Study of the hydrodynamic of a flapping foil at moderate angle of attack

► Mechanics of a rigid flapping foil animated by a heave translation and a pitch rotation. ► Numerical vs experimental (DPIV) investigation of the near wake. ► Hydrodynamic load at low and moderate St number: added terms induced by the flow. The mechanics of a rigid flapping foil animated by a combination of harmonic heave translation and pitch rotation is examined numerically and experimentally by Digital Particle Image Velocimetry (DPIV). The Arbitrary-Lagrangian–Eulerian (ALE) technique associated with a r-refinement grid adaption algorithm provides a good solution for studying the foil in a fixed reference frame while maintaining the grid quality over the whole simulation. Quantitative predictions were made, and showed very good agreement with the experimental data for a reduced frequency ranging in (0–0.6) and with the fixed values of Reynolds (4×103), heave amplitude (3c/4) and maximum angle of attack (20°). At low Strouhal numbers we observed a linear relationship between the thrust coefficient and the instantaneous angle of attack; for higher reduced frequency a non-linear behavior is observed, linked to added mass effects. Under the assumption of small angle of attack, these added mass effects correspond to a pitch stiffening and a heave damping.