  Supersonic jet noise reduction is important for high speed military aircraft.
Lower acoustic levels would reduce structural fatigue leading to longer
lifetime of the jet aircraft. It is not solely structural aspects which are of
importance, health issues of the pilot and the airfield per- sonnel are also
very important, as high acoustic levels may result in severe hearing damage. It
remains a major challenge to reduce the overall noise levels of the aircraft,
where the supersonic exhaust is the main noise source for near ground
operation. Fluidic injection into the supersonic jet at the nozzle exhaust has
been shown as a promising method for noise reduction. It has been shown to
speed up the mix- ing process of the main jet, hence reducing the kinetic
energy level of the jet and the power of the total acoustic radiation.
Furthermore, the interaction mechanism between the fluidic injection and the
shock structure in the jet exhaust plays a crucial role in the total noise
radia- tion. In this study, LES is used to investigate the change in flow
struc- tures of a supersonic (M=1.56) jet from a converging-diverging nozzle.
Six fluidic actuators, evenly distributed around the nozzle exit, inject air in
a radial direction towards the main flow axis with a total mass flow ratio of
3%. Steady injection is compared with flapping injection. With flapping
injection turned on, the injection angle of each injector is varied
sinusoidally in the nozzle exit plane and the variation is the same for all
injectors. This fluid dynamics video is submitted to the APS DFD Gallery of
Fluid Motion 2013 at the 66 the Annual Meeting of the American Physical
Society, Division of Fluid Dynamics (24-26 November, Pittsburgh, PA, USA).
