Geometrical influence on vortex shedding in turbulent axisymmetric wakes

Abstract

We investigate the structures generated by the vortex shedding mechanism in turbulent axisymmetric wakes of non-axisymmetric plates, including a square plate and a series of fractal plates, and compare the results to a disk. For a given characteristic length ℓ, all plates have the same frontal area A, since ℓ = A0.5, but the length of the perimeter and the irregularity of the perimeter were varied in a fractal manner thus allowing us to investigate the effect of boundary conditions. Measurements were taken over a large range of downstream and radial distances in order to obtain a more robust measure for the vortex shedding energy. It was found that the fractal plates are able to reduce the vortex shedding energy by as much as 60% compared to the disk and square plates. It was also found that the frequency at which the vortex shedding structures are generated and the manner in which they organise themselves in the wake are independent of the boundary conditions of the wake generator. The results suggest that the main function of the multi-scale segments around the perimeter of the plates is to re-distribute the energy to a broader range of scales in the flow, which could explain the previously observed increase in drag coefficient for the fractal edged plates.