Acoustic sources in a turbulent jet: A direct numerical simulation study

A direct numerical simulation of a Mach 0.9, Reynolds number 3600 jet is presented and analyzed. The simulation results are shown to be in excellent agreement with corresponding experiments conducted by Stromberg et al.¹ To understand noise mechanisms, Lighthill's theory is used to define a nominal acoustic source which is computed using Fourier methods to provide its streamwise wavenumber and frequency makeup. It is shown that the radiating portion of the source, defined by the sum of its components with supersonic phase velocities, has a significantly different character than the full Lighthill source. Similarly, the turbulent kinetic energy is shown to be poorly correlated with the radiating source. Streamwise phase velocities of the source are found to be distributed fairly evenly between 0.1a_∞and 0.5a_∞, where a_∞ is the ambient speed of sound. The “structure” of the radiating source is examined by filtering its non-radiating components. This reveals a simple modulated harmonic wave form as might be predicted by linear instability analysis. A simple model is used to demonstrate that the noise is extremely sensitive to minor disruptions of this form.