Machstämme

Machstämme (Mach stems) are a feature of shock-wave reflections observed in high-speed gas dynamics. They describe the portion of a reflected shock that forms a contiguous, nearly vertical front between a rigid boundary and the triple point where the incident, reflected, and Mach shocks meet. The Mach stem runs roughly normal to the boundary and concentrates pressure, producing higher stagnation pressures behind it than a simple regular reflection would.

The phenomenon occurs when a planar shock wave strikes a flat surface at a sufficiently large angle. At low incidence angles, the reflection is a regular reflection with two shocks meeting the boundary. Above a critical angle, the reflected shock cannot attach directly to the boundary, and a third shock—the Mach stem—emerges, along with a Mach front and a triple point that migrates along the surface as the flow evolves. The terms Mach stem and Mach reflection originate from studies of high-speed aerodynamics by Ernst Mach and subsequent experiments.

In practice, Machstämme influence peak overpressure and impulse in detonations and in explosive energy release, as well as loads on aerospace structures during hypersonic flight or blast interactions. They are analyzed with gas-dynamic theory, shock polar diagrams, and numerical simulations to estimate pressures and distances of the triple point.

The term is mainly used in German-language literature on gas dynamics, while English sources usually refer to Mach stems and Mach reflection.