Numerical Simulation Study On Flow Over A Tandem Of Disks

Tandem disks can be widely found in various engineering applications,such as air pollutants,red blood cells in the blood or flame stabilizers in the engine combustion chamber and so on.On the other hand,since the wake of two tandem disks interact with each other,their flow field characteristics are quite different and more complex compared with those of a single disk.In this paper,the large eddy simulation is used to simulate the flow over the tandem disks with the aspect ratio equal to 5 under the low Reynolds number(the upstream disk has a hole in center),mainly discuss the evolution of the near wake and the influence of the separation distance between tandem disks and the aperture of the upstream disk on the flow field characteristics under low Reynolds number,and the conclusions are drawn as follows.1)When the separation distance of two disks is small enough(l/d = 0.1),the placement of the upstream apertured disk(D/d = 0.2)delays the flow field transition.When Re<178,the wake is a steady-state axisymmetric mode.The wake is a steady-state symmetry mode when the Reynolds number varies from 178 to 212.When 207?Re<212,the stability of the flow field is destroyed with the large scale vortex shedding,the vortex loops are shedding along the same direction.Hence the wake is referred to as "Zig-zig" mode.When 212?Re<275,the wake is called"Zig-Zag" mode.The main difference between the current mode and the previous mode is that the vortex sheds from diametrically opposite locations of the disk.When Re?275,the symmetry plane is destroyed and the positive and negative vortices in the wake are entangled with each other.The large scale hair-vortex periodically shed in a randomly pattern and breaks into small scale vortex structure,thus the wake becomes chaos.2)When Re= 200 and D/d = 0.2,the flow field structure changes with the increase of disk separation distance.The wake is stable and plane-symmetry at l/d=1 and unstable but plane-symmetry at l/d=1.5.At l/d=2,however,the plane-symmetry structure is broken.The upstream wake resumes plane-symmetry at l/d=3 while the planar symmetry is resumed of both disks at l/d=6.When the separation distance between two disks is constant,the lift coefficient of the upstream disk is always smaller than that of the downstream disk.The drag coefficient of each disk in tandem is smaller than that of a single disk.With the increase of disk separation distance,the drag coefficient of upstream disk hardly changes while the drag coefficient of downstream disk increases but is always smaller than that of the upstream disk.3)When Re=200 and l/d = 1.5,as the upstream disk aperture diameter D/d increases from 0 to 0.6,the stability of the flow field increases and the drag coefficient of the upstream disk decreases while the drag coefficient of the downstream disk increases.