PIV Measurement Of The Flow Past Cactus-shaped Cylinder

Because of the practical importance and the wealth of interesting fluid flow phenomena, lots of progress has been made in understanding of the flow around a cactus-shaped cylinder. However, due to its physical complexity, the experimental results of unsteady characteristics and a complete and conclusive explanation of the physics are still lacking. The use of advanced techniques in experiments and data post-processing methodologies has made it possible to enhance our knowledge of the drag reduction phenomenon.In the present work, detailed results of time averaged flow properties as well as unsteady flow characteristics in the wake of a cactus-shaped cylinder were measured in a low-speed water tunnel. Particle image velocimetry(PIV) and time-resolved particle image velocimetry(TR-PIV) measurements were conducted for both smooth cylinder and cactus-shaped cylinder. The PIV measurements obtain some time-mean results such as velocity field, streamlines, reverse-flow distribution, shear stress distribution and swirl strength distribution. According to the comparison of these results in different shape of cylinders, the influence of cavities on vortex shedding and flow characteristics in the wake is clarified. The results obtain from TR-PIV measurements such as velocity spectrum, cross-correlation analysis and coherence analysis show the vortex shedding and transformation process, by which the different periodic character of the vortex shedding between smooth and cactus-shaped cylinder is revealed.Some new methods of PIV velocity data post-process were discussed in the present work. The swirling strength method was used to investigate the vortex structure instead of vorticity field. The turbulent integral length scale was also calculated. A revised phase averaging method based on cross-correlation of the instantaneous velocity field determined by TR-PIV measurement was proposed to extracting the quasi-periodic flow patterns related to the shedding coherent structures. At last, a revised method of calculating pressure field on the basis of the PIV data was developed.