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

Annexes
actuator is usually made of two thin electrodes asymmetrically flush mounted on a dielectric wall. When a highvoltage
is supplied to one of the electrode (the second being grounded), a plasma discharge is produced on the
dielectric surface. The plasma extension depends on the characteristic of the input electric signal [15]. In 1998,
Roth has demonstrated that a local flow can be produced in the plasma region, tangentially to the dielectric wall
surface [16]. Different authors have investigated the mechanism of the local airflow formation and it appears that
charged particles (ions and electrons) are produced in the inter electrode space and that the momentum transfer
from the charged particles to the neutral atmospheric air components produces a local flow of a few m.s -1 ,
usually called ‘ionic wind’ or ‘electric wind’ [17-21]. Further investigations have been performed in order to
optimize the electrode geometry, resulting in the improvement of the ‘electric wind’ produced by a single DBD
actuator and a local flow of approximately 7 m/s can be created [22-23]. Preliminary studies have demonstrated
the capability of a DBD actuator to be operated at low pressure or high humidity conditions corresponding to
real atmospheric flight conditions [24-26]. Each new publication related to DBD actuator tends to demonstrate
that this device can compete with others active control devices, such as blowing jets, synthetic jets or
piezoelectric devices.
Dielectric barrier discharge plasma actuators are used to impart momentum into moderate Reynolds number
flows and the achieved flow control take advantages of the wide operating frequency range of DBD devices.
Indeed, a previous study has reported that the produced airflow can impart momentum at the electric input
frequency [23, 27]. This property has been extensively used to promote partial or full airflow reattachment along
airfoils resulting in significant lift increase. For instance, Corke et al. [7] have demonstrated that a single DBD
operating at a reduced frequency equal to unity induces an improvement of NACA 0015 performances compared
to other reduced frequency values. The effects of the reduced frequency on the improvement of airfoils
performance were previously reported and summarized by other authors [28-29], but Corke et al. [7, 12] also
demonstrate by force balance measurements that a low duty-cycle value (typically lower than 10%) is sufficient
to initiate and sustain an enhancement of the lift. The significant effects of low duty-cycle values were recently
confirmed by Goksel et al. [10] or Greenblatt et al. [30], for instance. The improvement of the effectiveness of
the actuator when operating at reduced frequency equal to unity and low-duty-cycles is supposed to derive from
the excitation of the natural instabilities of the free shear layer located above the airfoil in post-stall regimes.
Although several studies demonstrated that flow reattachment can be forced by imparting low momentum close
to the separation point, the mechanisms responsible for the reattachment are still unclear. The manipulation of
the shedding of coherent structures occurring at the leading edge is often mentioned as the primary flow control
process. However, the interactions between the ionized airflow produced by the single DBD actuator and the
overall flow structures remains to be experimentally investigated and characterized.
In the present study the flow over a NACA 0015 in post-stall regime (α=16 degrees) is experimentally
investigated. A single DBD actuator is mounted at the leading edge of the airfoil and the profile is placed in an
open wind tunnel producing a free-stream velocity of 20 m/s. A time-resolved particle image velocimetry
(TRPIV, 10 kHz) system is used to observe and measure the baseline flow and also the forced flow reattachment
by steady actuation. The consequences of the airflow controlled by a non-thermal actuator are also introduced for
unsteady actuations performed at different reduced frequencies and duty-cycle values. The analysis proposed in
the present paper concerns the time-averaged velocity fields and non-stationary data such as the temporal
evolution along extraction lines, the instantaneous vorticity and cross correlation function dedicated to the
characterization of the natural or controlled vortex shedding. The last part of this study deals with the
reattachment process observed for steady and unsteady actuations. It is expected that the results of this study will
give a better insight in the control processes following the actuation produced by a single DBD actuator.
2. Experimental Setup
The NACA 0015 model has a chord length of 200 mm and a spanwise length of 296 mm. The model is
realized in two separated parts. The first, made of polyurethane, is the scaffold of the model (see figure 1). The
second is removable (made of PMMA) and constitutes 80% of the suction side and 20% of the lower camber
including the leading edge of the airfoil. As shown in figure 1, the single DBD consists in two thin aluminium
foils (0.1-mm-thick) flush mounted of each side of the 3-mm-thick removable part, acting as a dielectric. The
electrode stuck upside the dielectric material (active electrode) is connected to the high-voltage supply whereas
the second electrode located under the PMMA is grounded. The actuator covers 54 % of the span of the airfoil
and the edge of the active electrode is located at x/c= 0, upstream the expected separation point (Figure 1). The
active electrode is 20-mm-wide while the grounded electrode is 15-mm-wide. There is a small gap of 5 mm
between the active electrode and the grounded one (Figure 2). This asymmetric configuration is chosen
according to the optimization study recently established in our lab which defined this set-up as the most effective
in terms of production of electrohydrodynamic forces [23]. The electric wind produced by the actuator results in
a co-flow mode actuation.
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