Application of a phased array technique to DNS-generated turbulent subsonic jet data: source identification and comparison with an analytic model

摘要

Direct Numerical Simulation (DNS) solutions have previously been obtained for a fully developed turbulent subsonic flow exiting a long pipe into a laminar co-flow. Including the pipe/nozzle in the simulations with a fully turbulent flow upstream should ensure that all possible noise generation mechanisms are represented, although the geometry is unrealistic. A phased array technique called AFINDS has been applied to the sound radiation from the nozzle/jet configuration in order to separate out the distributed jet mixing noise source(s) from nozzle-based sources. The unsteady pressure inside the pipe and at 20 diameters in the far-field is first decomposed into its azimuthal modes and AFINDS applied to the first five modes (m=0, 1, 2, 3, & 4). AFINDS is unable to detect any jet mixing noise downstream of the nozzle for m=0 and it is confirmed that the weak velocity exponent (～4) of the radiated noise reported previously is due to the dominating influence of internal noise generated within the pipe. However, for m ＞ 0 the technique can separate jet noise sources downstream of the nozzle from nozzle-based noise sources and the jet noise spectral shapes and velocity exponents are more in line with that expected for a jet with a simulated flight stream. The nozzle-based sources can be attributed to internal noise and/or unsteady flow past the nozzle lip but the former cannot radiate below the cut-on Strouhal number of the first radial mode. Here a lip-noise mechanism akin to trailing edge noise is identified and a model is developed based on the DNS unsteady pressure differences across the pipe wall at and near the nozzle. The model already shows reasonable agreement with the far-field DNS data at frequencies where the internal noise is cut-off.