| Particle image velocimetry(PIV)technology is a full-field,non-contact,instantaneous measurement technology.With the rapid development of laser illumination technology and image acquisition technology,PIV technology has been applied in the measurement of hypersonic flow field.Compared with the low-speed incompressible flow field,the structures of hypersonic flow field are more complex,there are strong velocity gradient boundaries such as shock wave,and compressible flow field structures such as shear layer and recirculation vortex.The complex flow field structures make the particle concentration change sharply,and the particle image brightness distribution is not uniform,which puts forward higher requirements for PIV measurement method.However,the accuracy and spatial resolution of the cross-correlation PIV measurement method are difficult to be guaranteed in the hypersonic complex flow field measurement.Although the traditional variational optical flow PIV measurement method has higher spatial resolution,the assumptions of brightness conservation and smooth constraint are no strictly valid in the hypersonic flow field.At the same time,both of them do not consider the physical motion law of hypersonic flow field when building the model,so they could not be fully applicable to the measurement of hypersonic flow field.On the basis of the traditional variational optical flow method,this dissertation combines the characteristics of the hypersonic flow field,proposes a field-segmentation-based variational optical flow method and a velocity-decomposition-based variational optical flow method,develops PIV measurement instrument and software,and applies them to hypersonic flow field measurement.The main research results of this dissertation are as follows.(1)A field-segmentation-based variational optical flow method(FS-VOF)is proposed for particle image velocimetry.Due to the compressibility of air,the velocity distribution of the hypersonic flow field is often non-differentiable,and there are strong velocity gradient boundaries such as shock wave.To solve this problem,this dissertation segments the nondifferentiable flow field into multiple differentiable sub-regions.In the segmented sub-regions,the velocity changes smoothly,and the velocity changes non-smoothly at the segmented boundaries.On this basis,the FS-VOF method is proposed by improving the data term and regularization term in the traditional variational optical flow model,and the multi-scale and image warping iterative strategies are used to optimize the solution procedure.Compared with the cross-correlation method and traditional variational optical flow method,the proposed FSVOF method has higher accuracy in measuring the strong velocity gradient boundaries such as shock wave in the hypersonic flow field.(2)A velocity-decomposition-based variational optical flow method(VD-VOF)is proposed for particle image velocimetry.The basic assumptions of the cross-correlation method and the traditional variational optical flow method do not consider the compressible properties of the hypersonic flow field,so the measurement accuracy is low in compressible flow field structures such as shear layer and recirculation vortex.To solve this problem,this dissertation considers both the particle image information and the fluid physical model,and proposes the VD-VOF method,which decomposes the complex motion of the hypersonic fluid field into multiple motion components,and designs adaptive regularization parameters according to the characteristics of each motion component.Compared with the traditional variational optical flow method,this method is more robust to noise interference and illumination changes of particle image,and VD-VOF method has higher measurement accuracy for compressible flow field structures such as recirculation vortex in the hypersonic flow field.(3)Measurement and analysis of hypersonic shock wave/boundary layer interactions.In hypersonic flow,the shrinkage of the flow geometry will cause shock wave/boundary layer interactions.Shock wave/boundary layer interactions will affect the flight control of the aircraft and the wall thermal/mechanical loads.The measurement and research of shock wave/boundary layer interactions have great significance to the aerodynamic shape design and structural optimization of the aircraft.The self-developed PIV measurement instrument is used to carry out experimental research on three typical aerodynamic shape models,namely,the compression ramp model,the forward step model and the spherical cylinder flow around model.Proper orthogonal decomposition(POD)is used to extract and analyze the coherent structures in the flow field,and realize the high-resolution fine measurement and analysis of the hypersonic shock wave/boundary layer interactions.The hardware equipment of real-time adaptation(RTA)PIV instrument is developed for hypersonic wind tunnel,including particle generator,spectroscopic camera and image processor.The particle size range of the particle generator is 0.02～3μm,the maximum load pressure is10 MPa,and the maximum supply pressure is 8MPa.The imaging resolution of the spectroscopic camera is 2048×2048pixels,the maximum frame rate is 100 Hz,the minimum interval time is 10 ns,and the control accuracy of interval time is within 0.25 ns.The image processor can achieve a high-speed image processing frequency of up to one million pixels and1000 fps.On the basis of hardware equipment,RTA-PIV measurement software is developed,which can synchronously control the hardware equipment of RTA-PIV instrument,realize the time setting of spectroscopic camera and laser,real-time image acquisition and storage,space calibration,PIV image data processing,PIV results display and storage,etc.The RTA-PIV instrument has a measurement error of less than 1% in the 5Ma hypersonic experiment,which satisfies the use standard of the hypersonic wind tunnel experiment,and can be used in engineering applications of hypersonic flow field measurement. |
