Professor WANG Lianze and his group from the School of Aerospace, Tsinghua University, set out to investigate this problem. They developed novel plasma actuators using various winding-shaped electrodes to create plasma actuators which induce three-dimensional variations in the shear layer, offering significant flexibility in flow control. Their work, entitled "Study of Flow Induced by Sine Wave and Saw Tooth Plasma Actuators", was published in SCIENCE CHINA Physics, Mechanics & Astronomy.2011, Vol 54(11).
This is the 3-D view of the field induced by the saw tooth actuator (C1) at various z for Urms=10 Kv and frequency=8 KHz. The bold lines indicate the actuator location. Credit: Copyright Science China Press
Studies on flow control devices have focused primarily on adding momentum to the boundary layer or using trips to initiate a transition to leading edge devices. Thus, in general, manipulation of the boundary layer has been the main objective for flow control. To achieve this, certain devices, such as vortex generator jets or zero-net mass flux (ZNMF) synthetic jets and plasma actuators in various configurations have been used for flow control or thrust generation.
Plasma actuators, also known as dielectric barrier discharge (DBD) or devices, have been used as active flow control devices and received increasing interest in the past decade. Typically, a widely used type of plasma actuator [single DBD (SDBD)] refers to an asymmetric arrangement of two horizontally offset electrodes separated by a dielectric material. Under a high enough voltage input and a high-frequency AC or a pulsed DC signal, a region of weakly ionized air (plasma) is created over the covered electrode (typically less than one part per million of weakly ionized gas). The plasma appears blue and can be viewed easily with the naked eye in a darkened environment. In the presence of an electric field produced by high voltages between the electrodes, the electric field exceeds (the value needed to sustain electron–ion pairs in gas in the absence of space charge fields). The air is ionized, resulting in a body-force vector field that acts on the ambient (non-ionized and neutrally charged) air and, thus, inducing a ZNMF jet. This is the mechanism for active aerodynamic control. For conventional linear plasma actuator, the mechanism of the flow control is through a generated body-force vector field that couples with the momentum in the external flow. Previous researchers have made possible the exertion of significant electrohydrodymanic body forces in the boundary layer above the electrodes on aerodynamic surfaces, and the plasma actuators have been shown to control flow separation by adding near-wall flow momentum in low Re flow environments.
However, practical flow control applications demand robust actuators that have sufficient capability for operation in the Re and Mach number regimes, and thus, improvement of their control effect is highly sought after. Previous studies were conducted in the field of plasma actuator improvement to examine various aspects, such as the effects of dielectric material and thickness, voltage amplitude and frequency, voltage waveform, exposed electrode geometry, and the number of actuators in series.
Some studies have shown that the control effect of a plasma actuator is relatively strong in low Re and not very effective in high Re number regimes for various reasons, including inefficient energy transformation due to heat emission from the insulation board. However, some important aspects may have been neglected. The key point of these studies lies in the improvement of the tangential velocity or electric energy of the actuator. Thus, focusing on another aspect aimed at making appropriate changes to the actuator that extend the traditional two-dimensional (2-D) flow field to a three-dimensional (3-D) flow field, such as the introduction of streamwise vortex flow (traditional actuator can induce spanwise vortex flow), would result in a better energy mix and generate a stronger control effect. With the goal of improving actuator capability for applications in higher Re flows than previously possible, novel designs that use various electrode shapes (sawtooth and sine wave) were investigated in this study. These designs adjusted the plasma induced flow in the form of a ZNMF jet with streamwise and spanwise vortices, as shown in Fig. 1. These new designs render the plasma-induced flow in the form of streamwise and spanwise vortices upon actuation. These new DBD plasma actuators have the enhanced ability to induce a streamwise vortex and, at the same time, possess the advantages of the original linear actuator.
See the article: LIU Zhifeng, WANG Lianze, FU Song. Study of Flow Induced by Sine Wave and Saw Tooth Plasma Actuators. SCIENCE CHINA Physics, Mechanics & Astronomy.2011, Vol 54(11): 2033-2039.
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