A Study On Time-Resolution Real-Time Adaptive PIV Technology

The measurement of flow parameters in high-speed flow field is the foundation and key of developing major equipment such as high performance aero-engine,aerospace vehicle and cruise missile.The velocity of hypersonic flow field is above 5 Mach,which usually presents unsteady characteristics.The complex physical phenomena such as shock / shock,shock / surface interlayer interaction and turbulence in unsteady flow field are very challenging to the measurement of flow field.At present,the common method for measuring the hypersonic flow field is the Particle Image Velocimetry(PIV).In order to improve the measurement precision,it is necessary to adjust the time interval ?t of the two pictures of the current calculation so as to provide reference for the next frame image acquisition.In the traditional PIV measurement method ?t is constant.In order to meet the requirements of high accuracy and large dynamic range of unsteady flow field,the following work is done:(1)Aiming at the difficulty of accurate measurement of PIV in unsteady flow field,the principle of cross correlation and Calman filter algorithm is studied and the time resolution real-time adaptive adjustment algorithm is obtained.And the validity of the algorithm is verified by the simulation experiment.On this basis,combining with the principle of light splitting technology,a nanosecond sub-frame imaging device based on FPGA and double SCMOS is developed and debugged.The resolution of the imaging device is 2048 x 2048 and the frame rate is 2 x 500 fps.In the precision optical path system of the nanosecond sub-frame imaging device,a high precision mechanical slide platform is adopted to achieve a three degree of freedom adjustment with a precision of 1um,which ensures the same image brightness and field range of the left and right image sensors,and creates a good hardware condition for the collection of two particle images with very short time intervals.(2)The firmware system based on FPGA is developed for the high resolution,high frame rate and interframe interval of nanosecond sub-frame imaging device,which realizes the exposure and delay control of two image sensors.It realizes the transmission from high-speed to DDR3 cache and then through Ethernet to the upper computer or through the Cameralink protocol.The function of level processing module realizes the high speed and stable transmission with the highest speed of 10.88Gb/s.In the traditional PIV measurement,the interframe interval of the frame camera is not adjustable,and the exposure time interval of the left and right image sensor which is triggered by the nanosecond sub-frame imaging device is controlled in the 14ms~30ms,and the time interval between the two images is controlled in the 10ns~4ms range by the synchronous control laser and the imaging device.The adjustment precision is up to 10 ns.(3)Combined with high speed acquisition and high precision adjustment of time interval by the nanosecond sub-frame imaging device,the time resolution real-time adaptive adjustment algorithm uses two images of low speed flow field with time interval of 2us and high speed flow field with time interval of 100 ns for PIV and improves the accuracy of Mach 5 hypersonic flow field measurement.