Turbulent drag reduction using pulsed-DC plasma actuation

Abstract

In this paper, surface-mounted arrays of pulsed-DC dielectric barrier discharge plasma actuators are used to achieve skin friction drag reduction in turbulent boundary layers. The arrays are designed to produce a near-wall plasma-induced spanwise flow for the purpose of preventing the lift-up of low-speed streaks. Two array designs are demonstrated. One produces a unidirectional spanwise flow and the other a series of opposed wall jets. Both configurations are shown to produce unprecedented levels of drag reduction in excess of 70%. The amount of drag reduction achieved is shown to scale on the number of viscous wall units between adjacent surface electrodes which effectively sets the number of low-speed streaks under simultaneous control. In particular, the level of drag reduction increases logarithmically as the number of streaks under simultaneous control decreases. Furthermore, due to the low-power input required by the pulsed-DC actuator, this level of drag reduction is achieved with net power savings.

Characteristics of drag-reduced turbulent boundary layers with pulsed-direct-current plasma actuation

“…As a result, it reduces ∂u/∂z, and hence the wall-normal vorticity, ω y , which is the critical parameter governing the streak transient growth mechanism (Schoppa & Hussain 2002). Our previous results (Duong 2019;Duong et al 2019;Thomas et al 2019) have indicated the importance of confining the spanwise blowing to the sub-layer and buffer layer in achieving drag reduction.…”

“…This variation is the leading term in ω y , which as mentioned, is the critical parameter in the streak transient growth mechanism of Schoppa & Hussain (2002). Figure 2(b,c) shows two concepts that were investigated by Duong et al (2019) and Thomas et al (2019). Both are based on flush, surface mounted plasma actuators.…”

“…Recent active flow control experiments by Thomas et al (2019), Duong (2019) and Duong et al (2019) have demonstrated unprecedented levels of viscous drag reduction of up to 78 % in turbulent boundary layers that produce a significant net power savings. The approach has focused on actively intervening with an autonomous cycle involving lift-up and break-up of coherent streamwise vorticity that is associated with the low-speed wall streak structure, that can account for as much as 70 % of the [uv]-Reynolds stress (Corino & Brodkey 1969), and whose frequency of occurrence scales linearly with the wall skin friction, τ w (Kline et al 1967).…”

“…Our Thomas et al (2019), Duong (2019) and Duong et al (2019), embodiment of this approach consists of an array of dielectric barrier discharge (DBD) plasma actuators that are designed to produce a spanwise velocity component in the wall layer of a turbulent boundary layer. The DBD plasma actuators consist of a pair of electrodes that are separated by a dielectric insulator.…”

“…A significant result from the previous experiments (Duong 2019;Duong et al 2019;Thomas et al 2019) was the determination of boundary-layer-based scaling relations for the actuator design. For example, the degree of drag reduction strongly depends on the spanwise actuator spacing, which dictates the number of wall streak structures (nominally z + 100 apart) that are being simultaneously controlled.…”