When an otherwise-planar wavefront is made to propagate through a variable-index-of-refraction, relatively-thin turbulent flow, the wavefront becomes aberrated. The study of the effect on optical propagation through such flow fields is referred to as Aero-Optics. The rapidly time-varying aberrations imposed on the wavefront degrade the performance of an optical system attempting to make use of the optical signal.
Until 1995, a time-resolved time series of wavefronts for laser propagation through a Mach 0.8 free shear layer, representing propagation through a separated flow region over an aircraft, was not available. In that year, a joint project between the University of Notre Dame and the Arnold Air Development Center (AEDC) made the first ever high-bandwidth wavefront measurements for laser propagation through this simulated flight condition in AEDC’s Acoustic Research Tunnel modified specifically to make aero-optic measurements. These measurements revealed a much higher level of wavefront aberrations than had previously been presumed to exist for these conditions.
Finally, in 1995, Fitzgerald and Jumper realized that the widely accepted presumption that the unsteady pressure fluctuations, p’, in a free shear layer were negligible, the so-called Strong Reynolds Analogy (SRA), could not be correct. A new model, the Weakly Compressible Model (WCM), was then developed that was able to predict (with remarkable accuracy) wavefront amplitudes and character to those measured at AEDC. While this model began to gain acceptance on its own merit, its acceptance was resisted due to its most-controversial result of large pressure fluctuations within a free shear layer concomitant with the coherent vertical structures in the layer which form naturally under the influence of the Kelvin-Helmholtz instability. Not only were the data collected at the time the AEDC wavefront experiments absent extensive fluid-mechanic measurements that might have vindicated the predictions of the WCM, but also the use of the ART facility was no longer possible to revisit these issues.
These developments in the study of aero-optics led directly to the objectives of this thesis research. The objects were specifically:
1.Develop a compressible shear layer facility at Notre Dame that, on the one hand recreated as closely as possible the flow conditions of the earlier AEDC tests, and on the other, overcome some of the limitations of the ART facility (particularly its limited, 5-cm, aperture)
2.Design a method of making time-resolved, conditionally-sampled static pressure measurements through the core of the large-scale coherent vertical structures that form in the shear layer to specifically address the most-controversial implication of the previous work; that p’ is, in fact, not negligible as has been previously assumed.
In both these objectives, this thesis research was eminently successful. This thesis describes the construction of Notre Dames new Weakly-Compressible Shear Layer Facility. It then describes the method developed to capture the pressure profiles through the core of the coherent vertical structures in the shear layer at a location of 0.5 m down stream from the origin of the shear layer (i.e., the splitter plate). The results of the unsteady static pressure experiments are then presented along with a unique serendipitous flow-visualization method that allowed us to capture the actual form of the coherent structures. Both the static pressure data and the flow visualizations unequivocally vindicated the WCM’s predictions of large static pressure wells concomitant with the coherent character of the vertical structures. The work of this thesis has once and for all refuted the claim of the SRA that the static pressure fluctuations a relatively-high Mach number free shear layer are negligible.