Etude et développement d'un actionneur plasma à décharge à ...

Annexes
E. Control Process Scenario
The observation of the lift enhancement when the flow over an airfoil is forced by an plasma actuation is not
new. However, most of the studies are based on force measurements, proposing a global estimation of the
effectiveness of plasma actuators. No study has investigated flow reattachment process and only predictive
comments are usually introduced in the published researches. In the present study the repetition rate of the PIV
system is high enough to measure time-resolved PIV vectors fields and thus the control process can be
investigated. All the acquisitions (quasi-steady and unsteady cases) were carefully observed and it appears that a
repeatable process seems to be responsible of the partial flow reattachment reported in the present study. To
illustrate the temporal evolution of the reattachment, the instantaneous vectors of velocity are plotted in figure 20
for time varying from the beginning of the actuation up to 30 ms. Through the analysis of different flow
sequence, it appears that the local tangential electric wind induced by the BDD actuator is too low to create a
discrete vortex above the leading edge. The instantaneous velocity fields reveal that the actuation can promote
the coalescence of two successive discrete large scale structures as illustrated in figure 20 at t=0 and 3.3 ms
(Vort-1 and 2 in figure 20). The merging of vortices (Vort-1 and Vort-2) produces an energized large flow
structure (Vort-3 in figure 20). The size and the strength of this vortex is enhanced by the external airflow and
results in a momentum transfer from the outer flow to the boundary layer of the upside surface of the airfoil. This
vortex should modify the pressure gradient along the suction side and it results in a flow topology presenting a
front (velocity with a primary velocity component largely lower than the second one), perpendicular to the airfoil
chord and moving toward the trailing edge (see t=6.66, 10 and 13.3 ms in figure 20). The separation point at the
leading edge is not modified, the shear layer is still separated from the suction side and the vortex shedding at the
leading edge is still present. But, the vortex issuing from the separation point (vort-4 in figure 20) is not
convected along the shear layer axis due to the front of velocity which leads to a local pressure decrease and acts
like a virtual obstacle. The trajectory of this second vortex is moved along the suction side with a sense of
rotation promoting a momentum transfer to the boundary layer region. When the vortex “vort-3” is in the wake
of the airfoil, the mutual interaction between vort-3 and vort-4 disappears (time t=23.33 ms in figure 20). At this
time the flow is reattached over 70% of the chord length and remains attached despite the natural instabilities
occurring in the boundary layer and resulting in a vortex shedding at the trailing edge.
Finally, it appears that the reattachment process results from interacting vortices energized by the electric
wind because the actuation by DBD is not strong enough to produce vortices or streamwise vorticity by itself,
contrary to synthetic jets for instance. The plasma discharge acts as a catalyser for the flow reattachment and
requires to interact with natural flow structures. This point is essential and may explain the reason of the
improvement of the control authority when an unsteady actuation is applied as reported by other. But in this
case, the unsteady actuation requires to be synchronized with the natural vortex shedding to be operational in the
first milliseconds following the actuation. When the actuator is driven at a low frequency, the statistical
probability to act at the right time is decreased and could explain the lack of authority of the actuation at F + =0.5.
Moreover, the duty-cycle requires to be high enough to promote the vortex merging necessary to initiate the flow
reattachment. One can notice that one can not be sure that a similar process would be observed if the natural flow
was fully turbulent. However, it has been verified that the flow remains fully detached when the laminar-toturbulent
transition is obtained by a tripper placed at the leading edge. This demonstrates that the flow
reattachment is not due to laminar-to-turbulent transition.
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