Two tandem cylinders of different diameters in cross-flow: effect of an upstream cylinder on wake dynamics

This work aims to provide a systematic experimental study on the wake of two tandem cylinders of unequal diameters. The fluid dynamics around a circular cylinder of diameter D placed in the wake of another circular cylinder with a smaller diameter of d is investigated, including the time-mean drag coefficient (CD), the fluctuating drag and lift coefficients (C'(D) and C'(L)), the Strouhal number (St) and the flow structures. The Reynolds number based on D is kept constant at 4 : 27 x 10(4). The ratios d/D and L/d vary from 0.2 to 1.0 and 1.0 to 8.0 respectively, where L is the distance from the upstream cylinder centre to the forward stagnation point of the downstream cylinder. The ratios d/D and L/d are found, based on extensive hotwire, particle imaging velocimetry, pressure and flow visualization measurements, to have a marked influence on the wake dynamics behind the cylinders. As such, the flow is classified into the reattachment and co-shedding flow regimes, the latter being further subdivided into the lock-in, subharmonic lock-in and no lock-in regions. It is found that the critical spacing that divides the two regimes is dictated by the upstream-cylinder vortex formation length and becomes larger for smaller d/D. The characteristic flow properties are documented in each regime and subdivided region, including the flow structure, St, wake width, vortex formation length and the lateral width between the two gap shear layers. The variations in CD, C'(D), C'(L) and the pressure distribution around the downstream cylinder are connected to the flow physics.