MODIFICATIONS OF THE SHEAR LAYER DOWNSTREAM A BACKWARD FACING STEP BY DIELECTRIC BARRIER DISCHARGE PLASMA ACTUATOR

Abstract

The present article deals with a free shear layer induced by the separation of a turbulent boundary layer due to wall divergence. The investigated flow configuration is produced by a 30-mm-height backwardfacing-step mounted in a closed-loop wind tunnel. The experimental measurements are performed at 15 m/s , corresponding to a Reynolds number (based on this velocity and the step height) around 3x10. The modifications of the shear layer are achieved with a surface plasma actuator based on a single Dielectric Barrier Discharge (DBD). This actuator produces an electrohydrodynamic force, resulting in a flow called electric wind just upstream the flow separation. The plasma discharge is able to manipulate the first stages of the formation of the free shear layer and consequently to modify the flow dynamics, highlighting the control authority of plasma discharge. Time-averaged and time-resolved measurements techniques are used to investigate the influence of plasma device in two ways. The first one considers the modification of mean reattachment length whereas the second one studies the effect over large-scale structures. INTRODUCTION Free and bounded turbulent shear flows are intensively studied. The turbulent energy balance splits these flows in two generic configurations: wall-bounded and free shear layers. These two types of flow are usually found in the nature and also in a variety of engineering applications. Knowledge of the flow and the ability to control it are fundamental topics in turbulence. This paper is devoted to the characterization of a free shear layer produced by turbulent boundary layer separation and manipulated by a plasma actuator. The massively separated flow is yield by a sudden wall expansion. The sharp step corner is the fixed location where separation occurs and the location where KelvinHelmholtz instability mechanism begins. The whole process is dominated by the structures arising from this instability (Ho & Huerre, 1984). The effects presented here influence not only the mean flow but also these organized large-scale flow structures. In some extend, the particular ‘step’ flow can be compared with the canonical plane mixing layer case. Their characteristics are very similar in the initial region of the free shear layer but further downstream, the growth and evolution of the free shear layer are affected by the presence of the wall downstream in the case of the BFS. A recirculating region forms and feeds continuously the shear layer producing an increase on the overall turbulence (Adams & Johnston, 1988). Another important feature of a backward facing step flow is the unsteady and highly three-dimensional location of the reattachment point. The dynamic of the separated flow is directly linked with this unsteadiness (Driver et al., 1987). The objective of this study is to investigate the ability of plasma actuators as an effective device for modifying (manipulating) the shear layer, with the final objective of being able to control/reduce the parasitic effects such as unsteadiness, noise, etc. A surface non-thermal plasma discharge is used as flow control device which is now recognized for being effective in different aerodynamic configurations (Benard & Moreau, 2012). This device adds a volume force resulting in a secondary flow, usually called electric wind. Its amplitude and frequency are directly linked to the electrical input signal, this being of primary interest for studying the influence of localized periodic or non-periodic flow perturbations. After the details of the experimental configuration used and the inlet parameters, two main analyses are presented: a parametric study to determine the most effective actuation and a dynamic study of the natural step flow organization. The last part corresponds to a detailed analysis of the particular effective actuation.