Flow Physics of Nanosecond Dielectric Barrier Discharge Plasma Actuators

Abstract

This work examines the physics and scaling of nanosecond dielectric barrier discharge plasma actuators (ns-DBD) for active flow control (AFC). These actuators have been successfully employed in a number of high speed flow control applications in recent years, but many open questions remain. The ns-DBD flow control mechanism is thermal in nature which is fundamentally different than momentum-based devices (e.g. synthetic jets and ac-DBD plasma actuators). This research aims to shed light on ns-DBDs for AFC by studying the actuator in a canonical low-speed turbulent mixing layer. It is hypothesized that a key amplitude scaling parameter for ns-DBD plasma actuation is the initial incoming boundary or shear layer thickness (in contrast to the boundary layer edge velocity used for momentum-based devices). In this work, the shear layer thickness near the actuator is relatively large compared to other ns-DBD works in literature and it is shown that burst mode forcing, which increases the rate of energy deposition, is required to control the flow. These burst mode studies are leveraged to enable further understanding of AFC with ns-DBDs. The required fundamental forcing frequency at which burst mode forcing is carried out is well known from linear stability theory, but it is shown that the timescale of energy deposition (inverse of carrier frequency) should be shorter than the local flow convective time scale to accelerate mixing layer growth. However, increasing the level of energy deposition does not monotonically increase the mixing layer growth. Above a threshold pulse energy, the mixing layer growth relaxes back to the baseline level due to a stabilizing effect which is explained here by linear stability analysis (LSA) of the mixing layer flow subject to mean heating. Under high levels of mean heating the amplification rate of disturbances is shown to be reduced. The mechanism by which the ns-DBD actuator excites the mixing layer is related to the creation of a wake-like effect in the local velocity and/or density profile. This time varying disturbance introduces cross-stream perturbations which are amplified downstream leading to mixing layer growth. This behavior is directly contrasted with an ac-DBD plasma actuator, which is a momentum-based device, to further clarify the ns-DBD mechanisms. The mean flow response to actuation is matched for the two actuators at a single downstream location to gauge scaling of relevant parameters (carrier frequency, energy per pulse, etc). The similarity between local behavior is found to extend to the global flow, both in the mean and fluctuating components. However, the ns-DBD requires approximately six times more electrical energy to achieve the same control as the ac-DBD in this specific case. Hence, this work demonstrates that ns-DBD plasma actuation has greater efficacy in flows with a thin boundary/shear layer near the location of actuation.