Study On The Near Wake Of The Circular Disk And Disk With Centre Hole

The wake of a circular disk is not studied as extensively as the wake of the other typical axisymmetric bluff bodies, such as the circular cylinder, sphere. However, flow over a circular disk has been found in many aero- and hydrodynamic applications, such as car side view mirror, flame holder in industrial burners or gas turbine combustors. Compared with the circular cylinder or sphere, the circular disk has a more complex wake due to its sharp edge. Thus, study on the near wake of a circular disk helps in understanding not only dynamical processes, but also complex three dimensional coherent structures and their transport characteristics, their applications in engineering design.In real engineering applications, the inflow condition is not as simply as uniform one. In present study, the evolution and mean flow structure of a circular disk in a shear flow is numerical studied by large eddy simulation(LES). On the other hand, the circular disk with a centre hole is widely used as flame holder in combustors. Thus, the effect of hole diameter d/D on the wake is extensively investigated by PIV. The main work in this paper includes:1, The laminar wake of circular disk in the uniform shear flow is numerical studied by LES, with focus on the effect of shear rate on the flow structure.2, The near wake of the solid disk (D=40mm, D/W=5) and disk with a centre hole (d/D ranges from 0.1 to 0.5) are experimentally studied by PIV. The mean flow velocity, the Reynolds normal and shear stresses are presented and compared.3, Using PIV data, the snapshot POD is applied to decompose the velocity field of the solid disk and disk with a centre hole. The coherent structures are analyzed by comparison of differences between the characteristic modes of POD.Based on above study, the following conclusions are obtained:1, Under current shear rate inflow condition, it is found that the critical Reynolds number of Hopf bifurcation in shear flow, which characterized by planar symmetry steady state model to non-steady state with vortex shedding, is lower than that in uniform flow. The hairpin vortex increases in higher velocity side and is not well symmetric as it in uniform inflow wake. At the mean time, the frequency of nature vortex shedding is keep constant.2, Due to jet in the disk centre hole, there is a pair vortex inside the primary recirculation zone in the near wake. As increase of the hole’s diameter and thus the strength of the centre jet, the size of the secondary recirculation becomes bigger, while the primary recirculation region decreases in length. Further increase of the centre jet diameter, it is found that the primary recirculation region is separated by the jet. Two counter rotating vortex pairs are founded symmetry with the x axis. The length of recirculation region decreases rapidly.3, When the diameter of the centre hole is small, the jet is conducive to stabilize the vortex structure inside the disk recirculation region. It is founded that the energy contained in POD model 1 and 2 becomes larger, and the total energy in the top ten POD model is bigger than that in the solid disk wake. When the centre hole’s diameter is large enough, however, the stability of large scale vortex decreases due to separation of primary recirculation zone by jet. The energy contained in POD model 1 and 2 and total energy of the top ten modes decrease as well.