In this paper, surface-mounted arrays of pulsed-DC dielectric barrier discharge plasma actuators are used to achieve skin friction drag reduction in turbulent boundary layers. The arrays are designed to produce a near-wall plasma-induced spanwise flow for the purpose of preventing the lift-up of low-speed streaks. Two array designs are demonstrated. One produces a unidirectional spanwise flow and the other a series of opposed wall jets. Both configurations are shown to produce unprecedented levels of drag reduction in excess of 70%. The amount of drag reduction achieved is shown to scale on the number of viscous wall units between adjacent surface electrodes which effectively sets the number of low-speed streaks under simultaneous control. In particular, the level of drag reduction increases logarithmically as the number of streaks under simultaneous control decreases. Furthermore, due to the low-power input required by the pulsed-DC actuator, this level of drag reduction is achieved with net power savings.

An experimental investigation of an unsteady adverse pressure gradient turbulent boundary layer is described. It is demonstrated that the local flow physics is largely dominated by an inflectional instability which gives rise to an embedded shear layer contained within the boundary layer. Experimental measurements are presented which are fully consistent with the presence of clockwise spanwise-oriented coherent vorticity within the embedded shear layer. Using embedded shear layer scaling parameters in the form of the shear layer vorticity thickness and the velocity defect at the upper inflection point, both the mean and the phase-averaged boundary layer velocity profiles exhibit similarity in both space and time over a large wall-normal extent. In a similar manner, the profiles of the streamwise-component turbulence intensity and Reynolds stress also exhibit similarity when scaled with the embedded shear layer parameters. The embedded shear layer scaling of previously published adverse pressure gradient turbulent boundary layer measurements confirms its generic applicability in a wide range of flow-field geometries and extending to high Reynolds numbers.

Experiments were performed using an active flow control approach that has shown the ability to significantly reduce the viscous drag in turbulent boundary layers. The purpose of this work was to document the changes in the turbulence characteristics of the boundary layer with the drag reduction. The flow control involved generating a steady spanwise velocity component of the order of vortical structures, indicate a suppression of coherent features in the wall layer. These results are consistent with an underlying mechanism for drag reduction that comes from a suppression of the turbulence producing events in the wall layer associated with the wall streak structure.

The equation of motion used to derive the aerodynamic damping coefficient for a single-degree-of-freedom airfoil oscillating in pitch about its quarter-chord is rewritten in analytic signal form through application of the Hilbert transform. The results yield a mathematical framework that can be used to estimate the aerodynamic damping coefficient throughout the entire pitch cycle. The analysis is then applied to experimental data from attached, light, and deep dynamic stall conditions at freestream Mach numbers ranging from 0.2 to 0.6 and Reynolds numbers up to 3.5×106. The Hilbert-transform-based approach is used to demonstrate that the cycle-integrated aerodynamic damping coefficient masks the physics underlying the stabilizing and destabilizing mechanisms of the dynamic stall process. In particular, conditions that exhibit positive cycle-integrated aerodynamic damping may include time intervals of negative aerodynamic damping during the pitch cycle.