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. ... Read more

In the present study, the flow fields generated by two synthetic jets with a chevron and a conventional circular nozzle exits are studied and compared. For both configurations, the devices are operated at the same input electrical power, thus leading to Reynolds and Strouhal numbers equal to 5600 and 0.115 (for the circular exit) and 6000 and 0.106 (for the chevron exit). Phase-locked stereoscopic particle image velocimetry measurements are used to reconstruct the three-dimensional coherent vortex structures. Time-averaged and phase-averaged mean and turbulent statistics are analysed and discussed. The flow field strongly depends on the exit geometry. In presence of the chevron exit, the conventional vortex ring issued through the circular nozzle exit, is replaced by a non-circular vortex ring with additional streamwise vortices. The mutual interaction between these structures prevents the axis-switching of the non-circular vortex ring during its convection. These streamwise vortices disappear convecting downstream and the vortex ring assumes a circular shape. Comparing the two configurations, the chevron exit generates a larger time-averaged streamwise velocity along the centreline but with lower turbulent kinetic energy intensity. Differences are also present between the notch and the apex planes of the chevron exit. In the notch plane, both the time-averaged axial velocity component profile in the spanwise direction and the shearlayer width are wider than in the apex plane. Furthermore, the presence of the streamwise vortices causes a flow motion towards the jet axis in the apex plane and an opposite motion in the notch plane ... Read more