Particulate Entrainment and Acceleration

Abstract

We saw in the previous chapter that a blast wave traveling over a real surface can be modified significantly by even small surface roughness. In this chapter we will discuss the effects on the blast wave of a loose layer of particulates on the surface. The particulates share momentum and kinetic energy with the gasses in the flow. They exchange thermal energy with the gasses, either heating or cooling the particles. The particles that may be entrained in the flow have a density of the order 1–3 g/cm³, while the gasses have a density of 1–2 × 10⁻³g/cm³. It takes a very small volume of particulate matter to have the same mass as the gas in which they are entrained. If we take a dust particle with a 100 μm diameter, only 100 such particles will have a mass of 10% of the mass of air in a volume of 1 cm³. With such a density of opaque material, the optical depth is about 30 m. This is then the equivalent of a very severe dust storm with a visibility of only a 100 ft or so. It is very difficult for a gas to hold a mass of particulates in suspension when the mass of the particulates approaches the mass of the gas. The particulates will simply fall out of suspension under gravity.