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

Annexes
field for baseline flow confirms that, at an angle of attack of 16 degrees, the observed flow is in post-stall
regime with a large region naturally separated from the suction side resulting in a strong adverse pressure. The
results also demonstrate that the boundary layer separation occurs approximately at 5% of the chord length.
The location of the separation point agrees well with the literature data [31-32]. As usually observed at this
angle of attack, the flow separation results in the formation of a single separation bubble, delimited by a free
shear layer. This free shear layer presents high turbulent intensity levels (between 20 and 45% of turbulent
intensity), demonstrating a strong vortex dynamics in this region.
Figure 5 Baseline flow, (a) time-averaged non dimensional velocity norm and (b) turbulent intensity
An eddy localization algorithm is used to highlight the vortex dynamic occurring in the free shear layer.
This vortex detection is based on the flow topology and allows to identify the core of the large scale structures
of the flow as described by Graftieaux et al. [33]. This criterion is expressed in non dimensional form (bound
by unity) and negative values of the criterion correspond to counter clockwise vortices while positive values are
associated with clockwise vortices. This criterion is here applied to three PIV fields at successive times
separated by a delay of 3.3 milliseconds. The results, shown in figure 6, confirm the presence of a vortex
shedding starting from the leading edge of the airfoil and growing along the free shear layer axis. This figure
also demonstrates that the repetition rate of the high-speed PIV system (3 kHz here) is sufficient to obtain
time-resolved PIV fields as large scale flow structures can be detected and are followed along theirs time
history.
Figure 6 Eddy localization criterion applied to the baseline flow at (a) t0, (b) t0+3.3 ms and (c) t0+6.6 ms
To characterize the spatio-temporal dynamics of the flow over a long acquisition period (250 milliseconds
corresponding to 757 PIV fields), the evolution of the second velocity component (vy) extracted along
different y/C positions is plotted in figure 7. This figure presents the velocity contours as a function of x
position versus time for the baseline flow. At y/C=0.13 and 0.075, corresponding to the position of the free
shear layer as illustrated by the time-averaged flow fields, the flow presents a high dynamics with an
organized alternation of low and high velocity fluid packets. This demonstrates the existence of a vortex street in
the free shear layer. Similar data processing performed in the region located down to the free shear layer (i.e.
y/C=-0.025) highlights that the flow is disturbed but no large region of negative vy velocity component is
noticeable. This result confirms that the flow remains fully detached from the airfoil surface.
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