Shock detection technique based on light scattering by shock

When a narrow laser beam is brought to a grazing incidence on a shock, a small part of the beam scatters out in a thin, diverging sheet of light. The phenomenon disappears when the beam is moved to any other location where there is no shock or the beam pierces the shock surface, i.e., at a nongrazing incidence. Various experimental details indicate that the scattering is primarily due to diffraction of light bythe sudden jump in the refractive index created across a shock. The preceding optical phenomenon is conveniently used as the basis of a new shock detection technique. It depends on moving a laser beam from point to point in a shock-containing flowfield and sensing the presence of the scattered light using a photomultiplier tube (PMT). The incident beam locations, which correspond to a nonzero PMT signal, are the shock locations. It is demonstrated that the present method is able to provide both time-averaged and unsteady information in a very straightforward way.