Laser light scattering by shock waves

Abstract

Scattering of coherent light as it propagates parallel to a shock wave, formed in front of a bluff cylindrical body placed in a supersonic stream, is studied experimentally and numerically. Two incident optical fields are considered. First, a large diameter collimated beam is allowed to pass through the shock containing flow. The light intensity distribution in the resultant shadowgraph image, measured by a low light CCD camera, shows well-defined fringes upstream and downstream of the shadow cast by the shock. In the second situation, a narrow laser beam is brought to a grazing incidence on the shock and the scattered light, which appears as a diverging sheet from the point of interaction, is visualized and measured on a screen placed normal to the laser path. Experiments are conducted on shocks formed at various free-stream Mach numbers, M, and total pressures, P0. It is found that the widths of the shock shadows in a shadowgraph image become independent of M and P0 when plotted against the jump in the refractive index, Δn, created across the shock. The total scattered light measured from the narrow laser beam and shock interaction also follows the same trend. In the numerical part of the study, the shock is assumed to be a ‘‘phase object,’’ which introduces phase difference between the upstream and downstream propagating parts of the light disturbances. For a given shape and Δn of the bow shock the phase and amplitude modulations are first calculated by ray tracing. The wave front is then propagated to the screen using the Fresnel diffraction equation. The calculated intensity distribution, for both of the incident optical fields, shows good agreement with the experimental data.