text: Entropy-vorticity waves (or sometimes entropy-vortex waves) refer to small-amplitude waves carried by the gas within which entropy, vorticity, density but not pressure perturbations are propagated. Entropy-vortivity waves are essentially isobaric, incompressible, rotational perturbations along with entropy perturbations. This wave differs from the other well-known small-amplitude wave that is a sound wave, which propagates with respect to the gas within which density, pressure but not entropy perturbations are propagated. The classification of small disturbances into acoustic, entropy and vortex modes was introduced by Leslie S. G. Kovasznay.
  Entropy-vorticity waves are ubiquitous in supersonic problems, particularly those involving shock waves. Since these perturbations are carried by the gas, they are convected by the flow downstream of the shock wave, but they cannot be propagates in the upstream direction (behind the shock wave) unlike the acoustic wave, which can propagate upstream and can catch up the shock wave. As such, they are useful in understadning many high speed flows and are important in many applications such as in solid-propellant rockets and detonations.