Low-Reynolds-number wake of three tandem elliptic cylinders

Abstract

The flow around three elliptic cylinders with equal spacing and aspect ratio in tandem arrangements was numerically investigated through direct numerical simulation. The spacing ratio (L/D, where D and L are the major axis and the center-to-center distance of two adjacent elliptic cylinders, respectively) ranging from 1.5 to 10 and the Reynolds numbers of R e = 65 - 160 (based on D) are examined. The analysis aims at the effects of L/D and Re on wake structures, hydrodynamic forces, and Strouhal numbers and correlates them with the underlying flow physics. The flow is highly changeable to Re and L/D, classifying into five distinct regimes, namely, meandering, overshoot, reattachment, quasi-coshedding, and coshedding. Two vortex shedding frequencies for middle and downstream cylinders are observed in the latter two regimes, indicating the significant wake interference, where three vortex shedding modes are spatially observed including primary, two-layered, and secondary. The transition between two adjacent modes forms two boundaries. At the first boundary, vortices divert from the cylinder centerline and follow two layers, while vortices converge the cylinder centerline at the second boundary. The first boundary location is not stationary at R e = 65 - 100, while it is stationary at Re = 160. Otherwise, the second boundary location moves upstream with an increase in L/D, while the range of movement decreases with an increase in Re. The increase in Re advances the disturbance level and urges the transition between vortex shedding modes. The time-mean lift and drag coefficients for three cylinders are highly sensitive with an increase in L/D.