Predicting supersonic jet screech

Supersonic jets are crucial for air- and spacecraft, but they also play a role in other relevant scenarios, such as engine fuel injection, car airbag systems, and even volcanic eruptions. Their starting processes are especially crucial for the latter three applications and phenomena.

As underexpanded jets exhaust from a convergent circular nozzle, they usually emit a powerful acoustic tone. This tone is called screech and is an important component of overall supersonic jet noise. It contains information about physical properties of jets, such as flow stability, aeroacoustics, and aerodynamics.

While steady underexpanded free jets have been extensively studied with experiments and simulations, there is less literature on the starting processes of underexpanded jets. Most studies focus on the evolution of the flow structures without accounting for the evolution related to jet screech.

Sheng et al. used large-eddy simulations to model and analyze the starting process of free jets at different nozzle pressure ratios. They extracted key information about the acoustic feedback loop from the numerical results to better understand the evolution of the screech frequency and mode.

“Based on the upstream-propagating neutral acoustic modes that are energized by the interaction between the shock cells and the Kelvin-Helmholtz wavepacket, the screech frequencies of the starting jet at different stages can be predicted,” said author Xiangru Li.

The study includes the first prediction of screech frequency during the starting processes of jets. The team also provided the reason for the change of interaction mechanism during the starting process.

“We next intend to compare the similarities and differences between the starting processes of a free jet and impinging jet,” said Li.

Source: “Screech in transient supersonic jets,” by Juan Sheng, Xiangru Li, Yitao Wang, Pengfei Hao, Xiwen Zhang, and Feng He, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0102992 .