The potential benefits of active flow control are no more debated. Among many others applications, flow control provides an effective mean for manipulating turbulent separated flows. Here, a nonthermal surface plasma discharge (dielectric barrier discharge) is installed at the step corner of a backward-facing step ($U_{\circ }=15m/s,Re_{h}=30000,Re_{\theta }=1650$). Wall pressure sensors are used to estimate the reattaching location downstream of the step (objective function #1) and also to measure the wall pressure fluctuation coefficients (objective function #2). An autonomous multi-variable optimization by genetic algorithm is implemented in an experiment for optimizing simultaneously the voltage amplitude, the burst frequency and the duty cycle of the high-voltage signal producing the surface plasma discharge. The single-objective optimization problems concern alternatively the minimization of the objective function #1 and the maximization of the objective function #2. The present paper demonstrates that when coupled with the plasma actuator and the wall pressure sensors, the genetic algorithm can find the optimum forcing conditions in only a few generations. At the end of the iterative search process, the minimum reattaching position is achieved by forcing the flow at the shear layer mode where a large spreading rate is obtained by increasing the periodicity of the vortex street and by enhancing the vortex pairing process. The objective function #2 is maximized for an actuation at half the shear layer mode. In this specific forcing mode, time-resolved PIV shows that the vortex pairing is reduced and that the strong fluctuations of the wall pressure coefficients result from the periodic passages of flow structures whose size corresponds to the height of the step model.

B. Noack. Closed-Loop Turbulence Control-From Human to Machine Learning (and Retour). (2018) DOI 10.1007/978-981-10-7542-1_3

R. Preen, L. Bull. On Design Mining: Coevolution and Surrogate Models. Artificial Life 23(2) DOI 10.1162/artl_a_00225

S. Xu, J. Cai, Z. Zhang, S. Tang. Investigation of the electrode surface geometry effects driven by nanosecond-pulsed surface dielectric barrier discharge. J. Phys. D: Appl. Phys. 50(18) (2017) DOI 10.1088/1361-6463/aa66ad

D. Quinn, Y. van Halder, D. Lentink. Adaptive control of turbulence intensity is accelerated by frugal flow sampling. J. R. Soc. Interface 14(136) (2017) DOI 10.1098/rsif.2017.0621

S. XU, J. CAI, Y. LIAN. Investigation of nanosecond-pulsed dielectric barrier discharge actuators with powered electrodes of different exposures. Plasma Sci. Technol. 19(9) (2017) DOI 10.1088/2058-6272/aa6f59

B. Wang, H. Li. POD analysis of flow over a backward-facing step forced by right-angle-shaped plasma actuator. SpringerPlus 5(1) (2016) DOI 10.1186/s40064-016-2361-8

S. Xu, L. Kang, J. Cai, S. Tang. Experimental investigation on the optical emission spectroscopy of dielectric barrier discharge plasma actuators at different atmospheric pressures. AIP Advances 8(11) DOI 10.1063/1.5053602

C. Bingham, C. Raibaudo, C. Morton, R. Martinuzzi. Suppression of fluctuating lift on a cylinder via evolutionary algorithms: Control with interfering small cylinder. Physics of Fluids 30(12) DOI 10.1063/1.5055016