For an aero-engine in operation, the pressure distribution of the exhaust jet flow field is the main parameter of flow characteristics and temperature field. Therefore, the accurate measurement of the exhaust jet flow field pressure of the aero-engine is of great significance to study the state of the aero-engine in operation. As traditional speed and pressure measurement tools, sensors such as pitot tubes fail to be directly applied to high-temperature and high-speed complex flow fields such as combustion due to their shortcomings of destructive flow fields, single point measurement, and low temporal and spatial resolutions. With the development of visual measurement and image processing technologies, optical measurement methods have been gradually applied to measure the physical parameters of the flow fields. As a typical optical diagnosis method, the pressure field reconstruction method based on particle image velocimetry (PIV) is only applicable to the pressure field reconstruction of incompressible fluid. As a flow field visualization measurement technology, the schlieren method has the characteristics of a large measurement range, fast response speed, and simple test equipment. It is an effective method for real-time measurement of flow field parameters. By applying the schlieren method to reconstruct the pressure field of the jet flow field of the aero-engine, the real non-contact measurement can be realized, and the accuracy of measurement can be improved.
This paper proposes a method of reconstructing the pressure field distribution of high-speed airflow by using the schlieren method to decouple the velocity and density fields, so as to realize the real-time measurement and reconstruction of the density field, velocity field, and pressure field of the high-speed airflow. First, the relationship between the brightness of schlieren images and the light shift is calibrated by using the calibrated schlieren method. After obtaining the calibration curve, the light shift can be obtained according to the light and dark changes in the schlieren images, and then the density distribution of the flow field will be indirectly obtained. Meanwhile, the velocity distribution can be obtained by using an optical flow velocimetry algorithm through the schlieren images of continuous frames. Finally, the static and dynamic pressure distributions of the flow field can be obtained through a numerical calculation by using the obtained velocity and density information, and then the total pressure distribution can be obtained.
The density field (Fig. 11) and velocity field (Fig. 13) of the micro vortex jet wake field are reconstructed by the schlieren method, and the density field of the micro vortex jet wake field is reconstructed by using the obtained density and velocity information. In order to verify the feasibility and accuracy of the experimental method, the measured pressure at five points near the nozzle is selected and compared with the reconstructed pressure field (Fig. 18). The results show that the two results are close within the error range, and the maximum error is not more than 5%. The following factors are considered to determine error sources: when the density field of the flow field is measured, the model of the flow field is regarded as axisymmetric, and there is a certain error; when the pressure gradient distribution is calculated, the numerical calculation method is used, which has some errors; since the measurement time of pitot tubes and schlieren velocity-density field coupling reconstruction method is different, there will be some errors in the measurement results although the interval time is short. However, these errors are within the allowable range of measurement. Therefore, the reconstruction method of the high-speed airflow pressure field by using the schlieren method to decouple the velocity and density fields is feasible.
In this paper, the schlieren optical flow method is used to synchronously reconstruct the density and velocity fields of the axisymmetric flow field, which not only overcomes the shortcomings of the traditional single point measurement of pressure sensors, contact measurement, poor spatial and temporal resolutions but also compensates for the disadvantages of the PIV-based pressure field reconstruction technology. The technology requires the distribution of tracer particles and can only reconstruct the pressure field of incompressible fluid. Therefore, the proposed method is effective in accurately reconstructing the pressure distribution of high-speed flow fields, and it can extend the application scope of the schlieren method in the quantitative measurement of flow fields.
