Effect of fluidic injections near the end of potential core for a Mach 0.5 subsonic jet

Although jets are known to be most receptive at the nozzle

exit, the most energetic sound sources are located a few jet

diameters downstream: near the end of the jet potential core.

Hence, fluctuations introduced at the nozzle exit of a turbulent jet

do not necessarily reach its intended target.  In this work instead,

we introduce such disturbances via a fluidic injection system at

several locations downstream of the nozzle exit, including those

near the end of potenital core, to study their effect on the

radiated sound.  In our experiments, the fluidic injection is

achieved via a newly-designed array of six equispaced minijets

acting on a core Mach 0.5 jet.  A second injection system, located at

a fixed distance just upstream of the nozzle exit, with six

equispaced injectors is also used.  Both the injection systems are

operated in our experiments steadily for different mass flow ratios.  The latter

upstream injection system serves as the reference case and our goal

is to investigate how the downstream injections alter the radiated

sound over systems that perform fluidic injections near

the nozzle exit.  For our analyzed downstream injection cases,

fluidic injection alter the centreline velocity decay and its

turbulent fluctuations via reducing the corresponding Reynolds

stresses of the core jet.  This yields in a reduced peak sound at all the measured directivity

angles, but especially at the downstream shallow angles.  In

contrast, when the jet is injected via the upstream system, the

radiated sound at all angles only show minor changes.