Fluid Dynamics And Efficiency Of Colonial Swimming Via Multijet Propulsion At Intermediate Reynolds Numbers

Colonial physonect siphonophores swim via laterally distributed multijet propulsion at intermediate Reynolds numbers (Re's) on the orders of 1-1000. Here, a computational fluid dynamics approach that assumes steady axisymmetric flow is employed to investigate the underlying fluid mechanics and adaptive values of colonial swimming via laterally distributed multijet propulsion, with comparison with rear jetting single-jet propulsion. Results show that imposed flow fields, drag coefficients, powers, and efficiencies all vary significantly depending upon Re, jet angle, and way of jetting. For a given Re, two types of optimal jet angles are determined: one in the range of 61 degrees-70 degrees that maximizes the quasipropulsive efficiency (i.e., to minimize the jet power), and another in the range of 34 degrees-45 degrees that maximizes the Froude propulsion efficiency (i.e., to minimize the wake). Comparison with values for a documented siphonophore, Nanomia bijuga, indicates that siphonophores rely upon a spectrum of jet angles between these two theoretical optima. Multiple, laterally directed jets produced by colonial forms are less energetically efficient for propulsion than single, posteriorly directed jets produced by solitary individuals; however, colonial swimming achieves energetic benefits for jetting individuals within the colony because they require significantly lower per-module power than that required by a lone jet module swimming at the same speed. Hence, by sharing propulsive duties, colony formation helps alleviate inherent power constraints that characterize cnidarian muscles. Importantly, multiple jets that are directed obliquely away from the central body axis exert less impact on other colony members within the siphosome that is towed in the wake of the jetting aggregation.