Coherent Structures In Aero-optics Research, And Related Wave Front Aberration Measurement Techniques

Fluid-optic interaction problem historically have been separated into categories. The first is termed aero-optic and involved optical transmission through aberrating fluid layer whose thickness (propagation distance), L, is smaller than or comparable with the viewing aperture dimension, A, and the turbulence scales, A , are smaller than or the order as the layer's thickness:The second category, atmospheric propagation, involved optical transmission through the aberrating medium over very long propagation distance compare with the viewing aperture size, and the turbulent scales of note are generally less than or the same order as the propagation distance:Depending on the atmospheric conditions, for ground-based observatories, wave-front sensor operating at 300HZ can adequately resolved both the important spatial scales and the temporal evolution of the atmospheric-turbulence-induced aberrating wave front over viewing aperture associated with relatively large telescope. For aero-optic problem, however, even in the case of relatively slow (<10m/s) turbulent flow, a minimum wave-front-sensor bandwidth of approximately 2kHZ. For aberrating flow fields associated with airborne optical system, bandwidth ranging from 30 to 100kHZ are needed.The "coherent structure" is the most important discovery in the research of turbulence, it implied that the turbulence is not random and irregular at all. There is coherent structure in the turbulence that we can examined and visualized. Past treatments of the aerodynamic flows as being isotropic and homogeneous. However, it is known that turbulent shear layer contained large-scaled "coherent structure", which suggests that the assumption of isotropy and homogeneity may be too limiting. The results indicated, the turbulence "coherent structure"contained a large part of the energy. It is responsible for the flow convection and the aberration. If we change the initiated and boundary conditions, we can imposed a control on the flow field. These are very important to improve the system's performance. For example, to improve the beam qualify and the far field's light qualify etc..Both pulsed interferometer and fast-shear interferometers have been applied to aero-optical problems, a second commonly used technique of inferring optical degradation values involves an indirect measurement in which fluid-mechanic values are measured and related to optical-degradation. Both the indirect and indirect techniques are limited in that they are limited to quantify the rms of the optical-degradation, provided no information of frequency. Recently,the SABT(Small Aperture Beam Technique)can measure the time-resolved optical-degradation.