Chaos control in open flows: Experiments in a circular jet

We experimentally demonstrate a new chaos control strategy in an open flow: the axisymmetric jet. The near-field coherent structures (CS) of an initially laminar, circular air jet display low-dimensional chaos in the low-noise ambiance of a large anechoic chamber. The chaotic attractors reveal distinct trajectory bundles surrounding the (statistically) dominant unstable periodic orbits (UPO). We select an attractor for this study and apply a new “phase-averaging” method to its orbits to identify potential goal states for control. Specifically, we choose flow states without vortex pairings and then use two-point velocity measurements to determine the appropriate control perturbations, applied at the nozzle lip. The intrinsic convective instability spatiotemporally amplifies these low-level exit perturbations, steering the downstream attractor dynamics to the vicinity of the desired UPO. Near-field velocity signals show that control achieves the desired flow states. The use of chaos control for suppression of pairing suggests potential applications of our technique for the control of jet transition, hence aerodynamic noise and combustion. To our knowledge, this is the first experimental demonstration of chaos control in an open flow. Chaos control also appears promising for other open shear flows of technological significance, such as wakes, wall jets and boundary layers.