The present study compares the hydrodynamic performance of two rigid pitching foils arranged in specific tandem configurations- ‘tandem’ and ‘tandem staggered’. The two foil sections have symmetric NACA 0012 geometry and are placed in tandem at a distance equal to twice the chord length for both the configurations. In addition, the ‘tandem staggered’ configuration has a lateral spacing of 0.2 times the chord length. Numerical investigations are performed using the open-source Computational Fluid Dynamics (CFD) solver OpenFOAM (OF). For the two tandem configurations, pitching amplitudes of 10 and 15 degrees at two reduced frequency values are studied along with a phase difference in their pitching motion. A single-phase Unsteady Reynolds Averaged Navier Stokes (URANS) solver is selected to capture the physics of the problem with laminar flow assumption. Moving foil boundaries are handled by using the overset mesh method, involving a moving body mesh and a fixed background mesh.
The key objectives of the present work are to assess the influence of geometric configuration of the two tandem foils undergoing pitching motion on lift and thrust, and overall efficiency. The impact of the tandem configurations on the vortex street in the wake is also investigated. Another aspect of the study is to understand the influence of phase difference between the individual pitching motions on the integral hydrodynamic performance and resultant wake vortices. In order to explore this aspect, two cases are chosen: in-phase (0 degree), and out-of-phase (180 degrees). It is observed that the staggered arrangement changes the hydrodynamic interactions between the vortices generated by the forward flapping foil with the second foil placed downstream. This influences the resultant forces generated as well as the wake characteristics of the tandem pitching foils. An in-depth comparison is made between the vortex streets of the two tandem configurations in the phasing of the two foils’ pitching motion.