Interactions in micro-jets forced by dielectric barrier discharge
Creators: name:  T. FRIDLENDER 
name:   E. MOREAU 
name:   N. BENARD 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLOW CONTROL
AEROSPACE

description: Turbulent mixing downstream of an arrangement of interacting micro-jets is of major interest when it comes to flow control applied to aeronautics and combustion. A typical example is proper control over mixing that can lead to decreasing unburnt fuel quantities thereby limiting pollutant emissions. Only a few studies have investigated the interaction of micro-jets most often arranged linearly [1]. Here, mutual interactions between a central jet and 6 surrounding jets all with an initial diameter of 2 mm are experimentally investigated. Each individual jet is equipped with plasma actuator technology based on dielectric barrier discharge. In the present investigation, each and every micro-jet is under the influence of the plasma discharge and its associated momentum transfers. In this study, plasma actuation is intended to impart momentum fluctuations at controlled frequencies when the voltage is turned on [2]. Experiments are conducted in a circular open blowing wind-tunnel ended by a jet exit of 50 mm inner diameter (ReD=2600). A perforated ceramic plate is maintained at the exhaust of the wind-tunnel. Seven circular holes are considered. A conductive layer of Nickel with minimal protrusion of 10 m serves as an active electrode where high-voltage is applied. A counter electrode connected to the ground is embedded in the ceramic material. The electrode placement leads to a local periodic jet flow resulting from the electromechanical conversion at the edges of the holes. A low speed, high-resolution PIV system (4.4 Hz, CCD 16Mpx) is used to measure the time-averaged flow field in the wake of the perforated plate. The analysis will be based on mean and standard deviation velocity fields as well as on typical turbulent quantities such as TKE and Reynolds stresses components. A single plane extending from the initial region to the far-field wake is recorded. Here, as it has been observed in another configuration with multiple jets [3], the micro-jets quickly merge into a single main jet. In a first series of experiments, the electrical signal supplied to the plasma actuator is intended to interact with the instability of the jet preferred mode. Burst modulation at frequencies corresponding to Strouhal numbers of 0.05 to 0.6 based on jet diameter is applied. Two cases are considered. One where the Strouhal number is defined based on the characteristics of one micro-jet (i.e., diameter of 2 mm and velocity of 15 m/s) while in a second set of experiments the Strouhal number is based on the main jet scales (i.e., diameter of 7 mm for a centerline jet velocity of 12 m/s). The experimental campaign ended recently and the complete results are not yet available, but preliminary computations as well as raw images clearly show that excitation at the preferred main jets column mode significantly enhances turbulent mixing and conduces to a large reduction of the main potential core length. Furthermore, preliminary data also seem to indicate that aiming at the micro-jets preferred mode leads to more efficient mixing enhancement.