An experimental investigation about nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuator is presented in this thesis. This work aimed to answer fundamental questions on the actuation mechanism of this device. In order to do so, parametric studies in a quiescent air as well as laminar bounded of free shear layers were performed. Amplitude and location of the input with respect to the receptivity region as well as frequency of flow actuation were investigated. This work required the implementation of acquisition techniques such as Schlieren, Particle Image Velocimetry (PIV), infrared thermography, back current shunt technique and balancemeasurements. Moreover, tools of analysis were employed such as Linear Stability Theory (LST), Proper Orthogonal Decomposition (POD) and Inverse Heat Transfer Problem(IHTP). Results revealed that the effect of a ns-DBD is that of “enhancing” the development of natural hydrodynamic instabilities of the specific field of motion. Therefore, in case of a laminar boundary layer, the effect of a ns-DBD plasma actuator was to amplify Tollmien–Schlichting waves according to linear stability theory. Such results led to understand the influence of the actuator position on the achievement of a specific flow control task. A ns-DBD is capable of producing several effects: a shock wave, a small body force and a thermal gradient within the discharge volume. Thus, three were the possible causes of flow actuation. The shock wave was found to be too weak to be capable of introducing an appreciable disturbance. As the shock wave, also the momentum injection induced by the body force produced by the pulsed discharge was found to be relatively too small to justify a control authority based on momentum redistribution within the boundary layer, for cases of relatively high freestream velocity. Thus, the thermal gradient induced within the discharge volume by the energy deposition of a high voltage nanosecond discharge is the effect capable of inducing a relatively large disturbance into the field of motion. Nevertheless, a thermal gradient within a gaseous flow induces two effects, it reduces density and increases viscosity. At the moment it is still unclear which of these two effects is more relevant. Once identified the thermal gradient as the main cause of flow control mechanism, a characterization study was performed aimed to identify the properties of a ns-DBD plasma actuator (thermal, electrical and geometrical) important tomaximize the induced thermal gradient within the discharge volume. In general, a higher efficiency is achieved by a strong dielectric material concerning thermal energy deposition. A barrier of a ns-DBD plasma actuator should be as thin as possible. However, the thickness affects also the lifetime of the barrier itself. Nanosecond pulsed DBD plasma actuators have shown to have the capability to delay leading edge separation. However, in the relevant literature, an influence of the actuation frequency on the achieved results is always reported. In order to investigate this frequency effect, a parametric study on a Backward Facing Step was performed. This geometry was selected because it mimics a fixed point laminar separation, the flow sceixnario of interest. Such flow scenario is unstable at high frequencies close to the step and low frequencies downstream the step and it naturally develops a most unstable mode within it. However, when a flow is actuated, its stability changes, so do the most unstable frequencies naturally developed within it. Results showed that the effect of actuation is the redistribution of energy among modes and that the optimal frequency of actuation must be based on the new stability achieved by the flow due to the actuation itself. Moreover, results indicated that the optimal frequency of actuation is not related to the most unstable frequencies naturally present within the base non-actuated flow. A method to quantify the efficiency of ns-DBDs in depositing energy within the discharge volume is proposed. This energy is the one that eventually contributes to the formation of the thermal gradient responsible of the flow control capabilities shown by these devices. Such method is based on simultaneous implementation of infrared thermography and back-current shunt techniques. Results showed that the overall efficiency of a ns-DBD plasma actuator is inversely proportional to the thickness of the dielectric barrier. Last part of this thesis is concerned with a demonstrative application of a ns-DBD plasma actuator on a two element airfoil, at Reynolds numbers ranging between 0.2·106 and 2 ·106. Results demonstrated its capability to delay separation, increase lift and reduce drag in the post stall regime. Moreover, the plasma actuator showed the capability to eliminate both a laminar bubble separation for small angles of attack and the hysteresis behaviour of the selected airfoil. In conclusion, this work shed some light on the flow actuation mechanism of a ns- DBD plasma actuator and deepened its basic knowledge.