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18 International Journal of Plasma Environmental Science and Technology Vol.2, No.1, MARCH 2008 frequency for reattachment (that is to say the unsteady frequency for a minimum voltage applied) is, as it is usually expected for unsteady disturbances, close to the ratio of the free air stream velocity by the chord of the airfoil (a frequency corresponding to a Strouhal number equal to unity). Experiments using the trailing edge actuator with a 21 m/s free air stream velocity showed a ratio lift over drag enhanced by a factor of 2.8 to 2 for steady actuation in the range of 15° to 20° angle of attack and 2.7 to 2 for unsteady actuation in the range 15° to 18° angle of attack. 5) PulseDBDactuators Recently experiments have been performed on a NACA 0015 profile by the group of Starikovskii [48] using a set of DBD actuators (3 successive) supplied with a pulse generator. The chord length of the profile was 9 cm. The thickness of the dielectric (fluoroplastic) was 0.5mm and the frequency used was 6 kHz (figure 66). It seems that they succeeded to reattach for a 21° angle of attack and 75m/s airflow velocity. The outer electrodes were made of 0.04 mm thick copper foil tape and were 6.4 mm wide; they were stuck on the outer part of the cylinder. The inner electrodes were made of 0.04 mm thick lead foil tape and were 25.4 mm wide; they were stuck on the inner part of the cylinder. All the electrodes were around 50 cm long. Seven layer of 0.13 mm thick Kapton foil tape covered the inner electrodes. A small overlap was made between inner and outer electrodes. The outer electrodes were connected to a high voltage AC source providing 8.1 kV rms of sinusoidal form with a frequency of 10 kHz. The inner electrodes were grounded. Two kinds of actuations were done, steady and unsteady, in a similar way that those presented in § IV.D.4. Smoke flow visualization experiments were made for ReD=33000 (free air stream velocity around 5 m/s). For such velocity the effect of the actuators is clearly noticeable, the separated flow region is substantially reduced and the associated vortex shedding appeared to be virtually eliminated. With unsteady actuation it was found that the optimum unsteady frequency was 48 Hz. A duty cycle of only 25% still gave an important reduction of the near wake behind the cylinder. 7) DBDactuatorsperformingoscillatingflows It has been shown [50] that oscillating flows in the spanwise direction (perpendicular direction of the mean air stream) on an air plane wing could be an effective technique in turbulent boundary layer control. The two following actuators were performed in order to reach this goal. a)Three AC voltages DBD actuators with same amplitude but different frequencies Two arrangements have been tested by Wilkinson [51], they are made of one or two (figure 68 and figure 69) sets of DBD actuators. These arrangements were performed in order to produce oscillating flow for turbulent drag reduction. Each set was composed of one upper electrode made of 0.1 mm thick aluminum tape stuck on the upper side of a 0.75 mm thick epoxy-woven glass printed circuit board and two electrodes made of the same aluminum tape and stuck on the lower side of the printed circuit board. The lower electrodes were encapsulated between the printed circuit and a 1.2 mm thick float glass glued to the printed circuit with epoxy adhesive. The two lower electrodes were separated by a distance equal to the width of the upper electrode just located between the two lower ones. A reasonable distance separates the two sets. A.C. H.V. 1 A.C. H.V. 2 pulse H.V. Fig. 66. Three pulse DBD actuators on a NACA 0015 airfoil. 6) DBDactuatorsonacylinder Experiments have been made by Thomas and al on a quartz glass cylinder 100 mm in outer diameter and around 60 cm long [49]. The wall thickness of the cylinder was 2.5 mm. Two symmetric pairs of actuators were used, one pair was located on radii doing 90° with the free air stream direction; the other one was located on radii doing 135° with the free air stream direction (figure 67). 135° 90° Kapton foils glass cylinder A.C. H.V. Fig. 67. Side view of 2 DBD actuators on a cylinder. A.C. H.V. 3 Fig. 68. General view of the setup with two sets of DBD actuators.PDF Image | Plasma actuators for aeronautics applications
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