Diagnosis of velocity and density of a detonation exhaust flow using background-oriented schlieren technique

The exhaust flow of detonation engines, characterized by high temperatures, transience, and steep gradients, presents a significant challenge for diagnostic techniques. This study implements background-oriented schlieren (BOS) to quantitatively diagnose the external flow field generated by a laboratory-scale detonation tube. A random dot pattern was used as the background, and displacement fields were extracted from high-speed images using a window deformation cross-correlation algorithm in PIVlab (a Matlab application). The BOS technique successfully captured the evolution of shock structures, showing strong agreement with conventional schlieren images. The velocities in different regions of the flow were quantified by constructing an x–t diagram from the temporal image sequence and performing stripe-pattern fitting. Furthermore, two-dimensional density fields were reconstructed by solving the Poisson equation. The results indicate that the density field is dominated by a high-density region behind the leading shock wave and a low-density region of combustion products. The reconstructed density distributions and trends show close agreement with computational fluid dynamics simulations. This work demonstrates that BOS is a viable tool for obtaining high-resolution quantitative data in highly transient reactive flows, providing a valuable framework for validating simulations and guiding engine design.