Effect of an upstream cylinder on the wake dynamics of two tandem cylinders with different diameters at low Reynolds numbers

This paper presents a systematic numerical study on the fluid dynamics and flow structures around a cylinder with diameter D placed in the wake of another cylinder with a smaller diameter d. Reynolds numbers of Re = 100 and 150 (based on D) are considered so the flow is physically two-dimensional. The ratios d/D and L/D vary in the ranges of 0.4-1.0 and 1.0-8.0, respectively, where L is the distance from the center of the upstream cylinder to the forward stagnation point of the downstream cylinder. The analysis focuses on how d/D and L/D influence the Strouhal number St, wake topology, and fluid forces on the downstream cylinder and links them with the flow physics. The flow is classified into the reattachment and co-shedding flow regimes, with the latter being further subdivided into the prime vortex shedding, twolayer vortex shedding, and secondary vortex shedding (SVS) modes, and the detailed aspects of the three modes are discussed based on the time-averaged flow fields. The two vortex frequencies of the downstream cylinder can be detected only in the SVS mode, and in addition to the fundamental vortex frequency f(1), the shedding of the secondary vortex further results in the subharmonic frequency f(2). Only when the secondary shedding length L-s* is <10 does f(2) affect the downstream cylinder and lead to a pattern of alternating high- and low-amplitude peaks in the time history of the lift coefficient. A novel mechanism of secondary vortex formation is identified, and the critical spacing and the modulation of lift by f(2) are also discussed. Published under an exclusive license by AIP Publishing.

Automated summary

This study systematically investigates the fluid dynamics and flow structures around a cylinder with a smaller diameter placed in the wake of another cylinder with a larger diameter. The analysis focuses on how different ratios of d/D and L/D influence the Strouhal number, wake topology, and fluid forces on the downstream cylinder. The research identifies different vortex shedding modes and discusses the impact of secondary vortex shedding on the lift coefficient.