No abstract

Particle image velocimetry (PIV) was used to investigate the effect of 155 μm polystyrene particles on the wake structure of an isolated hemispherical roughness element placed in a laminar boundary layer of a flat plate. The main purpose of this study was to investigate the influence of particles on the large-scale vortices induced by the hemisphere. The turbulence statistics and the main modes of proper orthogonal decomposition (POD) in whole field of view (FOV) were comparatively analyzed. The shedding frequency was analyzed by the power spectral density (PSD) function, and the hairpin vortex head structure was conditionally detected by the spatial multi-scale local average function. The results showed that by introducing particles, the statistics in the near-wall region downstream of the hemisphere were significantly changed. PSD results showed that the existence of particles led to the shedding of structures with multiple frequencies and inhibited the shedding of primary structures. Further conclusions could be drawn through POD and hairpin vortex head extraction: the existence of particles changed the type of vortex shedding behind the hemisphere, thereby reducing the number and the vorticity intensity of hairpin vortex heads in the range near the peak, and promoting the scale of the prograde vortex structure.

No abstract

The flow of stirring process is complex and multiscale. To analyze the effects of baffle on the flow characteristics driven by paddle turbine, particle image velocimetry (PIV) experiments were performed for the unbaffled stirred vessel and the standard baffled stirred vessel stirred by the single paddle turbine. Under the baffle arrangement, the flow discharge was strengthened, and the range of high‐velocity region was obviously expanded, and the velocity significantly fluctuated near the blade. The swirling intensity behind the blade decreased significantly, and no significant wake structure was formed, thereby helping reduce the local velocity difference attributed to the wake structure and facilitate the uniform mixing of materials. The instantaneous velocity field was decomposed into four parts at different scales by employing quadruple Proper Orthogonal Decomposition method. The quantitative analysis of different scale flow structures in stirred process was realized. Under the baffle arrangement, the flow process was facilitated in the measurement plane. In addition, the fluctuation velocity increased significantly, and the mass transfer was enhanced by vortex motion of the fluid elements, which was conducive to the uniform mixing of materials.

The available workspace of parallel kinematics machine tool (PKM) is always very small, which is a common problem existing in PKM. In this paper, by making full use of the advantages of the series machine tool and PKM, a novel 5-DOF series-parallel machine tool has been proposed. The parallel mechanism of the series-parallel machine tool is a kind of 3-HSS mechanism with 3-DOF. The three ballscrews of the parallel mechanism are set horizontally. The series mechanism of the series-parallel machine tool, which is a turning mechanism with 2-DOF, is installed on the mobile platform of the parallel mechanism. The series-parallel machine tool has many advantages, such as easy building of the moving model, easy solving of direct and inverse kinematics, larger workspace in horizon direction, simple structure, easy controlling and so on. This paper mainly studies on its kinematics analysis and sets up the NC system with open modular architecture according to characteristics of the serial-parallel machine tool. Based on these, the prototype of the serial-parallel machine tool proposed in this paper has been manufactured. The correct tool trajectories have been achieved through the on-line manufacturing experiment, which verifies the reasonableness of the structure and the correctness of the NC system.