Active pitching of short splitters past a cylinder: Drag increase and wake

The flow and drag induced by active pitching of plates in the wake of a cylinder of diameter d were experimentally studied for various plate lengths L as well as pitching frequencies fp and amplitudes A0 at Reynolds number Re=1.6×104. Planar particle image velocimetry and a load cell were used to characterize the flow statistics and mean drag of a variety of cylinder-splitter assemblies. Results show the distinctive effect of active pitching on these quantities. In particular, flow recovery was significantly modulated by L, fp, or A0. Specific pitching settings resulted in a wake with dominant meandering patterns and faster flow recovery. We defined a modified version of the amplitude-based Strouhal number of the system StA to account for the effect of the cylinder in active pitching. It characterizes the drag coefficient Cd across all the cases studied, and reveals two regions intersecting at a critical value of StA≈0.035. Below this value, the Cd remained nearly constant; however, it exhibited a linear increase with increasing StA past this critical point. Inspection of the integral momentum equation showed the dominant role of velocity fluctuations in modulating Cd past the critical StA.