Turbulent drag reduction using an array of piezo-ceramic actuators

An experimental investigation on active control of turbulent boundary layer (TBL), aiming at reducing skin-friction drag, has been conducted in a wind tunnel at Re _θ = 1,000, based on the free-stream velocity (U _∞) and momentum thickness (θ). A spanwise-aligned piezo-ceramic (PZT) actuator array, consisting of 16 elements, was employed to generate wall-normal oscillations and, given a phase shift between two adjacent actuators, a transverse travelling wave along the wall surface. A wide range of parameters were examined, including the wavelength (41.6 ∼ ∞ wall units), oscillation frequency (0.13 ∼ 0.65 wall units) and amplitude (0.83 ∼ 2.77 wall units). Local skin-friction drag can be reduced as much as 50% at a location of 17 wall units downstream of the actuator tip, based on the change of hotwire-measured slope of mean streamwise velocity profile in the near-wall region, when the actuator array worked at a wavelength, oscillation amplitude and frequency of 416, 1.94 and 0.39, respectively, all in wall units. The near-wall flow structures with and without control were extensively measured using laser-illuminated smoke-wire flow visualization, hotwire and hot-film, and were compared with each other. Under the optimum control parameters, large-scale coherent structures were disturbed tremendously, resulting in comparatively steady small-scale structures. Moreover, results of two-point cross-correlation between wall shear stress and streamwise velocity reveal that near-wall shear layers were impaired significantly and pushed upward slightly, toward skin-friction drag reduction, during the perturbations.