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Cette thèse CIFRE est le fruit d’une collaboration entre Renault Trucks et le LMFA dans la perspective d’une évaluation de la pertinence du contrôle actif pour la réduction de traînée de véhicule industriel. Les deux principaux objectifs de ces travaux expérimentaux consistent à l’analyse de l’influence de l’écoulement de soubassement sur la dynamique du sillage et l’étude d’une stratégie de contrôle d’écoulement combinant des volets déflecteurs (positionnés sur les bords supérieurs et latéraux du culot) à des actionneurs de type jets pulsés dans l’optique d’une réduction de la traînée aérodynamique d’un corps non profilé à culot droit. Le développement du sillage pour différentes vitesses de soubassement évoluant de 10% à plus de 80% de la vitesse de l’écoulement infini amont est étudié sur une maquette simplifiée de véhicule poids lourd à l’échelle 1/43e. Des mesures de pression au culot permettent d’identifier quatre classes d’écoulement associées à des structures de sillage différentes mises en évidence par des mesures PIV 2D-3C. Le sillage de la première classe d’écoulement obtenu pour les très faibles vitesses de soubassement ressemble au sillage derrière une marche 3D. Pour des vitesses de soubassement plus élevées, l’écoulement de soubassement décolle au niveau du sol et impacte soit le culot ou soit la couche de cisaillement supérieure favorisant le développement des instabilités de type Kelvin-Helmoltz dans ce dernier cas. Enfin, la dernière classe est caractérisée par un sillage comparable à celui d’un corps d’Ahmed. L’ajout de volets déflecteurs à l’arrière du culot engendre une augmentation de la pression au culot pour l’ensemble des classes d’écoulement. Cette augmentation réside principalement dans l’effet de vectorisation de l’écoulement. Un système de contrôle actif est intégré sur une maquette 1/8e géométriquement identique à celle à l’échelle 1/43e et équipée de volets déflecteurs. Deux angles de volet supérieur sont testés afin d’obtenir en moyenne (i) un écoulement naturel attaché à la paroi du volet et (ii) un écoulement naturel détaché du volet. Par rapport au cas sans contrôle actif, des gains sur la traînée sont obtenus pour une certaine gamme de fréquence d’actionnement uniquement dans le cas (ii). Ces gains sont obtenus suite au recollement de l’écoulement sur le volet. Enfin, la robustesse des gains sur la pression au culot est testée avec succès en mettant la maquette en dérapage pour simuler un vent de travers. / This PhD thesis was realized in the scope of a collaboration with Renault Trucks and the LMFA in view of an evaluation of the relevance of active flow control for the drag reduction industrial vehicle. The two main objectives of this experimental work are to analyze the impact of the underbody flow on the wake dynamics and to study a flow control strategy combining inclined flaps (located on the upper and lateral edges of the rear base) with pulsed jet actuators for reducing the aerodynamic drag of a square-back bluff body. The wake development for several underbody velocities ranging from 10% to more than 80% of the free-stream velocity is studied on a simplified truck model at scale 1 :43. Rear base pressure measurements lead to the identification of four flow classes associated with different wake structures highlighted by 2D-3C PIV measurements. The wake of the first flow class obtained for very low underbody velocities looks like that of the wake of a 3D backward facing step. For higher underbody velocities, the underbody flow is separated from the ground impaging either the rear base or the upper shear layer triggering Kelvin-Helmoltz instabilities for this last case. Finally, the fourth class is characterized by a wake comparable to that of the Ahmed body. The implementation of inclined flaps at the rear base increases the base pressure for all classes. This increase is mainly due to the vectoring effect of the flow. An active control system is integrated to a 1 :8 scale model geometrically identical to that of the 1 :43 scale model with flaps. Two upper flap angles are tested to have (i) a natural flow attached to the flap and (ii) a natural flow detached from the flap. In comparison to the case without active flow control, drag reductions are obtained only for a specific range of actuation frequencies only in case (ii). These gains are associated with the reattachment of the flow on the flap. Finally, the robustness of the pressure gains is successfully tested in crosswind conditions.

A pulsed-plasma jet (called a "spark-jet" by other researchers), is a high-speed synthetic jet that is generated by striking an electrical discharge in a small cavity. The gas in the cavity pressurizes owing to the heating and is allowed to escape through a small orifice. A series of experiments were conducted to determine the characteristics of the pulsed-plasma jet issuing into stagnant air at a pressure of 45 Torr. These results show that typical jet exit velocities of about 250 m/s can be induced with discharge energies of about 30 mJ per jet. Furthermore, the maximum pulsing frequency was found to be about 5 kHz, because above this frequency the jet begins to misfire. The misfiring appears to be due to the finite time it takes for the cavity to be recharged with ambient air between discharge pulses. The velocity at the exit of the jet is found to be primarily dependent on the discharge current and independent of other discharge parameters such as cavity volume and orifice diameter. Temperature measurements are made using optical emission spectroscopy and reveal the presence of considerable non-equilibrium between rotational and vibrational modes. The gas heating efficiency was found to be 10% and this parameter is shown to have a direct effect on the plasma jet velocity. These results indicate that the pulsed-plasma jet creates a sufficiently strong flow perturbation that is holds great promise as a supersonic flow actuator. An experimental study is conducted to characterize the performance of a pulsed-plasma jet for potential use in supersonic flow control applications. To obtain an estimate of the relative strength of the pulsed-plasma jet, the jet is injected normally into a Mach 3 cross-flow and the penetration distance is measured by using schlieren imaging. These measurements show that the jet penetrates 1.5 [delta], where [delta] is the boundary layer thickness, into the cross-flow and the jet-to-crossflow momentum flux ratio is estimated to be 0.6. An array of pulsed-plasma jets was issued from different locations upstream of a 30-degree compression ramp in a Mach 3 flow. Furthermore, two different jet configurations were used: normal injection and pitched and skewed injection. The pitched and skewed configuration was used to see if the jets could act as high-bandwidth pulsed vortex generators. The interaction between the jets and the separation shock was studied using phase-locked schlieren imaging. Results show that the plasma jets cause a significant disturbance to the separation shock and clearly influence its unsteadiness. While all plasma jet configurations tested caused an upstream motion of the separation shock, pitched and skewed plasma jets caused an initial downstream shock motion before the upstream motion, demonstrating the potential use of these plasma jets as vortex generator jets. The effect of the plasma jet array on the separation shock unsteadiness is studied in a time-resolved manner by using 10 kHz schlieren imaging and fast-response wall pressure measurements. An array of three pulsed-plasma jets, in a pitched and skewed configuration, is used to force the unsteady motion of the interaction formed by a 24° compression ramp in a Mach 3 flow. The Reynolds number of the incoming boundary layer is Re[theta]=3300. Results show that when the pulsed jet array is placed upstream of the interaction, the jets cause the separation shock to move in a quasi-periodic manner, i.e., nearly in sync with the pulsing cycle. As the jet fluid convects across the separation shock, the shock responds by moving upstream, which is primarily due to the presence of hot gas and hence the lower effective Mach number of the incoming flow. Once the hot gases pass through the interaction, the separation shock recovers by moving downstream, and this recovery velocity is approximately 1% to 3% of the free stream velocity. With forcing, the low-frequency energy content of the pressure fluctuations at a given location under the intermittent region decreases significantly. This is believed to be a result of an increase in the mean scale of the interaction under forced conditions. Pulsed-jet injection are also employed within the separation bubble, but negligible changes to the separation shock motion were observed. These results indicate that influencing the dynamics of this compression ramp interaction is much more effective by placing the actuator in the upstream boundary layer. / text