Direct Numerical Simulation Investigation Of Gas-Solid Flow Around Square Cylinders

Flow around a bluff body has been considered a classical fluid dynamics problem, and researchers all over the world had put a lot of efforts on this issue. Gas-solid flow around a bluff body is a very popular phenomenon in industrial world and in the nature,-more and more researchers transferred into this research area. The irregular characteristics of gas-solid flow around a bluff body, the asymmetric distribution of particles in the flow field and the interaction of particles with gas made the study of Gas-solid flow around a bluff body phenomenon much more complicated. This paper mainly focused on the study of gas-solid flow around long and short squire cylinder. With advantages of high precision, high efficiency and easy for parallelization, High-order Finite Compact Difference numerical method combined with Virtual-body numerical method was adopted for the direct numerical simulation of three-dimensional flow of wake around single cylinder. The emphasis of the study is the instantaneous and time-averaged physical characteristics in the wake of pure gas flow around squire cylinders at the status of Re=225 cases. Lagrange method was used in the particle field for tracking the particles with Stokes number of 0.01,1 and 5. The influence of particles entering the flow field on the instantaneous and time-averaged physical characteristics of the wake has been studied.When Re=225, transition appears with flow around long squire cylinder. The wake vortex structure after the transition is constructed by two arrays of vortexes with relatively independence and opposite rotating direction. This formed the vortex street with the fishbone-like vortex, and it has great distinction with that of flow around a round cylinden With further analysis on the turbulent kinetic energy, distribution of spectrum energy and average velocity of the wake after long squire cylinder, it was found that the low oscillation frequency corresponding to the main flow field is the basic shedding frequency. With basic physical parameters (velocity, vorticity, drag coefficient, lift coefficient and average statistics) analysis, the initial position where the vortex shed from the cylinder, the length and height of the recirculation zone were ascertained. It was found that the maximum average velocity in flow direction, the minimum average velocity in vertical direction and the strongest turbulent flow field pulsating energy appeared at the end of the recirculation zone.With a number of vortex structures existed simultaneously with different time scales and spatial scales after entry of particles into the flow field, the response time of different particle St number varies with space and time, and the impacts of St number on instantaneous and time-averaged variables were different. When Stokes number equaled 0.01, the small particles in the flow field followed the vortex structure. They distributed more evenly at the core and edge of vortex area and the influences on instantaneous and time-averaged flow field variables were not obvious. When Stokes number equaled 1, the amplitude and frequency of the drag coefficient in wake of the flow field were strengthened. In the near flow field, the vortex shedding-cycle time increased and the average velocities in flow direction, vertical direction, outreach direction and average turbulent kinetic energy were strengthened, while it has less effects on mixed average Reynolds stress. Even though there is no significant impact on flow-direction average Reynolds stress, the vertical direction disturbance was increased and mixed average Reynolds stress diminished. When Stokes number equaled 5, the amplitude and frequency of the drag coefficient in wake of the flow field were weakened by medium sized particles. It increased span of the average flow velocity and the average velocity in vertical/outreach directions, while evened the changing trend of the average Reynolds stress. The average turbulent kinetic energy on the centerline was reduced, but that away from the regional centerline increased.By numerical study, it was found that the vortex structure flow around the short squire cylinder was totally different with those around ball and round cylinder. In the near-flow field, the vortex structure was composed of mainly the upper and lower surface of a breakdown relatively independent hair-clip vortex. The far-flow field was composed of mainly the band vortex structure generated by interaction of hair-clip vortex and fishbone. This vortex street was much different with that of flow around long squire cylinder. With further analysis on the turbulent kinetic energy, distribution of spectrum energy and average velocity of the wake after short squire cylinder, it was found that the low oscillation frequency corresponding to the main flow field is lower than that of long squire cylinder. With basic physical parameters (velocity, vorticity, drag coefficient, lift coefficient and average statistics) analysis, the initial position where the two vortexes merged and the interaction region were ascertained. It was found that the minimum average velocity in vertical direction and the strongest turbulent flow field pulsating energy appeared in this merge zone.With simultaneously existence of two completely different vortex structures after the entry of particles into the short squire cylinder wake flow field, the impacts of St number on instantaneous and time-averaged variables were different with that of long squire cylinder. When Stokes number equaled 0.01, the small particles in the flow field followed the structure movement of two vortexes. The influences on instantaneous and time-averaged flow field variables were found at the beginning but not obvious. When Stokes number equaled 1, the amplitude and frequency of the drag coefficient in wake of the flow field were reduced and the basic frequency of lift coefficient disappeared. The average Reynolds stress and average velocities in flow direction and vertical directions were diminished, while the average velocity in outreach direction increased. When Stokes number equaled 5, the amplitude and frequency of the drag coefficient in wake of the flow field were reduced. The average Reynolds stress and average velocities in vertical direction and outreach directions were diminished, while the average velocity in flow direction increased.