TR-PIV experiment
The TR-PIV system consists of a continuous solid Nd:YAG laser system (Kingder KSPL-05), a high-speed CMOS camera (CAMMC 1362), an image acquisition card and an image post-processing system. The resolution of the camera is 1280*1024 pixels, and the highest recording frequency is 506 frames per second. The TR-PIV system is equipped with 850 MB/s image acquisition card and the computer memory is 32GB. The images collected by the high-speed camera are transmitted to the image acquisition card with a speed of 850 MB/s. The continuous semiconductor laser has a power of 5.27 W with the wavelength of 532 nm. The thickness of the laser sheet is less than 1mm. The planer laser sheet is located on the plane of P45. Fluorescent particles of 6µm in diameter are used as the seeding particles.
Three typical planes, i.e. 10˚ before the baffle (P10), the middle plane between two adjacent baffles (P45), and 10˚ after the baffle (P-10) in the direction of rotation, as shown in Figures 2b and 2c, are chosen as the sampling planes. About 6000 images were captured in each experiment, corresponding to the data acquisition of about 12 s. Cross-correlation analysis was performed on two adjacent images to obtain the transient velocity distribution. The images were analyzed with the fast Fourier transformation (FFT) in 2 passes using interrogation area (IA)12. The first pass was 64*64 pixels, and the second pass was 32*32 pixels (each pass with 50% overlap between the neighboring interrogation areas). The camera frequency was fixed at 500 Hz (∆t =2 ms), which can ensure that the maximum in-plane and out-of-plane displacements of seeding particles are less than one-quarter of the size of IA and the diameter of the light sheet13. PIVlab in the commercial software of MATLAB tool is used to analyze the particle flow fields quantitatively14.