Wake and thrust characteristics of low Reynolds number, parallel twin single-pulsed round jets*

Individual single-pulsed jets are used for propulsion and locomotion by a number of aquatic animals such as squid and jellyfish. Although perhaps less known, some aquatic animals use arrangements of several pulsed jets for the same purpose. Salps and siphonophores form colonies that maneuver as a single organism by individually modulating their jet strength and timing. Arrangements of pulsed jets can therefore provide an alternative means of propulsion and maneuverability for aquatic vehicles. In general, we can define a pulsed jet as a fixed volume of fluid ejected through an orifice over a finite time interval on a periodic or aperiodic basis. In this work, we investigate the effect of interjet spacing and timing on the wake and thrust characteristics behind low Reynolds number (Re = 350), parallel twin single-pulsed round jets ejected from a wall. We conduct direct numerical simulations using the open source highly parallelized Xcompact3D solver (Bartholomew et al., 2020). The total thrust decreases as the interjet spacing is reduced due to the modification of the pressure field between the jets, as observed in the experimental work of Athanassiadis & Hart (2016). The generated vortex rings approach each other and decay throughout their streamwise evolution. By contrast, a vortex rebound effect is observed in the case of twin plane jets. Varying the interjet timing results in more complex vortex dynamics in the wake and induces a force couple at the wall that can be leveraged for rotational maneuvers.