M.Sc Thesis


M.Sc StudentYasmin Ribak
SubjectThe Aero-Optic Phenomenon in a Compressible Shear Layer
DepartmentDepartment of Aerospace Engineering
Supervisor Full Professors Cohen Jacob


Abstract

The present research is concerned with the aero-optic effect, associated with the transmission of a laser beam through compressible flow field. The aero-optic effect is a major topic of interest due to its relevance in applications involving interactions between propagated laser beams and inhomogeneous medium. An example for such medium can be the variable-density flow field evolving in compressible boundary layers or free shear layers. Among the various applications are: missiles with optical seekers, airborne telescopes, airborne free-space communication systems and high-powered lasers on combat air vehicles. Significant reductions in optical system performances are a direct result of the aero-optical aberrations, which result in a lower peak energy of the light beam and wider intensity distribution around its central spot.

In the present research, quantitative and qualitative methods are devised in order to evaluate the aero-optic effect in the mixing layers of a horizontal compressible free jet. The examined flow field included the two mixing layers formed between the transonic (or subsonic) jet and the stagnant ambient air. The aero-optic effect was experimented by transmitting a collimated 633nm laser beam through the mixing layers, perpendicular to the flow direction. Simultaneous static and stagnation pressure and temperature measurements were conducted.

Qualitative visualization experiments were conducted using Schlieren and Shadowgraph techniques which are based on the on the changes in the refractive index in inhomogeneous medium. The resulted image is the intensity variations of light rays which are transmitted through the flow. The experiments were held in different velocities where the mixing layer between the stagnant ambient air and the free jet were qualitatively characterized. The results show the different stages of the evolution of the mixing layer, such as the vortices roll up as a result of the Kelvin Helmholtz instability. The subsonic and the transonic mixing layers were characterized by their different vortical structures.

The spot of the laser beam was recorded under three different conditions: transmission through stagnant ambient air, through free subsonic and transonic mixing layers. An additional reference beam was transmitted through stagnant air, while the jet was activated, and its spot was recorded simultaneously. Both beams were transmitted to CCD cameras which recorded the images with a frame-rate of 30Hz. The recorded images were used to quantify the light intensity distribution of the spot and its time dependent center of intensity.

According to expectations, the results have shown lower peak intensity and wider light intensity distribution for high Mach numbers, associated with compressible flow. For low incompressible Mach numbers it was hardly changed in comparison with the results obtained under stagnant flow conditions. Additionally, the results for the reference beam were similar to those achieved under stagnant flow conditions, concluding that the beam was not influenced by the presence of the jet. Thus, confirming that the effect was a result of the different flow structures associated with compressible turbulent shear layers, and indicating that the designed optical setup is clearly capable of detecting the effect of compressibility on the laser beam.