Alonso M., Avital E.

The basic sound radiation generated by large-scale structures is calculated for circular and elliptical single jets. A hybrid method is used in which the hydrodynamics is computed using an incompressible Large-Eddy Simulation (LES) procedure and the generated sound field is calculated using Powell’s vortex sound formulation of Lighthill’s analogy. For the implementation of the acoustic analogy boundary corrections are required to account for the finite axial length of the computational domain. The latter can cause spurious sound calculation due to vortices leaving the domain and artificial dependency of the vortex sound formulation on the co-ordinates’ origin. New boundary corrections are developed to encounter these issues and are tested in the jets simulations. The simulations are performed for circular and elliptical jets with the same effective diameter and with a Reynolds number (Re) based on the diameter of the jet that is 6000. The inflow velocity profile provides a four-frequency excitation which is randomized in its phase and has a 2% peak amplitude. When compared with the circular jet the elliptical jet shows a shorter potential core and stronger mixing as expected. The circular jet shows a higher acoustic output for the lower range of the frequency spectrum which corresponds to the weaker mixing in the circular jet. The acoustic results based on the vortex sound formulation show a similar behaviour as those based on Lighthill’s original velocity source formulation. However the vortex sound formulation shows a higher sensitivity to the finite length of the computational domain. This sensitivity is minimised by the newly derived boundary conditions leading from fair to good quantitative agreement between the two acoustic formulations.