Flow characteristics around extremely low fineness-ratio circular cylinders

The accurate measurement of flows and aerodynamic characteristics around a bluff body has been a challenging task due to the existence of interference between the wake and mechanical model supports in wind-tunnel experiments. The present study focuses on a freestream-aligned circular cylinder with an extremely low fineness ratio (the ratio of the axial length to diameter) ranging from 0.30 to 0.50, which has never been investigated without interference from a mechanical model support. We employed a magnetic suspension and balance system to eliminate interference from the model support and measured the drag and velocity fields in the diameter-based Reynolds number between 2.0 x 10(4) and 7.7 x 10(4). As the fineness ratio decreases below 1.50, the size of the recirculation bubble increases and the velocity distribution on the central axis inside the bubble gradually converges to that of the circular disk. Furthermore, large-eddy simulations were performed in the Reynolds number of 4.0 x 10(4T), whose drag coefficient agrees well with experiments. Based on those results, it was found that the drag coefficient monotonically converges to that of the circular disk without local maximum. This study revealed that in the low-fineness-ratio regime (0.10-0.50), a critical geometry, at which the drag coefficient shows a local maximum, does not exist in the circular cylinder. Subsequently, unsteady flow analyses were performed, where two characteristic frequencies, i.e., St = 0.05 and 0.155, were identified from power spectral densities of the drag coefficient and the pitching moment coefficient. The associated flow structures are then extracted by a phase-averaging procedure, where the phase-averaged flows with St = 0.05 represent the recirculation bubble pumping while the phase-averaged flows with St = 0.155 show nonaxisymmetric structures inside the recirculation bubble.

Automated summary

This study focused on a freestream-aligned circular cylinder with a low fineness ratio and used a magnetic suspension and balance system to eliminate interference from a mechanical model support. The results showed that as the fineness ratio decreases below 1.50, the size of the recirculation bubble increases and the velocity distribution gradually converges to that of a circular disk. Large-eddy simulations also revealed that the drag coefficient of the cylinder monotonically converges to that of a circular disk without a local maximum in the low-fineness-ratio regime.