Exploring the hydrodynamic advantages of pleopod interaction in shrimp swimming

Shrimp possess closely branched appendages, with the abdomen housing five pairs of specialized appendages known as pleopods. During swimming, these pleopods provide propulsion by metachronal paddling. The close interaction among neighboring pleopods during swimming poses interesting questions regarding their hydrodynamic influence, given that their collective performance likely diverges from the behavior of an individual pleopod. While prevalent research often links these interactions to drag reduction, our findings suggest that the main benefit resides in lowering the power consumption needed to generate hydrodynamic force rather than in drag reduction. We demonstrate this observation by simulating a 3D high-fidelity shrimp model using an in-house immersed-boundary-method-based CFD solver. Our simulation results reveal that the flow interactions between neighboring appendages slightly reduced the thrust generation along the horizontal direction while saving the hydrodynamic power consumption by 21%. As a result, the metachronal movement of the five pairs of pleopods can enhance the cost of transport (COT) by approximately 22% compared to cases where each pleopod pair operates independently. Our research findings aim to guide for optimization of underwater locomotion in similar metachronal underwater robotic designs.