Roles of propulsor flexibility and aspect ratio in hybrid metachronal rowing*

Several crustaceans with widely varying body and appendage morphologies employ a swimming strategy called hybrid metachronal rowing, where closely spaced appendages (hereafter propulsors) perform an adlocomotory metachronal power stroke followed by a nearly synchronous recovery stroke. Many species of copepods use this strategy when performing escape maneuvers. A review of published images of the swimming legs of copepods showed that the range of aspect ratio (AR; paddle length/paddle width) varied between 1<=AR<=4. In addition, appendage flexibility also can vary across species. Previous studies have shown that the interaction of propulsor tip vortices plays a considerable role in metachronal rowing performance. This work examines the effects of varying AR and propulsor flexibility on tip vortex dynamics. We conducted 2D-2C PIV and three-dimensional PTV (shake-the-box) measurements on a dynamically scaled robotic model of hybrid metachronal rowing consisting of four propulsors. The strengths of propulsor tip vortices were observed to decrease with increase in AR. Additionally, vorticity was more diffuse along the length of flexible propulsors as compared to rigid propulsors. The implications of the tip vortex dynamics on rowing performance will be discussed.