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

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Annexes the vortex street is unstable at 25%. Thus, a duty-cycle of 25% should result in pressure fluctuations at the trailing edge. This may produce non-stationary lift and drag performances. However, previous publications have demonstrated that the most efficient actuation consists of using an unsteady actuation with a low duty-cycle value [10, 30]. The discrepancy with the present study may result from the location of the control device compared to the position of the natural separation point or from small differences in the local airflow induced by the plasma discharge. Figure 18 Unsteady actuation at F + =1 and duty-cycle of 25%, (a) instantaneous vectors, (b) streamlines and (c) vorticity Figure 19 Unsteady actuation at F + =1 and duty-cycle of 50%, (a) instantaneous vectors, (b) streamlines and (c) vorticity - 192 -
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. - 193 -
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THESE Pour l’obtention du Grade d
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Remerciements L’homme est un éte
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Remerciements Il va s’en dire que
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Table des matières La théorie, c
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Table des matières REMERCIEMENTS I
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Table des matières A.10. ARTICLE A
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Introduction Serons-nous capables d
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Introduction Le contrôle d’écou
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Introduction également des notions
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- L’actionneur plasma - - 7 -
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1. Revue bibliographique sur les d
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1.3. Le vent électrique 1. Revue b
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f j 1. Revue bibliographique sur le
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. Propriétés électriques 1. Revu
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Y (mm) 16 14 12 10 8 6 4 2 0 1. Rev
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(a) (b) 2. Optimisation du vent ind
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(a) (b) 2. Optimisation du vent ind
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3. Développement d’un actionneur
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3.2. Étude de la forme d’onde du
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4. Revue bibliographique sur le con
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4.1.2. Transition Laminaire - Turbu
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. Décollement inertiel 4. Revue bi
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4.3.1. Écoulement de plaque plane
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5. Contrôle du point de séparatio
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corps principal en bois 5. Contrôl
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Page 167 and 168: Annexes La science est peut être l
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Page 197 and 198: Annexes Figure 1 Sketch of the NACA
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Page 203 and 204: Annexes however the actuation is no
Page 205: Annexes flow is reattached at x/C
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Page 211 and 212: Bibliographie Une vérité cesse d
Page 213 and 214: Bibliographie [1] Hefner JN, 1988,
Page 215 and 216: Bibliographie [40] Pons J, Moreau E
Page 217 and 218: Bibliographie [73] Moreau E, Léger
Page 219 and 220: Bibliographie [108] Post ML et Cork
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