Chaos Control of an Open Shear Flow: A Novel Control Strategy and its Experimental Demonstration in an Axisymmetric Jet

A nonlinear dynamical systems approach to control the coherent structure dynamics and turbulence in the near field of an axisymmetric jet is presented. Experiments were performed in an initially laminar circular jet, housed in a low-noise anechoic chamber. Periodic flow states involving various sequences of vortex ring formation and their pairings were achieved using open-loop and closed-loop control. In contrast to conventional, linear, control approaches that use brute-force high-amplitude excitation, a systematic nonlinear control method is presented. Flow states that are inaccessible using prior control methods are achieved with small changes to control parameters. Measurements of velocity traces show the effectiveness of control of periodic states over a region extending beyond the end of the jet potential core. Control is demonstrated using flow visualization and measurements of the mean flow and turbulence intensity, and of the two-point spatial correlation. Measurements reveal effective amplification and suppression of nonlinear instabilities associated with the controlled vortex dynamics. Single- and two-point velocity measurements were used to describe the low dimensional (chaotic) jet flow dynamics, which have inherently periodic (but unstable) flow states, and to determine the control perturbations. A closed-loop, robust, method was also developed to control the flow for cases when the open-loop method was ineffective. Practical benefits from the jet control were assessed revealing turbulence enhancement (having nearly 20% jet spread increase) and turbulence suppression (having up to 70% turbulence intensity decrease). Finally, to address the applicability of the control approach to other open shear flows, the control method was demonstrated in a plane mixing layer, demonstrating extensibility to other open shear flows.