Experiments on the instability waves in a supersonic jet and their acoustic radiation

TLDR

The low‑Reynolds‑number axisymmetric supersonic jets studied here had Reynolds numbers between 8 000 and 107 000, far below the 1.3 × 10⁶ typical of atmospheric‑pressure jets. An experimental investigation using hot‑wire probes and microphones on jets at Mach ≈ 2 measured instability and acoustic radiation in the low‑Reynolds‑number regime. Hot‑wire data show discrete Strouhal‑number 0.18 modes that grow exponentially and propagate supersonically, matching Tam’s theoretical predictions, while microphone recordings confirm Mach‑wave acoustic radiation and reveal sound pressure levels comparable to high‑Reynolds‑number jets, supporting the conclusion that large‑scale instability dominates jet noise.

Abstract

An experimental investigation of the instability and the acoustic radiation of the low Reynolds number axisymmetric supersonic jet has been performed. Hot-wire measurements in the flow field and microphone measurements in the acoustic field were obtained from different size jets at Mach numbers of about 2. The Reynolds number ranged from 8000 to 107000, which contrasts with a Reynolds number of 1·3 × 10 6 for similar jets exhausting into atmospheric pressure. Hot-wire measurements indicate that the instability process in the perfectly expanded jet consists of numerous discrete frequency modes around a Strouhal number of 0·18. The waves grow almost exponentially and propagate downstream at a supersonic velocity with respect to the surrounding air. Measurements of the wavelength and wave speed of the St = 0·18 oscillation agree closely with Tam's theoretical predictions. Microphone measurements have shown that the wavelength, wave orientation and frequency of the acoustic radiation generated by the dominant instability agree with the Mach wave concept. The sound pressure levels measured in the low Reynolds number jet extrapolate to values approaching the noise levels measured by other experimenters in high Reynolds number jets. These measurements provide more evidence that the dominant noise generation mechanism in high Reynolds number jets is the large-scale instability.