On the origin of impinging tones at low supersonic flow

TL;DR

This paper investigates the origin of loud impinging tones in low supersonic jets, identifying vortex-shock interactions as the source mechanism distinct from screech feedback loops.

Contribution

Using direct numerical simulations, the study reveals two vortex-shock interaction types responsible for impinging tones, challenging previous screech-based explanations.

Findings

Identified vortex-shock interactions as tone sources

Distinguished from traditional screech mechanisms

Validated findings with DNS at specific flow conditions

Abstract

Impinging compressible jets may cause deafness and material fatigue due to immensely loud tonal noise. It is generally accepted that a feedback mechanism similar to the screech feedback loop is responsible for impinging tones. The close of the loop remained unclear. One hypothesis hold up in the literature explains the emanated sound with the direct interaction of vortices and the wall. Other explanations name the standoff shock oscillations as the origin of the tones. Using direct numerical simulations (DNS) we were able to identify the source mechanism for under-expanded impinging jets with a nozzle pressure ratio (NPR) of 2.15 and a plate distance of 5 diameters. We found two different types of interactions between vortices and shocks to be responsible for the generation of the impinging tones. They are not related to screech.