Projected Background-Oriented Schlieren Imaging

The Projected BOS imaging system developed at the NASA Langley Research Center provides a significant advancement over other BOS flow visualization techniques. Specifically, the present BOS imaging method removes the need for a physically patterned retroreflective background within the flow of interest and is therefore insensitive to the changing conditions due to the flow. For example, in a wind tunnel used for aerodynamics testing, there are vibrations and temperature changes that can affect the entire tunnel and anything inside it. Any patterned background within the wind tunnel will be subject to these changing conditions and those effects must be accounted for in the post-processing of the BOS image. This post-processing is not necessary in the Projected BOS process here. In the Projected BOS system, a pattern is projected onto a retroreflective background across the flow of interest (Figure 1). The imaged pattern in this configuration can be made physically (a pattern on a transparent slide) or can be digitally produced on an LCD screen. In this projection scheme, a reference image can be taken at the same time as the signal image, facilitating real-time BOS imaging and the pattern to be changed or optimized during the measurements. Thus far, the Projected BOS imaging technology has been proven to work by visualizing the air flow out of a compressed air canister taken with this new system (Figure 2).