Calculation of basic sound radiation of axisymmetric jets by direct numerical simulations

The basic radiation of sound from forced laminar axisymmetric jets (Re-D = 6 x 10(2) to 1.5 x 10(3)) is calculated by a hybrid approach, whereby the h ydrodynamic field is calculated by an incompressible stream function-vortic ity direct numerical simulation (DNS) and the sound field is calculated by the compact Lighthill and Mohring-Kambe formulations. It is shown that, bec ause of the oscillatory behavior of the source as a wave packet, the acoust ic results are more sensitive to numerical constraints such as the DNS box size than are the hydrodynamic results. Lighthill's formulation shows a str ong sensitivity to the radial boundary condition imposed on the velocity in the DNS, A zero second-order radial derivative for the stream function beh aves the best, and a zero radial velocity condition behaves the worst. The Mohring-Kambe formulation shows a strong dependence on the arbitrary locati on of the coordinate origin. A boundary correction developed to eliminate t his dependence is shown to work well by achieving a good agreement between the two formulations in the dominant features of the sound radiation. The e ffects of the inflow momentum thickness and Reynolds number are investigate d with reference to the directivity and frequency spectrum of the emitted s ound.