Investigation of a pulsed-plasma jet for shock / boundary layer control

Abstract

Abstract Pulsed jets with peak exit velocities as high as 250 m/s are generated by rapidly heating the air inside a chamber with an electrical discharge. The heated pressurized gas issues from a small orifice to form the pulsed plasma jet or 'spark jet'. Pulsing frequencies as high as 5 kHz are obtained. An array of these 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 =3300. The effect of the plasma jet array on the separation shock motion is studied by using 10 kHz Schlieren imaging and fast-response wall pressure measurements. 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 was 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