When Shock Waves Collide

Abstract

Supersonic outflows from objects as varied as stellar jets, massive stars and novae often exhibit multiple shock waves that overlap one another. When the intersection angle between two shock waves exceeds a critical value, the system reconfigures its geometry to create a normal shock known as a Mach stem where the shocks meet. Mach stems are important for interpreting emission-line images of shocked gas because a normal shock produces higher postshock temperatures and therefore a higher-excitation spectrum than an oblique one does. In this paper we summarize the results of a series of numerical simulations and laboratory experiments designed to quantify how Mach stems behave in supersonic plasmas that are the norm in astrophysical flows. The experiments test analytical predictions for critical angles where Mach stems should form, and quantify how Mach stems grow and decay as intersection angles between the incident shock and a surface change. While small Mach stems are destroyed by surface irregularities and subcritical angles, larger ones persist in these situations, and can regrow if the intersection angle changes to become more favorable. The experimental and numerical results show that although Mach stems occur only over a limited range of intersection angles and size scales, within these ranges they are relatively robust, and hence are a viable explanation for variable bright knots observed in HST images at the intersections of some bow shocks in stellar jets.

The Shock Dynamics of Heterogeneous YSO Jets:3D Simulations Meet Multi-epoch Observations

“…Furthermore, we have included a plot that shows how Mach stem size is affected by cooling strength. Our results confirm that the size of the Mach stem is always smaller than the cooling distance d cool , a conclusion consistent with predictions from Hartigan et al (2016).…”

“…However, a more realistic approach would be to keep an ideal equation of state, leave γ at 5/3, and incorporate radiative cooling. We ran 2D models to explore how using cooling affects Mach stem formation and size, and their parameters are similar to other 2D Mach stem simulations from Hartigan et al (2016). These simulations are of two stationary clumps in a moving ambient,which creates intersecting bow shocks.…”

Detection of the Red Supergiant Wind from the Progenitor of Cassiopeia A

“…9). Each of these ejecta knots would produce bow shocks in the CSM gas, perhaps merging into a large scale shock when they overlap (Hartigan et al 2016). Although speculative, such a shock wave complex might help to explain the high [O III]/Hα ratio for position E3, but this would require far more ejecta knots than visible in the HST images.…”

Enhancement of blast wave parameters due to shock focusing from multiple simultaneously detonated charges

“…Nonetheless, most of the work that has been done thus far is on shock waves using shock tubes (Perry and Kantrowitz, 1951), exploding wires (Fedotov et al, 2007) or micro explosives (Hosseini and Takayama, 2005). Another method to study colliding shock waves experimentally has been used by Hartigan et al (2016) where they sent a curved shock in a cylinder of low-density foam using laser. It is worth mentioning that experimental and numerical investigation of the effects of multiple charges using conventional explosives is scarce.…”