Wake structure and evolution of flow over a finned circular cylinder

Abstract

This paper reports the results of a numerical investigation into the flow over a circular cylinder evenly attached with rectangular ribs around its circumference and the associated near-wall vortex structure as well as the evolution of wake flow. The effect of the number of ribs (n) ranging from 1 to 12 is examined in a low Reynolds number range of 60–180. Five kinds of near-wall vortices are identified with the presence of rectangular ribs, including quasi-stagnation vortices, subordinate vortices, inter-rib vortices, inter-rib quasi-stagnation vortices, and dynamic inter-rib vortices. These near-wall vortices and their evolution are sensitive to Re as well as n. With the introduction of ribs, each boundary layer experiences multiple separations that increase from 2 to 4 as n increases from 1 to 12. The intermittent emergency of the last separation is attributed to the switching of the final separation between two adjacent ribs. Furthermore, the duration of the last separation depends on Re and the evolution of dynamic inter-rib vortices. The hydrodynamic coefficients and vortex shedding frequency are closely related to the vortex formation length (Lf*) and wake width (W*). The different variation of Lf* and W* with Re is possibly associated with the appearance and number of subordinate vortices. In addition, the fluctuation of W* is mainly attributed to the switching of boundary layer separation point. The wake flow experience two evolutions as the vortices are convected downstream: the transition from one-row primary vortex street to the two-layered vortices, and the transition from two-layered vortices to the secondary vortex street. The latter is only observed in the wake of a finned cylinder with 3–5 ribs at Re = 180, while the two-layered vortices are decayed in the far wake in other cases.