Research On Non-contact Optical Measurement Technology Of Flow Field

In aerodynamics research and test,especially in hypersonic wind tunnel test,in order to display the density change characteristics of the flow field in the test area,the flow field measurement system needs to be configured.Fluid motion is generally characterized by instantaneity,irregularity,pulsation,multi-scale,etc.The speed and direction at different positions change rapidly,and common gases and liquids are often transparent,which brings great challenges to their detection.In order to study these flow phenomena,it is often necessary to visually and non-contact measure the flow phenomena through certain measurement and observation means,visually demonstrate the flow around the model,and display the flow structure,so as to analyze some phenomena in the flow field.In view of these problems in flow field measurement,this paper has carried out the research on non-contact optical measurement technology of flow field.The schlieren imaging technology is studied in this paper.The focused schlieren system is used to record and display the structural information of the flow field around the model during the hypersonic wind tunnel test.At the same time,the four-wave transverse shear interference technology is studied,and the four-wave transverse shear interference system suitable for different fields of view is designed.On this basis,an experimental system for measuring flow field density field is designed and built,which verifies the effectiveness of the non-contact optical measurement system for flow field designed in this paper.The specific work is as follows:1.The basic theory of non-contact optical measurement technology for flow field is described,the principle and implementation method of schlieren imaging are studied,and the theory of transverse shearing interferometry is systematically analyzed,the shape of shearing interference fringes under different aberrations is studied,and the theory and method of shearing wavefront reconstruction are expounded.2.Aiming at the imaging requirements of hypersonic wind tunnel flow fields,a focused schlieren system is designed,which has the characteristics of large field of view,high resolution,and minimal depth of field.The design of each part of the focusing schlieren system is described in detail.Theoretical analysis and testing experiments have been carried out on the designed schlieren system.Focusing depth testing,schlieren image surface uniformity testing,and imaging resolution testing experiments have been carried out,proving that the system meets the requirements of the index.3.The boundary layer transition experiment of a flat plate model based on focused schlieren technique was conducted.A pyramid phase cross correlation image registration algorithm was proposed to solve the problem of small focused fringe fields and the inability to directly observe the gradual transition of the boundary layer from laminar flow to turbulent flow with a single observation.Simulation experiments have proved that this algorithm has good registration speed and accuracy,and using this algorithm to register and stitch the focusing schlieren images.Finally,the characteristics of boundary layer transition in a hypersonic flat plate model were successfully analyzed.4.The quadriwave lateral shearing interference theory,phase extraction method and interferometric wavefront reconstruction are studied,and according to the measurement requirements of the regional flow field in the hypersonic wind tunnel,the wide-field four-wave transverse shear interference system and the 25 mm wide-range disturbance area are designed respectively.The results of wavefront measurement experiments show that the wavefront detection error of the system is better than 1/40 λ.5.The density field measurement principle based on the quadriwave lateral shearing interferometry technology is studied,and the density field measurement experimental system suitable for the regional flow field in the hypersonic wind tunnel is designed.The experimental results show that the measurement error of the density field measurement system based on the four-wave transverse shear interference technology is better than 6%,which meets the measurement requirements of the regional flow field in the hypersonic wind tunnel.