Background-oriented schlieren and laser Rayleigh scattering complementary method for accurate density field visualization

Gas flow visualization is an essential technique for understanding the gas flow characteristics. Various quantitative distribution measurement methods have been proposed, each with its own advantages and disadvantages. For example, the background-oriented schlieren method provides the quantitative density distribution for wide areas with a simple optical setup, but it disadvantageously requires the appropriate boundary conditions need to be set when integrating the Poisson equation. The laser Rayleigh scattering method also provides quantitative density distribution, but it requires a high-power laser for wide-area measurements because laser intensity directly influences measurement accuracy. This study proposes a method that complements the weak points of the above two methods. First, a wide area is measured using the background-oriented schlieren method, and then, the laser Rayleigh scattering method is applied only for the boundary region to obtain the boundary condition. For a heated turbulent air jet with Reynolds number 3000, the results of the proposed method are compared with the numerical analysis and thermocouple temperature measurements. The results well match, indicating the applicability and usefulness of the proposed method. Furthermore, these results contribute to demonstrating the significance of boundary conditions in the background-oriented schlieren method and the establishment of setting guidelines.