Research On Background Schlieren Technology Based On Perspective Projection

Background Orient Schlieren(BOS)is a new method for density measurement of aerodynamic flow field.It has the advantages of wide field of view,high spatial and temporal resolution,simple structure and low cost.It has gradually become one of the research hotspots of flow visualization and quantitative measurement of aerodynamic flow field.The objective of complex aerodynamic flow field measurement is to obtain the instantaneous three-dimensional distribution of key parameters of the flow field.To achieve this goal,multi-directional BOS imaging devices are needed to reconstruct the three-dimensional density distribution of the flow field to be measured with CT algorithm.Aiming at the measurement of aerodynamic flow field by CT-BOS technology,the following aspects are studied in this paper:(1)Research on BOS projection model: By analyzing the imaging process of BOS and combining with the actual needs of wind tunnel measurement,BOS projection models based on parallel projection and perspective projection are discussed respectively.The analysis results show that perspective projection can give full play to the advantages of BOS technology for large field of view BOS measurement.(2)Research on Calibration Technology of BOS multi-camera: On the basis of BOS perspective projection model,through Zhang Zhengyou plane calibration method,the internal parameters of each camera and the external parameters of the multi-camera are correlated.Finally,the calibration technology of BOS multi-camera is realized by integrating the coordinate systems of each camera into the same reconstructed coordinate system.At the same time,an application program for estimating the area to be reconstructed based on the calibration parameters of multiphase cameras is developed,which can provide reference for the installation and layout of cameras in CT-BOS field experiments.(3)Research on reconstruction technology of BOS multiphase camera: Due to the location limitation of observation window in actual wind tunnel experiment,the cameras in CT-BOS experiment can not be placed on the same half-circle distribution plane.Therefore,the traditional method of decomposing three-dimensional CT reconstruction into two-dimensional parallel slices and reconstructing two-dimensional CT separately is totally inapplicable to CT-BOS technology.To solve this problem,this paper extends the local basis function expansion algorithm for2-D deflection CT reconstruction to 3-D.Combining with the BOS projection modelbased on perspective projection,the BOS reconstruction algorithm based on 3-D local basis function expansion is studied.On this basis,numerical simulation and reconstruction experiments are carried out to prove the feasibility and applicability of this method.Meanwhile,multi-directional CT-BOS reconstruction experiments were carried out with candle flame as the object,and the results were analyzed.