Experimental investigation of the relationship between the propulsive performance and vortex rings produced by bio-inspired pitching panels*

Many animals propel themselves through the water using either a caudal fin or a fluke. These propulsive appendages display wide diversity in planform, including those with different trailing edge shapes. In the current work, trailing edge shape and pitching amplitude are varied for bio-inspired pitching panels with a nominally trapezoidal planform. In total, five unique panel geometries, each with a different trailing edge shape, were pitched in a constant free stream flow at mulitple amplitudes. Experimental results are discussed in the context of changes to Strouhal number, which ranged between 0.09 and 0.66. Results show that the time-averaged performance of low aspect ratio panels with pointed trailing edges exceeds that of high aspect ratio panels with forked trailing edges. However, lower aspect ratio panels exhibit enhanced performance despite the presence of significant three-dimensional effects that manifest through the shedding of streamwise vorticity into the wake, which is typically associated with degraded performance. Through the actions of deforming vortex rings in the wake, streamwise vortices can be linked to the development of vortex-induced velocity fields that have a significant streamwise component. The development of accelerated streamwise flows in the wake, which are associated with the actions of streamwise vortices, is ultimately beneficial for increased time-averaged thrust and enhanced propulsive performance. The current work also focuses on the implications of these findings on the design of bio-inspired vehicles and the effects that propulsor geometry and kinematics may have on swimming animals.