Investigation of vortex dynamics downstream of moving leaflets using robust image velocimetry

The interaction of a sudden flow through a rectangular slot with moving leaflets, hinged at its border, was investigated experimentally in a Plexiglas vessel. This configuration resembles schematically some key features of many biological flows, e.g. in sea-animal propulsion, where the moving flaps control the flow, optimizing thrust, or in heart valves, where leaflets prevent backflow. Therefore, the comprehension of the basic mechanisms of the flow-structure interaction and of the features of the flow is of interest in a wide range of applications. Although some detail of the phenomenon could depend on the specific leaflet design, material and forcing, the objective of the present work is to investigate the overall dependence of the flow field on the leaflet arrangement. Specifically, three leaflet configurations have been tested at Reynolds number Re = 2,000 and Strouhal number St = 0.2: two symmetrical leaflets, two non-symmetrical leaflets, one being twice as wide as the other, and a single leaflet. Velocity fields were obtained using Robust Image Velocimetry in order to accurately resolve the structure of the vorticity field. The dynamics of the opening leaflets, the vorticity fields and the features of the vortices generated during the leaflet opening were investigated and compared in the different leaflet configurations. Advantages in the opening time, maximum aperture and closing time were observed in the two-leaflet non-symmetrical case in comparison to the other configurations.