On the unsteady dynamics of partially shrouded compressible jets

TL;DR

This study experimentally investigates the complex unsteady behavior and noise generation mechanisms of partially shrouded sonic jets, revealing their three-dimensional expansion, directional noise, and self-sustained oscillations caused by shock-induced flow separation.

Contribution

It provides new insights into the coupled unsteadiness and noise mechanisms of partially shrouded jets, highlighting the role of shock interactions and flow separation in acoustic emissions.

Findings

Jet unsteadiness is strongly coupled with emitted noise.

Noise is directional and linked to shock-induced separation.

Self-sustained oscillations arise from non-uniform shear layer forcing.

Abstract

We experimentally investigate a partially shrouded sonic jet (a sonic free-jet shielded by a solid wall-extension on one side) exiting from a planar nozzle at two different nozzle pressure ratio ($\zeta=4$ and $5$). We experimentally show that the inherent jet unsteadiness from the shock-induced flow separation on the wall and the emitted noise in the far-field is strongly coupled through a series of experiments like high-speed schlieren, wall-static pressure, unsteady pressure spectra, and microphone measurements. The partially shrouded jet's lateral free expansion is also identified to be complicated, three-dimensional, and the produced noise is directional. The emitted acoustic pulses from the flapping-jet, the radiated noise from the shock-induced separation on the wall, and the shock-shear layer interaction on the other side of the wall are responsible for the generated acoustic disturbances. The non-uniform aeroacoustic forcing on the top and bottom portion of the partially wall-bounded jet shear layer leads to a self-sustained jet oscillation and a discrete sound emission. The vital features are identified through the proper orthogonal decomposition of high-speed schlieren images and supplemented by other measurements.