The influence of spanwise-uniform (linear) forcing applied by a dielectric barrier discharge (DBD) plasma actuator on the growth of a plane mixing layer and the dynamics of large-scale spanwise vortices within, are investigated experimentally. Quantitative spatio-temporal measurement of the flow field is acquired using high-speed planar particle image velocimetry. The DBD actuator is used to impart perturbations into the mixing layer to force the fundamental Kelvin-Helmholtz instability and its first sub-harmonic. Forcing the fundamental instability resulted in the inhibition of vortex pairing due to the attenuation of sub-harmonic instabilities. Correspondingly, the growth of the mixing layer is halted initially. With sub-harmonic instability forcing, two vortices interact with each other and merge together. This results in a higher growth rate compared to the unforced mixing layer at the streamwise location of this vortex interaction. Eventually, the growth rate of the forced mixing layer becomes similar to that of the unforced mixing layer. These results demonstrate the influence of the applied forcing on the growth of the turbulent mixing layer and the dynamics of the coherent vortical structures within.

The influence of spanwise-uniform electro-fluid-dynamic forcing applied by a dielectric barrier discharge (DBD) plasma actuator on the growth of a plane mixing layer and the dynamics of large-scale spanwise vortices are investigated experimentally. A two-dimensional mixing layer formed between two streams of air with different velocities is employed for this study. Quantitative spatio-temporal measurement of the flow field is acquired using high-speed planar particle image velocimetry. The DBD actuator was constructed such that it imparts perturbations into the splitter-plate boundary layer formed by the high-velocity fluid stream, close to the trailing edge. Through this, the fundamental Kelvin-Helmholtz instability of the current mixing layer and its first sub-harmonic are forced. Forcing the fundamental instability results in the inhibition of vortex pairing due to the attenuation of sub-harmonic instabilities, and thus, mixing layer growth is halted in the vicinity of the trailing edge. With sub-harmonic instability forcing, neighboring vortices interact with each other and amalgamate together through mutual induction. This results in a higher growth rate compared to the unforced mixing layer at the streamwise location of this vortex interaction. Eventually, the growth rate of the forced mixing layers becomes similar to that of the unforced case. These results demonstrate the influence of the applied forcing on the spectral signature, growth, and stability characteristics of the plane mixing layer and the dynamics of the coherent vortical structures.

Active flow control is demanding for new actuation technologies as none of the actual available actuators has reached all the criterions for expecting an implementation in the coming years. Here, a new type of pulsed jet is designed and preliminary measurements of its performances in quiescent flow are conducted. Pulsed operation has been chosen because of the expected high efficiency of pulsed actuation in comparison to continuous blowing. The traditional pulsed jets being limited in term of frequency because of the use of a mechanical valve to achieve the desired pneumatic opening and closing of a jet provided by an external pressure source, the fast response of electrical discharge is exploited in the present investigation. The objective is to modulate the output of a small jet exhausting from a pressurized chamber. A spark discharge is used to affect the thermodynamic state of the gas in order to electrically achieve periodic cancellation of the chocked flow conditions at the throat upstream the jet exit. In the present study, such actuator with additional neck extension and jet diameter enlargement is investigated. The configuration results in a high-speed subsonic jet whose velocity amplitude is modified by an arc discharge with deposited energy from 18 to 780 mJ. Some characteristics of the jet are provided using optical methods such as high-speed Schlieren and PIV. In particular, it is shown that the jet flow velocity can be increased from 50 m.s-1 to 190 m.s-1.

The influence of linear (spanwise-uniform) forcing applied by a DBD plasma actuator on the growth of a turbulent mixing layer and the dynamics of large-scale spanwise vortices, are investigated experimentally. Furthermore, the freestream turbulence intensity in the low-velocity stream is maintained at a high level to examine its impact on the control-authority of the applied forcing. The influence of the applied forcing on the dynamics and interactions of the spanwise vortices in the mixing layer is apparent. However, the growth rate of the mixing layer is not affected, suggesting that the high level of freestream turbulence diminishes the control-authority of the applied forcing.