Research on density measurement based on background oriented schlieren method

Background Oriented Schlieren method (BOS )is a new technique for flow field measurement based on image with remarkable advantages,such as a big field of view,non-intru-sive,and quantitative measurement.It has extensive application prospects in flow field measure-ment.The BOS experimental setup mainly consists of a structured background pattern of random dots,a LED light source and a CCD camera.It needs two images.One is obtained without the density gradient effect as the reference image.The other one is obtained when there is disturb-ance in the flow and is stored as the refracted image.The displacement of the background dots is extracted by performing mathematical correlation analysis on a cluster of particles within each in-terrogation region between the two frames.The displacement vector field represents the gradient of the refractive index which is further integrated to calculate the line-of-sight integrated refrac-tion index via the solution of Poisson equation.This paper describes the implementation of the BOS technique to realize the quantitative visualization of the density in a flow.The basic principle of BOS is expatiated,and the sensitivity and resolution are analyzed.It is shown that the shift of the background dots and the sensitivity increase with increasing distance of the background field from the object of interest.But on the other hand,improving the sensitivity means reducing the physical resolution,as the interrogation size used in the correlation algorithm would have to be correspondingly larger.And the aero optics effect is getting more prominent at the same time. According to the principle of BOS,the background dot pattern is designed.In the experiment, the size of dots is about 3 pixels.In the later experiment,the dot pattern fits well with the dis-placement field’s extracting algorithm.And also a density measure system is constructed.Based on the flame flow and jet flow,through the detection of the deflection of a number of light beams passing the flow field,the density depending on the refractivity is obtained.And the preliminary results for the flame flow and the typical jet flow are presented,such as the refractive index field, the density field,and the temperature field.The experimental results show that the density and temperature distribution can be obtained quantitatively by BOS method expediently,effectively and in a compact way.