A study on drag reduction of a rotationally oscillating circular cylinder at low Reynolds number

Abstract

The flow field around a rotationally oscillating circular cylinder in a uniform flow is studied by using a particle image velocimetry to understand the mechanism of drag reduction and the corresponding suppression of vortex shedding in the cylinder wake at low Reynolds number. Experiments are conducted on the flow around the circular cylinder under rotational oscillation at forcing Strouhal number 1, rotational amplitude 2 and Reynolds number 2,000. It is found from the flow measurement by PIV that the width of the wake is narrowed and the velocity fluctuations are reduced by the rotational oscillation of the cylinder, which results in the drag reduction rate of 30%. The mechanism of drag reduction is studied by phase-averaged PIV measurement, which indicates the formation of periodic small-scale vortices from both sides of the cylinder. It is found from the cross-correlation measurement between the velocity fluctuations that the large-scale structure of vortex shedding is almost removed in the cylinder wake, when the small-scale vortices are generated at the unstable frequency of shear layer by the influence of rotational oscillation.

Visualization of aerodynamic noise source in the wake of a rotating cylinder

“…Figure 2 shows the aerodynamic noise from a rotating cylinder with Lighthill-Curle's equation. The predicted noise level does not depend on the Reynolds number, but depends strongly on the velocity ratio, α (Fujisawa et al, 2005). When the alternative vorticity structures exist, noise levels increase in comparison to the noise level of a non-rotating cylinder.…”

Vortex formation in a viscoelastic entry flow of asymmetric planar contraction

“…The flow observation was made by a monochrome CCD camera (1,018 9 1,008 pixels, 8 bit) with a microscope having a magnification of 2.7. The PIV analysis and the error analysis have been reported by Fujisawa et al (2005).…”

Numerical study on flow over a confined oscillating cylinder with a splitter plate

“…In the study, variations in length and porosity of plate were taken into account. Fujisawa et al (2005) investigated flow past a circular cylinder which had rotational pulsation in a uniform flow. It was concluded that rotational oscillation almost removes the large-scale structure of vortex shedding by generating small-scale vortices.…”