An accurate temperature field reconstruction method using schlieren imaging and optical flow for high-speed flow fields

The temperature distribution in the tail jet field of aircraft engines is crucial for reflecting combustion efficiency and understanding the formation mechanism of pollutants. The traditional temperature sensor measurement method has a series of shortcomings such as single point measurement, destruction of the flow field and often limited spatial and temporal resolution, so it is not suitable for the application scenarios of the tail jet temperature field measurement. Schlieren method, as a visual technique for measuring flow fields, is an effective method for real-time measurement of flow field parameters, with the characteristics of large measuring range, fast response speed and simple testing equipment. To improve the accuracy of the traditional schlieren method, this paper presents a temperature field distribution reconstruction method by decoupling flow velocity and density field. First, the light deflection angle of the schlieren image is obtained by the change of brightness and darkness in the picture, and then the density distribution of the flow field is obtained indirectly. Then, through the schlieren images of continuous frames, the flow velocity distribution is obtained by using the optical flow velocity measurement algorithm. The optical flow algorithm is based on constraint condition for brightness invariance and global smoothing assumption. After that, the obtained density and velocity information can be used to calculate the pressure distribution of the flow field by applying the central difference to the simplified Navier–Stokes equation. Finally, the temperature distribution of the flow field can be obtained by using the obtained flow velocity, density and pressure information through the numerical calculation of the energy equation.