Analysis and reduction of spurious displacements in high-framing-rate background-oriented Schlieren

Spurious displacements (SDs) associated with high-framing-rate background-oriented Schlieren (BOS) measurements were investigated experimentally in this study. To obtain the SDs, a multiple-pass cross-correlation algorithm was applied to reference/data images on a background printed with random dot pattern. Four sets of image pairs were dedicatedly selected to characterize the SDs. To isolate the SDs unique to the high-framing-rate BOS, measures were taken to minimize the contributions from other sources. Results showed that the SDs distinctive in high-framing-rate BOS measurements can be categorized according to their characteristic time: spatially uniform and fast oscillating SD (F-SD) that could be resolved during one recording event and non-uniform slowly varied SD (S-SD) that only become identifiable after several minutes time intervals between the two subsequent recordings. The proper boundary conditions (BCs) were proposed for solving the Poisson equation of the displacement fields contaminated by the SDs. A method allowing in-situ mitigation of both SDs was also proposed by introducing one additional light path into the BOS setup. Experiments on quasi-two-dimensional helium jets were conducted with and without acoustic excitations. The results confirmed the effectiveness of the proposed method to remove the SDs in both steady and unsteady flows. Graphic abstract