Experimental study of background-oriented schlieren for aero-optics

Abstract

In this research, a background-oriented schlieren (BOS)-based aero-optical measurement technique was improved and validated, and aero-optical phenomena around a wedge model in a Mach 6 high-speed flow and a static torch-heated window were experimentally studied. Compared to conventional Shack-Hartmann wavefront sensors, the BOS aero-optical measurement is flexible in spatial and temporal resolutions, affordable and easy to set up. The BOS technique’s measurement capacity was improved to include Strehl ratio and point spread function measurements, and the technique was directly compared with a Shack-Hartmann wavefront sensor for the first time to the author's best knowledge. The test case employed a hypersonic Ludwieg tube and a wedge model. BOS results showed agreement with the Shack-Hartmann wavefront sensor in terms of boresight error, optical path difference, Strehl ratio and point spread function. The applicability of BOS aero-optics in the hypersonic regime was demonstrated.

In case of the Mach 6 wedge model, boresight errors were below 20 μrad and rms optical path differences below 0.3 $\lambda$ at charge pressures of 10, 20 and 30 bar. Strehl ratios fluctuated between 1 and 0 due to its sensitive nature with respect to the optical path difference. A trend of intensified aero-optical phenomena with an increasing total pressure was observed. In case of the heated window, 18 different cases were tested based on three parameters of deformation state, attitude angle and heating time. The experimental results were compared with analytical results based on a 1D assumption and surface deformation measurements. The free-fix condition showed good agreement between analytical and experimental results while a notable difference arose in the fix-fix condition due to increased deformation complexity. Window deformation effects were shown to be significant for boresight error.

Moreover, the traditional BOS technique was applied to a Mach 6 hypersonic hemisphere model in a shock tunnel for visualization and density field measurement. The BOS effectively showed the position of a bow shock wave which was in agreement with shadowgraph and Eilmer 3 CFD code.