Simultaneous background-oriented schlieren and retroreflective shadowgraph visualization

This paper describes a large-scale visualization system that can simultaneously produce background-oriented schlieren (BOS) images and retroreflective shadowgrams (RS) of a compressible flow. The simultaneous measurements with BOS and RS in two optically equal systems allowed a direct comparison of the performance of the two methods and to quantify the relative differences, primarily with respect to identifying the broad range of length scales in a turbulent compressible flow. Further, direct comparisons are made between the schlieren image obtained from the BOS measurement, the displacement map obtained by subtracting a no-flow image from the unprocessed BOS record, the divergence of the BOS displacement field (the latter two are forms of a BOS-generated shadowgram), and the simultaneously obtained direct shadowgram from the RS setup. A qualitative comparison of the results indicated that all the three methods (BOS schlieren, direct difference, and divergence of the BOS-schlieren record) can capture the large-scale flow structures but that the smaller scales in the plume can only be resolved to a much lesser extent. These structures were, however, easily resolved in the RS images. The results show that, in the described system, the resolution of the BOS images is lower than that of the shadowgrams by a factor of about five. Further tests indicate that these differences in the spatial resolution of the BOS technique cannot be removed using a different design of the random-dot background pattern. These observations apply to the used large-scale system in which a minimum distance between the observed object and each of the two screens (background pattern/reflective) has to be maintained. This constraint also meant that neither system could be optimized with respect to spatial resolution.