low-viscosity carbon nanotube/epoxy fiber suspensions. The steady-state conductivities are independent of the initial state of the dispersion. The observed behavior is modeled phenomenologically by the competition between build-up and destruction of conductive filler network. A particle-level simulation of flowing fiber suspension also reflects the observed behavior. Our results show that properties of particulate suspensions can be controlled by steady shear. They should be considered to obtain reproducible properties in shear-based processing technologies as injection molding or resin transfer molding.     

Gun Muzzle Blast and Flash

Gun muzzle blast and flash phenomena are of importance since they are associated with the formation of large overpressures and intense muzzle flash. About 30% of the chemical energy released from the propellant used in a conventional gun is converted into kinetic energy of the projectile. The remaining energy is mainly contained in the propellant gas-particle mixture which escapes from the muzzle of the gun in a few milliseconds. The sudden discharge produces a blast wave because of the rapid displacement of air originally surrounding the gun. In addition, these gases are generally fuel-rich and mix with air turbulently entrained from the surroundings. Combustion of this mixture causes gun muzzle flash, usually associated with the formation of a secondary blast wave. The design of solid propellant charges, gun performance, muzzle attachments and chemical flash suppressants is guided by the need to keep the above hazards to safe limits. In this paper, blast and flash phenomena are characterized using data of recent investigations and showing illustrative examples of their development.