In
fluid dynamics, a
Mach wave is a pressure wave traveling with the
speed of sound caused by a slight change of
pressure added to a
compressible flow. These weak waves can combine in supersonic flow to become a
shock wave if sufficient Mach waves are present at any location. Such a shock wave is called a
Mach stem or
Mach front. Thus it is possible to have shockless compression or expansion in a supersonic flow by having the production of Mach waves sufficiently spaced (
cf. isentropic compression in supersonic flows). A Mach wave is the weak limit of an oblique shock wave (a normal shock is the other limit). A Mach wave propagates across the flow at the
Mach angle µ, which is the angle formed between the Mach wave wavefront and a vector that points opposite to the vector of motion. It is given by