Optical quantitative tomography of the three-dimensional density structure in aircraft flow fields

The density distributions of aircraft flow fields manifest energy conversion, pressure variation, and aerodynamic loading mechanisms. Thus, an accurate three-dimensional density structure of flow fields is crucial for an in-depth understanding of aerodynamics and enhanced aircraft performance optimization. Traditional contact measurements or two-dimensional optical diagnostics can only provide information at discrete points or along integral paths, making it difficult to capture the three-dimensional characteristics of the flow fields. Therefore, a non-intrusive tomographic imaging method is proposed for flow fields based on light-scattering echo signals. Ultra-narrow laser pulses and ultrashort exposure imaging are combined with photon-level high-sensitivity detection and radiometric calibration to achieve an optical quantitative measurement of the three-dimensional density structure of flow fields. Density field detection experiments were conducted on an aircraft exhaust plume, yielding absolute density distribution profiles with a spatial resolution of 3.75 cm. The results indicate that the exhaust plume exhibits a "hollow-tube" structure characterized by axial expansion and peripheral compression. The maximum and minimum recorded densities were 3.90 and 0.93 kg/m(3), which are 218.90% higher and 24.04% lower than the background mean, respectively. The fluctuation level or coefficient of variation reached 44.05%, which is 14.4 times higher than that of the environmental background. The proposed method effectively resolves the fine three-dimensional structure of aircraft flow fields and offers a new quantitative tool for fluid mechanics research and engineering applications.