Closed-loop enhancement of jet mixing with extremum-seeking and physics-based strategies

The closed-loop control of a turbulent round air jet is experimentally investigated based on two unsteady minijets, with a view to enhancing jet mixing. The two minijets are placed at diametrically opposite locations upstream of the nozzle exit. The open-loop control experiments are first performed. Given the mass flow rate ratio C_m of the minijets to that of the main jet, the decay rate K¯ of jet centerline mean velocity exhibits a maximum at the frequency ratio f_e/f₀ ≈ 1.0, where f_e and f₀ are the excitation frequency of minijets and the preferred mode frequency of the natural main jet, respectively. An extremum-seeking feedback control has been developed to achieve autonomously the optimal control performance. It has been found that, given C_m, this closed-loop control technique may obtain automatically and rapidly the optimal value of f_e and the desired or maximum K¯ , as achieved in the open-loop control. This control technique is robust and adaptable when the Reynolds number and initial excitation frequency are changed separately. A flow-physics-based feedback control strategy has also been investigated, which could achieve the optimal control performance automatically with a shorter convergence time than the extremum-seeking control, not robust though.