Conditioning of cross-flow instability modes using dielectric barrier discharge plasma actuators

Abstract

In the current study, selective forcing of cross-flow instability modes evolving on a 45• swept wing at Re = 2.17 · 10 6 is achieved by means of spanwise-modulated plasma actuators, positioned near the leading edge. In the perspective of laminar flow control, the followed methodology holds on the discrete roughness elements/upstream flow deformation (DRE/UFD) approach, thoroughly investigated by e.g. Saric et al. (1998);Malik et al. (1999) and Wassermann & Kloker (2002). The possibility of using active devices for UFD provides several advantages over passive means, allowing for a wider range of operating Re numbers and pressure distributions. In the present work, customised alternating current dielectric barrier discharge plasma actuators have been designed, manufactured and characterised. The authority of the actuators in forcing monochromatic stationary cross-flow modes at different spanwise wavelengths is assessed by means of infrared thermography. Moreover, quantitative spatio-temporal measurements of the boundary layer velocity field are performed using time-resolved particle image velocimetry. The results reveal distinct steady and unsteady forcing contributions of the plasma actuator on the boundary layer. It is shown that the actuators introduce unsteady fluctuations in the boundary layer, amplifying at frequencies significantly lower than the actuation frequency. In line with the DRE/UFD strategy, forcing a sub-critical stationary mode, with a shorter wavelength compared to the naturally selected mode, results in less amplified primary vortices and related fluctuations, compared to the critical forcing case. The effect of the forcing on the flow stability is further inspected by combining the measured actuators body-force with the numerical solution of the laminar boundary layer and linear stability theory. The simplified methodology yields fast and computationally cheap estimates on the effect of steady forcing (magnitude and direction) on the boundary layer stability.

Experimental control of swept-wing transition through base-flow modification by plasma actuators

“…The swept-wing model features an in-house designed airfoil shape (66018M3J ), streamwise chord of 1.27m and a 45 • sweep. Details of the employed swept-wing model and the wind tunnel are given in Serpieri & Kotsonis (2016) and Serpieri et al (2017). The flow over the wing's pressure side was investigated.…”

“…Different strategies have been proposed and investigated to control transition induced by stationary CFI modes. A brief review of these can be found in Serpieri et al (2017). A strategy that has received considerable attention is the use of discrete roughness elements (DRE), first proposed by Saric and co-workers (see Saric et al (2003)) and later generalised as upstream flow deformation (UFD) by Wassermann & Kloker (2002).…”

“…While the application of AC-DBD actuators on two-dimensional boundary layers has been extensively studied, utilisation of these devices to control CFI is rather limited (e.g. Schuele et al (2013); Dörr & Kloker (2015b); Shahriari (2016); Wang et al (2017); Serpieri et al (2017)). Prior to the experimental investigation, a preliminary theoretical/numerical study is performed.…”

“…Prior to the experimental investigation, a preliminary theoretical/numerical study is performed. The stability of the boundary layer, under the influence of the plasma body force, is numerically estimated using the simple model proposed by Serpieri et al (2017). Following the predictions of the model, experimental demonstration of the plasmabased base-flow modification is achieved by installing an AC-DBD plasma actuator that produces a two-dimensional body force, close to the leading edge of a swept-wing model.…”

“…A spanwise uniform body force is introduced using Dielectric Barrier Discharge plasma actuators, to either force against or along the local cross-flow component of the boundary layer. The effect of forcing on the stability of the boundary layer is analysed using a simplified model proposed by Serpieri et al (2017). A minimal thickness plasma actuator is fabricated using spray-on techniques and positioned near the leading edge of the swept-wing, while infrared thermography is used to detect and quantify transition location.…”