Rotating elliptic cylinders in a uniform cross flow

Numerical simulation of flow past a two-dimensional rotating cylinder of different thickness ratios at Re = 200 has been carried out to investigate how incremental changes in the cross-section of a cylinder from a circle to a thin ellipse affect their lift and drag characteristics as well as flow structures. In contrast to past studies, the present work considers velocity ratios up to 2.5, where vortex shedding of a circular cylinder completely ceases. Results show that a decreasing thickness generally leads to lower mean lift magnitude, but a non-monotonic change in mean drag with a local minimum at a thickness ratio of approximately 0.375. Interestingly, at velocity ratio 2.5, such minimum drag changes sign and turns into a positive thrust; this event occurs at a much smaller velocity ratio than the case of a circular cylinder. Detailed analysis on vorticity contours and surface pressure distributions reveals that the thrust generation is a result of the competition between a prevailing negative pressure below the cylinder due to rotational motion, and a suction effect from a “hovering vortex” formed above the cylinder. Additionally, as the thickness reduces, the wake transits from a von-Kármán type to a separated flow dictated by the two edges of the elliptic cylinder. And the crests and troughs in the transient aerodynamic forces become highly related to the instantaneous geometric angle of attack, which can be attributed to a projection effect.