A study of unstart and its delay using dielectric barrier discharges [electronic resource]
Keywords: Mechanical Engineering

In this study, unstart induced by mass injection was visualized by the planar laser Rayleigh scattering (PLRS) imaging technique. The dynamics of unstart induced by this sonic jet was investigated under various boundary layer conditions, jet pressures and the presence of incident shock trains. The PLRS visualization of unstart process and the pressure trace found that unstart originates from downstream of the jet and propagates to upstream followed by the propagation of boundary layer growth/separation and an unstart shock. The boundary layer condition prior to jet injection in the model inlet flow plays a significant role on the unstart process. In asymmetric inlet configurations, with a thick turbulent boundary layer on one wall and a thin (initially laminar) boundary layer on the other, an oblique unstart shock emerges, but only on the wall with the initially thicker boundary layer, independent from the boundary layer condition through which the jet is injected. In symmetric wall configurations, there is no oblique unstart shock. Instead, we can see the coalescence of relatively weak compression waves or a pseudo-shock which initially propagates upstream in advance of unstart and remains quasi-stable for a while, until a catastrophic breakdown in the structure occurs and the inlet flow unstarts completely. In relatively thin (initially laminar) boundary layer case, the pseudo-shock appears stable until 55 ms following jet injection—more than twice as long as the case in which the initial boundary layers are tripped (25 ms). Wall pressure measurements indicate that the speed of propagation of this pressure rise along the boundary layer increases when jet momentum ratio increases. While jet injection at any value of R, the jet momentum ratio, disrupts the initial flow structure and initiates unstart, it is found that there is a threshold value for R (~ 4.3) for the complete unstart of the inlet during in the facility flow time (~ 3 seconds), which corresponds to 0.01 kg/s mass injection into 0.03 kg/s mass flow through the channel (half of the tunnel). A dielectric barrier discharge (DBD) actuator is studied as a possible means of manipulating unstart process in a model inlet flow since particle image velocimetry (PIV) measurement, pitot probe velocity profile measurement and PLRS flow visualization reveals that the DBD actuator is able to thin subsonic/supersonic boundary layers. Flow unstart, initiated by mass injection, is studied for three model inlet flow configurations, distinguished by the initial conditions (untripped or tripped, plasma actuated or not) of the boundary layers. Unstart in the presence of thick, tripped boundary layers is characterized by the formation of an oblique unstart shock just upstream of a separation and propagating boundary layer. The presence of plasma actuation of this tripped boundary layer seems to arrest the boundary layer separation and leads to the formation of a quasi-stationary pseudo-shock which delays unstart. The flow generated with DBD actuation is characteristic of what is seen when unstart is generated in a model flow in which thin boundary layers grow naturally. PLRS visualizations suggest that the DBD actuation thins the tripped boundary layer over the exposed electrode region.