Coherent Structures in a Supersonic Jet Excited by Plasma Actuators

Simulations are used in conjunction with experimental measurements to understand the coherent structures generated by excitation of a Mach 1.3 jet by eight localized arc filament plasma actuators uniformly distributed just upstream of the nozzle exit.~ Several modes are excited, including the axisymmetric (m=0), helical (m=1-3), and mixed modes (m=$\pm $1, $\pm $2) modes.~ The Strouhal number for all cases is fixed at 0.3, which corresponds to the most amplified frequency.~The simulations reproduce the distinct coherent structures measured in the experiment for each azimuthal mode.~ Detailed analysis of instantaneous, time- and phase-averaged quantities highlights a complex coherent structure generation, evolution and dissipation process. A key feature observed is the initiation of hairpin-like structures with tips/heads in the outer region of the jet shear layer and legs extending forward and slightly inclined in the direction of the jet axis, where the velocity is higher.~The subsequent interactions of these structures yield different composite structures in the downstream region. For example, for m=0, adjacent hairpin structures merge to yield axisymmetric rings, with the legs connecting successive structures in the form of ribs in the braid region; and with m=1 and 2 mode excitation, distinct helical and double-helical structures are observed, respectively, with the hairpins forming substructures in the coils.