This experimental study demonstrates the feasibility of integrating particle swarm optimization (PSO) within a hardware-in-the-loop framework for real-time active flow control. The objective is the attenuation of Tollmien–Schlichting (TS) waves developing in an incompressible, two-dimensional laminar boundary layer, using dielectric barrier discharge plasma actuators. Single- and multi-frequency TS waves are artificially excited and amplify before reaching the control region. Control actions are optimized online using PSO, which iteratively updates a population of candidate solutions based on a pressure-based performance metric. In single-frequency scenarios, candidate solutions are parametrized as finite impulse response filters, convolved with an upstream pressure signal to generate the control voltage. In multi-frequency scenarios, the controller constructs a linear superposition of sinusoids at the triggered TS frequencies, identified online. The effectiveness of control is quantified by the attenuation of pressure fluctuations measured by wall-mounted microphones downstream of the actuator. Upon convergence, the best-performing control actions are re-tested in independent experiments, using two-component particle image velocimetry (PIV) to assess the effect of the controller on the velocity field inside the boundary layer. The results show that PSO consistently achieves appreciable TS waves suppression, as evidenced by pressure fluctuation levels reduced by 30%–40% relative to the uncontrolled case and by weakened phase-coherent velocity disturbances. Furthermore, phase-free PIV measurements demonstrate a downstream delay in the growth of velocity fluctuations of up to 15 δ₀*, where δ₀* is the displacement thickness at the control location. These findings confirm the feasibility of PSO-driven optimization for real-time flow control in transitional boundary layers.

The implementation of the Particle Swarm Optimization (PSO) algorithm is investigated to optimize the active attenuation of Tollmien–Schlichting (TS) waves developing in a two-dimensional zero pressure gradient boundary layer. This is done numerically, where the PSO algorithm optimizes the characteristics of harmonic suction and blowing jets, in a feedforward control framework. The PSO-based controller selects and modifies the phase and amplitude of the jets to minimize the pressure fluctuation amplitude downstream of the actuator. To allow for efficient simulation, the 2-dimensional incompressible Navier–Stokes equations are expanded in a harmonic perturbation form and solved in linear and nonlinear variants using harmonic balancing. This study explores the performance of control in both linear and nonlinear development regimes of TS waves through control of single and multi-frequency ensembles of instabilities. Respectively, linear and nonlinear controller design approaches are employed. The findings reveal that the integration of PSO into the control design produces an effective suppression of TS waves through opposition control. The linearly designed controller effectively attenuates single and multi-frequency disturbances. However, when applied in regions of strong nonlinear interactions among instability modes, performance degradation is observed. On the contrary, the nonlinearly designed controller proves effective in mitigating nonlinear multi-frequency instabilities dominating the later stages of growth. A near-complete elimination of TS waves is achieved by accounting for nonlinear interactions among harmonic modes detected by an input sensor. This highlights the benefit of integrating the PSO algorithm in control of TS waves, particularly in the nonlinear growth regime, where classical control methods are generally ineffective.