Experimental investigation of aero-optics induced by supersonic film based on near-field background-oriented schlieren

The effective acquisition of a distorted wavefront is the foundation of studying how to suppress and correct aero-optical effects. The background-oriented schlieren (BOS) technique, as a commonly quantitative measurement method of flow field, has great application potential in aero-optical effect measurement. With consideration of the thickness change of the refractive index field, introduction of near-field correction and construction of the double telecentric configuration, wavefront measurement accuracy was improved. A standard plano-convex lens was utilized to quantitatively evaluate the improved results, and the effectiveness of the improvement was verified. Determination methods of spatial resolution, sensitivity, and dynamic measurement range of wavefront measurement based on BOS were studied. The influences of cross-correlation interrogation window size and step length on the wavefront reconstruction accuracy were investigated. The step length should be less than the size of the interrogation window to make the reconstructed results have a satisfactory accuracy. Wavefront results of supersonic film at different positions and cases were obtained. The fitting results show that there is a good linear relationship between the root mean square of the optical path difference (OPDrms) and ρ1/ρ∞ at different locations. Under different experimental cases, the aero-optical distortion tends to become stronger first and then weaker along the flow direction, and the aero-optical distortion around Position 3 is relatively strong. The large aperture approximation can be used to predict Strehl ratio values effectively in a considerable range by OPDrms.