Lock-on to quasi-periodic flow transformation for a rotationally oscillating cylinder due to gust impulse

Abstract

A computational study is conducted to analyze the effects of an upstream gust impulse on lock-on flow of a cylinder, which is rotationally oscillating. Reynold numbers of 100, 110, and 120 and three discrete oscillation conditions based on rotational oscillation amplitudes are considered. The current solution scheme is validated for the cases of stationary and rotationally oscillating cylinders in a uniform flow. The oscillation Strouhal forcing frequency is varied with a resolution of 0.0001 for lock-on to quasi-periodic flow boundary identification. Flows are categorized as either lock-on or quasi-periodic based on frequency analysis of lift. Furthermore, three different gust impulses for each Reynolds number are selected by associating gust Strouhal frequency to natural shedding Strouhal frequency, whereas the Prandtl number is 7. These gust impulses were introduced in lock-on flow near lock-on to quasi-periodic flow boundary. Analysis of lift, drag, and vorticity contours confirms that the lock-on flow is transformed into quasi-periodic flow in conditions of high oscillation amplitude and low gust frequency. This transformation is observed to persist for four descending values of rotational oscillation Strouhal forcing frequency at the upper lock-on boundary.